@shorttitlepage The GNU C Library Reference Manual The GNU C Library
Reference Manual
Sandra Loosemore with Roland McGrath, Andrew Oram, and Richard M. Stallman
last updated 9 April 1993
for version 1.06 Beta Copyright (C) 1993 Free Software Foundation, Inc.
The C language provides no built-in facilities for performing such common operations as input/output, memory management, string manipulation, and the like. Instead, these facilities are defined in a standard library, which you compile and link with your programs.
The GNU C library, described in this document, defines all of the library functions that are specified by the ANSI C standard, as well as additional features specific to POSIX and other derivatives of the Unix operating system, and extensions specific to the GNU system.
The purpose of this manual is to tell you how to use the facilities of the GNU library. We have mentioned which features belong to which standards to help you identify things that are potentially nonportable to other systems. But the emphasis on this manual is not on strict portability.
This manual is written with the assumption that you are at least somewhat familiar with the C programming language and basic programming concepts. Specifically, familiarity with ANSI standard C (see section ANSI C), rather than "traditional" pre-ANSI C dialects, is assumed.
The GNU C library includes several header files, each of which provides definitions and declarations for a group of related facilities; this information is used by the C compiler when processing your program. For example, the header file `stdio.h' declares facilities for performing input and output, and the header file `string.h' declares string processing utilities. The organization of this manual generally follows the same division as the header files.
If you are reading this manual for the first time, you should read all of the introductory material and skim the remaining chapters. There are a lot of functions in the GNU C library and it's not realistic to expect that you will be able to remember exactly how to use each and every one of them. It's more important to become generally familiar with the kinds of facilities that the library provides, so that when you are writing your programs you can recognize when to make use of library functions, and where in this manual you can find more specific information about them.
This section discusses the various standards and other sources that the GNU C library is based upon. These sources include the ANSI C and POSIX standards, and the System V and Berkeley Unix implementations.
The primary focus of this manual is to tell you how to make effective use of the GNU library facilities. But if you are concerned about making your programs compatible with these standards, or portable to operating systems other than GNU, this can affect how you use the library. This section gives you an overview of these standards, so that you will know what they are when they are mentioned in other parts of the manual.
See section Summary of Library Facilities, for an alphabetical list of the functions and other symbols provided by the library. This list also states which standards each function or symbol comes from.
The GNU C library is compatible with the C standard adopted by the American National Standards Institute (ANSI): American National Standard X3.159-1989---"ANSI C". The header files and library facilities that make up the GNU library are a superset of those specified by the ANSI C standard.
If you are concerned about strict adherence to the ANSI C standard, you should use the `-ansi' option when you compile your programs with the GNU C compiler. This tells the compiler to define only ANSI standard features from the library header files, unless you explicitly ask for additional features. See section Feature Test Macros, for information on how to do this.
Being able to restrict the library to include only ANSI C features is important because ANSI C puts limitations on what names can be defined by the library implementation, and the GNU extensions don't fit these limitations. See section Reserved Names, for more information about these restrictions.
This manual does not attempt to give you complete details on the differences between ANSI C and older dialects. It gives advice on how to write programs to work portably under multiple C dialects, but does not aim for completeness.
The GNU library is also compatible with the IEEE POSIX family of standards, known more formally as the Portable Operating System Interface for Computer Environments. POSIX is derived mostly from various versions of the Unix operating system.
The library facilities specified by the POSIX standard are a superset of those required by ANSI C; POSIX specifies additional features for ANSI C functions, as well as specifying new additional functions. In general, the additional requirements and functionality defined by the POSIX standard are aimed at providing lower-level support for a particular kind of operating system environment, rather than general programming language support which can run in many diverse operating system environments.
The GNU C library implements all of the functions specified in IEEE Std 1003.1-1988, the POSIX System Application Program Interface, commonly referred to as POSIX.1. The primary extensions to the ANSI C facilities specified by this standard include file system interface primitives (see section File System Interface), device-specific terminal control functions (see section Low-Level Terminal Interface), and process control functions (see section Child Processes).
Some facilities from draft 11 of IEEE Std 1003.2, the POSIX Shell and Utilities standard (POSIX.2) are also implemented in the GNU library. These include utilities for dealing with regular expressions and other pattern matching facilities (see section Pattern Matching).
The GNU C library defines facilities from some other versions of Unix, specifically from the 4.2 BSD and 4.3 BSD Unix systems (also known as Berkeley Unix) and from SunOS (a popular 4.2 BSD derivative that includes some Unix System V functionality).
The BSD facilities include symbolic links (see section Symbolic Links), the
select function (see section Waiting for Input or Output), the BSD signal
functions (see section BSD Signal Handling), and sockets (see section Sockets).
The System V Interface Description (SVID) is a document describing the AT&T Unix System V operating system. It is to some extent a superset of the POSIX standard (see section POSIX (The Portable Operating System Interface)).
The GNU C library defines some of the facilities required by the SVID that are not also required by the ANSI or POSIX standards, for compatibility with System V Unix and other Unix systems (such as SunOS) which include these facilities. However, many of the more obscure and less generally useful facilities required by the SVID are not included. (In fact, Unix System V itself does not provide them all.)
Incomplete: Are there any particular System V facilities that ought to be mentioned specifically here?
This section describes some of the practical issues involved in using the GNU C library.
Libraries for use by C programs really consist of two parts: header files that define types and macros and declare variables and functions; and the actual library or archive that contains the definitions of the variables and functions.
(Recall that in C, a declaration merely provides information that a function or variable exists and gives its type. For a function declaration, information about the types of its arguments might be provided as well. The purpose of declarations is to allow the compiler to correctly process references to the declared variables and functions. A definition, on the other hand, actually allocates storage for a variable or says what a function does.)
In order to use the facilities in the GNU C library, you should be sure that your program source files include the appropriate header files. This is so that the compiler has declarations of these facilities available and can correctly process references to them. Once your program has been compiled, the linker resolves these references to the actual definitions provided in the archive file.
Header files are included into a program source file by the `#include' preprocessor directive. The C language supports two forms of this directive; the first,
#include "header"
is typically used to include a header file header that you write yourself; this would contain definitions and declarations describing the interfaces between the different parts of your particular application. By contrast,
#include <file.h>
is typically used to include a header file `file.h' that contains definitions and declarations for a standard library. This file would normally be installed in a standard place by your system administrator. You should use this second form for the C library header files.
Typically, `#include' directives are placed at the top of the C source file, before any other code. If you begin your source files with some comments explaining what the code in the file does (a good idea), put the `#include' directives immediately afterwards, following the feature test macro definition (see section Feature Test Macros).
For more information about the use of header files and `#include' directives, see section 'Header Files' in The GNU C Preprocessor Manual.
The GNU C library provides several header files, each of which contains the type and macro definitions and variable and function declarations for a group of related facilities. This means that your programs may need to include several header files, depending on exactly which facilities you are using.
Some library header files include other library header files automatically. However, as a matter of programming style, you should not rely on this; it is better to explicitly include all the header files required for the library facilities you are using. The GNU C library header files have been written in such a way that it doesn't matter if a header file is accidentally included more than once; including a header file a second time has no effect. Likewise, if your program needs to include multiple header files, the order in which they are included doesn't matter.
Compatibility Note: Inclusion of standard header files in any order and any number of times works in any ANSI C implementation. However, this has traditionally not been the case in many older C implementations.
Strictly speaking, you don't have to include a header file to use a function it declares; you could declare the function explicitly yourself, according to the specifications in this manual. But it is usually better to include the header file because it may define types and macros that are not otherwise available and because it may define more efficient macro replacements for some functions. It is also a sure way to have the correct declaration.
If we describe something as a function in this manual, it may have a macro definition as well. This normally has no effect on how your program runs--the macro definition does the same thing as the function would. In particular, macro equivalents for library functions evaluate arguments exactly once, in the same way that a function call would. The main reason for these macro definitions is that sometimes they can produce an inline expansion that is considerably faster than an actual function call.
Taking the address of a library function works even if it is also defined as a macro. This is because, in this context, the name of the function isn't followed by the left parenthesis that is syntactically necessary to recognize the a macro call.
You might occasionally want to avoid using the a macro definition of a function--perhaps to make your program easier to debug. There are two ways you can do this:
For example, suppose the header file `stdlib.h' declares a function
named abs with
extern int abs (int);
and also provides a macro definition for abs. Then, in:
#include <stdlib.h>
int f (int *i) { return (abs (++*i)); }
the reference to abs might refer to either a macro or a function.
On the other hand, in each of the following examples the reference is
to a function and not a macro.
#include <stdlib.h>
int g (int *i) { return ((abs)(++*i)); }
#undef abs
int h (int *i) { return (abs (++*i)); }
Since macro definitions that double for a function behave in exactly the same way as the actual function version, there is usually no need for any of these methods. In fact, removing macro definitions usually just makes your program slower.
The names of all library types, macros, variables and functions that come from the ANSI C standard are reserved unconditionally; your program may not redefine these names. All other library names are reserved if your programs explicitly includes the header file that defines or declares them. There are several reasons for these restrictions:
exit to do something completely different from
what the standard exit function does, for example. Preventing
this situation helps to make your programs easier to understand and
contributes to modularity and maintainability.
In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore (`_') and all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter are reserved names. This is so that the library and header files can define functions, variables, and macros for internal purposes without risk of conflict with names in user programs.
Some additional classes of identifier names are reserved for future extensions to the C language. While using these names for your own purposes right now might not cause a problem, they do raise the possibility of conflict with future versions of the C standard, so you should avoid these names.
float or long double arguments,
respectively.
In addition, some individual header files reserve names beyond those that they actually define. You only need to worry about these restrictions if your program includes that particular header file.
The exact set of features available when you compile a source file is controlled by which feature test macros you define.
If you compile your programs using `gcc -ansi', you get only the ANSI C library features, unless you explicitly request additional features by defining one or more of the feature macros. See section 'Options' in The GNU CC Manual, for more information about GCC options.
You should define these macros by using `#define' preprocessor directives at the top of your source code files. You could also use the `-D' option to GCC, but it's better if you make the source files indicate their own meaning in a self-contained way.
If you define this macro, then the functionality from the POSIX.1 standard (IEEE Standard 1003.1) is available, as well as all of the ANSI C facilities.
If you define this macro with a value of 1, then the
functionality from the POSIX.1 standard (IEEE Standard 1003.1) is made
available. If you define this macro with a value of 2, then both
the functionality from the POSIX.1 standard and the functionality from
the POSIX.2 standard (IEEE Standard 1003.2) are made available. This is
in addition to the ANSI C facilities.
If you define this macro, functionality derived from 4.3 BSD Unix is included as well as the ANSI C, POSIX.1, and POSIX.2 material.
Some of the features derived from 4.3 BSD Unix conflict with the corresponding features specified by the POSIX.1 standard. If this macro is defined, the 4.3 BSD definitions take precedence over the POSIX definitions.
If you define this macro, functionality derived from SVID is included as well as the ANSI C, POSIX.1, and POSIX.2 material.
If you define this macro, everything is included: ANSI C, POSIX.1, POSIX.2, BSD, SVID, and GNU extensions. In the cases where POSIX.1 conflicts with BSD, the POSIX definitions take precedence.
If you want to get the full effect of _GNU_SOURCE but make the
BSD definitions take precedence over the POSIX definitions, use this
sequence of definitions:
#define _GNU_SOURCE #define _BSD_SOURCE #define _SVID_SOURCE
We recommend you use _GNU_SOURCE in new programs.
If you don't specify the `-ansi' option to GCC and don't define
any of these macros explicitly, the effect as the same as defining
_GNU_SOURCE.
When you define a feature test macro to request a larger class of
features, it is harmless to define in addition a feature test macro for
a subset of those features. For example, if you define
_POSIX_C_SOURCE, then defining _POSIX_SOURCE as well has
no effect. Likewise, if you define _GNU_SOURCE, then defining
either _POSIX_SOURCE or _POSIX_C_SOURCE or
_SVID_SOURCE as well has no effect.
Note, however, that the features of _BSD_SOURCE are not a subset
of any of the other feature test macros supported. This is because it
defines BSD features that take precedence over the POSIX features that
are requested by the other macros. For this reason, defining
_BSD_SOURCE in addition to the other feature test macros does
have an effect: it causes the BSD features to take priority over the
conflicting POSIX features.
Here is an overview of the contents of the remaining chapters of this manual.
sizeof operator and the symbolic constant NULL, and how to
write functions accepting variable numbers of arguments.
isspace) and functions for
performing case conversion.
char data type.
FILE * objects). These are the normal C library functions
from `stdio.h'.
setjmp and
longjmp functions.
If you already know the name of the facility you are interested in, you can look it up in section Summary of Library Facilities. This gives you a summary of its syntax and a pointer to where you can find a more detailed description. This appendix is particularly useful if you just want to verify the order and type of arguments to a function, for example.
Many functions in the GNU C library detect and report error conditions, and sometimes your programs need to check for these error conditions. For example, when you open an input file, you should verify that the file was actually opened correctly, and print an error message or take other appropriate action if the call to the library function failed.
This chapter describes how the error reporting facility works. Your program should include the header file `errno.h' to use this facility.
Most library functions return a special value to indicate that they have
failed. The special value is typically -1, a null pointer, or a
constant such as EOF that is defined for that purpose. But this
return value tells you only that an error has occurred. To find out
what kind of error it was, you need to look at the error code stored in the
variable errno. This variable is declared in the header file
`errno.h'.
The variable errno contains the system error number. You can
change the value of errno.
Since errno is declared volatile, it might be changed
asynchronously by a signal handler; see section Defining Signal Handlers.
However, a properly written signal handler saves and restores the value
of errno, so you generally do not need to worry about this
possibility except when writing signal handlers.
The initial value of errno at program startup is zero. Many
library functions are guaranteed to set it to certain nonzero values
when they encounter certain kinds of errors. These error conditions are
listed for each function. These functions do not change errno
when they succeed; thus, the value of errno after a successful
call is not necessarily zero, and you should not use errno to
determine whether a call failed. The proper way to do that is
documented for each function. If the call the failed, you can
examine errno.
Many library functions can set errno to a nonzero value as a
result of calling other library functions which might fail. You should
assume that any library function might alter errno.
Portability Note: ANSI C specifies errno as a
"modifiable lvalue" rather than as a variable, permitting it to be
implemented as a macro. For example, its expansion might involve a
function call, like *_errno (). In fact, that is what it is
on the GNU system itself. The GNU library, on non-GNU systems, does
whatever is right for the particular system.
There are a few library functions, like sqrt and atan,
that return a perfectly legitimate value in case of an error, but also
set errno. For these functions, if you want to check to see
whether an error occurred, the recommended method is to set errno
to zero before calling the function, and then check its value afterward.
All the error codes have symbolic names; they are macros defined in `errno.h'. The names start with `E' and an upper-case letter or digit; you should consider names of this form to be reserved names. See section Reserved Names.
The error code values are all positive integers and are all distinct.
(Since the values are distinct, you can use them as labels in a
switch statement, for example.) Your program should not make any
other assumptions about the specific values of these symbolic constants.
The value of errno doesn't necessarily have to correspond to any
of these macros, since some library functions might return other error
codes of their own for other situations. The only values that are
guaranteed to be meaningful for a particular library function are the
ones that this manual lists for that function.
On non-GNU systems, almost any system call can return EFAULT if
it is given an invalid pointer as an argument. Since this could only
happen as a result of a bug in your program, and since it will not
happen on the GNU system, we have saved space by not mentioning
EFAULT in the descriptions of individual functions.
The error code macros are defined in the header file `errno.h'. All of them expand into integer constant values. Some of these error codes can't occur on the GNU system, but they can occur using the GNU library on other systems.
Operation not permitted; only the owner of the file (or other resource) or processes with special privileges can perform the operation.
No such file or directory. This is a "file doesn't exist" error for ordinary files that are referenced in contexts where they are expected to already exist.
No process matches the specified process ID.
Interrupted function call; an asynchronous signal occured and prevented completion of the call. When this happens, you should try the call again.
You can choose to have functions resume after a signal that is handled,
rather than failing with EINTR; see section Primitives Interrupted by Signals.
Input/output error; usually used for physical read or write errors.
No such device or address. Typically, this means that a file representing a device has been installed incorrectly, and the system can't find the right kind of device driver for it.
Argument list too long; used when the arguments passed to a new program
being executed with one of the exec functions (see section Executing a File) occupy too much memory space. This condition never arises in the
GNU system.
Invalid executable file format. This condition is detected by the
exec functions; see section Executing a File.
Bad file descriptor; for example, I/O on a descriptor that has been closed or reading from a descriptor open only for writing (or vice versa).
There are no child processes. This error happens on operations that are supposed to manipulate child processes, when there aren't any processes to manipulate.
Deadlock avoided; allocating a system resource would have resulted in a deadlock situation. For an example, See section File Locks.
No memory available. The system cannot allocate more virtual memory because its capacity is full.
Permission denied; the file permissions do not allow the attempted operation.
Bad address; an invalid pointer was detected.
A file that isn't a block special file was given in a situation that requires one. For example, trying to mount an ordinary file as a file system in Unix gives this error.
Resource busy; a system resource that can't be shared is already in use. For example, if you try to delete a file that is the root of a currently mounted filesystem, you get this error.
File exists; an existing file was specified in a context where it only makes sense to specify a new file.
An attempt to make an improper link across file systems was detected.
The wrong type of device was given to a function that expects a particular sort of device.
A file that isn't a directory was specified when a directory is required.
File is a directory; attempting to open a directory for writing gives this error.
Invalid argument. This is used to indicate various kinds of problems with passing the wrong argument to a library function.
There are too many distinct file openings in the entire system. Note that any number of linked channels count as just one file opening; see section Linked Channels.
The current process has too many files open and can't open any more. Duplicate descriptors do count toward this limit.
Inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file.
An attempt to execute a file that is currently open for writing, or write to a file that is currently being executed. (The name stands for "text file busy".) This is not an error in the GNU system; the text is copied as necessary.
File too big; the size of a file would be larger than allowed by the system.
No space left on device; write operation on a file failed because the disk is full.
Invalid seek operation (such as on a pipe).
An attempt was made to modify a file on a read-only file system.
Too many links; the link count of a single file is too large.
Broken pipe; there is no process reading from the other end of a pipe.
Every library function that returns this error code also generates a
SIGPIPE signal; this signal terminates the program if not handled
or blocked. Thus, your program will never actually see EPIPE
unless it has handled or blocked SIGPIPE.
Domain error; used by mathematical functions when an argument value does not fall into the domain over which the function is defined.
Range error; used by mathematical functions when the result value is not representable because of overflow or underflow.
Resource temporarily unavailable; the call might work if you try again
later. Only fork returns error code EAGAIN for such a
reason.
An operation that would block was attempted on an object that has non-blocking mode selected.
Portability Note: In 4.4BSD and GNU, EWOULDBLOCK and
EAGAIN are the same. Earlier versions of BSD (see section Berkeley Unix) have two distinct codes, and use EWOULDBLOCK to indicate
an I/O operation that would block on an object with non-blocking mode
set, and EAGAIN for other kinds of errors.
An operation that cannot complete immediately was initiated on an object that has non-blocking mode selected.
An operation is already in progress on an object that has non-blocking mode selected.
A file that isn't a socket was specified when a socket is required.
No destination address was supplied on a socket operation.
The size of a message sent on a socket was larger than the supported maximum size.
The socket type does not support the requested communications protocol.
You specified a socket option that doesn't make sense for the particular protocol being used by the socket. See section Socket Options.
The socket domain does not support the requested communications protocol. See section Creating a Socket.
The socket type is not supported.
The operation you requested is not supported. Some socket functions don't make sense for all types of sockets, and others may not be implemented for all communications protocols.
The socket communications protocol family you requested is not supported.
The address family specified for a socket is not supported; it is inconsistent with the protocol being used on the socket. See section Sockets.
The requested socket address is already in use. See section Socket Addresses.
The requested socket address is not available; for example, you tried to give a socket a name that doesn't match the local host name. See section Socket Addresses.
A socket operation failed because the network was down.
A socket operation failed because the subnet containing the remost host was unreachable.
A network connection was reset because the remote host crashed.
A network connection was aborted locally.
A network connection was closed for reasons outside the control of the local host, such as by the remote machine rebooting.
The kernel's buffers for I/O operations are all in use.
You tried to connect a socket that is already connected. See section Making a Connection.
The socket is not connected to anything. You get this error when you try to transmit data over a socket, without first specifying a destination for the data.
The socket has already been shut down.
A socket operation with a specified timeout received no response during the timeout period.
A remote host refused to allow the network connection (typically because it is not running the requested service).
Too many levels of symbolic links were encountered in looking up a file name. This often indicates a cycle of symbolic links.
Filename too long (longer than PATH_MAX; see section Limits on File System Capacity) or host name too long (in gethostname or
sethostname; see section Host Identification).
The remote host for a requested network connection is down.
The remote host for a requested network connection is not reachable.
Directory not empty, where an empty directory was expected. Typically, this error occurs when you are trying to delete a directory.
The file quota system is confused because there are too many users.
The user's disk quota was exceeded.
Stale NFS file handle. This indicates an internal confusion in the NFS system which is due to file system rearrangements on the server host. Repairing this condition usually requires unmounting and remounting the NFS file system on the local host.
An attempt was made to NFS-mount a remote file system with a file name that already specifies an NFS-mounted file. (This is an error on some operating systems, but we expect it to work properly on the GNU system, making this error code impossible.)
No locks available. This is used by the file locking facilities; see section File Locks.
Function not implemented. Some functions have commands or options defined that might not be supported in all implementations, and this is the kind of error you get if you request them and they are not supported.
The experienced user will know what is wrong.
This error code has no purpose.
The library has functions and variables designed to make it easy for
your program to report informative error messages in the customary
format about the failure of a library call. The functions
strerror and perror give you the standard error message
for a given error code; the variable
program_invocation_short_name gives you convenient access to the
name of the program that encountered the error.
Function: char * strerror (int errnum)
The strerror function maps the error code (see section Checking for Errors) specified by the errnum argument to a descriptive error
message string. The return value is a pointer to this string.
The value errnum normally comes from the variable errno.
You should not modify the string returned by strerror. Also, if
you make subsequent calls to strerror, the string might be
overwritten. (But it's guaranteed that no library function ever calls
strerror behind your back.)
The function strerror is declared in `string.h'.
Function: void perror (const char *message)
This function prints an error message to the stream stderr;
see section Standard Streams.
If you call perror with a message that is either a null
pointer or an empty string, perror just prints the error message
corresponding to errno, adding a trailing newline.
If you supply a non-null message argument, then perror
prefixes its output with this string. It adds a colon and a space
character to separate the message from the error string corresponding
to errno.
The function perror is declared in `stdio.h'.
strerror and perror produce the exact same message for any
given error code; the precise text varies from system to system. On the
GNU system, the messages are fairly short; there are no multi-line
messages or embedded newlines. Each error message begins with a capital
letter and does not include any terminating punctuation.
Compatibility Note: The strerror function is a new
feature of ANSI C. Many older C systems do not support this function
yet.
Many programs that don't read input from the terminal are designed to
exit if any system call fails. By convention, the error message from
such a program should start with the program's name, sans directories.
You can find that name in the variable
program_invocation_short_name; the full file name is stored the
variable program_invocation_name:
Variable: char * program_invocation_name
This variable's value is the name that was used to invoke the program
running in the current process. It is the same as argv[0].
Variable: char * program_invocation_short_name
This variable's value is the name that was used to invoke the program
running in the current process, with directory names removed. (That is
to say, it is the same as program_invocation_name minus
everything up to the last slash, if any.)
Both program_invocation_name and
program_invocation_short_name are set up by the system before
main is called.
Portability Note: These two variables are GNU extensions. If
you want your program to work with non-GNU libraries, you must save the
value of argv[0] in main, and then strip off the directory
names yourself. We added these extensions to make it possible to write
self-contained error-reporting subroutines that require no explicit
cooperation from main.
Here is an example showing how to handle failure to open a file
correctly. The function open_sesame tries to open the named file
for reading and returns a stream if successful. The fopen
library function returns a null pointer if it couldn't open the file for
some reason. In that situation, open_sesame constructs an
appropriate error message using the strerror function, and
terminates the program. If we were going to make some other library
calls before passing the error code to strerror, we'd have to
save it in a local variable instead, because those other library
functions might overwrite errno in the meantime.
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
FILE *
open_sesame (char *name)
{
FILE *stream;
errno = 0;
stream = fopen (name, "r");
if (!stream) {
fprintf (stderr, "%s: Couldn't open file %s; %s\n",
program_invocation_short_name, name, strerror (errno));
exit (EXIT_FAILURE);
} else
return stream;
}
The GNU system provides several methods for allocating memory space under explicit program control. They vary in generality and in efficiency.
malloc facility allows fully general dynamic allocation.
See section Unconstrained Allocation.
malloc but more
efficient and convenient for stacklike allocation. See section Obstacks.
alloca lets you allocate storage dynamically that
will be freed automatically. See section Automatic Storage with Variable Size.
Dynamic memory allocation is a technique in which programs determine as they are running where to store some information. You need dynamic allocation when the number of memory blocks you need, or how long you continue to need them, depends on the data you are working on.
For example, you may need a block to store a line read from an input file; since there is no limit to how long a line can be, you must allocate the storage dynamically and make it dynamically larger as you read more of the line.
Or, you may need a block for each record or each definition in the input data; since you can't know in advance how many there will be, you must allocate a new block for each record or definition as you read it.
When you use dynamic allocation, the allocation of a block of memory is an action that the program requests explicitly. You call a function or macro when you want to allocate space, and specify the size with an argument. If you want to free the space, you do so by calling another function or macro. You can do these things whenever you want, as often as you want.
The C language supports two kinds of memory allocation through the variables in C programs:
In GNU C, the length of the automatic storage can be an expression that varies. In other C implementations, it must be a constant.
Dynamic allocation is not supported by C variables; there is no storage class "dynamic", and there can never be a C variable whose value is stored in dynamically allocated space. The only way to refer to dynamically allocated space is through a pointer. Because it is less convenient, and because the actual process of dynamic allocation requires more computation time, programmers use dynamic allocation only when neither static nor automatic allocation will serve.
For example, if you want to allocate dynamically some space to hold a
struct foobar, you cannot declare a variable of type struct
foobar whose contents are the dynamically allocated space. But you can
declare a variable of pointer type struct foobar * and assign it the
address of the space. Then you can use the operators `*' and
`->' on this pointer variable to refer to the contents of the space:
{
struct foobar *ptr
= (struct foobar *) malloc (sizeof (struct foobar));
ptr->name = x;
ptr->next = current_foobar;
current_foobar = ptr;
}
The most general dynamic allocation facility is malloc. It
allows you to allocate blocks of memory of any size at any time, make
them bigger or smaller at any time, and free the blocks individually at
any time (or never).
To allocate a block of memory, call malloc. The prototype for
this function is in `stdlib.h'.
Function: void * malloc (size_t size)
This function returns a pointer to a newly allocated block size bytes long, or a null pointer if the block could not be allocated.
The contents of the block are undefined; you must initialize it yourself
(or use calloc instead; see section Allocating Cleared Space).
Normally you would cast the value as a pointer to the kind of object
that you want to store in the block. Here we show an example of doing
so, and of initializing the space with zeros using the library function
memset (see section Copying and Concatenation):
struct foo *ptr; ... ptr = (struct foo *) malloc (sizeof (struct foo)); if (ptr == 0) abort (); memset (ptr, 0, sizeof (struct foo));
You can store the result of malloc into any pointer variable
without a cast, because ANSI C automatically converts the type
void * to another type of pointer when necessary. But the cast
is necessary in contexts other than assignment operators or if you might
want your code to run in traditional C.
Remember that when allocating space for a string, the argument to
malloc must be one plus the length of the string. This is
because a string is terminated with a null character that doesn't count
in the "length" of the string but does need space. For example:
char *ptr; ... ptr = (char *) malloc (length + 1);
See section Representation of Strings, for more information about this.
malloc
If no more space is available, malloc returns a null pointer.
You should check the value of every call to malloc. It is
useful to write a subroutine that calls malloc and reports an
error if the value is a null pointer, returning only if the value is
nonzero. This function is conventionally called xmalloc. Here
it is:
void *
xmalloc (size_t size)
{
register void *value = malloc (size);
if (value == 0)
fatal ("virtual memory exhausted");
return value;
}
Here is a real example of using malloc (by way of xmalloc).
The function savestring will copy a sequence of characters into
a newly allocated null-terminated string:
char *
savestring (const char *ptr, size_t len)
{
register char *value = (char *) xmalloc (len + 1);
memcpy (value, ptr, len);
value[len] = 0;
return value;
}
The block that malloc gives you is guaranteed to be aligned so
that it can hold any type of data. In the GNU system, the address is
always a multiple of eight; if the size of block is 16 or more, then the
address is always a multiple of 16. Only rarely is any higher boundary
(such as a page boundary) necessary; for those cases, use
memalign or valloc (see section Allocating Aligned Memory Blocks).
Note that the memory located after the end of the block is likely to be
in use for something else; perhaps a block already allocated by another
call to malloc. If you attempt to treat the block as longer than
you asked for it to be, you are liable to destroy the data that
malloc uses to keep track of its blocks, or you may destroy the
contents of another block. If you have already allocated a block and
discover you want it to be bigger, use realloc (see section Changing the Size of a Block).
malloc
When you no longer need a block that you got with malloc, use the
function free to make the block available to be allocated again.
The prototype for this function is in `stdlib.h'.
Function: void free (void *ptr)
The free function deallocates the block of storage pointed at
by ptr.
Function: void cfree (void *ptr)
This function does the same thing as free. It's provided for
backward compatibility with SunOS; you should use free instead.
Freeing a block alters the contents of the block. Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it. Copy whatever you need out of the block before freeing it! Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to:
struct chain
{
struct chain *next;
char *name;
}
void
free_chain (struct chain *chain)
{
while (chain != 0)
{
struct chain *next = chain->next;
free (chain->name);
free (chain);
chain = next;
}
}
Occasionally, free can actually return memory to the operating
system and make the process smaller. Usually, all it can do is allow a
later later call to malloc to reuse the space. In the mean time,
the space remains in your program as part of a free-list used internally
by malloc.
There is no point in freeing blocks at the end of a program, because all of the program's space is given back to the system when the process terminates.
Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block. For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer.
You can make the block longer by calling realloc. This function
is declared in `stdlib.h'.
Function: void * realloc (void *ptr, size_t newsize)
The realloc function changes the size of the block whose address is
ptr to be newsize.
Since the space after the end of the block may be in use, realloc
may find it necessary to copy the block to a new address where more free
space is available. The value of realloc is the new address of the
block. If the block needs to be moved, realloc copies the old
contents.
Like malloc, realloc may return a null pointer if no
memory space is available to make the block bigger. When this happens,
the original block is untouched; it has not been modified or relocated.
In most cases it makes no difference what happens to the original block
when realloc fails, because the application program cannot continue
when it is out of memory, and the only thing to do is to give a fatal error
message. Often it is convenient to write and use a subroutine,
conventionally called xrealloc, that takes care of the error message
as xmalloc does for malloc:
void *
xrealloc (void *ptr, size_t size)
{
register void *value = realloc (ptr, size);
if (value == 0)
fatal ("Virtual memory exhausted");
return value;
}
You can also use realloc to make a block smaller. The reason you
would do this is to avoid tying up a lot of memory space when only a little
is needed. Making a block smaller sometimes necessitates copying it, so it
can fail if no other space is available.
If the new size you specify is the same as the old size, realloc
is guaranteed to change nothing and return the same address that you gave.
The function calloc allocates memory and clears it to zero. It
is declared in `stdlib.h'.
Function: void * calloc (size_t count, size_t eltsize)
This function allocates a block long enough to contain a vector of
count elements, each of size eltsize. Its contents are
cleared to zero before calloc returns.
You could define calloc as follows:
void *
calloc (size_t count, size_t eltsize)
{
size_t size = count * eltsize;
void *value = malloc (size);
if (value != 0)
memset (value, 0, size);
return value;
}
We rarely use calloc today, because it is equivalent to such a
simple combination of other features that are more often used. It is a
historical holdover that is not quite obsolete.
malloc
To make the best use of malloc, it helps to know that the GNU
version of malloc always dispenses small amounts of memory in
blocks whose sizes are powers of two. It keeps separate pools for each
power of two. This holds for sizes up to a page size. Therefore, if
you are free to choose the size of a small block in order to make
malloc more efficient, make it a power of two.
Once a page is split up for a particular block size, it can't be reused for another size unless all the blocks in it are freed. In many programs, this is unlikely to happen. Thus, you can sometimes make a program use memory more efficiently by using blocks of the same size for many different purposes.
When you ask for memory blocks of a page or larger, malloc uses a
different strategy; it rounds the size up to a multiple of a page, and
it can coalesce and split blocks as needed.
The reason for the two strategies is that it is important to allocate and free small blocks as fast as possible, but speed is less important for a large block since the program normally spends a fair amount of time using it. Also, large blocks are normally fewer in number. Therefore, for large blocks, it makes sense to use a method which takes more time to minimize the wasted space.
The address of a block returned by malloc or realloc in
the GNU system is always a multiple of eight. If you need a block whose
address is a multiple of a higher power of two than that, use
memalign or valloc. These functions are declared in
`stdlib.h'.
With the GNU library, you can use free to free the blocks that
memalign and valloc return. That does not work in BSD,
however--BSD does not provide any way to free such blocks.
Function: void * memalign (size_t size, int boundary)
The memalign function allocates a block of size bytes whose
address is a multiple of boundary. The boundary must be a
power of two! The function memalign works by calling
malloc to allocate a somewhat larger block, and then returning an
address within the block that is on the specified boundary.
Function: void * valloc (size_t size)
Using valloc is like using memalign and passing the page size
as the value of the second argument.
You can ask malloc to check the consistency of dynamic storage by
using the mcheck function. This function is a GNU extension,
declared in `malloc.h'.
Function: void mcheck (void (*abortfn) (void))
Calling mcheck tells malloc to perform occasional
consistency checks. These will catch things such as writing
past the end of a block that was allocated with malloc.
The abortfn argument is the function to call when an inconsistency
is found. If you supply a null pointer, the abort function is
used.
It is too late to begin allocation checking once you have allocated
anything with malloc. So mcheck does nothing in that
case. The function returns -1 if you call it too late, and
0 otherwise (when it is successful).
The easiest way to arrange to call mcheck early enough is to use
the option `-lmcheck' when you link your program.
The GNU C library lets you modify the behavior of malloc,
realloc, and free by specifying appropriate hook
functions. You can use these hooks to help you debug programs that use
dynamic storage allocation, for example.
The hook variables are declared in `malloc.h'.
The value of this variable is a pointer to function that malloc
uses whenever it is called. You should define this function to look
like malloc; that is, like:
void *function (size_t size)
The value of this variable is a pointer to function that realloc
uses whenever it is called. You should define this function to look
like realloc; that is, like:
void *function (void *ptr, size_t size)
The value of this variable is a pointer to function that free
uses whenever it is called. You should define this function to look
like free; that is, like:
void function (void *ptr)
You must make sure that the function you install as a hook for one of these functions does not call that function recursively without restoring the old value of the hook first! Otherwise, your program will get stuck in an infinite recursion.
Here is an example showing how to use __malloc_hook properly. It
installs a function that prints out information every time malloc
is called.
static void *(*old_malloc_hook) (size_t);
static void *
my_malloc_hook (size_t size)
{
void *result;
__malloc_hook = old_malloc_hook;
result = malloc (size);
__malloc_hook = my_malloc_hook;
printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
return result;
}
main ()
{
...
old_malloc_hook = __malloc_hook;
__malloc_hook = my_malloc_hook;
...
}
The mcheck function (see section Heap Consistency Checking) works by
installing such hooks.
malloc
You can get information about dynamic storage allocation by calling the
mstats function. This function and its associated data type are
declared in `malloc.h'; they are a GNU extension.
This structure type is used to return information about the dynamic storage allocator. It contains the following members:
size_t bytes_total
size_t chunks_used
malloc requests; see section Efficiency Considerations for malloc.)
size_t bytes_used
size_t chunks_free
size_t bytes_free
Function: struct mstats mstats (void)
This function returns information about the current dynamic memory usage
in a structure of type struct mstats.
malloc-Related Functions
Here is a summary of the functions that work with malloc:
void *malloc (size_t size)
void free (void *addr)
malloc. See section Freeing Memory Allocated with malloc.
void *realloc (void *addr, size_t size)
malloc larger or smaller,
possibly by copying it to a new location. See section Changing the Size of a Block.
void *calloc (size_t count, size_t eltsize)
malloc, and set its contents to zero. See section Allocating Cleared Space.
void *valloc (size_t size)
void *memalign (size_t size, size_t boundary)
void mcheck (void (*abortfn) (void))
malloc to perform occasional consistency checks on
dynamically allocated memory, and to call abortfn when an
inconsistency is found. See section Heap Consistency Checking.
void *(*__malloc_hook) (size_t size)
malloc uses whenever it is called.
void *(*__realloc_hook) (void *ptr, size_t size)
realloc uses whenever it is called.
void (*__free_hook) (void *ptr)
free uses whenever it is called.
void struct mstats mstats (void)
malloc.
An obstack is a pool of memory containing a stack of objects. You can create any number of separate obstacks, and then allocate objects in specified obstacks. Within each obstack, the last object allocated must always be the first one freed, but distinct obstacks are independent of each other.
Aside from this one constraint of order of freeing, obstacks are totally general: an obstack can contain any number of objects of any size. They are implemented with macros, so allocation is usually very fast as long as the objects are usually small. And the only space overhead per object is the padding needed to start each object on a suitable boundary.
The utilities for manipulating obstacks are declared in the header file `obstack.h'.
An obstack is represented by a data structure of type struct
obstack. This structure has a small fixed size; it records the status
of the obstack and how to find the space in which objects are allocated.
It does not contain any of the objects themselves. You should not try
to access the contents of the structure directly; use only the functions
described in this chapter.
You can declare variables of type struct obstack and use them as
obstacks, or you can allocate obstacks dynamically like any other kind
of object. Dynamic allocation of obstacks allows your program to have a
variable number of different stacks. (You can even allocate an
obstack structure in another obstack, but this is rarely useful.)
All the functions that work with obstacks require you to specify which
obstack to use. You do this with a pointer of type struct obstack
*. In the following, we often say "an obstack" when strictly
speaking the object at hand is such a pointer.
The objects in the obstack are packed into large blocks called
chunks. The struct obstack structure points to a chain of
the chunks currently in use.
The obstack library obtains a new chunk whenever you allocate an object
that won't fit in the previous chunk. Since the obstack library manages
chunks automatically, you don't need to pay much attention to them, but
you do need to supply a function which the obstack library should use to
get a chunk. Usually you supply a function which uses malloc
directly or indirectly. You must also supply a function to free a chunk.
These matters are described in the following section.
Each source file in which you plan to use the obstack functions must include the header file `obstack.h', like this:
#include <obstack.h>
Also, if the source file uses the macro obstack_init, it must
declare or define two functions or macros that will be called by the
obstack library. One, obstack_chunk_alloc, is used to allocate the
chunks of memory into which objects are packed. The other,
obstack_chunk_free, is used to return chunks when the objects in
them are freed.
Usually these are defined to use malloc via the intermediary
xmalloc (see section Unconstrained Allocation). This is done with
the following pair of macro definitions:
#define obstack_chunk_alloc xmalloc #define obstack_chunk_free free
Though the storage you get using obstacks really comes from malloc,
using obstacks is faster because malloc is called less often, for
larger blocks of memory. See section Obstack Chunks, for full details.
At run time, before the program can use a struct obstack object
as an obstack, it must initialize the obstack by calling
obstack_init.
Function: void obstack_init (struct obstack *obstack_ptr)
Initialize obstack obstack_ptr for allocation of objects.
Here are two examples of how to allocate the space for an obstack and initialize it. First, an obstack that is a static variable:
struct obstack myobstack; ... obstack_init (&myobstack);
Second, an obstack that is itself dynamically allocated:
struct obstack *myobstack_ptr = (struct obstack *) xmalloc (sizeof (struct obstack)); obstack_init (myobstack_ptr);
The most direct way to allocate an object in an obstack is with
obstack_alloc, which is invoked almost like malloc.
Function: void * obstack_alloc (struct obstack *obstack_ptr, size_t size)
This allocates an uninitialized block of size bytes in an obstack
and returns its address. Here obstack_ptr specifies which obstack
to allocate the block in; it is the address of the struct obstack
object which represents the obstack. Each obstack function or macro
requires you to specify an obstack_ptr as the first argument.
For example, here is a function that allocates a copy of a string str
in a specific obstack, which is the variable string_obstack:
struct obstack string_obstack;
char *
copystring (char *string)
{
char *s = (char *) obstack_alloc (&string_obstack,
strlen (string) + 1);
memcpy (s, string, strlen (string));
return s;
}
To allocate a block with specified contents, use the function
obstack_copy, declared like this:
Function: void * obstack_copy (struct obstack *obstack_ptr, void *address, size_t size)
This allocates a block and initializes it by copying size bytes of data starting at address.
Function: void * obstack_copy0 (struct obstack *obstack_ptr, void *address, size_t size)
Like obstack_copy, but appends an extra byte containing a null
character. This extra byte is not counted in the argument size.
The obstack_copy0 function is convenient for copying a sequence
of characters into an obstack as a null-terminated string. Here is an
example of its use:
char *
obstack_savestring (char *addr, size_t size)
{
return obstack_copy0 (&myobstack, addr, size);
}
Contrast this with the previous example of savestring using
malloc (see section Basic Storage Allocation).
To free an object allocated in an obstack, use the function
obstack_free. Since the obstack is a stack of objects, freeing
one object automatically frees all other objects allocated more recently
in the same obstack.
Function: void obstack_free (struct obstack *obstack_ptr, void *object)
If object is a null pointer, everything allocated in the obstack is freed. Otherwise, object must be the address of an object allocated in the obstack. Then object is freed, along with everything allocated in obstack since object.
Note that if object is a null pointer, the result is an
uninitialized obstack. To free all storage in an obstack but leave it
valid for further allocation, call obstack_free with the address
of the first object allocated on the obstack:
obstack_free (obstack_ptr, first_object_allocated_ptr);
Recall that the objects in an obstack are grouped into chunks. When all the objects in a chunk become free, the obstack library automatically frees the chunk (see section Preparing for Using Obstacks). Then other obstacks, or non-obstack allocation, can reuse the space of the chunk.
The interfaces for using obstacks may be defined either as functions or as macros, depending on the compiler. The obstack facility works with all C compilers, including both ANSI C and traditional C, but there are precautions you must take if you plan to use compilers other than GNU C.
If you are using an old-fashioned non-ANSI C compiler, all the obstack "functions" are actually defined only as macros. You can call these macros like functions, but you cannot use them in any other way (for example, you cannot take their address).
Calling the macros requires a special precaution: namely, the first operand (the obstack pointer) may not contain any side effects, because it may be computed more than once. For example, if you write this:
obstack_alloc (get_obstack (), 4);
you will find that get_obstack may be called several times.
If you use *obstack_list_ptr++ as the obstack pointer argument,
you will get very strange results since the incrementation may occur
several times.
In ANSI C, each function has both a macro definition and a function definition. The function definition is used if you take the address of the function without calling it. An ordinary call uses the macro definition by default, but you can request the function definition instead by writing the function name in parentheses, as shown here:
char *x; void *(*funcp) (); /* Use the macro. */ x = (char *) obstack_alloc (obptr, size); /* Call the function. */ x = (char *) (obstack_alloc) (obptr, size); /* Take the address of the function. */ funcp = obstack_alloc;
This is the same situation that exists in ANSI C for the standard library functions. See section Macro Definitions of Functions.
Warning: When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in ANSI C.
If you use the GNU C compiler, this precaution is not necessary, because various language extensions in GNU C permit defining the macros so as to compute each argument only once.
Because storage in obstack chunks is used sequentially, it is possible to build up an object step by step, adding one or more bytes at a time to the end of the object. With this technique, you do not need to know how much data you will put in the object until you come to the end of it. We call this the technique of growing objects. The special functions for adding data to the growing object are described in this section.
You don't need to do anything special when you start to grow an object.
Using one of the functions to add data to the object automatically
starts it. However, it is necessary to say explicitly when the object is
finished. This is done with the function obstack_finish.
The actual address of the object thus built up is not known until the object is finished. Until then, it always remains possible that you will add so much data that the object must be copied into a new chunk.
While the obstack is in use for a growing object, you cannot use it for ordinary allocation of another object. If you try to do so, the space already added to the growing object will become part of the other object.
Function: void obstack_blank (struct obstack *obstack_ptr, size_t size)
The most basic function for adding to a growing object is
obstack_blank, which adds space without initializing it.
Function: void obstack_grow (struct obstack *obstack_ptr, void *data, size_t size)
To add a block of initialized space, use obstack_grow, which is
the growing-object analogue of obstack_copy. It adds size
bytes of data to the growing object, copying the contents from
data.
Function: void obstack_grow0 (struct obstack *obstack_ptr, void *data, size_t size)
This is the growing-object analogue of obstack_copy0. It adds
size bytes copied from data, followed by an additional null
character.
Function: void obstack_1grow (struct obstack *obstack_ptr, char c)
To add one character at a time, use the function obstack_1grow.
It adds a single byte containing c to the growing object.
Function: void * obstack_finish (struct obstack *obstack_ptr)
When you are finished growing the object, use the function
obstack_finish to close it off and return its final address.
Once you have finished the object, the obstack is available for ordinary allocation or for growing another object.
When you build an object by growing it, you will probably need to know
afterward how long it became. You need not keep track of this as you grow
the object, because you can find out the length from the obstack just
before finishing the object with the function obstack_object_size,
declared as follows:
Function: size_t obstack_object_size (struct obstack *obstack_ptr)
This function returns the current size of the growing object, in bytes.
Remember to call this function before finishing the object.
After it is finished, obstack_object_size will return zero.
If you have started growing an object and wish to cancel it, you should finish it and then free it, like this:
obstack_free (obstack_ptr, obstack_finish (obstack_ptr));
This has no effect if no object was growing.
You can use obstack_blank with a negative size argument to make
the current object smaller. Just don't try to shrink it beyond zero
length--there's no telling what will happen if you do that.
The usual functions for growing objects incur overhead for checking whether there is room for the new growth in the current chunk. If you are frequently constructing objects in small steps of growth, this overhead can be significant.
You can reduce the overhead by using special "fast growth" functions that grow the object without checking. In order to have a robust program, you must do the checking yourself. If you do this checking in the simplest way each time you are about to add data to the object, you have not saved anything, because that is what the ordinary growth functions do. But if you can arrange to check less often, or check more efficiently, then you make the program faster.
The function obstack_room returns the amount of room available
in the current chunk. It is declared as follows:
Function: size_t obstack_room (struct obstack *obstack_ptr)
This returns the number of bytes that can be added safely to the current growing object (or to an object about to be started) in obstack obstack using the fast growth functions.
While you know there is room, you can use these fast growth functions for adding data to a growing object:
Function: void obstack_1grow_fast (struct obstack *obstack_ptr, char c)
The function obstack_1grow_fast adds one byte containing the
character c to the growing object in obstack obstack_ptr.
Function: void obstack_blank_fast (struct obstack *obstack_ptr, size_t size)
The function obstack_blank_fast adds size bytes to the
growing object in obstack obstack_ptr without initializing them.
When you check for space using obstack_room and there is not
enough room for what you want to add, the fast growth functions
are not safe. In this case, simply use the corresponding ordinary
growth function instead. Very soon this will copy the object to a
new chunk; then there will be lots of room available again.
So, each time you use an ordinary growth function, check afterward for
sufficient space using obstack_room. Once the object is copied
to a new chunk, there will be plenty of space again, so the program will
start using the fast growth functions again.
Here is an example:
void
add_string (struct obstack *obstack, char *ptr, size_t len)
{
while (len > 0)
{
if (obstack_room (obstack) > len)
{
/* We have enough room: add everything fast. */
while (len-- > 0)
obstack_1grow_fast (obstack, *ptr++);
}
else
{
/* Not enough room. Add one character slowly,
which may copy to a new chunk and make room. */
obstack_1grow (obstack, *ptr++);
len--;
}
}
}
Here are functions that provide information on the current status of allocation in an obstack. You can use them to learn about an object while still growing it.
Function: void * obstack_base (struct obstack *obstack_ptr)
This function returns the tentative address of the beginning of the currently growing object in obstack_ptr. If you finish the object immediately, it will have that address. If you make it larger first, it may outgrow the current chunk--then its address will change!
If no object is growing, this value says where the next object you allocate will start (once again assuming it fits in the current chunk).
Function: void * obstack_next_free (struct obstack *obstack_ptr)
This function returns the address of the first free byte in the current
chunk of obstack obstack_ptr. This is the end of the currently
growing object. If no object is growing, obstack_next_free
returns the same value as obstack_base.
Function: size_t obstack_object_size (struct obstack *obstack_ptr)
This function returns the size in bytes of the currently growing object. This is equivalent to
obstack_next_free (obstack_ptr) - obstack_base (obstack_ptr)
Each obstack has an alignment boundary; each object allocated in the obstack automatically starts on an address that is a multiple of the specified boundary. By default, this boundary is 4 bytes.
To access an obstack's alignment boundary, use the macro
obstack_alignment_mask, whose function prototype looks like
this:
Macro: int obstack_alignment_mask (struct obstack *obstack_ptr)
The value is a bit mask; a bit that is 1 indicates that the corresponding bit in the address of an object should be 0. The mask value should be one less than a power of 2; the effect is that all object addresses are multiples of that power of 2. The default value of the mask is 3, so that addresses are multiples of 4. A mask value of 0 means an object can start on any multiple of 1 (that is, no alignment is required).
The expansion of the macro obstack_alignment_mask is an lvalue,
so you can alter the mask by assignment. For example, this statement:
obstack_alignment_mask (obstack_ptr) = 0;
has the effect of turning off alignment processing in the specified obstack.
Note that a change in alignment mask does not take effect until
after the next time an object is allocated or finished in the
obstack. If you are not growing an object, you can make the new
alignment mask take effect immediately by calling obstack_finish.
This will finish a zero-length object and then do proper alignment for
the next object.
Obstacks work by allocating space for themselves in large chunks, and then parceling out space in the chunks to satisfy your requests. Chunks are normally 4096 bytes long unless you specify a different chunk size. The chunk size includes 8 bytes of overhead that are not actually used for storing objects. Regardless of the specified size, longer chunks will be allocated when necessary for long objects.
The obstack library allocates chunks by calling the function
obstack_chunk_alloc, which you must define. When a chunk is no
longer needed because you have freed all the objects in it, the obstack
library frees the chunk by calling obstack_chunk_free, which you
must also define.
These two must be defined (as macros) or declared (as functions) in each
source file that uses obstack_init (see section Creating Obstacks).
Most often they are defined as macros like this:
#define obstack_chunk_alloc xmalloc #define obstack_chunk_free free
Note that these are simple macros (no arguments). Macro definitions with
arguments will not work! It is necessary that obstack_chunk_alloc
or obstack_chunk_free, alone, expand into a function name if it is
not itself a function name.
The function that actually implements obstack_chunk_alloc cannot
return "failure" in any fashion, because the obstack library is not
prepared to handle failure. Therefore, malloc itself is not
suitable. If the function cannot obtain space, it should either
terminate the process (see section Program Termination) or do a nonlocal
exit using longjmp (see section Non-Local Exits).
If you allocate chunks with malloc, the chunk size should be a
power of 2. The default chunk size, 4096, was chosen because it is long
enough to satisfy many typical requests on the obstack yet short enough
not to waste too much memory in the portion of the last chunk not yet used.
Macro: size_t obstack_chunk_size (struct obstack *obstack_ptr)
This returns the chunk size of the given obstack.
Since this macro expands to an lvalue, you can specify a new chunk size by assigning it a new value. Doing so does not affect the chunks already allocated, but will change the size of chunks allocated for that particular obstack in the future. It is unlikely to be useful to make the chunk size smaller, but making it larger might improve efficiency if you are allocating many objects whose size is comparable to the chunk size. Here is how to do so cleanly:
if (obstack_chunk_size (obstack_ptr) < new_chunk_size) obstack_chunk_size (obstack_ptr) = new_chunk_size;
Here is a summary of all the functions associated with obstacks. Each
takes the address of an obstack (struct obstack *) as its first
argument.
void obstack_init (struct obstack *obstack_ptr)
void *obstack_alloc (struct obstack *obstack_ptr, size_t size)
void *obstack_copy (struct obstack *obstack_ptr, void *address, size_t size)
void *obstack_copy0 (struct obstack *obstack_ptr, void *address, size_t size)
void obstack_free (struct obstack *obstack_ptr, void *object)
void obstack_blank (struct obstack *obstack_ptr, size_t size)
void obstack_grow (struct obstack *obstack_ptr, void *address, size_t size)
void obstack_grow0 (struct obstack *obstack_ptr, void *address, size_t size)
void obstack_1grow (struct obstack *obstack_ptr, char data_char)
void *obstack_finish (struct obstack *obstack_ptr)
size_t obstack_object_size (struct obstack *obstack_ptr)
void obstack_blank_fast (struct obstack *obstack_ptr, size_t size)
void obstack_1grow_fast (struct obstack *obstack_ptr, char data_char)
size_t obstack_room (struct obstack *obstack_ptr)
int obstack_alignment_mask (struct obstack *obstack_ptr)
size_t obstack_chunk_size (struct obstack *obstack_ptr)
void *obstack_base (struct obstack *obstack_ptr)
void *obstack_next_free (struct obstack *obstack_ptr)
The function alloca supports a kind of half-dynamic allocation in
which blocks are allocated dynamically but freed automatically.
Allocating a block with alloca is an explicit action; you can
allocate as many blocks as you wish, and compute the size at run time. But
all the blocks are freed when you exit the function that alloca was
called from, just as if they were automatic variables declared in that
function. There is no way to free the space explicitly.
The prototype for alloca is in `stdlib.h'. This function is
a BSD extension.
Function: void * alloca (size_t size);
The return value of alloca is the address of a block of size
bytes of storage, allocated in the stack frame of the calling function.
Do not use alloca inside the arguments of a function call--you
will get unpredictable results, because the stack space for the
alloca would appear on the stack in the middle of the space for
the function arguments. An example of what to avoid is foo (x,
alloca (4), y).
alloca Example
As an example of use of alloca, here is a function that opens a file
name made from concatenating two argument strings, and returns a file
descriptor or minus one signifying failure:
int
open2 (char *str1, char *str2, int flags, int mode)
{
char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
strcpy (name, str1);
strcat (name, str2);
return open (name, flags, mode);
}
Here is how you would get the same results with malloc and
free:
int
open2 (char *str1, char *str2, int flags, int mode)
{
char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
int desc;
if (name == 0)
fatal ("virtual memory exceeded");
strcpy (name, str1);
strcat (name, str2);
desc = open (name, flags, mode);
free (name);
return desc;
}
As you can see, it is simpler with alloca. But alloca has
other, more important advantages, and some disadvantages.
alloca
Here are the reasons why alloca may be preferable to malloc:
alloca wastes very little space and is very fast. (It is
open-coded by the GNU C compiler.)
alloca does not have separate pools for different sizes of
block, space used for any size block can be reused for any other size.
alloca does not cause storage fragmentation.
longjmp (see section Non-Local Exits)
automatically free the space allocated with alloca when they exit
through the function that called alloca. This is the most
important reason to use alloca.
To illustrate this, suppose you have a function
open_or_report_error which returns a descriptor, like
open, if it succeeds, but does not return to its caller if it
fails. If the file cannot be opened, it prints an error message and
jumps out to the command level of your program using longjmp.
Let's change open2 (see section alloca Example) to use this
subroutine:
int
open2 (char *str1, char *str2, int flags, int mode)
{
char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
strcpy (name, str1);
strcat (name, str2);
return open_or_report_error (name, flags, mode);
}
Because of the way alloca works, the storage it allocates is
freed even when an error occurs, with no special effort required.
By contrast, the previous definition of open2 (which uses
malloc and free) would develop a storage leak if it were
changed in this way. Even if you are willing to make more changes to
fix it, there is no easy way to do so.
alloca
These are the disadvantages of alloca in comparison with
malloc:
alloca, so it is less
portable. However, a slower emulation of alloca written in C
is available for use on systems with this deficiency.
In GNU C, you can replace most uses of alloca with an array of
variable size. Here is how open2 would look then:
int open2 (char *str1, char *str2, int flags, int mode)
{
char name[strlen (str1) + strlen (str2) + 1];
strcpy (name, str1);
strcat (name, str2);
return open (name, flags, mode);
}
But alloca is not always equivalent to a variable-sized array, for
several reasons:
alloca usually
remains until the end of the function.
alloca within a loop, allocating an
additional block on each iteration. This is impossible with
variable-sized arrays. On the other hand, this is also slightly
unclean.
Note: If you mix use of alloca and variable-sized arrays
within one function, exiting a scope in which a variable-sized array was
declared frees all blocks allocated with alloca during the
execution of that scope.
Any system of dynamic memory allocation has overhead: the amount of space it uses is more than the amount the program asks for. The relocating memory allocator achieves very low overhead by moving blocks in memory as necessary, on its own initiative.
When you allocate a block with malloc, the address of the block
never changes unless you use realloc to change its size. Thus,
you can safely store the address in various places, temporarily or
permanently, as you like. This is not safe when you use the relocating
memory allocator, because any and all relocatable blocks can move
whenever you allocate memory in any fashion. Even calling malloc
or realloc can move the relocatable blocks.
For each relocatable block, you must make a handle---a pointer object in memory, designated to store the address of that block. The relocating allocator knows where each block's handle is, and updates the address stored there whenever it moves the block, so that the handle always points to the block. Each time you access the contents of the block, you should fetch its address anew from the handle.
To call any of the relocating allocator functions from a signal handler is almost certainly incorrect, because the signal could happen at any time and relocate all the blocks. The only way to make this safe is to block the signal around any access to the contents of any relocatable block--not a convenient mode of operation. See section Signal Handling and Nonreentrant Functions.
In the descriptions below, handleptr designates the address of the handle. All the functions are declared in `malloc.h'; all are GNU extensions.
Function: void * r_alloc (void **handleptr, size_t size)
This function allocates a relocatable block of size size. It
stores the block's address in *handleptr and returns
a non-null pointer to indicate success.
If r_alloc can't get the space needed, it stores a null pointer
in *handleptr, and returns a null pointer.
Function: void r_alloc_free (void **handleptr)
This function is the way to free a relocatable block. It frees the
block that *handleptr points to, and stores a null pointer
in *handleptr to show it doesn't point to an allocated
block any more.
Function: void * r_re_alloc (void **handleptr, size_t size)
The function r_re_alloc adjusts the size of the block that
*handleptr points to, making it size bytes long. It
stores the address of the resized block in *handleptr and
returns a non-null pointer to indicate success.
If enough memory is not available, this function returns a null pointer
and does not modify *handleptr.
You can ask for warnings as the program approaches running out of memory
space, by calling memory_warnings. This is a GNU extension
declared in `malloc.h'.
Function: void memory_warnings (void *start, void (*warn_func) (char *))
Call this function to request warnings for nearing exhaustion of virtual memory.
The argument start says where data space begins, in memory. The allocator compares this against the last address used and against the limit of data space, to determine the fraction of available memory in use. If you supply zero for start, then a default value is used which is right in most circumstances.
For warn_func, supply a function that malloc can call to
warn you. It is called with a string (a warning message) as argument.
Normally it ought to display the string for the user to read.
The warnings come when memory becomes 75% full, when it becomes 85% full, and when it becomes 95% full. Above 95% you get another warning each time memory usage increases.
Programs that work with characters and strings often need to classify a character--is it alphabetic, is it a digit, is it whitespace, and so on--and perform case conversion operations on characters. The functions in the header file `ctype.h' are provided for this purpose.
Since the choice of locale and character set can alter the
classifications of particular character codes, all of these functions
are affected by the current locale. (More precisely, they are affected
by the locale currently selected for character classification--the
LC_CTYPE category; see section Categories of Activities that Locales Affect.)
This section explains the library functions for classifying characters.
For example, isalpha is the function to test for an alphabetic
character. It takes one argument, the character to test, and returns a
nonzero integer if the character is alphabetic, and zero otherwise. You
would use it like this:
if (isalpha (c))
printf ("The character `%c' is alphabetic.\n", c);
Each of the functions in this section tests for membership in a
particular class of characters; each has a name starting with `is'.
Each of them takes one argument, which is a character to test, and
returns an int which is treated as a boolean value. The
character argument is passed as an int, and it may be the
constant value EOF instead of a real character.
The attributes of any given character can vary between locales. See section Locales and Internationalization, for more information on locales.
These functions are declared in the header file `ctype.h'.
Returns true if c is a lower-case letter.
Returns true if c is an upper-case letter.
Returns true if c is an alphabetic character (a letter). If
islower or isupper is true of a character, then
isalpha is also true.
In some locales, there may be additional characters for which
isalpha is true--letters which are neither upper case nor lower
case. But in the standard "C" locale, there are no such
additional characters.
Returns true if c is a decimal digit (`0' through `9').
Returns true if c is an alphanumeric character (a letter or
number); in other words, if either isalpha or isdigit is
true of a character, then isalnum is also true.
Function: int isxdigit (int c)
Returns true if c is a hexadecimal digit. Hexadecimal digits include the normal decimal digits `0' through `9' and the letters `A' through `F' and `a' through `f'.
Returns true if c is a punctuation character. This means any printing character that is not alphanumeric or a space character.
Returns true if c is a whitespace character. In the standard
"C" locale, isspace returns true for only the standard
whitespace characters:
' '
'\f'
'\n'
'\r'
'\t'
'\v'
Returns true if c is a blank character; that is, a space or a tab. This function is a GNU extension.
Returns true if c is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic.
Returns true if c is a printing character. Printing characters include all the graphic characters, plus the space (` ') character.
Returns true if c is a control character (that is, a character that is not a printing character).
Returns true if c is a 7-bit unsigned char value that fits
into the US/UK ASCII character set. This function is a BSD extension
and is also an SVID extension.
This section explains the library functions for performing conversions
such as case mappings on characters. For example, toupper
converts any character to upper case if possible. If the character
can't be converted, toupper returns it unchanged.
These functions take one argument of type int, which is the
character to convert, and return the converted character as an
int. If the conversion is not applicable to the argument given,
the argument is returned unchanged.
Compatibility Note: In pre-ANSI C dialects, instead of
returning the argument unchanged, these functions may fail when the
argument is not suitable for the conversion. Thus for portability, you
may need to write islower(c) ? toupper(c) : c rather than just
toupper(c).
These functions are declared in the header file `ctype.h'.
If c is an upper-case letter, tolower returns the corresponding
lower-case letter. If c is not an upper-case letter,
c is returned unchanged.
If c is a lower-case letter, tolower returns the corresponding
upper-case letter. Otherwise c is returned unchanged.
This function converts c to a 7-bit unsigned char value
that fits into the US/UK ASCII character set, by clearing the high-order
bits. This function is a BSD extension and is also an SVID extension.
Function: int _tolower (int c)
This is identical to tolower, and is provided for compatibility
with the SVID. See section SVID (The System V Interface Description).
Function: int _toupper (int c)
This is identical to toupper, and is provided for compatibility
with the SVID.
Operations on strings (or arrays of characters) are an important part of
many programs. The GNU C library provides an extensive set of string
utility functions, including functions for copying, concatenating,
comparing, and searching strings. Many of these functions can also
operate on arbitrary regions of storage; for example, the memcpy
function can be used to copy the contents of any kind of array.
It's fairly common for beginning C programmers to "reinvent the wheel" by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability.
For instance, you could easily compare one string to another in two
lines of C code, but if you use the built-in strcmp function,
you're less likely to make a mistake. And, since these library
functions are typically highly optimized, your program may run faster
too.
This section is a quick summary of string concepts for beginning C programmers. It describes how character strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section.
A string is an array of char objects. But string-valued
variables are usually declared to be pointers of type char *.
Such variables do not include space for the text of a string; that has
to be stored somewhere else--in an array variable, a string constant,
or dynamically allocated memory (see section Memory Allocation). It's up to
you to store the address of the chosen memory space into the pointer
variable. Alternatively you can store a null pointer in the
pointer variable. The null pointer does not point anywhere, so
attempting to reference the string it points to gets an error.
By convention, a null character, '\0', marks the end of a
string. For example, in testing to see whether the char *
variable p points to a null character marking the end of a string,
you can write !*p or *p == '\0'.
A null character is quite different conceptually from a null pointer,
although both are represented by the integer 0.
String literals appear in C program source as strings of
characters between double-quote characters (`"'). In ANSI C,
string literals can also be formed by string concatenation:
"a" "b" is the same as "ab". Modification of string
literals is not allowed by the GNU C compiler, because literals
are placed in read-only storage.
Character arrays that are declared const cannot be modified
either. It's generally good style to declare non-modifiable string
pointers to be of type const char *, since this often allows the
C compiler to detect accidental modifications as well as providing some
amount of documentation about what your program intends to do with the
string.
The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string. In this document, the term allocation size is always used to refer to the total amount of memory allocated for the string, while the term length refers to the number of characters up to (but not including) the terminating null character.
A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions do not do this for you! Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string.
This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters.
Functions that operate on arbitrary blocks of memory have names
beginning with `mem' (such as memcpy) and invariably take an
argument which specifies the size (in bytes) of the block of memory to
operate on. The array arguments and return values for these functions
have type void *, and as a matter of style, the elements of these
arrays are referred to as "bytes". You can pass any kind of pointer
to these functions, and the sizeof operator is useful in
computing the value for the size argument.
In contrast, functions that operate specifically on strings have names
beginning with `str' (such as strcpy) and look for a null
character to terminate the string instead of requiring an explicit size
argument to be passed. (Some of these functions accept a specified
maximum length, but they also check for premature termination with a
null character.) The array arguments and return values for these
functions have type char *, and the array elements are referred
to as "characters".
In many cases, there are both `mem' and `str' versions of a function. The one that is more appropriate to use depends on the exact situation. When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the `mem' functions. On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the `str' functions, unless you already know the length of the string in advance.
You can get the length of a string using the strlen function.
This function is declared in the header file `string.h'.
Function: size_t strlen (const char *s)
The strlen function returns the length of the null-terminated
string s. (In other words, it returns the offset of the terminating
null character within the array.)
For example,
strlen ("hello, world")
=> 12
When applied to a character array, the strlen function returns
the length of the string stored there, not its allocation size. You can
get the allocation size of the character array that holds a string using
the sizeof operator:
char string[32] = "hello, world";
sizeof (string)
=> 32
strlen (string)
=> 12
You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another. These functions are declared in the header file `string.h'.
A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array. All of these functions return the address of the destination array.
Most of these functions do not work properly if the source and destination arrays overlap. For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied. Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program.
All functions that have problems copying between overlapping arrays are
explicitly identified in this manual. In addition to functions in this
section, there are a few others like sprintf (see section Formatted Output Functions) and scanf (see section Formatted Input Functions).
Function: void * memcpy (void *to, const void *from, size_t size)
The memcpy function copies size bytes from the object
beginning at from into the object beginning at to. The
behavior of this function is undefined if the two arrays to and
from overlap; use memmove instead if overlapping is possible.
The value returned by memcpy is the value of to.
Here is an example of how you might use memcpy to copy the
contents of a struct:
struct foo *old, *new; ... memcpy (new, old, sizeof(struct foo));
Function: void * memmove (void *to, const void *from, size_t size)
memmove copies the size bytes at from into the
size bytes at to, even if those two blocks of space
overlap. In the case of overlap, memmove is careful to copy the
original values of the bytes in the block at from, including those
bytes which also belong to the block at to.
Function: void * memccpy (void *to, const void *from, int c, size_t size)
This function copies no more than size bytes from from to to, stopping if a byte matching c is found. The return value is a pointer into to one byte past where c was copied, or a null pointer if no byte matching c appeared in the first size bytes of from.
Function: void * memset (void *block, int c, size_t size)
This function copies the value of c (converted to an
unsigned char) into each of the first size bytes of the
object beginning at block. It returns the value of block.
Function: char * strcpy (char *to, const char *from)
This copies characters from the string from (up to and including
the terminating null character) into the string to. Like
memcpy, this function has undefined results if the strings
overlap. The return value is the value of to.
Function: char * strncpy (char *to, const char *from, size_t size)
This function is similar to strcpy but always copies exactly
size characters into to.
If the length of from is more than size, then strncpy
copies just the first size characters.
If the length of from is less than size, then strncpy
copies all of from, followed by enough null characters to add up
to size characters in all. This behavior is rarely useful, but it
is specified by the ANSI C standard.
The behavior of strncpy is undefined if the strings overlap.
Using strncpy as opposed to strcpy is a way to avoid bugs
relating to writing past the end of the allocated space for to.
However, it can also make your program much slower in one common case:
copying a string which is probably small into a potentially large buffer.
In this case, size may be large, and when it is, strncpy will
waste a considerable amount of time copying null characters.
Function: char * strdup (const char *s)
This function copies the null-terminated string s into a newly
allocated string. The string is allocated using malloc; see
section Unconstrained Allocation. If malloc cannot allocate space
for the new string, strdup returns a null pointer. Otherwise it
returns a pointer to the new string.
Function: char * stpcpy (char *to, const char *from)
This function is like strcpy, except that it returns a pointer to
the end of the string to (that is, the address of the terminating
null character) rather than the beginning.
For example, this program uses stpcpy to concatenate `foo'
and `bar' to produce `foobar', which it then prints.
#include <string.h>
int
main (void)
{
char *to = buffer;
to = stpcpy (to, "foo");
to = stpcpy (to, "bar");
printf ("%s\n", buffer);
}
This function is not part of the ANSI or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOG.
Its behavior is undefined if the strings overlap.
Function: char * strcat (char *to, const char *from)
The strcat function is similar to strcpy, except that the
characters from from are concatenated or appended to the end of
to, instead of overwriting it. That is, the first character from
from overwrites the null character marking the end of to.
An equivalent definition for strcat would be:
char *
strcat (char *to, const char *from)
{
strcpy (to + strlen (to), from);
return to;
}
This function has undefined results if the strings overlap.
Function: char * strncat (char *to, const char *from, size_t size)
This function is like strcat except that not more than size
characters from from are appended to the end of to. A
single null character is also always appended to to, so the total
allocated size of to must be at least size + 1 bytes
longer than its initial length.
char *
strncat (char *to, const char *from, size_t size)
{
strncpy (to + strlen (to), from, size);
return to;
}
The behavior of strncat is undefined if the strings overlap.
Here is an example showing the use of strncpy and strncat.
Notice how, in the call to strncat, the size parameter
is computed to avoid overflowing the character array buffer.
#include <string.h>
#include <stdio.h>
#define SIZE 10
static char buffer[SIZE];
main ()
{
strncpy (buffer, "hello", SIZE);
printf ("%s\n", buffer);
strncat (buffer, ", world", SIZE - strlen (buffer) - 1);
printf ("%s\n", buffer);
}
The output produced by this program looks like:
hello hello, wo
Function: void * bcopy (void *from, const void *to, size_t size)
This is a partially obsolete alternative for memmove, derived from
BSD. Note that it is not quite equivalent to memmove, because the
arguments are not in the same order.
Function: void * bzero (void *block, size_t size)
This is a partially obsolete alternative for memset, derived from
BSD. Note that it is not as general as memset, because the only
value it can store is zero.
You can use the functions in this section to perform comparisons on the contents of strings and arrays. As well as checking for equality, these functions can also be used as the ordering functions for sorting operations. See section Searching and Sorting, for an example of this.
Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are not equivalent rather than if they are. The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent: a negative value indicates that the first string is "less" than the second, while a positive value indicates that the first string is "greater".
If you are using these functions only to check for equality, you might find it makes for a cleaner program to hide them behind a macro definition, like this:
#define str_eq(s1,s2) (!strcmp ((s1),(s2)))
All of these functions are declared in the header file `string.h'.
Function: int memcmp (const void *a1, const void *a2, size_t size)
The function memcmp compares the size bytes of memory
beginning at a1 against the size bytes of memory beginning
at a2. The value returned has the same sign as the difference
between the first differing pair of bytes (interpreted as unsigned
char objects, then promoted to int).
If the contents of the two blocks are equal, memcmp returns
0.
On arbitrary arrays, the memcmp function is mostly useful for
testing equality. It usually isn't meaningful to do byte-wise ordering
comparisons on arrays of things other than bytes. For example, a
byte-wise comparison on the bytes that make up floating-point numbers
isn't likely to tell you anything about the relationship between the
values of the floating-point numbers.
You should also be careful about using memcmp to compare objects
that can contain "holes", such as the padding inserted into structure
objects to enforce alignment requirements, extra space at the end of
unions, and extra characters at the ends of strings whose length is less
than their allocated size. The contents of these "holes" are
indeterminate and may cause strange behavior when performing byte-wise
comparisons. For more predictable results, perform an explicit
component-wise comparison.
For example, given a structure type definition like:
struct foo
{
unsigned char tag;
union
{
double f;
long i;
char *p;
} value;
};
you are better off writing a specialized comparison function to compare
struct foo objects instead of comparing them with memcmp.
Function: int strcmp (const char *s1, const char *s2)
The strcmp function compares the string s1 against
s2, returning a value that has the same sign as the difference
between the first differing pair of characters (interpreted as
unsigned char objects, then promoted to int).
If the two strings are equal, strcmp returns 0.
A consequence of the ordering used by strcmp is that if s1
is an initial substring of s2, then s1 is considered to be
"less than" s2.
Function: int strcasecmp (const char *s1, const char *s2)
This function is like strcmp, except that differences in case
are ignored.
strcasecmp is derived from BSD.
Function: int strncasecmp (const char *s1, const char *s2, size_t n)
This function is like strncmp, except that differences in case
are ignored.
strncasecmp is a GNU extension.
Function: int strncmp (const char *s1, const char *s2, size_t size)
This function is the similar to strcmp, except that no more than
size characters are compared. In other words, if the two strings are
the same in their first size characters, the return value is zero.
Here are some examples showing the use of strcmp and strncmp.
These examples assume the use of the ASCII character set. (If some
other character set--say, EBCDIC--is used instead, then the glyphs
are associated with different numeric codes, and the return values
and ordering may differ.)
strcmp ("hello", "hello")
=> 0 /* These two strings are the same. */
strcmp ("hello", "Hello")
=> 32 /* Comparisons are case-sensitive. */
strcmp ("hello", "world")
=> -15 /* The character 'h' comes before 'w'. */
strcmp ("hello", "hello, world")
=> -44 /* Comparing a null character against a comma. */
strncmp ("hello", "hello, world"", 5)
=> 0 /* The initial 5 characters are the same. */
strncmp ("hello, world", "hello, stupid world!!!", 5)
=> 0 /* The initial 5 characters are the same. */
Function: int bcmp (const void *a1, const void *a2, size_t size)
This is an obsolete alias for memcmp, derived from BSD.
In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence `ll' is treated as a single letter that is collated immediately after `l'.
You can use the functions strcoll and strxfrm (declared in
the header file `string.h') to compare strings using a collation
ordering appropriate for the current locale. The locale used by these
functions in particular can be specified by setting the locale for the
LC_COLLATE category; see section Locales and Internationalization.
In the standard C locale, the collation sequence for strcoll is
the same as that for strcmp.
Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence.
The function strcoll performs this translation implicitly, in
order to do one comparison. By contrast, strxfrm performs the
mapping explicitly. If you are making multiple comparisons using the
same string or set of strings, it is likely to be more efficient to use
strxfrm to transform all the strings just once, and subsequently
compare the transformed strings with strcmp.
Function: int strcoll (const char *s1, const char *s2)
The strcoll function is similar to strcmp but uses the
collating sequence of the current locale for collation (the
LC_COLLATE locale).
Here is an example of sorting an array of strings, using strcoll
to compare them. The actual sort algorithm is not written here; it
comes from qsort (see section Array Sort Function). The job of the
code shown here is to say how to compare the strings while sorting them.
(Later on in this section, we will show a way to do this more
efficiently using strxfrm.)
/* This is the comparison function used withqsort. */ int compare_elements (char **p1, char **p2) { return strcoll (*p1, *p2); } /* This is the entry point--the function to sort strings using the locale's collating sequence. */ void sort_strings (char **array, int nstrings) { /* Sorttemp_arrayby comparing the strings. */ qsort (array, sizeof (char *), nstrings, compare_elements); }
Function: size_t strxfrm (char *to, const char *from, size_t size)
The function strxfrm transforms string using the collation
transformation determined by the locale currently selected for
collation, and stores the transformed string in the array to. Up
to size characters (including a terminating null character) are
stored.
The behavior is undefined if the strings to and from overlap; see section Copying and Concatenation.
The return value is the length of the entire transformed string. This
value is not affected by the value of size, but if it is greater
than size, it means that the transformed string did not entirely
fit in the array to. In this case, only as much of the string as
actually fits was stored. To get the whole transformed string, call
strxfrm again with a bigger output array.
The transformed string may be longer than the original string, and it may also be shorter.
If size is zero, no characters are stored in to. In this
case, strxfrm simply returns the number of characters that would
be the length of the transformed string. This is useful for determining
what size string to allocate. It does not matter what to is if
size is zero; to may even be a null pointer.
Here is an example of how you can use strxfrm when
you plan to do many comparisons. It does the same thing as the previous
example, but much faster, because it has to transform each string only
once, no matter how many times it is compared with other strings. Even
the time needed to allocate and free storage is much less than the time
we save, when there are many strings.
struct sorter { char *input; char *transformed; };
/* This is the comparison function used with qsort
to sort an array of struct sorter. */
int
compare_elements (struct sorter *p1, struct sorter *p2)
{
return strcmp (p1->transformed, p2->transformed);
}
/* This is the entry point--the function to sort
strings using the locale's collating sequence. */
void
sort_strings_fast (char **array, int nstrings)
{
struct sorter temp_array[nstrings];
int i;
/* Set up temp_array. Each element contains
one input string and its transformed string. */
for (i = 0; i < nstrings; i++)
{
size_t length = strlen (array[i]) * 2;
temp_array[i].input = array[i];
/* Transform array[i].
First try a buffer probably big enough. */
while (1)
{
char *transformed = (char *) xmalloc (length);
if (strxfrm (transformed, array[i], length) < length)
{
temp_array[i].transformed = transformed;
break;
}
/* Try again with a bigger buffer. */
free (transformed);
length *= 2;
}
}
/* Sort temp_array by comparing transformed strings. */
qsort (temp_array, sizeof (struct sorter),
nstrings, compare_elements);
/* Put the elements back in the permanent array
in their sorted order. */
for (i = 0; i < nstrings; i++)
array[i] = temp_array[i].input;
/* Free the strings we allocated. */
for (i = 0; i < nstrings; i++)
free (temp_array[i].transformed);
}
Compatibility Note: The string collation functions are a new feature of ANSI C. Older C dialects have no equivalent feature.
This section describes library functions which perform various kinds of searching operations on strings and arrays. These functions are declared in the header file `string.h'.
Function: void * memchr (const void *block, int c, size_t size)
This function finds the first occurrence of the byte c (converted
to an unsigned char) in the initial size bytes of the
object beginning at block. The return value is a pointer to the
located byte, or a null pointer if no match was found.
Function: char * strchr (const char *string, int c)
The strchr function finds the first occurrence of the character
c (converted to a char) in the null-terminated string
beginning at string. The return value is a pointer to the located
character, or a null pointer if no match was found.
For example,
strchr ("hello, world", 'l')
=> "llo, world"
strchr ("hello, world", '?')
=> NULL
The terminating null character is considered to be part of the string, so you can use this function get a pointer to the end of a string by specifying a null character as the value of the c argument.
Function: char * strrchr (const char *string, int c)
The function strrchr is like strchr, except that it searches
backwards from the end of the string string (instead of forwards
from the front).
For example,
strrchr ("hello, world", 'l')
=> "ld"
Function: char * strstr (const char *haystack, const char *needle)
This is like strchr, except that it searches haystack for a
substring needle rather than just a single character. It
returns a pointer into the string haystack that is the first
character of the substring, or a null pointer if no match was found. If
needle is an empty string, the function returns haystack.
For example,
strstr ("hello, world", "l")
=> "llo, world"
strstr ("hello, world", "wo")
=> "world"
Function: void * memmem (const void *needle, size_t needle_len,
const void *haystack, size_t haystack_len)
This is like strstr, but needle and haystack are byte
arrays rather than null-terminated strings. needle_len is the
length of needle and haystack_len is the length of
haystack.
This function is a GNU extension.
Function: size_t strspn (const char *string, const char *skipset)
The strspn ("string span") function returns the length of the
initial substring of string that consists entirely of characters that
are members of the set specified by the string skipset. The order
of the characters in skipset is not important.
For example,
strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
=> 5
Function: size_t strcspn (const char *string, const char *stopset)
The strcspn ("string complement span") function returns the length
of the initial substring of string that consists entirely of characters
that are not members of the set specified by the string stopset.
(In other words, it returns the offset of the first character in string
that is a member of the set stopset.)
For example,
strcspn ("hello, world", " \t\n,.;!?")
=> 5
Function: char * strpbrk (const char *string, const char *stopset)
The strpbrk ("string pointer break") function is related to
strcspn, except that it returns a pointer to the first character
in string that is a member of the set stopset instead of the
length of the initial substring. It returns a null pointer if no such
character from stopset is found.
For example,
strpbrk ("hello, world", " \t\n,.;!?")
=> ", world"
It's fairly common for programs to have a need to do some simple kinds
of lexical analysis and parsing, such as splitting a command string up
into tokens. You can do this with the strtok function, declared
in the header file `string.h'.
Function: char * strtok (char *newstring, const char *delimiters)
A string can be split into tokens by making a series of calls to the
function strtok.
The string to be split up is passed as the newstring argument on
the first call only. The strtok function uses this to set up
some internal state information. Subsequent calls to get additional
tokens from the same string are indicated by passing a null pointer as
the newstring argument. Calling strtok with another
non-null newstring argument reinitializes the state information.
It is guaranteed that no other library function ever calls strtok
behind your back (which would mess up this internal state information).
The delimiters argument is a string that specifies a set of delimiters that may surround the token being extracted. All the initial characters that are members of this set are discarded. The first character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next character that is a member of the delimiter set. This character in the original string newstring is overwritten by a null character, and the pointer to the beginning of the token in newstring is returned.
On the next call to strtok, the searching begins at the next
character beyond the one that marked the end of the previous token.
Note that the set of delimiters delimiters do not have to be the
same on every call in a series of calls to strtok.
If the end of the string newstring is reached, or if the remainder of
string consists only of delimiter characters, strtok returns
a null pointer.
Warning: Since strtok alters the string it is parsing,
you always copy the string to a temporary buffer before parsing it with
strtok. If you allow strtok to modify a string that came
from another part of your program, you are asking for trouble; that
string may be part of a data structure that could be used for other
purposes during the parsing, when alteration by strtok makes the
data structure temporarily inaccurate.
The string that you are operating on might even be a constant. Then
when strtok tries to modify it, your program will get a fatal
signal for writing in read-only memory. See section Program Error Signals.
This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily.
The function strtok is not reentrant. See section Signal Handling and Nonreentrant Functions, for
a discussion of where and why reentrancy is important.
Here is a simple example showing the use of strtok.
#include <string.h> #include <stddef.h> ... char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *token; ... token = strtok (string, delimiters); /* token => "words" */ token = strtok (NULL, delimiters); /* token => "separated" */ token = strtok (NULL, delimiters); /* token => "by" */ token = strtok (NULL, delimiters); /* token => "spaces" */ token = strtok (NULL, delimiters); /* token => "and" */ token = strtok (NULL, delimiters); /* token => "punctuation" */ token = strtok (NULL, delimiters); /* token => NULL */
A number of languages use character sets that are larger than the range
of values of type char. Japanese and Chinese are probably the
most familiar examples.
The GNU C library includes support for two mechanisms for dealing with extended character sets: multibyte characters and wide characters. This chapter describes how to use these mechanisms, and the functions for converting between them.
The behavior of the functions in this chapter is affected by the current
locale for character classification--the LC_CTYPE category; see
section Categories of Activities that Locales Affect. This choice of locale selects which multibyte
code is used, and also controls the meanings and characteristics of wide
character codes.
You can represent extended characters in either of two ways:
char objects. Their advantage is that many
programs and operating systems can handle occasional multibyte
characters scattered among ordinary ASCII characters, without any
change.
wchar_t,
has a range large enough to hold extended character codes as well as
old-fashioned ASCII codes.
An advantage of wide characters is that each character is a single data object, just like ordinary ASCII characters. There are a few disadvantages:
Typically, you use the multibyte character representation as part of the
external program interface, such as reading or writing text to files.
However, it's usually easier to perform internal manipulations on
strings containing extended characters on arrays of wchar_t
objects, since the uniform representation makes most editing operations
easier. If you do use multibyte characters for files and wide
characters for internal operations, you need to convert between them
when you read and write data.
If your system supports extended characters, then it supports them both as multibyte characters and as wide characters. The library includes functions you can use to convert between the two representations. These functions are described in this chapter.
A computer system can support more than one multibyte character code, and more than one wide character code. The user controls the choice of codes through the current locale for character classification (see section Locales and Internationalization). Each locale specifies a particular multibyte character code and a particular wide character code. The choice of locale influences the behavior of the conversion functions in the library.
Some locales support neither wide characters nor nontrivial multibyte characters. In these locales, the library conversion functions still work, even though what they do is basically trivial.
If you select a new locale for character classification, the internal shift state maintained by these functions can become confused, so it's not a good idea to change the locale while you are in the middle of processing a string.
In the ordinary ASCII code, a sequence of characters is a sequence of bytes, and each character is one byte. This is very simple, but allows for only 256 distinct characters.
In a multibyte character code, a sequence of characters is a sequence of bytes, but each character may occupy one or more consecutive bytes of the sequence.
There are many different ways of designing a multibyte character code; different systems use different codes. To specify a particular code means designating the basic byte sequences--those which represent a single character--and what characters they stand for. A code that a computer can actually use must have a finite number of these basic sequences, and typically none of them is more than a few characters long.
These sequences need not all have the same length. In fact, many of
them are just one byte long. Because the basic ASCII characters in the
range from 0 to 0177 are so important, they stand for
themselves in all multibyte character codes. That is to say, a byte
whose value is 0 through 0177 is always a character in
itself. The characters which are more than one byte must always start
with a byte in the range from 0200 through 0377.
The byte value 0 can be used to terminated a string, just as it
is often used in a string of ASCII characters.
Specifying the basic byte sequences that represent single characters
automatically gives meanings to many longer byte sequences, as more than
one character. For example, if the two byte sequence 0205 049
stands for the Greek letter alpha, then 0205 049 065 must stand
for an alpha followed by an `A' (ASCII code 065), and 0205 049
0205 049 must stand for two alphas in a row.
If any byte sequence can have more than one meaning as a sequence of characters, then the multibyte code is ambiguous--and no good. The codes that systems actually use are all unambiguous.
In most codes, there are certain sequences of bytes that have no meaning as a character or characters. These are called invalid.
The simplest possible multibyte code is a trivial one:
The basic sequences consist of single bytes.
This particular code is equivalent to not using multibyte characters at all. It has no invalid sequences. But it can handle only 256 different characters.
Here is another possible code which can handle 9376 different characters:
The basic sequences consist of
- single bytes with values in the range
0through0237.
- two-byte sequences, in which both of the bytes have values in the range from
0240through0377.
This code or a similar one is used on some systems to represent Japanese
characters. The invalid sequences are those which consist of an odd
number of consecutive bytes in the range from 0240 through
0377.
Here is another multibyte code which can handle more distinct extended characters--in fact, almost thirty million:
The basic sequences consist of
- single bytes with values in the range
0through0177.
- sequences of up to four bytes in which the first byte is in the range from
0200through0237, and the remaining bytes are in the range from0240through0377.
In this code, any sequence that starts with a byte in the range
from 0240 through 0377 is invalid.
And here is another variant which has the advantage that removing the last byte or bytes from a valid character can never produce another valid character. (This property is convenient when you want to search strings for particular characters.)
The basic sequences consist of
- single bytes with values in the range
0through0177.
- two-byte sequences in which the first byte is in the range from
0200through0207, and the second byte is in the range from0240through0377.
- three-byte sequences in which the first byte is in the range from
0210through0217, and the other bytes are in the range from0240through0377.
- four-byte sequences in which the first byte is in the range from
0220through0227, and the other bytes are in the range from0240through0377.
The list of invalid sequences for this code is long and not worth
stating in full; examples of invalid sequences include 0240 and
0220 0300 065.
The number of possible multibyte codes is astronomical. But a given computer system will support at most a few different codes. (One of these codes may allow for thousands of different characters.) Another computer system may support a completely different code. The library facilities described in this chapter are helpful because they package up the knowledge of the details of a particular computer system's multibyte code, so your programs need not know them.
You can use special standard macros to find out the maximum possible
number of bytes in a character in the currently selected multibyte
code with MB_CUR_MAX, and the maximum for any multibyte
code supported on your computer with MB_LEN_MAX.
This is the maximum length of a multibyte character for any supported locale. It is defined in `limits.h'.
This macro expands into a (possibly non-constant) positive integer
expression that is the maximum number of bytes in a multibyte character
in the current locale. The value is never greater than MB_LEN_MAX.
MB_CUR_MAX is defined in `stdlib.h'.
Normally, each basic sequence in a particular character code stands for one character, the same character regardless of context. Some multibyte character codes have a concept of shift state; certain codes, called shift sequences, change to a different shift state, and the meaning of some or all basic sequences varies according to the current shift state. In fact, the set of basic sequences might even be different depending on the current shift state. See section Multibyte Codes Using Shift Sequences, for more information on handling this sort of code.
What happens if you try to pass a string containing multibyte characters to a function that doesn't know about them? Normally, such a function treats a string as a sequence of bytes, and interprets certain byte values specially; all other byte values are "ordinary". As long as a multibyte character doesn't contain any of the special byte values, the function should pass it through as if it were several ordinary characters.
For example, let's figure out what happens if you use multibyte
characters in a file name. The functions such as open and
unlink that operate on file names treat the name as a sequence of
byte values, with `/' as the only special value. Any other byte
values are copied, or compared, in sequence, and all byte values are
treated alike. Thus, you may think of the file name as a sequence of
bytes or as a string containing multibyte characters; the same behavior
makes sense equally either way, provided no multibyte character contains
a `/'.
Wide characters are much simpler than multibyte characters. They
are simply characters with more than eight bits, so that they have room
for more than 256 distinct codes. The wide character data type,
wchar_t, has a range large enough to hold extended character
codes as well as old-fashioned ASCII codes.
An advantage of wide characters is that each character is a single data object, just like ordinary ASCII characters. Wide characters also have some disadvantages:
Wide character values 0 through 0177 are always identical
in meaning to the ASCII character codes. The wide character value zero
is often used to terminate a string of wide characters, just as a single
byte with value zero often terminates a string of ordinary characters.
This is the "wide character" type, an integer type whose range is large enough to represent all distinct values in any extended character set in the supported locales. See section Locales and Internationalization, for more information about locales. This type is defined in the header file `stddef.h'.
If your system supports extended characters, then each extended character has both a wide character code and a corresponding multibyte basic sequence.
In this chapter, the term code is used to refer to a single
extended character object to emphasize the distinction from the
char data type.
The mbstowcs function converts a string of multibyte characters
to a wide character array. The wcstombs function does the
reverse. These functions are declared in the header file
`stdlib.h'.
In most programs, these functions are the only ones you need for conversion between wide strings and multibyte character strings. But they have limitations. If your data is not null-terminated or is not all in core at once, you probably need to use the low-level conversion functions to convert one character at a time. See section Conversion of Extended Characters One by One.
Function: size_t mbstowcs (wchar_t *wstring, const char *string, size_t size)
The mbstowcs ("multibyte string to wide character string")
function converts the null-terminated string of multibyte characters
string to an array of wide character codes, storing not more than
size wide characters into the array beginning at wstring.
The terminating null character counts towards the size, so if size
is less than the actual number of wide characters resulting from
string, no terminating null character is stored.
The conversion of characters from string begins in the initial shift state.
If an invalid multibyte character sequence is found, this function
returns a value of -1. Otherwise, it returns the number of wide
characters stored in the array wstring. This number does not
include the terminating null character, which is present if the number
is less than size.
Here is an example showing how to convert a string of multibyte characters, allocating enough space for the result.
wchar_t *
mbstowcs_alloc (char *string)
{
int size = strlen (string) + 1;
wchar_t *buffer = (wchar_t) xmalloc (size * sizeof (wchar_t));
size = mbstowcs (buffer, string, size);
if (size < 0)
return NULL;
return (wchar_t) xrealloc (buffer, (size + 1) * sizeof (wchar_t));
}
Function: size_t wcstombs (char *string, const wchar_t wstring, size_t size)
The wcstombs ("wide character string to multibyte string")
function converts the null-terminated wide character array wstring
into a string containing multibyte characters, storing not more than
size bytes starting at string, followed by a terminating
null character if there is room. The conversion of characters begins in
the initial shift state.
The terminating null character counts towards the size, so if size is less than or equal to the number of bytes needed in wstring, no terminating null character is stored.
If a code that does not correspond to a valid multibyte character is
found, this function returns a value of -1. Otherwise, the
return value is the number of bytes stored in the array string.
This number does not include the terminating null character, which is
present if the number is less than size.
This section describes how to scan a string containing multibyte
characters, one character at a time. The difficulty in doing this
is to know how many bytes each character contains. Your program
can use mblen to find this out.
Function: int mblen (const char *string, size_t size)
The mblen function with non-null string returns the number
of bytes that make up the multibyte character beginning at string,
never examining more than size bytes. (The idea is to supply
for size the number of bytes of data you have in hand.)
The return value of mblen distinguishes three possibilities: the
first size bytes at string start with valid multibyte
character, they start with an invalid byte sequence or just part of a
character, or string points to an empty string (a null character).
For a valid multibyte character, mblen returns the number of
bytes in that character (always at least 1, and never more than
size). For an invalid byte sequence, mblen returns
-1. For an empty string, it returns 0.
If the multibyte character code uses shift characters, then mblen
maintains and updates a shift state as it scans. If you call
mblen with a null pointer for string, that initializes the
shift state to its standard initial value. It also returns nonzero if
the multibyte character code in use actually has a shift state.
See section Multibyte Codes Using Shift Sequences.
The function mblen is declared in `stdlib.h'.
You can convert multibyte characters one at a time to wide characters
with the mbtowc function. The wctomb function does the
reverse. These functions are declared in `stdlib.h'.
Function: int mbtowc (wchar_t *result, const char *string, size_t size)
The mbtowc ("multibyte to wide character") function when called
with non-null string converts the first multibyte character
beginning at string to its corresponding wide character code. It
stores the result in *result.
mbtowc never examines more than size bytes. (The idea is
to supply for size the number of bytes of data you have in hand.)
mbtowc with non-null string distinguishes three
possibilities: the first size bytes at string start with
valid multibyte character, they start with an invalid byte sequence or
just part of a character, or string points to an empty string (a
null character).
For a valid multibyte character, mbtowc converts it to a wide
character and stores that in *result, and returns the
number of bytes in that character (always at least 1, and never
more than size).
For an invalid byte sequence, mbtowc returns -1. For an
empty string, it returns 0, also storing 0 in
*result.
If the multibyte character code uses shift characters, then
mbtowc maintains and updates a shift state as it scans. If you
call mbtowc with a null pointer for string, that
initializes the shift state to its standard initial value. It also
returns nonzero if the multibyte character code in use actually has a
shift state. See section Multibyte Codes Using Shift Sequences.
Function: int wctomb (char *string, wchar_t wchar)
The wctomb ("wide character to multibyte") function converts
the wide character code wchar to its corresponding multibyte
character sequence, and stores the result in bytes starting at
string. At most MB_CUR_MAX characters are stored.
wctomb with non-null string distinguishes three
possibilities for wchar: a valid wide character code (one that can
be translated to a multibyte character), an invalid code, and 0.
Given a valid code, wctomb converts it to a multibyte character,
storing the bytes starting at string. Then it returns the number
of bytes in that character (always at least 1, and never more
than MB_CUR_MAX).
If wchar is an invalid wide character code, wctomb returns
-1. If wchar is 0, it returns 0, also
storing 0 in *string.
If the multibyte character code uses shift characters, then
wctomb maintains and updates a shift state as it scans. If you
call wctomb with a null pointer for string, that
initializes the shift state to its standard initial value. It also
returns nonzero if the multibyte character code in use actually has a
shift state. See section Multibyte Codes Using Shift Sequences.
Calling this function with a wchar argument of zero when
string is not null has the side-effect of reinitializing the
stored shift state as well as storing the multibyte character
0 and returning 0.
Here is an example that reads multibyte character text from descriptor
input and writes the corresponding wide characters to descriptor
output. We need to convert characters one by one for this
example because mbstowcs is unable to continue past a null
character, and cannot cope with an apparently invalid partial character
by reading more input.
int
file_mbstowcs (int input, int output)
{
char buffer[BUFSIZ + MB_LEN_MAX];
int filled = 0;
int eof = 0;
while (!eof)
{
int nread;
int nwrite;
char *inp = buffer;
wchar_t outbuf[BUFSIZ];
wchar_t *outp = outbuf;
/* Fill up the buffer from the input file. */
nread = read (input, buffer + filled, BUFSIZ);
if (nread < 0) {
perror ("read");
return 0;
}
/* If we reach end of file, make a note to read no more. */
if (nread == 0)
eof = 1;
/* filled is now the number of bytes in buffer. */
filled += nread;
/* Convert those bytes to wide characters--as many as we can. */
while (1)
{
int thislen = mbtowc (outp, inp, filled);
/* Stop converting at invalid character;
this can mean we have read just the first part
of a valid character. */
if (thislen == -1)
break;
/* Treat null character like any other,
but also reset shift state. */
if (thislen == 0) {
thislen = 1;
mbtowc (NULL, NULL, 0);
}
/* Advance past this character. */
inp += thislen;
filled -= thislen;
outp++;
}
/* Write the wide characters we just made. */
nwrite = write (output, outbuf,
(outp - outbuf) * sizeof (wchar_t));
if (nwrite < 0)
{
perror ("write");
return 0;
}
/* See if we have a real invalid character. */
if ((eof && filled > 0) || filled >= MB_CUR_MAX)
{
error ("invalid multibyte character");
return 0;
}
/* If any characters must be carried forward,
put them at the beginning of buffer. */
if (filled > 0)
memcpy (inp, buffer, filled);
}
}
return 1;
}
In some multibyte character codes, the meaning of any particular byte sequence is not fixed; it depends on what other sequences have come earlier in the same string. Typically there are just a few sequences that can change the meaning of other sequences; these few are called shift sequences and we say that they set the shift state for other sequences that follow.
To illustrate shift state and shift sequences, suppose we decide that
the sequence 0200 (just one byte) enters Japanese mode, in which
pairs of bytes in the range from 0240 to 0377 are single
characters, while 0201 enters Latin-1 mode, in which single bytes
in the range from 0240 to 0377 are characters, and
interpreted according to the ISO Latin-1 character set. This is a
multibyte code which has two alternative shift states ("Japanese mode"
and "Latin-1 mode"), and two shift sequences that specify particular
shift states.
When the multibyte character code in use has shift states, then
mblen, mbtowc and wctomb must maintain and update
the current shift state as they scan the string. To make this work
properly, you must follow these rules:
mblen (NULL,
0). This initializes the shift state to its standard initial value.
Here is an example of using mblen following these rules:
void
scan_string (char *s)
{
int length = strlen (s);
/* Initialize shift state. */
mblen (NULL, 0);
while (1)
{
int thischar = mblen (s, length);
/* Deal with end of string and invalid characters. */
if (thischar == 0)
break;
if (thischar == -1)
{
error ("invalid multibyte character");
break;
}
/* Advance past this character. */
s += thischar;
length -= thischar;
}
}
The functions mblen, mbtowc and wctomb are not
reentrant when using a multibyte code that uses a shift state. However,
no other library functions call these functions, so you don't have to
worry that the shift state will be changed mysteriously.
Different countries and cultures have varying conventions for how to communicate. These conventions range from very simple ones, such as the format for representing dates and times, to very complex ones, such as the language spoken.
Internationalization of software means programming it to be able to adapt to the user's favorite conventions. In ANSI C, internationalization works by means of locales. Each locale specifies a collection of conventions, one convention for each purpose. The user chooses a set of conventions by specifying a locale (via environment variables).
All programs inherit the chosen locale as part of their environment. Provided the programs are written to obey the choice of locale, they will follow the conventions preferred by the user.
Each locale specifies conventions for several purposes, including the following:
Some aspects of adapting to the specified locale are handled
automatically by the library subroutines. For example, all your program
needs to do in order to use the collating sequence of the chosen locale
is to use strcoll or strxfrm to compare strings.
Other aspects of locales are beyond the comprehension of the library. For example, the library can't automatically translate your program's output messages into other languages. The only way you can support output in the user's favorite language is to program this more or less by hand. (Eventually, we hope to provide facilities to make this easier.)
This chapter discusses the mechanism by which you can modify the current locale. The effects of the current locale on specific library functions are discussed in more detail in the descriptions of those functions.
The simplest way for the user to choose a locale is to set the
environment variable LANG. This specifies a single locale to use
for all purposes. For example, a user could specify a hypothetical
locale named `espana-castellano' to use the standard conventions of
most of Spain.
The set of locales supported depends on the operating system you are using, and so do their names. We can't make any promises about what locales will exist, except for one standard locale called `C' or `POSIX'.
A user also has the option of specifying different locales for different purposes--in effect, choosing a mixture of two locales.
For example, the user might specify the locale `espana-castellano' for most purposes, but specify the locale `usa-english' for currency formatting. This might make sense if the user is a Spanish-speaking American, working in Spanish, but representing monetary amounts in US dollars.
Note that both locales `espana-castellano' and `usa-english', like all locales, would include conventions for all of the purposes to which locales apply. However, the user can choose to use each locale for a particular subset of those purposes.
The purposes that locales serve are grouped into categories, so
that a user or a program can choose the locale for each category
independently. Here is a table of categories; each name is both an
environment variable that a user can set, and a macro name that you can
use as an argument to setlocale.
LC_COLLATE
strcoll
and strxfrm); see section Collation Functions.
LC_CTYPE
LC_MONETARY
LC_NUMERIC
LC_TIME
LC_ALL
setlocale to set a single locale for all purposes.
LANG
A C program inherits its locale environment variables when it starts up.
This happens automatically. However, these variables do not
automatically control the locale used by the library functions, because
ANSI C says that all programs start by default in the standard `C'
locale. To use the locales specified by the environment, you must call
setlocale. Call it as follows:
setlocale (LC_ALL, "");
to select a locale based on the appropriate environment variables.
You can also use setlocale to specify a particular locale, for
general use or for a specific category.
The symbols in this section are defined in the header file `locale.h'.
Function: char * setlocale (int category, const char *locale)
The function setlocale sets the current locale for
category category to locale.
If category is LC_ALL, this specifies the locale for all
purposes. The other possible values of category specify an
individual purpose (see section Categories of Activities that Locales Affect).
You can also use this function to find out the current locale by passing
a null pointer as the locale argument. In this case,
setlocale returns a string that is the name of the locale
currently selected for category category.
The string returned by setlocale can be overwritten by subsequent
calls, so you should make a copy of the string (see section Copying and Concatenation) if you want to save it past any further calls to
setlocale. (The standard library is guaranteed never to call
setlocale itself.)
You should not modify the string returned by setlocale.
It might be the same string that was passed as an argument in a
previous call to setlocale.
When you read the current locale for category LC_ALL, the value
encodes the entire combination of selected locales for all categories.
In this case, the value is not just a single locale name. In fact, we
don't make any promises about what it looks like. But if you specify
the same "locale name" with LC_ALL in a subsequent call to
setlocale, it restores the same combination of locale selections.
When the locale argument is not a null pointer, the string returned
by setlocale reflects the newly modified locale.
If you specify an empty string for locale, this means to read the appropriate environment variable and use its value to select the locale for category.
If you specify an invalid locale name, setlocale returns a null
pointer and leaves the current locale unchanged.
Here is an example showing how you might use setlocale to
temporarily switch to a new locale.
#include <stddef.h>
#include <locale.h>
#include <stdlib.h>
#include <string.h>
void
with_other_locale (char *new_locale,
void (*subroutine) (int),
int argument)
{
char *old_locale, *saved_locale;
/* Get the name of the current locale. */
old_locale = setlocale (LC_ALL, NULL);
/* Copy the name so it won't be clobbered by setlocale. */
saved_locale = strdup (old_locale);
if (old_locale == NULL)
fatal ("Out of memory");
/* Now change the locale and do some stuff with it. */
setlocale (LC_ALL, new_locale);
(*subroutine) (argument);
/* Restore the original locale. */
setlocale (LC_ALL, saved_locale);
free (saved_locale);
}
Portability Note: Some ANSI C systems may define additional locale categories. For portability, assume that any symbol beginning with `LC_' might be defined in `locale.h'.
The only locale names you can count on finding on all operating systems are these three standard ones:
"C"
"POSIX"
""
Defining and installing named locales is normally a responsibility of the system administrator at your site (or the person who installed the GNU C library). Some systems may allow users to create locales, but we don't discuss that here.
If your program needs to use something other than the `C' locale, it will be more portable if you use the whatever locale the user specifies with the environment, rather than trying to specify some non-standard locale explicitly by name. Remember, different machines might have different sets of locales installed.
When you want to format a number or a currency amount using the
conventions of the current locale, you can use the function
localeconv to get the data on how to do it. The function
localeconv is declared in the header file `locale.h'.
Function: struct lconv * localeconv (void)
The localeconv function returns a pointer to a structure whose
components contain information about how numeric and monetary values
should be formatted in the current locale.
You shouldn't modify the structure or its contents. The structure might
be overwritten by subsequent calls to localeconv, or by calls to
setlocale, but no other function in the library overwrites this
value.
This is the data type of the value returned by localeconv.
If a member of the structure struct lconv has type char,
and the value is CHAR_MAX, it means that the current locale has
no value for that parameter.
These are the standard members of struct lconv; there may be
others.
char *decimal_point
char *mon_decimal_point
decimal_point is ".", and the value of
mon_decimal_point is "".
char *thousands_sep
char *mon_thousands_sep
"" (the empty string).
char *grouping
char *mon_grouping
grouping applies to non-monetary quantities
and mon_grouping applies to monetary quantities. Use either
thousands_sep or mon_thousands_sep to separate the digit
groups.
Each string is made up of decimal numbers separated by semicolons. Successive numbers (from left to right) give the sizes of successive groups (from right to left, starting at the decimal point). The last number in the string is used over and over for all the remaining groups.
If the last integer is -1, it means that there is no more
grouping--or, put another way, any remaining digits form one large
group without separators.
For example, if grouping is "4;3;2", the number
123456787654321 should be grouped into `12', `34',
`56', `78', `765', `4321'. This uses a group of 4
digits at the end, preceded by a group of 3 digits, preceded by groups
of 2 digits (as many as needed). With a separator of `,', the
number would be printed as `12,34,56,78,765,4321'.
A value of "3" indicates repeated groups of three digits, as
normally used in the U.S.
In the standard `C' locale, both grouping and
mon_grouping have a value of "". This value specifies no
grouping at all.
char int_frac_digits
char frac_digits
In the standard `C' locale, both of these members have the value
CHAR_MAX, meaning "unspecified". The ANSI standard doesn't say
what to do when you find this the value; we recommend printing no
fractional digits. (This locale also specifies the empty string for
mon_decimal_point, so printing any fractional digits would be
confusing!)
These members of the struct lconv structure specify how to print
the symbol to identify a monetary value--the international analog of
`$' for US dollars.
Each country has two standard currency symbols. The local currency symbol is used commonly within the country, while the international currency symbol is used internationally to refer to that country's currency when it is necessary to indicate the country unambiguously.
For example, many countries use the dollar as their monetary unit, and when dealing with international currencies it's important to specify that one is dealing with (say) Canadian dollars instead of U.S. dollars or Australian dollars. But when the context is known to be Canada, there is no need to make this explicit--dollar amounts are implicitly assumed to be in Canadian dollars.
char *currency_symbol
In the standard `C' locale, this member has a value of ""
(the empty string), meaning "unspecified". The ANSI standard doesn't
say what to do when you find this value; we recommend you simply print
the empty string as you would print any other string found in the
appropriate member.
char *int_curr_symbol
The value of int_curr_symbol should normally consist of a
three-letter abbreviation determined by the international standard
ISO 4217 Codes for the Representation of Currency and Funds,
followed by a one-character separator (often a space).
In the standard `C' locale, this member has a value of ""
(the empty string), meaning "unspecified". We recommend you simply
print the empty string as you would print any other string found in the
appropriate member.
char p_cs_precedes
char n_cs_precedes
1 if the currency_symbol string should
precede the value of a monetary amount, or 0 if the string should
follow the value. The p_cs_precedes member applies to positive
amounts (or zero), and the n_cs_precedes member applies to
negative amounts.
In the standard `C' locale, both of these members have a value of
CHAR_MAX, meaning "unspecified". The ANSI standard doesn't say
what to do when you find this value, but we recommend printing the
currency symbol before the amount. That's right for most countries.
In other words, treat all nonzero values alike in these members.
The POSIX standard says that these two members apply to the
int_curr_symbol as well as the currency_symbol. The ANSI
C standard seems to imply that they should apply only to the
currency_symbol---so the int_curr_symbol should always
preceed the amount.
We can only guess which of these (if either) matches the usual conventions for printing international currency symbols. Our guess is that they should always preceed the amount. If we find out a reliable answer, we will put it here.
char p_sep_by_space
char n_sep_by_space
1 if a space should appear between the
currency_symbol string and the amount, or 0 if no space
should appear. The p_sep_by_space member applies to positive
amounts (or zero), and the n_sep_by_space member applies to
negative amounts.
In the standard `C' locale, both of these members have a value of
CHAR_MAX, meaning "unspecified". The ANSI standard doesn't say
what you should do when you find this value; we suggest you treat it as
one (print a space). In other words, treat all nonzero values alike in
these members.
These members apply only to currency_symbol. When you use
int_curr_symbol, you never print an additional space, because
int_curr_symbol itself contains the appropriate separator.
The POSIX standard says that these two members apply to the
int_curr_symbol as well as the currency_symbol. But an
example in the ANSI C standard clearly implies that they should apply
only to the currency_symbol---that the int_curr_symbol
contains any appropriate separator, so you should never print an
additional space.
Based on what we know now, we recommend you ignore these members when printing international currency symbols, and print no extra space.
These members of the struct lconv structure specify how to print
the sign (if any) in a monetary value.
char *positive_sign
char *negative_sign
In the standard `C' locale, both of these members have a value of
"" (the empty string), meaning "unspecified".
The ANSI standard doesn't say what to do when you find this value; we
recommend printing positive_sign as you find it, even if it is
empty. For a negative value, print negative_sign as you find it
unless both it and positive_sign are empty, in which case print
`-' instead. (Failing to indicate the sign at all seems rather
unreasonable.)
char p_sign_posn
char n_sign_posn
positive_sign or negative_sign.) The possible values are
as follows:
0
1
2
3
4
CHAR_MAX
The ANSI standard doesn't say what you should do when the value is
CHAR_MAX. We recommend you print the sign after the currency
symbol.
It is not clear whether you should let these members apply to the international currency format or not. POSIX says you should, but intuition plus the examples in the ANSI C standard suggest you should not. We hope that someone who knows well the conventions for formatting monetary quantities will tell us what we should recommend.
This chapter describes functions for searching and sorting arrays of arbitrary objects. You pass the appropriate comparison function to be applied as an argument, along with the size of the objects in the array and the total number of elements.
In order to use the sorted array library functions, you have to describe how to compare the elements of the array.
To do this, you supply a comparison function to compare two elements of
the array. The library will call this function, passing as arguments
pointers to two array elements to be compared. Your comparison function
should return a value the way strcmp (see section String/Array Comparison) does: negative if the first argument is "less" than the
second, zero if they are "equal", and positive if the first argument
is "greater".
Here is an example of a comparison function which works with an array of
numbers of type double:
int
compare_doubles (const double *a, const double *b)
{
double temp = *a - *b;
if (temp > 0)
return 1;
else if (temp < 0)
return -1;
else
return 0;
}
The header file `stdlib.h' defines a name for the data type of comparison functions. This is a GNU extension and thus defined only if you request the GNU extensions.
int comparison_fn_t (const void *, const void *);
To search a sorted array for an element matching the key, use the
bsearch function. The prototype for this function is in
the header file `stdlib.h'.
Function: void * bsearch (const void *key, const void *array, size_t count, size_t size, comparison_fn_t compare)
The bsearch function searches the sorted array array for an object
that is equivalent to key. The array contains count elements,
each of which is of size size.
The compare function is used to perform the comparison. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument. The elements of the array must already be sorted in ascending order according to this comparison function.
The return value is a pointer to the matching array element, or a null pointer if no match is found. If the array contains more than one element that matches, the one that is returned is unspecified.
This function derives its name from the fact that it is implemented using the binary search.
To sort an array using an arbitrary comparison function, use the
qsort function. The prototype for this function is in
`stdlib.h'.
Function: void qsort (void *array, size_t count, size_t size, comparison_fn_t compare)
The qsort function sorts the array array. The array contains count elements, each of which is of size size.
The compare function is used to perform the comparison on the array elements. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument.
Warning: If two objects compare as equal, their order after sorting is unpredictable. That is to say, the sorting is not stable. This can make a difference when the comparison considers only part of the elements. Two elements with the same sort key may differ in other respects.
If you want the effect of a stable sort, you can get this result by writing the comparison function so that, lacking other reason distinguish between two elements, it compares them by their addresses.
Here is a simple example of sorting an array of doubles in numerical order, using the comparison function defined above (see section Defining the Comparison Function):
{
double *array;
int size;
...
qsort (array, size, sizeof (double), compare_doubles);
}
The qsort function derives its name from the fact that it was
originally implemented using the algorithm "quick sort".
Here is an example showing the use of qsort and bsearch
with an array of structures. The objects in the array are sorted
by comparing their name fields with the strcmp function.
Then, we can look up individual objects based on their names.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
/* Define an array of critters to sort. */
struct critter
{
char *name;
char *species;
};
struct critter muppets[]=
{
{"Kermit", "frog"},
{"Piggy", "pig"},
{"Gonzo", "whatever"},
{"Fozzie", "bear"},
{"Sam", "eagle"},
{"Robin", "frog"},
{"Animal", "animal"},
{"Camilla", "chicken"},
{"Sweetums", "monster"},
{"Dr. Strangepork", "pig"},
{"Link Hogthrob", "pig"},
{"Zoot", "human"},
{"Dr. Bunsen Honeydew", "human"},
{"Beaker", "human"},
{"Swedish Chef", "human"}};
int count = sizeof (muppets) / sizeof (struct critter);
/* This is the comparison function used for sorting and searching. */
int
critter_cmp (const struct critter *c1, const struct critter *c2)
{
return strcmp (c1->name, c2->name);
}
/* Print information about a critter. */
void
print_critter (const struct critter *c)
{
printf ("%s, the %s\n", c->name, c->species);
}
/* Do the lookup into the sorted array. */
void
find_critter (char *name)
{
struct critter target, *result;
target.name = name;
result = bsearch (&target, muppets, count, sizeof (struct critter),
critter_cmp);
if (result)
print_critter (result);
else
printf ("Couldn't find %s.\n", name);
}
/* Main program. */
int
main (void)
{
int i;
for (i = 0; i < count; i++)
print_critter (&muppets[i]);
printf ("\n");
qsort (muppets, count, sizeof (struct critter), critter_cmp);
for (i = 0; i < count; i++)
print_critter (&muppets[i]);
printf ("\n");
find_critter ("Kermit");
find_critter ("Gonzo");
find_critter ("Janice");
return 0;
}
The output from this program looks like:
Animal, the animal Beaker, the human Camilla, the chicken Dr. Bunsen Honeydew, the human Dr. Strangepork, the pig Fozzie, the bear Gonzo, the whatever Kermit, the frog Link Hogthrob, the pig Piggy, the pig Robin, the frog Sam, the eagle Swedish Chef, the human Sweetums, the monster Zoot, the human Kermit, the frog Gonzo, the whatever Couldn't find Janice.
The GNU C Library provides pattern matching facilities for two kinds of patterns: regular expressions and file-name wildcards.
This section describes how to match a wildcard pattern against a particular string. The result is a yes or no answer: does the string fit the pattern or not. The symbols described here are all declared in `fnmatch.h'.
Function: int fnmatch (const char *pattern, const char *string, int flags)
This function tests whether the string string matches the pattern
pattern. It returns 0 if they do match; otherwise, it
returns the nonzero value FNM_NOMATCH. The arguments
pattern and string are both strings.
The argument flags is a combination of flag bits that alter the details of matching. See below for a list of the defined flags.
In the GNU C Library, fnmatch cannot experience an "error"---it
always returns an answer for whether the match succeeds. However, other
implementations of fnmatch might sometimes report "errors".
They would do so by returning nonzero values that are not equal to
FNM_NOMATCH.
These are the available flags for the flags argument:
FNM_FILE_NAME
FNM_PATHNAME
FNM_FILE_NAME; it comes from POSIX.2. We
don't recommend this name because we don't use the term "pathname" for
file names.
FNM_PERIOD
If you set both FNM_PERIOD and FNM_FILE_NAME, then the
special treatment applies to `.' following `/' as well as
to `.' at the beginning of string.
FNM_NOESCAPE
If you use FNM_NOESCAPE, then `\' is an ordinary character.
FNM_LEADING_DIR
If this flag is set, either `foo*' or `foobar' as a pattern would match the string `foobar/frobozz'.
FNM_CASEFOLD
The archetypal use of wildcards is for matching against the files in a directory, and making a list of all the matches. This is called globbing.
You could do this using fnmatch, by reading the directory entries
one by one and testing each one with fnmatch. But that would be
slow (and complex, since you would have to handle subdirectories by
hand).
The library provides a function glob to make this particular use
of wildcards convenient. glob and the other symbols in this
section are declared in `glob.h'.
glob
The result of globbing is a vector of file names (strings). To return
this vector, glob uses a special data type, glob_t, which
is a structure. You pass glob the address of the structure, and
it fills in the structure's fields to tell you about the results.
This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size.
gl_pathc
gl_pathv
char **.
gl_offs
gl_pathv field. Unlike the other fields, this
is always an input to glob, rather than an output from it.
If you use a nonzero offset, then that many elements at the beginning of
the vector are left empty. (The glob function fills them with
null pointers.)
The gl_offs field is meaningful only if you use the
GLOB_DOOFFS flag. Otherwise, the offset is always zero
regardless of what is in this field, and the first real element comes at
the beginning of the vector.
Function: int glob (const char *pattern, int flags, int (*errfunc) (const char *filename, int error-code), glob_t *vector_ptr)
The function glob does globbing using the pattern pattern
in the current directory. It puts the result in a newly allocated
vector, and stores the size and address of this vector into
*vector-ptr. The argument flags is a combination of
bit flags; see section Flags for Globbing, for details of the flags.
The result of globbing is a sequence of file names. The function
glob allocates a string for each resulting word, then
allocates a vector of type char ** to store the addresses of
these strings. The last element of the vector is a null pointer.
This vector is called the word vector.
To return this vector, glob stores both its address and its
length (number of elements, not counting the terminating null pointer)
into *vector-ptr.
Normally, glob sorts the file names alphabetically before
returning them. You can turn this off with the flag GLOB_NOSORT
if you want to get the information as fast as possible. Usually it's
a good idea to let glob sort them--if you process the files in
alphabetical order, the users will have a feel for the rate of progress
that your application is making.
If glob succeeds, it returns 0. Otherwise, it returns one
of these error codes:
GLOB_ABORTED
GLOB_ERR or your specified errfunc returned a nonzero
value.
GLOB_NOMATCH
GLOB_NOCHECK flag, then you never get this error code, because
that flag tells glob to pretend that the pattern matched
at least one file.
GLOB_NOSPACE
In the event of an error, glob stores information in
*vector-ptr about all the matches it has found so far.
This section describes the flags that you can specify in the
flags argument to glob. Choose the flags you want,
and combine them with the C operator |.
GLOB_APPEND
glob. This way you can effectively expand
several words as if they were concatenated with spaces between them.
In order for appending to work, you must not modify the contents of the
word vector structure between calls to glob. And, if you set
GLOB_DOOFFS in the first call to glob, you must also
set it when you append to the results.
GLOB_DOOFFS
gl_offs field says how many slots to leave.
The blank slots contain null pointers.
GLOB_ERR
glob tries its best to keep
on going despite any errors, reading whatever directories it can.
You can exercise even more control than this by specifying an error-handler
function errfunc when you call glob. If errfunc is
nonzero, then glob doesn't give up right away when it can't read
a directory; instead, it calls errfunc with two arguments, like
this:
(*errfunc) (filename, error-code)
The argument filename is the name of the directory that
glob couldn't open or couldn't read, and error-code is the
errno value that was reported to glob.
If the error handler function returns nonzero, then glob gives up
right away. Otherwise, it continues.
GLOB_MARK
GLOB_NOCHECK
glob returns that there were no
matches.)
GLOB_NOSORT
GLOB_NOESCAPE
If you use GLOB_NOESCAPE, then `\' is an ordinary character.
glob does its work by calling the function fnmatch
repeatedly. It handles the flag GLOB_NOESCAPE by turning on the
FNM_NOESCAPE flag in calls to fnmatch.
The GNU C library supports two interfaces for matching regular expressions. One is the standard POSIX.2 interface, and the other is what the GNU system has had for many years.
Both interfaces are declared in the header file `regex.h'.
If you define _GNU_SOURCE, then the GNU functions, structures
and constants are declared. Otherwise, only the POSIX names are
declared.
Before you can actually match a regular expression, you must compile it. This is not true compilation--it produces a special data structure, not machine instructions. But it is like ordinary compilation in that its purpose is to enable you to "execute" the pattern fast. (See section Matching a Compiled POSIX Regular Expression, for how to use the compiled regular expression for matching.)
There is a special data type for compiled regular expressions:
This type of object holds a compiled regular expression. It is actually a structure. It has just one field that your programs should look at:
re_nsub
There are several other fields, but we don't describe them here, because only the functions in the library should use them.
After you create a regex_t object, you can compile a regular
expression into it by calling regcomp.
Function: int regcomp (regex_t *compiled, const char *pattern, int cflags)
The function regcomp "compiles" a regular expression into a
data structure that you can use with regexec to match against a
string. The compiled regular expression format is designed for
efficient matching. regcomp stores it into *compiled.
It's up to you to allocate an object of type regex_t and pass its
address to regcomp.
The argument cflags lets you specify various options that control the syntax and semantics of regular expressions. See section Flags for POSIX Regular Expressions.
If you use the flag REG_NOSUB, then regcomp omits from
the compiled regular expression the information necessary to record
how subexpressions actually match. In this case, you might as well
pass 0 for the matchptr and nmatch arguments when
you call regexec.
If you don't use REG_NOSUB, then the compiled regular expression
does have the capacity to record how subexpressions match. Also,
regcomp tells you how many subexpressions pattern has, by
storing the number in compiled->re_nsub. You can use that
value to decide how long an array to allocate to hold information about
subexpression matches.
regcomp returns 0 if it succeeds in compiling the regular
expression; otherwise, it returns a nonzero error code (see the table
below). You can use regerror to produce an error message string
describing the reason for a nonzero value; see section POSIX Regexp Matching Cleanup.
Here are the possible nonzero values that regcomp can return:
REG_BADBR
REG_BADPAT
REG_BADRPT
REG_ECOLLATE
REG_ECTYPE
REG_EESCAPE
REG_ESUBREG
REG_EBRACK
REG_EPAREN
REG_EBRACE
REG_ERANGE
REG_ESPACE
regcomp or regexec ran out of memory.
These are the bit flags that you can use in the cflags operand when
compiling a regular expression with regcomp.
REG_EXTENDED
REG_ICASE
REG_NOSUB
REG_NEWLINE
Otherwise, newline acts like any other ordinary character.
Once you have compiled a regular expression, as described in section POSIX Regular Expression Compilation, you can match it against strings using
regexec. A match anywhere inside the string counts as success,
unless the regular expression contains anchor characters (`^' or
`$').
Function: int regexec (regex_t *compiled, char *string, size_t nmatch, regmatch_t matchptr [], int eflags)
This function tries to match the compiled regular expression
*compiled against string.
regexec returns 0 if the regular expression matches;
otherwise, it returns a nonzero value. See the table below for
what nonzero values mean. You can use regerror to produce an
error message string describing the reason for a nonzero value;
see section POSIX Regexp Matching Cleanup.
The argument eflags is a word of bit flags that enable various options.
If you want to get information about what part of string actually
matched the regular expression or its subexpressions, use the arguments
matchptr and nmatch. Otherwise, pass 0 for
nmatch, and NULL for matchptr. See section Subexpressions Match Results.
You must match the regular expression with the same set of current locales that were in effect when you compiled the regular expression.
The function regexec accepts the following flags in the
eflags argument:
REG_NOTBOL
REG_NOTEOL
Here are the possible nonzero values that regexec can return:
REG_NOMATCH
REG_ESPACE
regcomp or regexec ran out of memory.
When regexec matches parenthetical subexpressions of
pattern, it records which parts of string they match. It
returns that information by storing the offsets into an array whose
elements are structures of type regmatch_t. The first element of
the array records the part of the string that matched the entire regular
expression. Each other element of the array records the beginning and
end of the part that matched a single parenthetical subexpression.
This is the data type of the matcharray array that you pass to
regexec. It containes two structure fields, as follows:
rm_so
rm_eo
regoff_t is an alias for another signed integer type.
The fields of regmatch_t have type regoff_t.
The regmatch_t elements correspond to subexpressions
positionally; the first element records where the first subexpression
matched, the second element records the second subexpression, and so on.
The order of the subexpressions is the order in which they begin.
When you call regexec, you specify how long the matchptr
array is, with the nmatch argument. This tells regexec how
many elements to store. If the actual regular expression has more than
nmatch subexpressions, then you won't get offset information about
the rest of them. But this doesn't alter whether the pattern matches a
particular string or not.
If you don't want regexec to return any information about where
the subexpressions matched, you can either supply 0 for
nmatch, or use the flag REG_NOSUB when you compile the
pattern with regcomp.
Sometimes a subexpression matches a substring of no characters. This
happens when `f\(o*\)' matches the string `fum'. (It really
matches just the `f'.) In this case, both of the offsets identify
the point in the string where the null substring was found. In this
example, the offsets are both 1.
Sometimes the entire regular expression can match without using some of
its subexpressions at all--for example, when `ba\(na\)*' matches the
string `ba', the parenthetical subexpression is not used. When
this happens, regexec stores -1 in both fields of the
element for that subexpression.
Sometimes matching the entire regular expression can match a particular
subexpression more than once--for example, when `ba\(na\)*'
matches the string `bananana', the parenthetical subexpression
matches three times. When this happens, regexec usually stores
the offsets of the last part of the string that matched the
subexpression. In the case of `bananana', these offsets are
6 and 8.
But the last match is not always the one that is chosen. It's more
accurate to say that the last opportunity to match is the one
that takes precedence. What this means is that when one subexpression
appears within another, then the results reported for the inner
subexpression reflect whatever happened on the last match of the outer
subexpression. For an example, consider `\(ba\(na\)*s \)' matching
the string `bananas bas '. The last time the inner expression
actually matches is near the end of the first word. But it is
considered again in the second word, and fails to match there.
regexec reports nonuse of the "na" subexpression.
Another place where this rule applies is when `\(ba\(na\)*s
\|nefer\(ti\)* \)*' matches `bananas nefertiti'. The "na"
subexpression does match in the first word, but it doesn't match in the
second word because the other alternative is used there. Once again,
the second repetition of the outer subexpression overrides the first,
and within that second repetition, the "na" subexpression is not used.
So regexec reports nonuse of the "na" subexpression.
When you are finished using a compiled regular expression, you can
free the storage it uses by calling regfree.
Function: void regfree (regex_t *compiled)
Calling regfree frees all the storage that *compiled
points to. This includes various internal fields of the regex_t
structure that aren't documented in this manual.
regfree does not free the object *compiled itself.
You should always free the space in a regex_t structure with
regfree before using the structure to compile another regular
expression.
When regcomp or regexec reports an error, you can use
the function regerror to turn it into an error message string.
Function: size_t regerror (int errcode, regex_t *compiled, char *buffer, size_t length)
This function produces an error message string for the error code
errcode, and stores the string in length bytes of memory
starting at buffer. For the compiled argument, supply the
same compiled regular expression structure that regcomp or
regexec was working with when it got the error. Alternatively,
you can supply NULL for compiled; you will still get a
meaningful error message, but it might not be as detailed.
If the error message can't fit in length bytes (including a
terminating null character), then regerror truncates it.
The string that regerror stores is always null-terminated
even if it has been truncated.
The return value of regerror is the minimum length needed to
store the entire error message. If this is less than length, then
the error message was not truncated, and you can use it. Otherwise, you
should call regerror again with a larger buffer.
char *get_regerror (int errcode, regex_t *compiled)
{
size_t length = regerror (errcode, compiled, NULL, 0);
char *buffer = xmalloc (length);
(void) regerror (errcode, compiled, buffer, length);
return buffer;
}
Word expansion means the process of splitting a string into words and substituting for variables, commands, and wildcards just as the shell does.
For example, when you write `ls -l foo.c', this string is split into three separate words---`ls', `-l' and `foo.c'. This is the most basic function of word expansion.
When you write `ls *.c', this can become many words, because the word `*.c' can be replaced with any number of file names. This is called wildcard expansion, and it is also a part of word expansion.
When you use `echo $PATH' to print your path, you are taking advantage of variable substitution, which is also part of word expansion.
Ordinary programs can perform word expansion just like the shell by
calling the library function wordexp.
When word expansion is applied to a sequence of words, it performs the following transformations in the order shown here:
For the details of these transformations, and how to write the constructs that use them, see The BASH Manual (to appear).
wordexpAll the functions, constants and data types for word expansion are declared in the header file `wordexp.h'.
Word expansion produces a vector of words (strings). To return this
vector, wordexp uses a special data type, wordexp_t, which
is a structure. You pass wordexp the address of the structure,
and it fills in the structure's fields to tell you about the results.
This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size.
we_wordc
we_wordv
char **.
we_offs
we_wordv field. Unlike the other fields, this
is always an input to wordexp, rather than an output from it.
If you use a nonzero offset, then that many elements at the beginning of
the vector are left empty. (The wordexp function fills them with
null pointers.)
The we_offs field is meaningful only if you use the
WRDE_DOOFFS flag. Otherwise, the offset is always zero
regardless of what is in this field, and the first real element comes at
the beginning of the vector.
Function: int wordexp (const char *words, wordexp_t *word-vector-ptr, int flags)
Perform word expansion on the string words, putting the result in
a newly allocated vector, and store the size and address of this vector
into *word-vector-ptr. The argument flags is a
combination of bit flags; see section Flags for Word Expansion, for details of
the flags.
You shouldn't use any of the characters `|&;<>' in the string
words unless they are quoted; likewise for newline. If you use
these characters unquoted, you will get the WRDE_BADCHAR error
code. Don't use parentheses or braces unless they are quoted or part of
a word expansion construct. If you use quotation characters `'"`',
they should come in pairs that balance.
The results of word expansion are a sequence of words. The function
wordexp allocates a string for each resulting word, then
allocates a vector of type char ** to store the addresses of
these strings. The last element of the vector is a null pointer.
This vector is called the word vector.
To return this vector, wordexp stores both its address and its
length (number of elements, not counting the terminating null pointer)
into *word-vector-ptr.
If wordexp succeeds, it returns 0. Otherwise, it returns one
of these error codes:
WRDE_BADCHAR
WRDE_BADVAL
WRDE_UNDEF to forbid such references.
WRDE_CMDSUB
WRDE_NOCMD to forbid command substitution.
WRDE_NOSPACE
wordexp can store part of the results--as much as it could
allocate room for.
WRDE_SYNTAX
Function: void wordfree (wordexp_t *word-vector-ptr)
Free the storage used for the word-strings and vector that
*word-vector-ptr points to. This does not free the
structure *word-vector-ptr itself--only the other
data it points to.
This section describes the flags that you can specify in the
flags argument to wordexp. Choose the flags you want,
and combine them with the C operator |.
WRDE_APPEND
wordexp. This way you can effectively expand
several words as if they were concatenated with spaces between them.
In order for appending to work, you must not modify the contents of the
word vector structure between calls to wordexp. And, if you set
WRDE_DOOFFS in the first call to wordexp, you must also
set it when you append to the results.
WRDE_DOOFFS
we_offs field says how many slots to leave.
The blank slots contain null pointers.
WRDE_NOCMD
WRDE_REUSE
wordexp.
Instead of allocating a new vector of words, this call to wordexp
will use the vector that already exists (making it larger if necessary).
WRDE_SHOWERR
wordexp gives these
commands a standard error stream that discards all output.
WRDE_UNDEF
wordexp Example
Here is an example of using wordexp to expand several strings
and use the results to run a shell command. It also shows the use of
WRDE_APPEND to concatenate the expansions and of wordfree
to free the space allocated by wordexp.
int
expand_and_execute (const char *program, const char *options)
{
wordexp_t result;
pid_t pid
int status, i;
/* Expand the string for the program to run. */
switch (wordexp (program, &result, 0))
{
case 0: /* Successful. */
break;
case WRDE_NOSPACE:
/* If the error was WRDE_NOSPACE,
then perhaps part of the result was allocated. */
wordfree (&result);
default: /* Some other error. */
return -1;
}
/* Expand the strings specified for the arguments. */
for (i = 0; args[i]; i++)
{
if (wordexp (options, &result, WRDE_APPEND))
{
wordfree (&result);
return -1;
}
}
pid = fork ();
if (pid == 0)
{
/* This is the child process. Execute the command. */
execv (result.we_wordv[0], result.we_wordv);
exit (EXIT_FAILURE);
}
else if (pid < 0)
/* The fork failed. Report failure. */
status = -1;
else
/* This is the parent process. Wait for the child to complete. */
if (waitpid (pid, &status, 0) != pid)
status = -1;
wordfree (&result);
return status;
}
In practice, since wordexp is executed by running a subshell, it
would be faster to do this by concatenating the strings with spaces
between them and running that as a shell command using `sh -c'.
Most programs need to do either input (reading data) or output (writing data), or most frequently both, in order to do anything useful. The GNU C library provides such a large selection of input and output functions that the hardest part is often deciding which function is most appropriate!
This chapter introduces concepts and terminology relating to input and output. Other chapters relating to the GNU I/O facilities are:
Before you can read or write the contents of a file, you must establish a connection or communications channel to the file. This process is called opening the file. You can open a file for reading, writing, or both.
The connection to an open file is represented either as a stream or as a file descriptor. You pass this as an argument to the functions that do the actual read or write operations, to tell them which file to operate on. Certain functions expect streams, and others are designed to operate on file descriptors.
When you have finished reading to or writing from the file, you can terminate the connection by closing the file. Once you have closed a stream or file descriptor, you cannot do any more input or output operations on it.
When you want to do input or output to a file, you have a choice of two
basic mechanisms for representing the connection between your program
and the file: file descriptors and streams. File descriptors are
represented as objects of type int, while streams are represented
as FILE * objects.
File descriptors provide a primitive, low-level interface to input and output operations. Both file descriptors and streams can represent a connection to a device (such as a terminal), or a pipe or socket for communicating with another process, as well as a normal file. But, if you want to do control operations that are specific to a particular kind of device, you must use a file descriptor; there are no facilities to use streams in this way. You must also use file descriptors if your program needs to do input or output in special modes, such as nonblocking (or polled) input (see section File Status Flags).
Streams provide a higher-level interface, layered on top of the primitive file descriptor facilities. The stream interface treats all kinds of files pretty much alike--the sole exception being the three styles of buffering that you can choose (see section Stream Buffering).
The main advantage of using the stream interface is that the set of
functions for performing actual input and output operations (as opposed
to control operations) on streams is much richer and more powerful than
the corresponding facilities for file descriptors. The file descriptor
interface provides only simple functions for transferring blocks of
characters, but the stream interface also provides powerful formatted
input and output functions (printf and scanf) as well as
functions for character- and line-oriented input and output.
Since streams are implemented in terms of file descriptors, you can extract the file descriptor from a stream and perform low-level operations directly on the file descriptor. You can also initially open a connection as a file descriptor and then make a stream associated with that file descriptor.
In general, you should stick with using streams rather than file descriptors, unless there is some specific operation you want to do that can only be done on a file descriptor. If you are a beginning programmer and aren't sure what functions to use, we suggest that you concentrate on the formatted input functions (see section Formatted Input) and formatted output functions (see section Formatted Output).
If you are concerned about portability of your programs to systems other than GNU, you should also be aware that file descriptors are not as portable as streams. You can expect any system running ANSI C to support streams, but non-GNU systems may not support file descriptors at all, or may only implement a subset of the GNU functions that operate on file descriptors. Most of the file descriptor functions in the GNU library are included in the POSIX.1 standard, however.
One of the attributes of an open file is its file position that keeps track of where in the file the next character is to be read or written. In the GNU system, the file position is simply an integer representing the number of bytes from the beginning of the file.
The file position is normally set to the beginning of the file when it is opened, and each time a character is read or written, the file position is incremented. In other words, access to the file is normally sequential.
Ordinary files permit read or write operations at any position within
the file. Some other kinds of files may also permit this. Files which
do permit this are sometimes referred to as random-access files.
You can change the file position using the fseek function on a
stream (see section File Positioning) or the lseek function on a file
descriptor (see section Input and Output Primitives). If you try to change the file
position on a file that doesn't support random access, you get an error.
Streams and descriptors that are opened for append access are treated specially for output: output to such files is always appended sequentially to the end of the file, regardless of the file position. But, the file position is still used to control where in the file reading is done.
If you think about it, you'll realize that several programs can read a given file at the same time. In order for each program to be able to read the file at its own pace, each program must have its own file pointer, which is not affected by anything the other programs do.
In fact, each opening of a file creates a separate file position. Thus, if you open a file twice even in the same program, you get two streams or descriptors with independent file positions.
By contrast, if you open a descriptor and then duplicate it to get another descriptor, these two descriptors share the same file position: changing the file position of one descriptor will affect the other.
In order to open a connection to a file, or to perform other operations such as deleting a file, you need some way to refer to the file. Nearly all files have names that are strings--even files which are actually devices such as tape drives or terminals. These strings are called file names. You specify the file name to say which file you want to open or operate on.
This section describes the conventions for file names and how the operating system works with them.
In order to understand the syntax of file names, you need to understand how the file system is organized into a hierarchy of directories.
A directory is a file that contains information to associate other files with names; these associations are called links or directory entries. Sometimes, people speak of "files in a directory", but in reality, a directory only contains pointers to files, not the files themselves.
The name of a file contained in a directory entry is called a file name component. In general, a file name consists of a sequence of one or more such components, separated by the slash character (`/'). A file name which is just one component names a file with respect to its directory. A file name with multiple components names a directory, and then a file in that directory, and so on.
Some other documents, such as the POSIX standard, use the term pathname for what we call a file name, and either filename or pathname component for what this manual calls a file name component. We don't use this terminology because a "path" is something completely different (a list of directories to search), and we think that "pathname" used for something else will confuse users. We always use "file name" and "file name component" (or sometimes just "component", where the context is obvious) in GNU documentation.
You can find more detailed information about operations on directories in section File System Interface.
A file name consists of file name components separated by slash (`/') characters. On the systems that that GNU library supports, multiple successive `/' characters are equivalent to a single `/' character.
The process of determining what file a file name refers to is called file name resolution. This is performed by examining the components that make up a file name in left-to-right order, and locating each successive component in the directory named by the previous component. Of course, each of the files that are referenced as directories must actually exist, be directories instead of regular files, and have the appropriate permissions to be accessible by the process; otherwise the file name resolution fails.
If a file name begins with a `/', the first component in the file name is located in the root directory of the process. Such a file name is called an absolute file name.
Otherwise, the first component in the file name is located in the current working directory (see section Working Directory). This kind of file name is called a relative file name.
The file name components `.' ("dot") and `..' ("dot-dot") have special meanings. Every directory has entries for these file name components. The file name component `.' refers to the directory itself, while the file name component `..' refers to its parent directory (the directory that contains the link for the directory in question).
Here are some examples of file names:
A file name that names a directory may optionally end in a `/'. You can specify a file name of `/' to refer to the root directory, but the empty string is not a meaningful file name. If you want to refer to the current working directory, use a file name of `.' or `./'.
Unlike some other operating systems, the GNU system doesn't have any built-in support for file types (or extensions) or file versions as part of its file name syntax. Many programs and utilities use conventions for file names--for example, files containing C source code usually have names suffixed with `.c'---but there is nothing in the file system itself that enforces this kind of convention.
Functions that accept file name arguments usually detect these
errno error conditions relating to file name syntax. These
errors are referred to throughout this manual as the usual file
name syntax errors.
EACCES
ENAMETOOLONG
PATH_MAX, or when an individual file name component
has a length greater than NAME_MAX. See section Limits on File System Capacity.
In the GNU system, there is no imposed limit on overall file name length, but some file systems may place limits on the length of a component.
ENOENT
ENOTDIR
The rules for the syntax of file names discussed in section File Names, are the rules normally used by the GNU system and by other POSIX systems. However, other operating systems may use other conventions.
There are two reasons why it can be important for you to be aware of file name portability issues:
The ANSI C standard says very little about file name syntax, only that file names are strings. In addition to varying restrictions on the length of file names and what characters can validly appear in a file name, different operating systems use different conventions and syntax for concepts such as structured directories and file types or extensions. Some concepts such as file versions might be supported in some operating systems and not by others.
The POSIX.1 standard allows implementations to put additional restrictions on file name syntax, concerning what characters are permitted in file names and on the length of file name and file name component strings. However, in the GNU system, you do not need to worry about these restrictions; any character except the null character is permitted in a file name string, and there are no limits on the length of file name strings.
This chapter describes the functions for creating streams and performing input and output operations on them. As discussed in section Input/Output Overview, a stream is a fairly abstract, high-level concept representing a communications channel to a file, device, or process.
For historical reasons, the type of the C data structure that represents
a stream is called FILE rather than "stream". Since most of
the library functions deal with objects of type FILE *, sometimes
the term file pointer is also used to mean "stream". This leads
to unfortunate confusion over terminology in many books on C. This
manual, however, is careful to use the terms "file" and "stream"
only in the technical sense.
The FILE type is declared in the header file `stdio.h'.
This is the data type is used to represent stream objects. A
FILE object holds all of the internal state information about the
connection to the associated file, including such things as the file
position indicator and buffering information. Each stream also has
error and end-of-file status indicators that can be tested with the
ferror and feof functions; see section End-Of-File and Errors.
FILE objects are allocated and managed internally by the
input/output library functions. Don't try to create your own objects of
type FILE; let the library do it. Your programs should
deal only with pointers to these objects (that is, FILE * values)
rather than the objects themselves.
When the main function of your program is invoked, it already has
three predefined streams open and available for use. These represent
the "standard" input and output channels that have been established
for the process.
These streams are declared in the header file `stdio.h'.
The standard input stream, which is the normal source of input for the program.
The standard output stream, which is used for normal output from the program.
The standard error stream, which is used for error messages and diagnostics issued by the program.
In the GNU system, you can specify what files or processes correspond to these streams using the pipe and redirection facilities provided by the shell. (The primitives shells use to implement these facilities are described in section File System Interface.) Most other operating systems provide similar mechanisms, but the details of how to use them can vary.
It is probably not a good idea to close any of the standard streams.
But you can use freopen to get te effect of closing one and
reopening it. See section Opening Streams.
Opening a file with the fopen function creates a new stream and
establishes a connection between the stream and a file. This may
involve creating a new file.
Everything described in this section is declared in the header file `stdio.h'.
Function: FILE * fopen (const char *filename, const char *opentype)
The fopen function opens a stream for I/O to the file
filename, and returns a pointer to the stream.
The opentype argument is a string that controls how the file is opened and specifies attributes of the resulting stream. It must begin with one of the following sequences of characters:
As you can see, `+' requests a stream that can do both input and
output. When using such a stream, you must call fflush
(see section Stream Buffering) or a file positioning function such as
fseek (see section File Positioning) when switching from reading to
writing or vice versa. Otherwise, internal buffers might not be emptied
properly.
The GNU C library defines one additional character for use in
opentype: the character `x' insists on creating a new
file--if a file filename already exists, fopen fails
rather than opening it. This is equivalent to the O_EXCL option
to the open function (see section File Status Flags).
The character `b' in opentype has a standard meaning; it requests a binary stream rather than a text stream. But this makes no difference in POSIX systems (including the GNU system). If both `+' and `b' are specified, they can appear in either order. See section Text and Binary Streams.
Any other characters in opentype are simply ignored. They may be meaningful in other systems.
If the open fails, fopen returns a null pointer.
You can have multiple streams (or file descriptors) pointing to the same file open at the same time. If you do only input, this works straightforwardly, but you must be careful if any output streams are included. See section Precautions for Mixing Streams and Descriptors. This is equally true whether the streams are in one program (not usual) or in several programs (which can easily happen). It may be advantageous to use the file locking facilities to avoid simultaneous access. See section File Locks.
The value of this macro is an integer constant expression that
represents the minimum number of streams that the implementation
guarantees can be open simultaneously. The value of this constant is at
least eight, which includes the three standard streams stdin,
stdout, and stderr.
Function: FILE * freopen (const char *filename, const char *opentype, FILE *stream)
This function is like a combination of fclose and fopen.
It first closes the stream referred to by stream, ignoring any
errors that are detected in the process. (Because errors are ignored,
you should not use freopen on an output stream if you have
actually done any output using the stream.) Then the file named by
filename is opened with mode opentype as for fopen,
and associated with the same stream object stream.
If the operation fails, a null pointer is returned; otherwise,
freopen returns stream.
The main use of freopen is to connect a standard stream such as
stdir with a file of your own choice. This is useful in programs
in which use of a standard stream for certain purposes is hard-coded.
When a stream is closed with fclose, the connection between the
stream and the file is cancelled. After you have closed a stream, you
cannot perform any additional operations on it any more.
Function: int fclose (FILE *stream)
This function causes stream to be closed and the connection to
the corresponding file to be broken. Any buffered output is written
and any buffered input is discarded. The fclose function returns
a value of 0 if the file was closed successfully, and EOF
if an error was detected.
It is important to check for errors when you call fclose to close
an output stream, because real, everyday errors can be detected at this
time. For example, when fclose writes the remaining buffered
output, it might get an error because the disk is full. Even if you you
know the buffer is empty, errors can still occur when closing a file if
you are using NFS.
The function fclose is declared in `stdio.h'.
If the main function to your program returns, or if you call the
exit function (see section Normal Termination), all open streams are
automatically closed properly. If your program terminates in any other
manner, such as by calling the abort function (see section Aborting a Program) or from a fatal signal (see section Signal Handling), open streams
might not be closed properly. Buffered output may not be flushed and
files may not be complete. For more information on buffering of
streams, see section Stream Buffering.
This section describes functions for performing character- and
line-oriented output. Largely for historical compatibility, there are
several variants of these functions, but as a matter of style (and for
simplicity!) we suggest you stick with using fputc and
fputs, and perhaps putc and putchar.
These functions are declared in the header file `stdio.h'.
Function: int fputc (int c, FILE *stream)
The fputc function converts the character c to type
unsigned char, and writes it to the stream stream.
EOF is returned if a write error occurs; otherwise the
character c is returned.
Function: int putc (int c, FILE *stream)
This is just like fputc, except that most systems implement it as
a macro, making it faster. One consequence is that it may evaluate the
stream argument more than once.
The putchar function is equivalent to fputc with
stdout as the value of the stream argument.
Function: int fputs (const char *s, FILE *stream)
The function fputs writes the string s to the stream
stream. The terminating null character is not written.
This function does not add a newline character, either.
It outputs only the chars in the string.
This function returns EOF if a write error occurs, and otherwise
a non-negative value.
For example:
fputs ("Are ", stdout);
fputs ("you ", stdout);
fputs ("hungry?\n", stdout);
outputs the text `Are you hungry?' followed by a newline.
Function: int puts (const char *s)
The puts function writes the string s to the stream
stdout followed by a newline. The terminating null character of
the string is not written.
Function: int putw (int w, FILE *stream)
This function writes the word w (that is, an int) to
stream. It is provided for compatibility with SVID, but we
recommend you use fwrite instead (see section Block Input/Output).
This section describes functions for performing character- and
line-oriented input. Again, there are several variants of these
functions, some of which are considered obsolete stylistically. It's
suggested that you stick with fgetc, getline, and maybe
getc, getchar and fgets.
These functions are declared in the header file `stdio.h'.
Function: int fgetc (FILE *stream)
This function reads the next character as an unsigned char from
the stream stream and returns its value, converted to an
int. If an end-of-file condition or read error occurs,
EOF is returned instead.
Function: int getc (FILE *stream)
This is just like fgetc, except that it is permissible (and typical)
for it to be implemented as a macro that evaluates the stream
argument more than once.
The getchar function is equivalent to fgetc with stdin
as the value of the stream argument.
Here is an example of a function that does input using fgetc. It
would work just as well using getc instead, or using
getchar () instead of fgetc (stdin).
int
y_or_n_p (const char *question)
{
fputs (question, stdout);
while (1) {
int c, answer;
/* Write a space to separate answer from question. */
fputc (' ', stdout);
/* Read the first character of the line.
This should be the answer character, but might not be. */
c = tolower (fgetc (stdin));
answer = c;
/* Discard rest of input line. */
while (c != '\n')
c = fgetc (stdin);
/* Obey the answer if it was valid. */
if (answer == 'y')
return 1;
if (answer == 'n')
return 0;
/* Answer was invalid: ask for valid answer. */
fputs ("Please answer y or n:", stdout);
}
}
Function: int getw (FILE *stream)
This function reads a word (that is, an int) from stream.
It's provided for compatibility with SVID. We recommend you use
fread instead (see section Block Input/Output).
Since many programs interpret input on the basis of lines, it's convenient to have functions to read a line of text from a stream.
Standard C has functions to do this, but they aren't very safe: null
characters and even (for gets) long lines can confuse them. So
the GNU library provides the nonstandard getline function that
makes it easy to read lines reliably.
Another GNU extension, getdelim, generalizes getline. It
reads a delimited record, defined as everything through the next
occurrence of a specified delimeter character.
All these functions are declared in `stdio.h'.
Function: ssize_t getline (char **lineptr, size_t *n, FILE *stream)
This function reads an entire line from stream, storing the text
(including the newline and a terminating null character) in a buffer
and storing the buffer address in *lineptr.
Before calling getline, you should place in *lineptr
the address of a buffer *n bytes long. If this buffer is
long enough to hold the line, getline stores the line in this
buffer. Otherwise, getline makes the buffer bigger using
realloc, storing the new buffer address back in
*lineptr and the increased size back in *n.
In either case, when getline returns, *lineptr is
a char * which points to the text of the line.
When getline is successful, it returns the number of characters
read (including the newline, but not including the terminating null).
This value enables you to distinguish null characters that are part of
the line from the null character inserted as a terminator.
This function is a GNU extension, but it is the recommended way to read lines from a stream. The alternative standard functions are unreliable.
If an error occurs or end of file is reached, getline returns
-1.
Function: ssize_t getdelim (char **lineptr, size_t *n, int delimiter, FILE *stream)
This function is like getline except that the character which
tells it to stop reading is not necessarily newline. The argument
delimeter specifies the delimeter character; getdelim keeps
reading until it sees that character (or end of file).
The text is stored in lineptr, including the delimeter character
and a terminating null. Like getline, getdelim makes
lineptr bigger if it isn't big enough.
Function: char * fgets (char *s, int count, FILE *stream)
The fgets function reads characters from the stream stream
up to and including a newline character and stores them in the string
s, adding a null character to mark the end of the string. You
must supply count characters worth of space in s, but the
number of characters read is at most count - 1. The extra
character space is used to hold the null character at the end of the
string.
If the system is already at end of file when you call fgets, then
the contents of the array s are unchanged and a null pointer is
returned. A null pointer is also returned if a read error occurs.
Otherwise, the return value is the pointer s.
Warning: If the input data has a null character, you can't tell.
So don't use fgets unless you know the data cannot contain a null.
Don't use it to read files edited by the user because, if the user inserts
a null character, you should either handle it properly or print a clear
error message. We recommend using getline instead of fgets.
Deprecated function: char * gets (char *s)
The function gets reads characters from the stream stdin
up to the next newline character, and stores them in the string s.
The newline character is discarded (note that this differs from the
behavior of fgets, which copies the newline character into the
string).
Warning: The gets function is very dangerous
because it provides no protection against overflowing the string s.
The GNU library includes it for compatibility only. You should
always use fgets or getline instead.
In parser programs it is often useful to examine the next character in the input stream without removing it from the stream. This is called "peeking ahead" at the input because your program gets a glimpse of the input it will read next.
Using stream I/O, you can peek ahead at input by first reading it and
then unreading it (also called pushing it back on the stream).
Unreading a character makes it available to be input again from the stream,
by the next call to fgetc or other input function on that stream.
Here is a pictorial explanation of unreading. Suppose you have a stream reading a file that contains just six characters, the letters `foobar'. Suppose you have read three characters so far. The situation looks like this:
f o o b a r
^
so the next input character will be `b'.
If instead of reading `b' you unread the letter `o', you get a situation like this:
f o o b a r
|
o--
^
so that the next input characters will be `o' and `b'.
If you unread `9' instead of `o', you get this situation:
f o o b a r
|
9--
^
so that the next input characters will be `9' and `b'.
ungetc To Do Unreadingungetc, because it
reverses the action of fgetc.
Function: int ungetc (int c, FILE *stream)
The ungetc function pushes back the character c onto the
input stream stream. So the next input from stream will
read c before anything else.
The character that you push back doesn't have to be the same as the last
character that was actually read from the stream. In fact, it isn't
necessary to actually read any characters from the stream before
unreading them with ungetc! But that is a strange way to write
a program; usually ungetc is used only to unread a character
that was just read from the same stream.
The GNU C library only supports one character of pushback--in other
words, it does not work to call ungetc twice without doing input
in between. Other systems might let you push back multiple characters;
then reading from the stream retrieves the characters in the reverse
order that they were pushed.
Pushing back characters doesn't alter the file; only the internal
buffering for the stream is affected. If a file positioning function
(such as fseek or rewind; see section File Positioning) is
called, any pending pushed-back characters are discarded.
Unreading a character on a stream that is at end of file clears the end-of-file indicator for the stream, because it makes the character of input available. Reading that character will set the end-of-file indicator again.
Here is an example showing the use of getc and ungetc to
skip over whitespace characters. When this function reaches a
non-whitespace character, it unreads that character to be seen again on
the next read operation on the stream.
#include <stdio.h>
void
skip_whitespace (FILE *stream)
{
int c;
do
/* No need to check for EOF because it is not
isspace, and ungetc ignores EOF. */
c = getc (stream);
while (isspace (c));
ungetc (c, stream);
}
The functions described in this section (printf and related
functions) provide a convenient way to perform formatted output. You
call printf with a format string or template string
that specifies how to format the values of the remaining arguments.
Unless your program is a filter that specifically performs line- or
character-oriented processing, using printf or one of the other
related functions described in this section is usually the easiest and
most concise way to perform output. These functions are especially
useful for printing error messages, tables of data, and the like.
The printf function can be used to print any number of arguments.
The template string argument you supply in a call provides
information not only about the number of additional arguments, but also
about their types and what style should be used for printing them.
Ordinary characters in the template string are simply written to the output stream as-is, while conversion specifications introduced by a `%' character in the template cause subsequent arguments to be formatted and written to the output stream. For example,
int pct = 37;
char filename[] = "foo.txt";
printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n",
filename, pct);
produces output like
Processing of `foo.txt' is 37% finished. Please be patient.
This example shows the use of the `%d' conversion to specify that
an int argument should be printed in decimal notation, the
`%s' conversion to specify printing of a string argument, and
the `%%' conversion to print a literal `%' character.
There are also conversions for printing an integer argument as an unsigned value in octal, decimal, or hexadecimal radix (`%o', `%u', or `%x', respectively); or as a character value (`%c').
Floating-point numbers can be printed in normal, fixed-point notation using the `%f' conversion or in exponential notation using the `%e' conversion. The `%g' conversion uses either `%e' or `%f' format, depending on what is more appropriate for the magnitude of the particular number.
You can control formatting more precisely by writing modifiers between the `%' and the character that indicates which conversion to apply. These slightly alter the ordinary behavior of the conversion. For example, most conversion specifications permit you to specify a minimum field width and a flag indicating whether you want the result left- or right-justified within the field.
The specific flags and modifiers that are permitted and their interpretation vary depending on the particular conversion. They're all described in more detail in the following sections. Don't worry if this all seems excessively complicated at first; you can almost always get reasonable free-format output without using any of the modifiers at all. The modifiers are mostly used to make the output look "prettier" in tables.
This section provides details about the precise syntax of conversion
specifications that can appear in a printf template
string.
Characters in the template string that are not part of a conversion specification are printed as-is to the output stream. Multibyte character sequences (see section Extended Characters) are permitted in a template string.
The conversion specifications in a printf template string have
the general form:
% flags width [ . precision ] type conversion
For example, in the conversion specifier `%-10.8ld', the `-'
is a flag, `10' specifies the field width, the precision is
`8', the letter `l' is a type modifier, and `d' specifies
the conversion style. (This particular type specifier says to
print a long int argument in decimal notation, with a minimum of
8 digits left-justified in a field at least 10 characters wide.)
In more detail, output conversion specifications consist of an initial `%' character followed in sequence by:
The GNU library's version of printf also allows you to specify a
field width of `*'. This means that the next argument in the
argument list (before the actual value to be printed) is used as the
field width. The value must be an int. Other C library versions may
not recognize this syntax.
The GNU library's version of printf also allows you to specify a
precision of `*'. This means that the next argument in the
argument list (before the actual value to be printed) is used as the
precision. The value must be an int. If you specify `*'
for both the field width and precision, the field width argument
precedes the precision argument. Other C library versions may not
recognize this syntax.
int,
but you can specify `h', `l', or `L' for other integer
types.)
The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they use.
Here is a table summarizing what all the different conversions do:
scanf for input
(see section Table of Input Conversions).
size_t. See section Integer Conversions, for details.
errno.
See section Other Output Conversions.
If the syntax of a conversion specification is invalid, unpredictable things will happen, so don't do this. If there aren't enough function arguments provided to supply values for all the conversion specifications in the template string, or if the arguments are not of the correct types, the results are unpredictable. If you supply more arguments than conversion specifications, the extra argument values are simply ignored; this is sometimes useful.
This section describes the options for the `%d', `%i', `%o', `%u', `%x', `%X', and `%Z' conversion specifications. These conversions print integers in various formats.
The `%d' and `%i' conversion specifications both print an
int argument as a signed decimal number; while `%o',
`%u', and `%x' print the argument as an unsigned octal,
decimal, or hexadecimal number (respectively). The `%X' conversion
specification is just like `%x' except that it uses the characters
`ABCDEF' as digits instead of `abcdef'. `%Z' is like
`%u' but expects an argument of type size_t.
The following flags are meaningful:
If a precision is supplied, it specifies the minimum number of digits to appear; leading zeros are produced if necessary. If you don't specify a precision, the number is printed with as many digits as it needs. If you convert a value of zero with an explicit precision of zero, then no characters at all are produced.
Without a type modifier, the corresponding argument is treated as an
int (for the signed conversions `%i' and `%d') or
unsigned int (for the unsigned conversions `%o', `%u',
`%x', and `%X'). Recall that since printf and friends
are variadic, any char and short arguments are
automatically converted to int by the default argument
promotions. For arguments of other integer types, you can use these
modifiers:
short int or unsigned
short int, as appropriate. A short argument is converted to an
int or unsigned int by the default argument promotions
anyway, but the `h' modifier says to convert it back to a
short again.
long int or unsigned long
int, as appropriate.
long long int. (This type is
an extension supported by the GNU C compiler. On systems that don't
support extra-long integers, this is the same as long int.)
The modifiers for argument type are not applicable to `%Z', since
the sole purpose of `%Z' is to specify the data type
size_t.
Here is an example. Using the template string:
|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n"
to print numbers using the different options for the `%d' conversion gives results like:
| 0|0 | +0|+0 | 0|00000| | 00|0| | 1|1 | +1|+1 | 1|00001| 1| 01|1| | -1|-1 | -1|-1 | -1|-0001| -1| -01|-1| |100000|100000|+100000| 100000|100000|100000|100000|100000|
In particular, notice what happens in the last case where the number is too large to fit in the minimum field width specified.
Here are some more examples showing how unsigned integers print under various format options, using the template string:
"|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n"
| 0| 0| 0| 0| 0| 0x0| 0X0|0x00000000| | 1| 1| 1| 1| 01| 0x1| 0X1|0x00000001| |100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|
This section discusses the conversion specifications for floating-point numbers: the `%f', `%e', `%E', `%g', and `%G' conversions.
The `%f' conversion prints its argument in fixed-point notation,
producing output of the form
[-]ddd.ddd,
where the number of digits following the decimal point is controlled
by the precision you specify.
The `%e' conversion prints its argument in exponential notation,
producing output of the form
[-]d.ddde[+|-]dd.
Again, the number of digits following the decimal point is controlled by
the precision. The exponent always contains at least two digits. The
`%E' conversion is similar but the exponent is marked with the letter
`E' instead of `e'.
The `%g' and `%G' conversions print the argument in the style of `%e' or `%E' (respectively) if the exponent would be less than -4 or greater than or equal to the precision; otherwise they use the `%f' style. Trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit.
The following flags can be used to modify the behavior:
The precision specifies how many digits follow the decimal-point
character for the `%f', `%e', and `%E' conversions. For
these conversions, the default is 6. If the precision is
explicitly 0, this has the rather strange effect of suppressing
the decimal point character entirely! For the `%g' and `%G'
conversions, the precision specifies how many significant digits to
print; if 0 or not specified, it is treated like a value of
1.
Without a type modifier, the floating-point conversions use an argument
of type double. (By the default argument promotions, any
float arguments are automatically converted to double.)
The following type modifier is supported:
long
double.
Here are some examples showing how numbers print using the various floating-point conversions. All of the numbers were printed using this template string:
"|%12.4f|%12.4e|%12.4g|\n"
Here is the output:
| 0.0000| 0.0000e+00| 0| | 1.0000| 1.0000e+00| 1| | -1.0000| -1.0000e+00| -1| | 100.0000| 1.0000e+02| 100| | 1000.0000| 1.0000e+03| 1000| | 10000.0000| 1.0000e+04| 1e+04| | 12345.0000| 1.2345e+04| 1.234e+04| | 100000.0000| 1.0000e+05| 1e+05| | 123456.0000| 1.2346e+05| 1.234e+05|
Notice how the `%g' conversion drops trailing zeros.
This section describes miscellaneous conversions for printf.
The `%c' conversion prints a single character. The int
argument is first converted to an unsigned char. The `-'
flag can be used to specify left-justification in the field, but no
other flags are defined, and no precision or type modifier can be given.
For example:
printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o');
prints `hello'.
The `%s' conversion prints a string. The corresponding argument
must be of type char *. A precision can be specified to indicate
the maximum number of characters to write; otherwise characters in the
string up to but not including the terminating null character are
written to the output stream. The `-' flag can be used to specify
left-justification in the field, but no other flags or type modifiers
are defined for this conversion. For example:
printf ("%3s%-6s", "no", "where");
prints ` nowhere '.
If you accidentally pass a null pointer as the argument for a `%s' conversion, the GNU library prints it as `(null)'. We think this is more useful than crashing. But it's not good practice to pass a null argument intentionally.
The `%m' conversion prints the string corresponding to the error
code in errno. See section Error Messages. Thus:
fprintf (stderr, "can't open `%s': %m\n", filename);
is equivalent to:
fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));
The `%m' conversion is a GNU C library extension.
The `%p' conversion prints a pointer value. The corresponding
argument must be of type void *. In practice, you can use any
type of pointer.
In the GNU system, non-null pointers are printed as unsigned integers, as if a `%#x' conversion were used. Null pointers print as `(nil)'. (Pointers might print differently in other systems.)
For example:
printf ("%p", "testing");
prints `0x' followed by a hexadecimal number--the address of the
string constant "testing". It does not print the word
`testing'.
You can supply the `-' flag with the `%p' conversion to specify left-justification, but no other flags, precision, or type modifiers are defined.
The `%n' conversion is unlike any of the other output conversions.
It uses an argument which must be a pointer to an int, but
instead of printing anything it stores the number of characters printed
so far by this call at that location. The `h' and `l' type
modifiers are permitted to specify that the argument is of type
short int * or long int * instead of int *, but no
flags, field width, or precision are permitted.
For example,
int nchar;
printf ("%d %s%n\n", 3, "bears", &nchar);
prints:
3 bears
and sets nchar to 7, because `3 bears' is seven
characters.
The `%%' conversion prints a literal `%' character. This conversion doesn't use an argument, and no flags, field width, precision, or type modifiers are permitted.
This section describes how to call printf and related functions.
Prototypes for these functions are in the header file `stdio.h'.
Function: int printf (const char *template, ...)
The printf function prints the optional arguments under the
control of the template string template to the stream
stdout. It returns the number of characters printed, or a
negative value if there was an output error.
Function: int fprintf (FILE *stream, const char *template, ...)
This function is just like printf, except that the output is
written to the stream stream instead of stdout.
Function: int sprintf (char *s, const char *template, ...)
This is like printf, except that the output is stored in the character
array s instead of written to a stream. A null character is written
to mark the end of the string.
The sprintf function returns the number of characters stored in
the array s, not including the terminating null character.
The behavior of this function is undefined if copying takes place between objects that overlap--for example, if s is also given as an argument to be printed under control of the `%s' conversion. See section Copying and Concatenation.
Warning: The sprintf function can be dangerous
because it can potentially output more characters than can fit in the
allocation size of the string s. Remember that the field width
given in a conversion specification is only a minimum value.
To avoid this problem, you can use snprintf or asprintf,
described below.
Function: int snprintf (char *s, size_t size, const char *template, ...)
The snprintf function is similar to sprintf, except that
the size argument specifies the maximum number of characters to
produce. The trailing null character is counted towards this limit, so
you should allocate at least size characters for the string s.
The return value is the number of characters stored, not including the terminating null. If this value equals size, then there was not enough space in s for all the output. You should try again with a bigger output string. Here is an example of doing this:
/* Construct a message describing the value of a variable
whose name is name and whose value is value. */
char *
make_message (char *name, char *value)
{
/* Guess we need no more than 100 chars of space. */
int size = 100;
char *buffer = (char *) xmalloc (size);
while (1)
{
/* Try to print in the allocated space. */
int nchars = snprintf (buffer, size,
"value of %s is %s", name, value);
/* If that worked, return the string. */
if (nchars < size)
return buffer;
/* Else try again with twice as much space. */
size *= 2;
buffer = (char *) xrealloc (size, buffer);
}
}
In practice, it is often easier just to use asprintf, below.
The functions in this section do formatted output and place the results in dynamically allocated memory.
Function: int asprintf (char **ptr, const char *template, ...)
This function is similar to sprintf, except that it dynamically
allocates a string (as with malloc; see section Unconstrained Allocation) to hold the output, instead of putting the output in a
buffer you allocate in advance. The ptr argument should be the
address of a char * object, and asprintf stores a pointer
to the newly allocated string at that location.
Here is how to use asprint to get the same result as the
snprintf example, but more easily:
/* Construct a message describing the value of a variable
whose name is name and whose value is value. */
char *
make_message (char *name, char *value)
{
char *result;
asprintf (&result, "value of %s is %s", name, value);
return result;
}
Function: int obstack_printf (struct obstack *obstack, const char *template, ...)
This function is similar to asprintf, except that it uses the
obstack obstack to allocate the space. See section Obstacks.
The characters are written onto the end of the current object.
To get at them, you must finish the object with obstack_finish
(see section Growing Objects).
The functions vprintf and friends are provided so that you can
define your own variadic printf-like functions that make use of
the same internals as the built-in formatted output functions.
The most natural way to define such functions would be to use a language
construct to say, "Call printf and pass this template plus all
of my arguments after the first five." But there is no way to do this
in C, and it would be hard to provide a way, since at the C language
level there is no way to tell how many arguments your function received.
Since that method is impossible, we provide alternative functions, the
vprintf series, which lets you pass a va_list to describe
"all of my arguments after the first five."
Before calling vprintf or the other functions listed in this
section, you must call va_start (see section Variadic Functions) to initialize a pointer to the variable arguments. Then you
can call va_arg to fetch the arguments that you want to handle
yourself. This advances the pointer past those arguments.
Once your va_list pointer is pointing at the argument of your
choice, you are ready to call vprintf. That argument and all
subsequent arguments that were passed to your function are used by
vprintf along with the template that you specified separately.
In some other systems, the va_list pointer may become invalid
after the call to vprintf, so you must not use va_arg
after you call vprintf. Instead, you should call va_end
to retire the pointer from service. However, you can safely call
va_start on another pointer variable and begin fetching the
arguments again through that pointer. Calling vfprintf does
not destroy the argument list of your function, merely the particular
pointer that you passed to it.
The GNU library does not have such restrictions. You can safely continue
to fetch arguments from a va_list pointer after passing it to
vprintf, and va_end is a no-op.
Prototypes for these functions are declared in `stdio.h'.
Function: int vprintf (const char *template, va_list ap)
This function is similar to printf except that, instead of taking
a variable number of arguments directly, it takes an argument list
pointer ap.
Function: int vfprintf (FILE *stream, const char *template, va_list ap)
This is the equivalent of fprintf with the variable argument list
specified directly as for vprintf.
Function: int vsprintf (char *s, const char *template, va_list ap)
This is the equivalent of sprintf with the variable argument list
specified directly as for vprintf.
Function: int vsnprintf (char *s, size_t size, const char *template, va_list ap)
This is the equivalent of snprintf with the variable argument list
specified directly as for vprintf.
Function: int vasprintf (char **ptr, const char *template, va_list ap)
The vasprintf function is the equivalent of asprintf with the
variable argument list specified directly as for vprintf.
Function: int obstack_vprintf (struct obstack *obstack, const char *template, va_list ap)
The obstack_vprintf function is the equivalent of
obstack_printf with the variable argument list specified directly
as for vprintf.
Here's an example showing how you might use vfprintf. This is a
function that prints error messages to the stream stderr, along
with a prefix indicating the name of the program
(see section Error Messages, for a description of
program_invocation_short_name).
#include <stdio.h>
#include <stdarg.h>
void
eprintf (char *template, ...)
{
va_list ap;
extern char *program_invocation_short_name;
fprintf (stderr, "%s: ", program_invocation_short_name);
va_start (ap, count);
vfprintf (stderr, template, ap);
va_end (ap);
}
You could call eprintf like this:
eprintf ("file `%s' does not exist\n", filename);
You can use the function parse_printf_format to obtain
information about the number and types of arguments that are expected by
a given template string. This function permits interpreters that
provide interfaces to printf to avoid passing along invalid
arguments from the user's program, which could cause a crash.
All the symbols described in this section are declared in the header file `printf.h'.
Function: size_t parse_printf_format (const char *template, size_t n, int *argtypes)
This function returns information about the number and types of
arguments expected by the printf template string template.
The information is stored in the array argtypes; each element of
this array describes one argument. This information is encoded using
the various `PA_' macros, listed below.
The n argument specifies the number of elements in the array
argtypes. This is the most elements that
parse_printf_format will try to write.
parse_printf_format returns the total number of arguments required
by template. If this number is greater than n, then the
information returned describes only the first n arguments. If you
want information about more than that many arguments, allocate a bigger
array and call parse_printf_format again.
The argument types are encoded as a combination of a basic type and modifier flag bits.
This macro is a bitmask for the type modifier flag bits. You can write
the expression (argtypes[i] & PA_FLAG_MASK) to extract just the
flag bits for an argument, or (argtypes[i] & ~PA_FLAG_MASK) to
extract just the basic type code.
Here are symbolic constants that represent the basic types; they stand for integer values.
PA_INT
int.
PA_CHAR
int, cast to char.
PA_STRING
char *, a null-terminated string.
PA_POINTER
void *, an arbitrary pointer.
PA_FLOAT
float.
PA_DOUBLE
double.
PA_LAST
PA_LAST. For example, if you have data types `foo'
and `bar' with their own specialized printf conversions,
you could define encodings for these types as:
#define PA_FOO PA_LAST #define PA_BAR (PA_LAST + 1)
Here are the flag bits that modify a basic type. They are combined with the code for the basic type using inclusive-or.
PA_FLAG_PTR
PA_FLAG_SHORT
short. (This corresponds to the `h' type modifier.)
PA_FLAG_LONG
long. (This corresponds to the `l' type modifier.)
PA_FLAG_LONG_LONG
long long. (This corresponds to the `L' type modifier.)
PA_FLAG_LONG_DOUBLE
PA_FLAG_LONG_LONG, used by convention with
a base type of PA_DOUBLE to indicate a type of long double.
Here is an example of decoding argument types for a format string. We
assume this is part of an interpreter which contains arguments of type
NUMBER, CHAR, STRING and STRUCTURE (and
perhaps others which are not valid here).
/* Test whether the nargs specified objects
in the vector args are valid
for the format string format:
if so, return 1.
If not, return 0 after printing an error message. */
int
validate_args (char *format, int nargs, OBJECT *args)
{
int nelts = 20;
int *argtypes;
int nwanted;
/* Get the information about the arguments. */
while (1) {
argtypes = (int *) alloca (nelts * sizeof (int));
nwanted = parse_printf_format (string, nelts, argtypes);
if (nwanted <= nelts)
break;
nelts *= 2;
}
/* Check the number of arguments. */
if (nwanted > nargs) {
error ("too few arguments (at least %d required)", nwanted);
return 0;
}
/* Check the C type wanted for each argument
and see if the object given is suitable. */
for (i = 0; i < nwanted; i++) {
int wanted;
if (argtypes[i] & PA_FLAG_PTR)
wanted = STRUCTURE;
else
switch (argtypes[i] & ~PA_FLAG_MASK) {
case PA_INT:
case PA_FLOAT:
case PA_DOUBLE:
wanted = NUMBER;
break;
case PA_CHAR:
wanted = CHAR;
break;
case PA_STRING:
wanted = STRING;
break;
case PA_POINTER:
wanted = STRUCTURE;
break;
}
if (TYPE (args[i]) != wanted) {
error ("type mismatch for arg number %d", i);
return 0;
}
}
return 1;
}
printf
The GNU C library lets you define your own custom conversion specifiers
for printf template strings, to teach printf clever ways
to print the important data structures of your program.
The way you do this is by registering the conversion with
register_printf_function; see section Registering New Conversions.
One of the arguments you pass to this function is a pointer to a handler
function that produces the actual output; see section Defining the Output Handler, for information on how to write this function.
You can also install a function that just returns information about the number and type of arguments expected by the conversion specifier. See section Parsing a Template String, for information about this.
The facilities of this section are declared in the header file `printf.h'.
Portability Note: The ability to extend the syntax of
printf template strings is a GNU extension. ANSI standard C has
nothing similar.
The function to register a new output conversion is
register_printf_function, declared in `printf.h'.
Function: int register_printf_function (int spec, printf_function handler_function, printf_arginfo_function arginfo_function)
This function defines the conversion specifier character spec.
Thus, if spec is 'q', it defines the conversion `%q'.
The handler_function is the function called by printf and
friends when this conversion appears in a template string.
See section Defining the Output Handler, for information about how to define
a function to pass as this argument. If you specify a null pointer, any
existing handler function for spec is removed.
The arginfo_function is the function called by
parse_printf_format when this conversion appears in a
template string. See section Parsing a Template String, for information
about this.
Normally, you install both functions for a conversion at the same time,
but if you are never going to call parse_printf_format, you do
not need to define an arginfo function.
The return value is 0 on success, and -1 on failure
(which occurs if spec is out of range).
You can redefine the standard output conversions, but this is probably not a good idea because of the potential for confusion. Library routines written by other people could break if you do this.
If you define a meaning for `%q', what if the template contains `%+Sq' or `%-#q'? To implement a sensible meaning for these, the handler when called needs to be able to get the options specified in the template.
Both the handler_function and arginfo_function arguments
to register_printf_function accept an argument of type
struct print_info, which contains information about the options
appearing in an instance of the conversion specifier. This data type
is declared in the header file `printf.h'.
This structure is used to pass information about the options appearing
in an instance of a conversion specifier in a printf template
string to the handler and arginfo functions for that specifier. It
contains the following members:
int prec
-1 if no precision
was specified. If the precision was given as `*', the
printf_info structure passed to the handler function contains the
actual value retrieved from the argument list. But the structure passed
to the arginfo function contains a value of INT_MIN, since the
actual value is not known.
int width
0 if no
width was specified. If the field width was given as `*', the
printf_info structure passed to the handler function contains the
actual value retrieved from the argument list. But the structure passed
to the arginfo function contains a value of INT_MIN, since the
actual value is not known.
char spec
unsigned int is_long_double
unsigned int is_short
unsigned int is_long
unsigned int alt
unsigned int space
unsigned int left
unsigned int showsign
char pad
'0' if the `0' flag was specified, and
' ' otherwise.
Now let's look at how to define the handler and arginfo functions
which are passed as arguments to register_printf_function.
You should define your handler functions with a prototype like:
int function (FILE *stream, const struct printf_info *info,
va_list *ap_pointer)
The stream argument passed to the handler function is the stream to
which it should write output.
The info argument is a pointer to a structure that contains
information about the various options that were included with the
conversion in the template string. You should not modify this structure
inside your handler function. See section Conversion Specifier Options, for
a description of this data structure.
The ap_pointer argument is used to pass the tail of the variable
argument list containing the values to be printed to your handler.
Unlike most other functions that can be passed an explicit variable
argument list, this is a pointer to a va_list, rather than
the va_list itself. Thus, you should fetch arguments by
means of va_arg (type, *ap_pointer).
(Passing a pointer here allows the function that calls your handler
function to update its own va_list variable to account for the
arguments that your handler processes. See section Variadic Functions.)
The return value from your handler function should be the number of
argument values that it processes from the variable argument list. You
can also return a value of -1 to indicate an error.
This is the data type that a handler function should have.
If you are going to use parse_printf_format in your
application, you should also define a function to pass as the
arginfo_function argument for each new conversion you install with
register_printf_function.
You should define these functions with a prototype like:
int function (const struct printf_info *info,
size_t n, int *argtypes)
The return value from the function should be the number of arguments the
conversion expects, up to a maximum of n. The function should
also fill in the argtypes array with information about the types
of each of these arguments. This information is encoded using the
various `PA_' macros. (You will notice that this is the same
calling convention parse_printf_format itself uses.)
Data Type: printf_arginfo_function
This type is used to describe functions that return information about the number and type of arguments used by a conversion specifier.
printf Extension Example
Here is an example showing how to define a printf handler function.
This program defines a data structure called a Widget and
defines the `%W' conversion to print information about Widget *
arguments, including the pointer value and the name stored in the data
structure. The `%W' conversion supports the minimum field width and
left-justification options, but ignores everything else.
#include <stdio.h>
#include <printf.h>
#include <stdarg.h>
typedef struct
{
char *name;
} Widget;
int
print_widget (FILE *stream, const struct printf_info *info, va_list *app)
{
Widget *w;
char *buffer;
int len;
/* Format the output into a string. */
w = va_arg (*app, Widget *);
len = asprintf (&buffer, "<Widget %p: %s>", w, w->name);
if (len == -1)
return -1;
/* Pad to the minimum field width and print to the stream. */
len = fprintf (stream, "%*s",
(info->left ? - info->width : info->width),
buffer);
/* Clean up and return. */
free (buffer);
return len;
}
int
main (void)
{
/* Make a widget to print. */
Widget mywidget;
mywidget.name = "mywidget";
/* Register the print function for widgets. */
register_printf_function ('W', print_widget, NULL); /* No arginfo. */
/* Now print the widget. */
printf ("|%W|\n", &mywidget);
printf ("|%35W|\n", &mywidget);
printf ("|%-35W|\n", &mywidget);
return 0;
}
The output produced by this program looks like:
|<Widget 0xffeffb7c: mywidget>| | <Widget 0xffeffb7c: mywidget>| |<Widget 0xffeffb7c: mywidget> |
The functions described in this section (scanf and related
functions) provide facilities for formatted input analogous to the
formatted output facilities. These functions provide a mechanism for
reading arbitrary values under the control of a format string or
template string.
Calls to scanf are superficially similar to calls to
printf in that arbitrary arguments are read under the control of
a template string. While the syntax of the conversion specifications in
the template is very similar to that for printf, the
interpretation of the template is oriented more towards free-format
input and simple pattern matching, rather than fixed-field formatting.
For example, most scanf conversions skip over any amount of
"white space" (including spaces, tabs, and newlines) in the input
file, and there is no concept of precision for the numeric input
conversions as there is for the corresponding output conversions.
Ordinarily, non-whitespace characters in the template are expected to
match characters in the input stream exactly, but a matching failure is
distinct from an input error on the stream.
Another area of difference between scanf and printf is
that you must remember to supply pointers rather than immediate values
as the optional arguments to scanf; the values that are read are
stored in the objects that the pointers point to. Even experienced
programmers tend to forget this occasionally, so if your program is
getting strange errors that seem to be related to scanf, you
might want to double-check this.
When a matching failure occurs, scanf returns immediately,
leaving the first non-matching character as the next character to be
read from the stream. The normal return value from scanf is the
number of values that were assigned, so you can use this to determine if
a matching error happened before all the expected values were read.
The scanf function is typically used for things like reading in
the contents of tables. For example, here is a function that uses
scanf to initialize an array of double:
void
readarray (double *array, int n)
{
int i;
for (i=0; i<n; i++)
if (scanf (" %lf", &(array[i])) != 1)
invalid_input_error ();
}
The formatted input functions are not used as frequently as the formatted output functions. Partly, this is because it takes some care to use them properly. Another reason is that it is difficult to recover from a matching error.
If you are trying to read input that doesn't match a single, fixed
pattern, you may be better off using a tool such as Bison to generate a
parser, rather than using scanf. For more information about
this, see section 'Bison' in The Bison Reference Manual.
A scanf template string is a string that contains ordinary
multibyte characters interspersed with conversion specifications that
start with `%'.
Any whitespace character (as defined by the isspace function;
see section Classification of Characters) in the template causes any number
of whitespace characters in the input stream to be read and discarded.
The whitespace characters that are matched need not be exactly the same
whitespace characters that appear in the template string. For example,
write ` , ' in the template to recognize a comma with optional
whitespace before and after.
Other characters in the template string that are not part of conversion specifications must match characters in the input stream exactly; if this is not the case, a matching failure occurs.
The conversion specifications in a scanf template string
have the general form:
% flags width type conversion
In more detail, an input conversion specification consists of an initial `%' character followed in sequence by:
scanf finds a conversion
specification that uses this flag, it reads input as directed by the
rest of the conversion specification, but it discards this input, does
not use a pointer argument, and does not increment the count of
successful assignments.
long int
rather than a pointer to an int.
The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they allow.
Here is a table that summarizes the various conversion specifications:
printf. See section Other Input Conversions.
If the syntax of a conversion specification is invalid, the behavior is undefined. If there aren't enough function arguments provided to supply addresses for all the conversion specifications in the template strings that perform assignments, or if the arguments are not of the correct types, the behavior is also undefined. On the other hand, extra arguments are simply ignored.
This section describes the scanf conversions for reading numeric
values.
The `%d' conversion matches an optionally signed integer in decimal
radix. The syntax that is recognized is the same as that for the
strtol function (see section Parsing of Integers) with the value
10 for the base argument.
The `%i' conversion matches an optionally signed integer in any of
the formats that the C language defines for specifying an integer
constant. The syntax that is recognized is the same as that for the
strtol function (see section Parsing of Integers) with the value
0 for the base argument.
For example, any of the strings `10', `0xa', or `012'
could be read in as integers under the `%i' conversion. Each of
these specifies a number with decimal value 10.
The `%o', `%u', and `%x' conversions match unsigned
integers in octal, decimal, and hexadecimal radices, respectively. The
syntax that is recognized is the same as that for the strtoul
function (see section Parsing of Integers) with the appropriate value
(8, 10, or 16) for the base argument.
The `%X' conversion is identical to the `%x' conversion. They both permit either uppercase or lowercase letters to be used as digits.
The default type of the corresponding argument for the %d and
%i conversions is int *, and unsigned int * for the
other integer conversions. You can use the following type modifiers to
specify other sizes of integer:
short int * or unsigned
short int *.
long int * or unsigned
long int *.
long long int * or unsigned long long int *. (The long long type is an extension supported by the
GNU C compiler. For systems that don't provide extra-long integers, this
is the same as long int.)
All of the `%e', `%f', `%g', `%E', and `%G'
input conversions are interchangeable. They all match an optionally
signed floating point number, in the same syntax as for the
strtod function (see section Parsing of Floats).
For the floating-point input conversions, the default argument type is
float *. (This is different from the corresponding output
conversions, where the default type is double; remember that
float arguments to printf are converted to double
by the default argument promotions, but float * arguments are
not promoted to double *.) You can specify other sizes of float
using these type modifiers:
double *.
long double *.
This section describes the scanf input conversions for reading
string and character values: `%s', `%[', and `%c'.
You have two options for how to receive the input from these conversions:
char *.
Warning: To make a robust program, you must make sure that the input (plus its terminating null) cannot possibly exceed the size of the buffer you provide. In general, the only way to do this is to specify a maximum field width one less than the buffer size. If you provide the buffer, always specify a maximum field width to prevent overflow.
scanf to allocate a big enough buffer, by specifying the
`a' flag character. This is a GNU extension. You should provide
an argument of type char ** for the buffer address to be stored
in. See section Dynamically Allocating String Conversions.
The `%c' conversion is the simplest: it matches a fixed number of characters, always. The maximum field with says how many characters to read; if you don't specify the maximum, the default is 1. This conversion doesn't append a null character to the end of the text it reads. It also does not skip over initial whitespace characters. It reads precisely the next n characters, and fails if it cannot get that many. Since there is always a maximum field width with `%c' (whether specified, or 1 by default), you can always prevent overflow by making the buffer long enough.
The `%s' conversion matches a string of non-whitespace characters. It skips and discards initial whitespace, but stops when it encounters more whitespace after having read something. It stores a null character at the end of the text that it reads.
For example, reading the input:
hello, world
with the conversion `%10c' produces " hello, wo", but
reading the same input with the conversion `%10s' produces
"hello,".
Warning: If you do not specify a field width for `%s', then the number of characters read is limited only by where the next whitespace character appears. This almost certainly means that invalid input can make your program crash--which is a bug.
To read in characters that belong to an arbitrary set of your choice, use the `%[' conversion. You specify the set between the `[' character and a following `]' character, using the same syntax used in regular expressions. As special cases:
The `%[' conversion does not skip over initial whitespace characters.
Here are some examples of `%[' conversions and what they mean:
One more reminder: the `%s' and `%[' conversions are dangerous if you don't specify a maximum width or use the `a' flag, because input too long would overflow whatever buffer you have provided for it. No matter how long your buffer is, a user could supply input that is longer. A well-written program reports invalid input with a comprehensible error message, not with a crash.
A GNU extension to formatted input lets you safely read a string with no
maximum size. Using this feature, you don't supply a buffer; instead,
scanf allocates a buffer big enough to hold the data and gives
you its address. To use this feature, write `a' as a flag
character, as in `%as' or `%a[0-9a-z]'.
The pointer argument you supply for where to store the input should have
type char **. The scanf function allocates a buffer and
stores its address in the word that the argument points to. You should
free the buffer with free when you no longer need it.
Here is an example of using the `a' flag with the `%[...]' conversion specification to read a "variable assignment" of the form `variable = value'.
{
char *variable, *value;
if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n",
&variable, &value))
{
invalid_input_error ();
return 0;
}
...
}
This section describes the miscellaneous input conversions.
The `%p' conversion is used to read a pointer value. It recognizes
the same syntax as is used by the `%p' output conversion for
printf. The corresponding argument should be of type void **;
that is, the address of a place to store a pointer.
The resulting pointer value is not guaranteed to be valid if it was not originally written during the same program execution that reads it in.
The `%n' conversion produces the number of characters read so far
by this call. The corresponding argument should be of type int *.
This conversion works in the same way as the `%n' conversion for
printf; see section Other Output Conversions, for an example.
The `%n' conversion is the only mechanism for determining the
success of literal matches or conversions with suppressed assignments.
If the `%n' follows the locus of a matching failure, then no value
is stored for it since scanf returns before processing the
`%n'. If you store -1 in that argument slot before calling
scanf, the presence of -1 after scanf indicates an
error occurred before the `%n' was reached.
Finally, the `%%' conversion matches a literal `%' character in the input stream, without using an argument. This conversion does not permit any flags, field width, or type modifier to be specified.
Here are the descriptions of the functions for performing formatted input. Prototypes for these functions are in the header file `stdio.h'.
Function: int scanf (const char *template, ...)
The scanf function reads formatted input from the stream
stdin under the control of the template string template.
The optional arguments are pointers to the places which receive the
resulting values.
The return value is normally the number of successful assignments. If
an end-of-file condition is detected before any matches are performed
(including matches against whitespace and literal characters in the
template), then EOF is returned.
Function: int fscanf (FILE *stream, const char *template, ...)
This function is just like scanf, except that the input is read
from the stream stream instead of stdin.
Function: int sscanf (const char *s, const char *template, ...)
This is like scanf, except that the characters are taken from the
null-terminated string s instead of from a stream. Reaching the
end of the string is treated as an end-of-file condition.
The behavior of this function is undefined if copying takes place between objects that overlap--for example, if s is also given as an argument to receive a string read under control of the `%s' conversion.
The functions vscanf and friends are provided so that you can
define your own variadic scanf-like functions that make use of
the same internals as the built-in formatted output functions.
These functions are analogous to the vprintf series of output
functions. See section Variable Arguments Output Functions, for important
information on how to use them.
Portability Note: The functions listed in this section are GNU extensions.
Function: int vscanf (const char *template, va_list ap)
This function is similar to scanf except that, instead of taking
a variable number of arguments directly, it takes an argument list
pointer ap of type va_list (see section Variadic Functions).
Function: int vfscanf (FILE *stream, const char *template, va_list ap)
This is the equivalent of fscanf with the variable argument list
specified directly as for vscanf.
Function: int vsscanf (const char *s, const char *template, va_list ap)
This is the equivalent of sscanf with the variable argument list
specified directly as for vscanf.
This section describes how to do input and output operations on blocks of data. You can use these functions to read and write binary data, as well as to read and write text in fixed-size blocks instead of by characters or lines.
Binary files are typically used to read and write blocks of data in the same format as is used to represent the data in a running program. In other words, arbitrary blocks of memory--not just character or string objects--can be written to a binary file, and meaningfully read in again by the same program.
Storing data in binary form is often considerably more efficient than using the formatted I/O functions. Also, for floating-point numbers, the binary form avoids possible loss of precision in the conversion process. On the other hand, binary files can't be examined or modified easily using many standard file utilities (such as text editors), and are not portable between different implementations of the language, or different kinds of computers.
These functions are declared in `stdio.h'.
Function: size_t fread (void *data, size_t size, size_t count, FILE *stream)
This function reads up to count objects of size size into the array data, from the stream stream. It returns the number of objects actually read, which might be less than count if a read error occurs or the end of the file is reached. This function returns a value of zero (and doesn't read anything) if either size or count is zero.
If fread encounters end of file in the middle of an object, it
returns the number of complete objects read, and discards the partial
object. Therefore, the stream remains at the actual end of the file.
Function: size_t fwrite (const void *data, size_t size, size_t count, FILE *stream)
This function writes up to count objects of size size from the array data, to the stream stream. The return value is normally count, if the call succeeds. Any other value indicates some sort of error, such as running out of space.
Many of the functions described in this chapter return the value of the
macro EOF to indicate unsuccessful completion of the operation.
Since EOF is used to report both end of file and random errors,
it's often better to use the feof function to check explicitly
for end of file and ferror to check for errors. These functions
check indicators that are part of the internal state of the stream
object, indicators set if the appropriate condition was detected by a
previous I/O operation on that stream.
These symbols are declared in the header file `stdio.h'.
This macro is an integer value that is returned
by a number of functions to indicate an end-of-file condition, or some
other error situation. With the GNU library, EOF is -1.
In other libraries, its value may be some other negative number.
Function: void clearerr (FILE *stream)
This function clears the end-of-file and error indicators for the stream stream.
The file positioning functions (see section File Positioning) also clear the end-of-file indicator for the stream.
Function: int feof (FILE *stream)
The feof function returns nonzero if and only if the end-of-file
indicator for the stream stream is set.
Function: int ferror (FILE *stream)
The ferror function returns nonzero if and only if the error
indicator for the stream stream is set, indicating that an error
has occurred on a previous operation on the stream.
In addition to setting the error indicator associated with the stream,
the functions that operate on streams also set errno in the same
way as the corresponding low-level functions that operate on file
descriptors. For example, all of the functions that perform output to a
stream--such as fputc, printf, and fflush---are
implemented in terms of write, and all of the errno error
conditions defined for write are meaningful for these functions.
For more information about the descriptor-level I/O functions, see
section Low-Level Input/Output.
The GNU system and other POSIX-compatible operating systems organize all files as uniform sequences of characters. However, some other systems make a distinction between files containing text and files containing binary data, and the input and output facilities of ANSI C provide for this distinction. This section tells you how to write programs portable to such systems.
When you open a stream, you can specify either a text stream or a
binary stream. You indicate that you want a binary stream by
specifying the `b' modifier in the opentype argument to
fopen; see section Opening Streams. Without this
option, fopen opens the file as a text stream.
Text and binary streams differ in several ways:
'\n') characters, while a binary stream is
simply a long series of characters. A text stream might on some systems
fail to handle lines more than 254 characters long (including the
terminating newline character).
Since a binary stream is always more capable and more predictable than a text stream, you might wonder what purpose text streams serve. Why not simply always use binary streams? The answer is that on these operating systems, text and binary streams use different file formats, and the only way to read or write "an ordinary file of text" that can work with other text-oriented programs is through a text stream.
In the GNU library, and on all POSIX systems, there is no difference between text streams and binary streams. When you open a stream, you get the same kind of stream regardless of whether you ask for binary. This stream can handle any file content, and has none of the restrictions that text streams sometimes have.
The file position of a stream describes where in the file the stream is currently reading or writing. I/O on the stream advances the file position through the file. In the GNU system, the file position is represented as an integer, which counts the number of bytes from the beginning of the file. See section File Position.
During I/O to an ordinary disk file, you can change the file position whenever you wish, so as to read or write any portion of the file. Some other kinds of files may also permit this. Files which support changing the file position are sometimes referred to as random-access files.
You can use the functions in this section to examine or modify the file position indicator associated with a stream. The symbols listed below are declared in the header file `stdio.h'.
Function: long int ftell (FILE *stream)
This function returns the current file position of the stream stream.
This function can fail if the stream doesn't support file positioning,
or if the file position can't be represented in a long int, and
possibly for other reasons as well. If a failure occurs, a value of
-1 is returned.
Function: int fseek (FILE *stream, long int offset, int whence)
The fseek function is used to change the file position of the
stream stream. The value of whence must be one of the
constants SEEK_SET, SEEK_CUR, or SEEK_END, to
indicate whether the offset is relative to the beginning of the
file, the current file position, or the end of the file, respectively.
This function returns a value of zero if the operation was successful,
and a nonzero value to indicate failure. A successful call also clears
the end-of-file indicator of stream and discards any characters
that were "pushed back" by the use of ungetc.
fseek either flushes any buffered output before setting the file
position or else remembers it so it will be written later in its proper
place in the file.
Portability Note: In non-POSIX systems, ftell and
fseek might work reliably only on binary streams. See section Text and Binary Streams.
The following symbolic constants are defined for use as the whence
argument to fseek. They are also used with the lseek
function (see section Input and Output Primitives) and to specify offsets for file locks
(see section Control Operations on Files).
This is an integer constant which, when used as the whence
argument to the fseek function, specifies that the offset
provided is relative to the beginning of the file.
This is an integer constant which, when used as the whence
argument to the fseek function, specifies that the offset
provided is relative to the current file position.
This is an integer constant which, when used as the whence
argument to the fseek function, specifies that the offset
provided is relative to the end of the file.
Function: void rewind (FILE *stream)
The rewind function positions the stream stream at the
begining of the file. It is equivalent to calling fseek on the
stream with an offset argument of 0L and a
whence argument of SEEK_SET, except that the return
value is discarded and the error indicator for the stream is reset.
These three aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'.
On the GNU system, the file position is truly a character count. You
can specify any character count value as an argument to fseek and
get reliable results for any random access file. However, some ANSI C
systems do not represent file positions in this way.
On some systems where text streams truly differ from binary streams, it is impossible to represent the file position of a text stream as a count of characters from the beginning of the file. For example, the file position on some systems must encode both a record offset within the file, and a character offset within the record.
As a consequence, if you want your programs to be portable to these systems, you must observe certain rules:
ftell on a text stream has no predictable
relationship to the number of characters you have read so far. The only
thing you can rely on is that you can use it subsequently as the
offset argument to fseek to move back to the same file
position.
fseek on a text stream, either the offset must
either be zero; or whence must be SEEK_SET and the
offset must be the result of an earlier call to ftell on
the same stream.
ungetc
that haven't been read or discarded. See section Unreading.
But even if you observe these rules, you may still have trouble for long
files, because ftell and fseek use a long int value
to represent the file position. This type may not have room to encode
all the file positions in a large file.
So if you do want to support systems with peculiar encodings for the
file positions, it is better to use the functions fgetpos and
fsetpos instead. These functions represent the file position
using the data type fpos_t, whose internal representation varies
from system to system.
These symbols are declared in the header file `stdio.h'.
This is the type of an object that can encode information about the
file position of a stream, for use by the functions fgetpos and
fsetpos.
In the GNU system, fpos_t is equivalent to off_t or
long int. In other systems, it might have a different internal
representation.
Function: int fgetpos (FILE *stream, fpos_t *position)
This function stores the value of the file position indicator for the
stream stream in the fpos_t object pointed to by
position. If successful, fgetpos returns zero; otherwise
it returns a nonzero value and stores an implementation-defined positive
value in errno.
Function: int fsetpos (FILE *stream, const fpos_t position)
This function sets the file position indicator for the stream stream
to the position position, which must have been set by a previous
call to fgetpos on the same stream. If successful, fsetpos
clears the end-of-file indicator on the stream, discards any characters
that were "pushed back" by the use of ungetc, and returns a value
of zero. Otherwise, fsetpos returns a nonzero value and stores
an implementation-defined positive value in errno.
Characters that are written to a stream are normally accumulated and transmitted asynchronously to the file in a block, instead of appearing as soon as they are output by the application program. Similarly, streams often retrieve input from the host environment in blocks rather than on a character-by-character basis. This is called buffering.
If you are writing programs that do interactive input and output using streams, you need to understand how buffering works when you design the user interface to your program. Otherwise, you might find that output (such as progress or prompt messages) doesn't appear when you intended it to, or that input typed by the user is made available by lines instead of by single characters, or other unexpected behavior.
This section deals only with controlling when characters are transmitted between the stream and the file or device, and not with how things like echoing, flow control, and the like are handled on specific classes of devices. For information on common control operations on terminal devices, see section Low-Level Terminal Interface.
You can bypass the stream buffering facilities altogether by using the low-level input and output functions that operate on file descriptors instead. See section Low-Level Input/Output.
There are three different kinds of buffering strategies:
Newly opened streams are normally fully buffered, with one exception: a stream connected to an interactive device such as a terminal is initially line buffered. See section Controlling Which Kind of Buffering, for information on how to select a different kind of buffering.
The use of line buffering for interactive devices implies that output
messages ending in a newline will appear immediately--which is usually
what you want. Output that doesn't end in a newline might or might not
show up immediately, so if you want them to appear immediately, you
should flush buffered output explicitly with fflush, as described
in section Flushing Buffers.
Line buffering is a good default for terminal input as well, because most interactive programs read commands that are normally single lines. The program should be able to execute each line right away. A line buffered stream permits this, whereas a fully buffered stream would always read enough text to fill the buffer before allowing the program to read any of it. Line buffering also fits in with the usual input-editing facilities of most operating systems, which work within a line of input.
Some programs need an unbuffered terminal input stream. These include programs that read single-character commands (like Emacs) and programs that do their own input editing (such as those that use readline). In order to read a character at a time, it is not enough to turn off buffering in the input stream; you must also turn off input editing in the operating system. This requires changing the terminal mode (see section Terminal Modes). If you want to change the terminal modes, you have to do this separately--merely using an unbuffered stream does not change the modes.
Flushing output on a buffered stream means transmitting all accumulated characters to the file. There are many circumstances when buffered output on a stream is flushed automatically:
exit.
See section Normal Termination.
If you want to flush the buffered output at another time, call
fflush, which is declared in the header file `stdio.h'.
Function: int fflush (FILE *stream)
This function causes any buffered output on stream to be delivered
to the file. If stream is a null pointer, then
fflush causes buffered output on all open output streams
to be flushed.
This function returns EOF if a write error occurs, or zero
otherwise.
Compatibility Note: Some brain-damaged operating systems have been known to be so thoroughly fixated on line-oriented input and output that flushing a line buffered stream causes a newline to be written! Fortunately, this "feature" seems to be becoming less common. You do not need to worry about this in the GNU system.
After opening a stream (but before any other operations have been
performed on it), you can explicitly specify what kind of buffering you
want it to have using the setvbuf function.
The facilities listed in this section are declared in the header file `stdio.h'.
Function: int setvbuf (FILE *stream, char *buf, int mode, size_t size)
This function is used to specify that the stream stream should
have the buffering mode mode, which can be either _IOFBF
(for full buffering), _IOLBF (for line buffering), or
_IONBF (for unbuffered input/output).
If you specify a null pointer as the buf argument, then setvbuf
allocates a buffer itself using malloc. This buffer will be freed
when you close the stream.
Otherwise, buf should be a character array that can hold at least
size characters. You should not free the space for this array as
long as the stream remains open and this array remains its buffer. You
should usually either allocate it statically, or malloc
(see section Unconstrained Allocation) the buffer. Using an automatic array
is not a good idea unless you close the file before exiting the block
that declares the array.
While the array remains a stream buffer, the stream I/O functions will use the buffer for their internal purposes. You shouldn't try to access the values in the array directly while the stream is using it for buffering.
The setvbuf function returns zero on success, or a nonzero value
if the value of mode is not valid or if the request could not
be honored.
The value of this macro is an integer constant expression that can be
used as the mode argument to the setvbuf function to
specify that the stream should be fully buffered.
The value of this macro is an integer constant expression that can be
used as the mode argument to the setvbuf function to
specify that the stream should be line buffered.
The value of this macro is an integer constant expression that can be
used as the mode argument to the setvbuf function to
specify that the stream should be unbuffered.
The value of this macro is an integer constant expression that is good
to use for the size argument to setvbuf. This value is
guaranteed to be at least 256.
The value of BUFSIZ is chosen on each system so as to make stream
I/O efficient. So it is a good idea to use BUFSIZ as the size
for the buffer when you call setvbuf.
Actually, you can get an even better value to use for the buffer size
by means of the fstat system call: it is found in the
st_blksize field of the file attributes. See section What the File Attribute Values Mean.
Sometimes people also use BUFSIZ as the allocation size of
buffers used for related purposes, such as strings used to receive a
line of input with fgets (see section Character Input). There is no
particular reason to use BUFSIZ for this instead of any other
integer, except that it might lead to doing I/O in chunks of an
efficient size.
Function: void setbuf (FILE *stream, char *buf)
If buf is a null pointer, the effect of this function is
equivalent to calling setvbuf with a mode argument of
_IONBF. Otherwise, it is equivalent to calling setvbuf
with buf, and a mode of _IOFBF and a size
argument of BUFSIZ.
The setbuf function is provided for compatibility with old code;
use setvbuf in all new programs.
Function: void setbuffer (FILE *stream, char *buf, size_t size)
If buf is a null pointer, this function makes stream unbuffered. Otherwise, it makes stream fully buffered using buf as the buffer. The size argument specifies the length of buf.
This function is provided for compatibility with old BSD code. Use
setvbuf instead.
Function: void setlinebuf (FILE *stream)
This function makes stream be line buffered, and allocates the buffer for you.
This function is provided for compatibility with old BSD code. Use
setvbuf instead.
If you need to use a temporary file in your program, you can use the
tmpfile function to open it. Or you can use the tmpnam
function make a name for a temporary file and then open it in the usual
way with fopen.
These facilities are declared in the header file `stdio.h'.
Function: FILE * tmpfile (void)
This function creates a temporary binary file for update mode, as if by
calling fopen with mode "wb+". The file is deleted
automatically when it is closed or when the program terminates. (On
some other ANSI C systems the file may fail to be deleted if the program
terminates abnormally).
Function: char * tmpnam (char *result)
This function constructs and returns a file name that is a valid file
name and that does not name any existing file. If the result
argument is a null pointer, the return value is a pointer to an internal
static string, which might be modified by subsequent calls. Otherwise,
the result argument should be a pointer to an array of at least
L_tmpnam characters, and the result is written into that array.
It is possible for tmpnam to fail if you call it too many times.
This is because the fixed length of a temporary file name gives room for
only a finite number of different names. If tmpnam fails, it
returns a null pointer.
The value of this macro is an integer constant expression that represents
the minimum allocation size of a string large enough to hold the
file name generated by the tmpnam function.
The macro TMP_MAX is a lower bound for how many temporary names
you can create with tmpnam. You can rely on being able to call
tmpnam at least this many times before it might fail saying you
have made too many temporary file names.
With the GNU library, you can create a very large number of temporary
file names--if you actually create the files, you will probably run out
of disk space before you run out of names. Some other systems have a
fixed, small limit on the number of temporary files. The limit is never
less than 25.
Function: char * tempnam (const char *dir, const char *prefix)
This function generates a unique temporary filename. If prefix is not a null pointer, up to five characters of this string are used as a prefix for the file name.
The directory prefix for the temporary file name is determined by testing each of the following, in sequence. The directory must exist and be writable.
TMPDIR, if it is defined.
P_tmpdir macro.
This function is defined for SVID compatibility.
This macro is the name of the default directory for temporary files.
The GNU library provides ways for you to define additional kinds of streams that do not necessarily correspond to an open file.
One such type of stream takes input from or writes output to a string.
These kinds of streams are used internally to implement the
sprintf and sscanf functions. You can also create such a
stream explicitly, using the functions described in section String Streams.
More generally, you can define streams that do input/output to arbitrary objects using functions supplied by your program. This protocol is discussed in section Programming Your Own Custom Streams.
Portability Note: The facilities described in this section are specific to GNU. Other systems or C implementations might or might not provide equivalent functionality.
The fmemopen and open_memstream functions allow you to do
I/O to a string or memory buffer. These facilities are declared in
`stdio.h'.
Function: FILE * fmemopen (void *buf, size_t size, const char *opentype)
This function opens a stream that allows the access specified by the opentype argument, that reads from or writes to the buffer specified by the argument buf. This array must be at least size bytes long.
If you specify a null pointer as the buf argument, fmemopen
dynamically allocates (as with malloc; see section Unconstrained Allocation) an array size bytes long. This is really only useful
if you are going to write things to the buffer and then read them back
in again, because you have no way of actually getting a pointer to the
buffer (for this, try open_memstream, below). The buffer is
freed when the stream is open.
The argument opentype is the same as in fopen
(See section Opening Streams). If the opentype specifies
append mode, then the initial file position is set to the first null
character in the buffer. Otherwise the initial file position is at the
beginning of the buffer.
When a stream open for writing is flushed or closed, a null character (zero byte) is written at the end of the buffer if it fits. You should add an extra byte to the size argument to account for this. Attempts to write more than size bytes to the buffer result in an error.
For a stream open for reading, null characters (zero bytes) in the buffer do not count as "end of file". Read operations indicate end of file only when the file position advances past size bytes. So, if you want to read characters from a null-terminated string, you should supply the length of the string as the size argument.
Here is an example of using fmemopen to create a stream for
reading from a string:
#include <stdio.h>
static char buffer[] = "foobar";
int
main (void)
{
int ch;
FILE *stream;
stream = fmemopen (buffer, strlen (buffer), "r");
while ((ch = fgetc (stream)) != EOF)
printf ("Got %c\n", ch);
fclose (stream);
return 0;
}
This program produces the following output:
Got f Got o Got o Got b Got a Got r
Function: FILE * open_memstream (char **ptr, size_t *sizeloc)
This function opens a stream for writing to a buffer. The buffer is
allocated dynamically (as with malloc; see section Unconstrained Allocation) and grown as necessary.
When the stream is closed with fclose or flushed with
fflush, the locations ptr and sizeloc are updated to
contain the pointer to the buffer and its size. The values thus stored
remain valid only as long as no further output on the stream takes
place. If you do more output, you must flush the stream again to store
new values before you use them again.
A null character is written at the end of the buffer. This null character is not included in the size value stored at sizeloc.
You can move the stream's file position with fseek (see section File Positioning). Moving the file position past the end of the data
already written fills the intervening space with zeroes.
Here is an example of using open_memstream:
#include <stdio.h>
int
main (void)
{
char *bp;
size_t size;
FILE *stream;
stream = open_memstream (&bp, &size);
fprintf (stream, "hello");
fflush (stream);
printf ("buf = %s, size = %d\n", bp, size);
fprintf (stream, ", world");
fclose (stream);
printf ("buf = %s, size = %d\n", bp, size);
return 0;
}
This program produces the following output:
buf = `hello', size = 5 buf = `hello, world', size = 12
You can open an output stream that puts it data in an obstack. See section Obstacks.
Function: FILE * open_obstack_stream (struct obstack *obstack)
This function opens a stream for writing data into the obstack obstack. This starts an object in the obstack and makes it grow as data is written (see section Growing Objects).
Calling fflush on this stream updates the current size of the
object to match the amount of data that has been written. After a call
to fflush, you can examine the object temporarily.
You can move the file position of an obstack stream with fseek
(see section File Positioning). Moving the file position past the end of
the data written fills the intervening space with zeros.
To make the object permanent, update the obstack with fflush, and
then use obstack_finish to finalize the object and get its address.
The following write to the stream starts a new object in the obstack,
and later writes add to that object until you do another fflush
and obstack_finish.
But how do you find out how long the object is? You can get the length
in bytes by calling obstack_object_size (see section Status of an Obstack), or you can null-terminate the object like this:
obstack_1grow (obstack, 0);
Whichever one you do, you must do it before calling
obstack_finish. (You can do both if you wish.)
Here is a sample function that uses open_obstack_stream:
char *
make_message_string (const char *a, int b)
{
FILE *stream = open_obstack_stream (&message_obstack);
output_task (stream);
fprintf (stream, ": ");
fprintf (stream, a, b);
fprintf (stream, "\n");
fclose (stream);
obstack_1grow (&message_obstack, 0);
return obstack_finish (&message_obstack);
}
This section describes how you can make a stream that gets input from an arbitrary data source or writes output to an arbitrary data sink programmed by you. We call these custom streams.
Inside every custom stream is a special object called the cookie.
This is an object supplied by you which records where to fetch or store
the data read or written. It is up to you to define a data type to use
for the cookie. The stream functions in the library never refer
directly to its contents, and they don't even know what the type is;
they record its address with type void *.
To implement a custom stream, you must specify how to fetch or store the data in the specified place. You do this by defining hook functions to read, write, change "file position", and close the stream. All four of these functions will be passed the stream's cookie so they can tell where to fetch or store the data. The library functions don't know what's inside the cookie, but your functions will know.
When you create a custom stream, you must specify the cookie pointer,
and also the four hook functions stored in a structure of type
struct cookie_io_functions.
These facilities are declared in `stdio.h'.
Data Type: struct cookie_io_functions
This is a structure type that holds the functions that define the communications protocol between the stream and its cookie. It has the following members:
cookie_read_function *read
EOF.
cookie_write_function *write
cookie_seek_function *seek
fseek on this stream return an ESPIPE error.
cookie_close_function *close
Function: FILE * fopencookie (void *cookie, const char *opentype, struct cookie_functions io_functions)
This function actually creates the stream for communicating with the
cookie using the functions in the io_functions argument.
The opentype argument is interpreted as for fopen;
see section Opening Streams. (But note that the "truncate on
open" option is ignored.) The new stream is fully buffered.
The fopencookie function returns the newly created stream, or a null
pointer in case of an error.
Here are more details on how you should define the four hook functions that a custom stream needs.
You should define the function to read data from the cookie as:
ssize_t reader (void *cookie, void *buffer, size_t size)
This is very similar to the read function; see section Input and Output Primitives. Your function should transfer up to size bytes into
the buffer, and return the number of bytes read, or zero to
indicate end-of-file. You can return a value of -1 to indicate
an error.
You should define the function to write data to the cookie as:
ssize_t writer (void *cookie, const void *buffer, size_t size)
This is very similar to the write function; see section Input and Output Primitives. Your function should transfer up to size bytes from
the buffer, and return the number of bytes written. You can return a
value of -1 to indicate an error.
You should define the function to perform seek operations on the cookie as:
int seeker (void *cookie, fpos_t *position, int whence)
For this function, the position and whence arguments are
interpreted as for fgetpos; see section Portable File-Position Functions. In
the GNU library, fpos_t is equivalent to off_t or
long int, and simply represents the number of bytes from the
beginning of the file.
After doing the seek operation, your function should store the resulting
file position relative to the beginning of the file in position.
Your function should return a value of 0 on success and -1
to indicate an error.
You should define the function to do cleanup operations on the cookie appropriate for closing the stream as:
int cleaner (void *cookie)
Your function should return -1 to indicate an error, and 0
otherwise.
Data Type: cookie_read_function
This is the data type that the read function for a custom stream should have. If you declare the function as shown above, this is the type it will have.
Data Type: cookie_write_function
The data type of the write function for a custom stream.
Data Type: cookie_seek_function
The data type of the seek function for a custom stream.
Data Type: cookie_close_function
The data type of the close function for a custom stream.
This chapter describes functions for performing low-level input/output operations on file descriptors. These functions include the primitives for the higher-level I/O functions described in section Input/Output on Streams, as well as functions for performing low-level control operations for which there are no equivalents on streams.
Stream-level I/O is more flexible and usually more convenient; therefore, programmers generally use the descriptor-level functions only when necessary. These are some of the usual reasons:
fileno to get the descriptor
corresponding to a stream.)
This section describes the primitives for opening and closing files
using file descriptors. The open and creat functions are
declared in the header file `fcntl.h', while close is
declared in `unistd.h'.
Function: int open (const char *filename, int flags[, mode_t mode])
The open function creates and returns a new file descriptor
for the file named by filename. Initially, the file position
indicator for the file is at the beginning of the file. The argument
mode is used only when a file is created, but it doesn't hurt
to supply the argument in any case.
The flags argument controls how the file is to be opened. This is a bit mask; you create the value by the bitwise OR of the appropriate parameters (using the `|' operator in C).
The flags argument must include exactly one of these values to specify the file access mode:
O_RDONLY
O_WRONLY
O_RDWR
The flags argument can also include any combination of these flags:
O_APPEND
write operations write the data at the end of
the file, extending it, regardless of the current file position.
O_CREAT
O_EXCL
O_CREAT and O_EXCL are set, then open fails
if the specified file already exists.
O_NOCTTY
O_NONBLOCK
open blocks until
the file is "ready". If O_NONBLOCK is set, open
returns immediately.
The O_NONBLOCK bit also affects read and write: It
permits them to return immediately with a failure status if there is no
input immediately available (read), or if the output can't be
written immediately (write).
O_TRUNC
For more information about these symbolic constants, see section File Status Flags.
The normal return value from open is a non-negative integer file
descriptor. In the case of an error, a value of -1 is returned
instead. In addition to the usual file name syntax errors (see section File Name Errors), the following errno error conditions are defined
for this function:
EACCES
EEXIST
O_CREAT and O_EXCL are set, and the named file already
exists.
EINTR
open operation was interrupted by a signal.
See section Primitives Interrupted by Signals.
EISDIR
EMFILE
ENFILE
ENOENT
O_CREAT is not specified.
ENOSPC
ENXIO
O_NONBLOCK and O_WRONLY are both set in the flags
argument, the file named by filename is a FIFO (see section Pipes and FIFOs), and no process has the file open for reading.
EROFS
O_WRONLY,
O_RDWR, O_CREAT, and O_TRUNC are set in the
flags argument.
The open function is the underlying primitive for the fopen
and freopen functions, that create streams.
Obsolete function: int creat (const char *filename, mode_t mode)
This function is obsolete. The call
creat (filename, mode)
is equivalent to
open (filename, O_WRONLY | O_CREAT | O_TRUNC, mode)
Function: int close (int filedes)
The function close closes the file descriptor filedes.
Closing a file has the following consequences:
The normal return value from close is 0; a value of -1
is returned in case of failure. The following errno error
conditions are defined for this function:
EBADF
EINTR
EINTR properly:
TEMP_FAILURE_RETRY (close (desc));
To close a stream, call fclose (see section Closing Streams) instead
of trying to close its underlying file descriptor with close.
This flushes any buffered output and updates the stream object to
indicate that it is closed.
This section describes the functions for performing primitive input and
output operations on file descriptors: read, write, and
lseek. These functions are declared in the header file
`unistd.h'.
This data type is used to represent the sizes of blocks that can be
read or written in a single operation. It is similar to size_t,
but must be a signed type.
Function: ssize_t read (int filedes, void *buffer, size_t size)
The read function reads up to size bytes from the file
with descriptor filedes, storing the results in the buffer.
(This is not necessarily a character string and there is no terminating
null character added.)
The return value is the number of bytes actually read. This might be less than size; for example, if there aren't that many bytes left in the file or if there aren't that many bytes immediately available. The exact behavior depends on what kind of file it is. Note that reading less than size bytes is not an error.
A value of zero indicates end-of-file (except if the value of the
size argument is also zero). This is not considered an error.
If you keep calling read while at end-of-file, it will keep
returning zero and doing nothing else.
If read returns at least one character, there is no way you can
tell whether end-of-file was reached. But if you did reach the end, the
next read will return zero.
In case of an error, read returns -1. The following
errno error conditions are defined for this function:
EAGAIN
read waits for
some input. But if the O_NONBLOCK flag is set for the file
(see section File Status Flags), read returns immediately without
reading any data, and reports this error.
Compatibility Note: Most versions of BSD Unix use a different
error code for this: EWOULDBLOCK. In the GNU library,
EWOULDBLOCK is an alias for EAGAIN, so it doesn't matter
which name you use.
On some systems, reading a large amount of data from a character special
file can also fail with EAGAIN if the kernel cannot find enough
physical memory to lock down the user's pages. This is limited to
devices that transfer with direct memory access into the user's memory,
which means it does not include terminals, since they always use
separate buffers inside the kernel.
EBADF
EINTR
read was interrupted by a signal while it was waiting for input.
See section Primitives Interrupted by Signals.
EIO
EIO also occurs when a background process tries to read from the
controlling terminal, and the normal action of stopping the process by
sending it a SIGTTIN signal isn't working. This might happen if
signal is being blocked or ignored, or because the process group is
orphaned. See section Job Control, for more information about job control,
and section Signal Handling, for information about signals.
The read function is the underlying primitive for all of the
functions that read from streams, such as fgetc.
Function: ssize_t write (int filedes, const void *buffer, size_t size)
The write function writes up to size bytes from
buffer to the file with descriptor filedes. The data in
buffer is not necessarily a character string and a null character
output like any other character.
The return value is the number of bytes actually written. This is normally the same as size, but might be less (for example, if the physical media being written to fills up).
In the case of an error, write returns -1. The following
errno error conditions are defined for this function:
EAGAIN
write blocks until the write operation is complete.
But if the O_NONBLOCK flag is set for the file (see section Control Operations on Files), it returns immediately without writing any data, and
reports this error. An example of a situation that might cause the
process to block on output is writing to a terminal device that supports
flow control, where output has been suspended by receipt of a STOP
character.
Compatibility Note: Most versions of BSD Unix use a different
error code for this: EWOULDBLOCK. In the GNU library,
EWOULDBLOCK is an alias for EAGAIN, so it doesn't matter
which name you use.
On some systems, writing a large amount of data from a character special
file can also fail with EAGAIN if the kernel cannot find enough
physical memory to lock down the user's pages. This is limited to
devices that transfer with direct memory access into the user's memory,
which means it does not include terminals, since they always use
separate buffers inside the kernel.
EBADF
EFBIG
EINTR
write operation was interrupted by a signal while it was
blocked waiting for completion. See section Primitives Interrupted by Signals.
EIO
EIO also occurs when a background process tries to write to the
controlling terminal, and the normal action of stopping the process by
sending it a SIGTTOU signal isn't working. This might happen if
the signal is being blocked or ignored. See section Job Control, for more
information about job control, and section Signal Handling, for
information about signals.
ENOSPC
EPIPE
SIGPIPE
signal is also sent to the process; see section Signal Handling.
Unless you have arranged to prevent EINTR failures, you should
check errno after each failing call to write, and if the
error was EINTR, you should simply repeat the call.
See section Primitives Interrupted by Signals. The easy way to do this is with the
macro TEMP_FAILURE_RETRY, as follows:
nbytes = TEMP_FAILURE_RETRY (write (desc, buffer, count));
The write function is the underlying primitive for all of the
functions that write to streams, such as fputc.
Just as you can set the file position of a stream with fseek, you
can set the file position of a descriptor with lseek. This
specifies the position in the file for the next read or
write operation. See section File Positioning, for more information
on the file position and what it means.
To read the current file position value from a descriptor, use
lseek (desc, 0, SEEK_CUR).
Function: off_t lseek (int filedes, off_t offset, int whence)
The lseek function is used to change the file position of the
file with descriptor filedes.
The whence argument specifies how the offset should be
interpreted in the same way as for the fseek function, and can be
one of the symbolic constants SEEK_SET, SEEK_CUR, or
SEEK_END.
SEEK_SET
SEEK_CUR
SEEK_END
The return value from lseek is normally the resulting file
position, measured in bytes from the beginning of the file.
You can use this feature together with SEEK_CUR to read the
current file position.
You can set the file position past the current end of the file. This
does not by itself make the file longer; lseek never changes the
file. But subsequent output at that position will extend the file's
size.
If the file position cannot be changed, or the operation is in some way
invalid, lseek returns a value of -1. The following
errno error conditions are defined for this function:
EBADF
EINVAL
ESPIPE
The lseek function is the underlying primitive for the
fseek, ftell and rewind functions, which operate on
streams instead of file descriptors.
You can have multiple descriptors for the same file if you open the file
more than once, or if you duplicate a descriptor with dup.
Descriptors that come from separate calls to open have independent
file positions; using lseek on one descriptor has no effect on the
other. For example,
{
int d1, d2;
char buf[4];
d1 = open ("foo", O_RDONLY);
d2 = open ("foo", O_RDONLY);
lseek (d1, 1024, SEEK_SET);
read (d2, buf, 4);
}
will read the first four characters of the file `foo'. (The error-checking code necessary for a real program has been omitted here for brevity.)
By contrast, descriptors made by duplication share a common file position with the original descriptor that was duplicated. Anything which alters the file position of one of the duplicates, including reading or writing data, affects all of them alike. Thus, for example,
{
int d1, d2, d3;
char buf1[4], buf2[4];
d1 = open ("foo", O_RDONLY);
d2 = dup (d1);
d3 = dup (d2);
lseek (d3, 1024, SEEK_SET);
read (d1, buf1, 4);
read (d2, buf2, 4);
}
will read four characters starting with the 1024'th character of `foo', and then four more characters starting with the 1028'th character.
This is an arithmetic data type used to represent file sizes.
In the GNU system, this is equivalent to fpos_t or long int.
These three aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'.
L_SET
SEEK_SET.
L_INCR
SEEK_CUR.
L_XTND
SEEK_END.
Given an open file descriptor, you can create a stream for it with the
fdopen function. You can get the underlying file descriptor for
an existing stream with the fileno function. These functions are
declared in the header file `stdio.h'.
Function: FILE * fdopen (int filedes, const char *opentype)
The fdopen function returns a new stream for the file descriptor
filedes.
The opentype argument is interpreted in the same way as for the
fopen function (see section Opening Streams), except that
the `b' option is not permitted; this is because GNU makes no
distinction between text and binary files. Also, "w" and
"w+" do not cause truncation of the file; these have affect only
when opening a file, and in this case the file has already been opened.
You must make sure that the opentype argument matches the actual
mode of the open file descriptor.
The return value is the new stream. If the stream cannot be created (for example, if the modes for the file indicated by the file descriptor do not permit the access specified by the opentype argument), a null pointer is returned instead.
For an example showing the use of the fdopen function,
see section Creating a Pipe.
Function: int fileno (FILE *stream)
This function returns the file descriptor associated with the stream
stream. If an error is detected (for example, if the stream
is not valid) or if stream does not do I/O to a file,
fileno returns -1.
There are also symbolic constants defined in `unistd.h' for the
file descriptors belonging to the standard streams stdin,
stdout, and stderr; see section Standard Streams.
STDIN_FILENO
0, which is the file descriptor for
standard input.
STDOUT_FILENO
1, which is the file descriptor for
standard output.
STDERR_FILENO
2, which is the file descriptor for
standard error output.
You can have multiple file descriptors and streams (let's call both streams and descriptors "channels" for short) connected to the same file, but you must take care to avoid confusion between channels. There are two cases to consider: linked channels that share a single file position value, and independent channels that have their own file positions.
It's best to use just one channel in your program for actual data
transfer to any given file, except when all the access is for input.
For example, if you open a pipe (something you can only do at the file
descriptor level), either do all I/O with the descriptor, or construct a
stream from the descriptor with fdopen and then do all I/O with
the stream.
Channels that come from a single opening share the same file position;
we call them linked channels. Linked channels result when you
make a stream from a descriptor using fdopen, when you get a
descriptor from a stream with fileno, and when you copy a
descriptor with dup or dup2. For files that don't support
random access, such as terminals and pipes, all channels are
effectively linked. On random-access files, all append-type output
streams are effectively linked to each other.
If you have been using a stream for I/O, and you want to do I/O using another channel (either a stream or a descriptor) that is linked to it, you must first clean up the stream that you have been using. See section Cleaning Streams.
Terminating a process, or executing a new program in the process, destroys all the streams in the process. If descriptors linked to these streams persist in other processes, their file positions become undefined as a result. To prevent this, you must clean up the streams before destroying them.
When you open channels (streams or descriptors) separately on a seekable file, each channel has its own file position. These are called independent channels.
The system handles each channel independently. Most of the time, this is quite predictable and natural (especially for input): each channel can read or write sequentially at its own place in the file. The precautions you should take are these:
If you do output to one channel at the end of the file, this will certainly leave the other independent channels positioned somewhere before the new end. If you want them to output at the end, you must set their file positions to end of file, first. (This is not necessary if you use an append-type descriptor or stream; they always output at the current end of the file.) In order to make the end-of-file position accurate, you must clean the output channel you were using, if it is a stream. (This is necessary even if you plan to use an append-type channel next.)
It's impossible for two channels to have separate file pointers for a file that doesn't support random access. Thus, channels for reading or writing such files are always linked, never independent. Append-type channels are also always linked. For these channels, follow the rules for linked channels; see section Linked Channels.
On the GNU system, you can clean up any stream with fclean:
Clean up the stream stream so that its buffer is empty. If stream is doing output, force it out. If stream is doing input, give the data in the buffer back to the system, arranging to reread it.
On other systems, you can use fflush to clean a stream in most
cases.
You can skip the fclean or fflush if you know the stream
is already clean. A stream is clean whenever its buffer is empty. For
example, an unbuffered stream is always clean. An input stream that is
at end-of-file is clean. A line-buffered stream is clean when the last
character output was a newline.
There is one case in which cleaning a stream is impossible on most
systems. This is when the stream is doing input from a file that is not
random-access. Such streams typically read ahead, and when the file is
not random access, there is no way to give back the excess data already
read. When an input stream reads from a random-access file,
fflush does clean the stream, but leaves the file pointer at an
unpredictable place; you must set the file pointer before doing any
further I/O. On the GNU system, using fclean avoids both of
these problems.
Closing an output-only stream also does fflush, so this is a
valid way of cleaning an output stream. On the GNU system, closing an
input stream does fclean.
You need not clean a stream before using its descriptor for control operations such as setting terminal modes; these operations don't affect the file position and are not affected by it. You can use any descriptor for these operations, and all channels are affected simultaneously. However, text already "output" to a stream but still buffered by the stream will be subject to the new terminal modes when subsequently flushed. To make sure "past" output is covered by the terminal settings that were in effect at the time, flush the output streams for that terminal before setting the modes. See section Terminal Modes.
Sometimes a program needs to accept input on multiple input channels whenever input arrives. For example, some workstations may have devices such as a digitizing tablet, function button box, or dial box that are connected via normal asynchronous serial interfaces; good user interface style requires responding immediately to input on any device. Another example is a program that acts as a server to several other processes via pipes or sockets.
You cannot normally use read for this purpose, because this
blocks the program until input is available on one particular file
descriptor; input on other channels won't wake it up. You could set
nonblocking mode and poll each file descriptor in turn, but this is very
inefficient.
A better solution is to use the select function. This blocks the
program until input or output is ready on a specified set of file
descriptors, or until timer expires, whichever comes first. This
facility is declared in the header file `sys/types.h'.
The file descriptor sets for the select function are specified
as fd_set objects. Here is the description of the data type
and some macros for manipulating these objects.
The fd_set data type represents file descriptor sets for the
select function. It is actually a bit array.
The value of this macro is the maximum number of file descriptors that a
fd_set object can hold information about. On systems with a
fixed maximum number, FD_SETSIZE is at least that number. On
some systems, including GNU, there is no absolute limit on the number of
descriptors open, but this macro still has a constant value which
controls the number of bits in an fd_set.
Macro: void FD_ZERO (fd_set *set)
This macro initializes the file descriptor set set to be the empty set.
Macro: void FD_SET (int filedes, fd_set *set)
This macro adds filedes to the file descriptor set set.
Macro: void FD_CLR (int filedes, fd_set *set)
This macro removes filedes from the file descriptor set set.
Macro: int FD_ISSET (int filedes, fd_set *set)
This macro returns a nonzero value (true) if filedes is a member of the the file descriptor set set, and zero (false) otherwise.
Next, here is the description of the select function itself.
Function: int select (int nfds, fd_set *read_fds, fd_set *write_fds, fd_set *except_fds, struct timeval *timeout)
The select function blocks the calling process until there is
activity on any of the specified sets of file descriptors, or until the
timeout period has expired.
The file descriptors specified by the read_fds argument are checked to see if they are ready for reading; the write_fds file descriptors are checked to see if they are ready for writing; and the except_fds file descriptors are checked for exceptional conditions. You can pass a null pointer for any of these arguments if you are not interested in checking for that kind of condition.
"Exceptional conditions" does not mean errors--errors are reported immediately when an erroneous system call is executed, and do not constitute a state of the descriptor. Rather, they include conditions such as the presence of an urgent message on a socket. (See section Sockets, for information on urgent messages.)
The select function checks only the first nfds file
descriptors. The usual thing is to pass FD_SETSIZE as the value
of this argument.
The timeout specifies the maximum time to wait. If you pass a
null pointer for this argument, it means to block indefinitely until one
of the file descriptors is ready. Otherwise, you should provide the
time in struct timeval format; see section High-Resolution Calendar. Specify zero as the time (a struct timeval containing
all zeros) if you want to find out which descriptors are ready without
waiting if none are ready.
The normal return value from select is the total number of ready file
descriptors in all of the sets. Each of the argument sets is overwritten
with information about the descriptors that are ready for the corresponding
operation. Thus, to see if a particular descriptor desc has input,
use FD_ISSET (desc, read_fds) after select returns.
If select returns because the timeout period expires, it returns
a value of zero.
Any signal will cause select to return immediately. So if your
program uses signals, you can't rely on select to keep waiting
for the full time specified. If you want to be sure of waiting for a
particular amount of time, you must check for EINTR and repeat
the select with a newly calculated timeout based on the current
time. See the example below. See also section Primitives Interrupted by Signals.
If an error occurs, select returns -1 and does not modify
the argument file descriptor sets. The following errno error
conditions are defined for this function:
EBADF
EINTR
EINVAL
Portability Note: The select function is a BSD Unix
feature.
Here is an example showing how you can use select to establish a
timeout period for reading from a file descriptor. The input_timeout
function blocks the calling process until input is available on the
file descriptor, or until the timeout period expires.
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>
int
input_timeout (int filedes, unsigned int seconds)
{
fd_set set;
struct timeval timeout;
/* Initialize the file descriptor set. */
FD_ZERO (&set);
FD_SET (filedes, &set);
/* Initialize the timeout data structure. */
timeout.tv_sec = seconds;
timeout.tv_usec = 0;
/* select returns 0 if timeout, 1 if input available, -1 if error. */
return TEMP_FAILURE_RETRY (select (FD_SETSIZE, &set, NULL, NULL, &timeout));
}
int
main (void)
{
fprintf (stderr, "select returned %d.\n", input_timeout (STDIN_FILENO, 5));
return 0;
}
There is another example showing the use of select to multiplex
input from multiple sockets in section Byte Stream Connection Server Example.
This section describes how you can perform various other operations on
file descriptors, such as inquiring about or setting flags describing
the status of the file descriptor, manipulating record locks, and the
like. All of these operations are performed by the function fcntl.
The second argument to the fcntl function is a command that
specifies which operation to perform. The function and macros that name
various flags that are used with it are declared in the header file
`fcntl.h'. (Many of these flags are also used by the open
function; see section Opening and Closing Files.)
Function: int fcntl (int filedes, int command, ...)
The fcntl function performs the operation specified by
command on the file descriptor filedes. Some commands
require additional arguments to be supplied. These additional arguments
and the return value and error conditions are given in the detailed
descriptions of the individual commands.
Briefly, here is a list of what the various commands are.
F_DUPFD
F_GETFD
F_SETFD
F_GETFL
F_SETFL
F_GETLK
F_SETLK
F_SETLKW
F_SETLK, but wait for completion. See section File Locks.
F_GETOWN
SIGIO signals.
See section Interrupt-Driven Input.
F_SETOWN
SIGIO signals.
See section Interrupt-Driven Input.
You can duplicate a file descriptor, or allocate another file descriptor that refers to the same open file as the original. Duplicate descriptors share one file position and one set of file status flags (see section File Status Flags), but each has its own set of file descriptor flags (see section File Descriptor Flags).
The major use of duplicating a file descriptor is to implement redirection of input or output: that is, to change the file or pipe that a particular file descriptor corresponds to.
You can perform this operation using the fcntl function with the
F_DUPFD command, but there are also convenient functions
dup and dup2 for duplicating descriptors.
The fcntl function and flags are declared in `fcntl.h',
while prototypes for dup and dup2 are in the header file
`unistd.h'.
This function copies descriptor old to the first available
descriptor number (the first number not currently open). It is
equivalent to fcntl (old, F_DUPFD, 0).
Function: int dup2 (int old, int new)
This function copies the descriptor old to descriptor number new.
If old is an invalid descriptor, then dup2 does nothing; it
does not close new. Otherwise, the new duplicate of old
replaces any previous meaning of descriptor new, as if new
were closed first.
If old and new are different numbers, and old is a
valid descriptor number, then dup2 is equivalent to:
close (new); fcntl (old, F_DUPFD, new)
However, dup2 does this atomically; there is no instant in the
middle of calling dup2 at which new is closed and not yet a
duplicate of old.
This macro is used as the command argument to fcntl, to
copy the file descriptor given as the first argument.
The form of the call in this case is:
fcntl (old, F_DUPFD, next_filedes)
The next_filedes argument is of type int and specifies that
the file descriptor returned should be the next available one greater
than or equal to this value.
The return value from fcntl with this command is normally the value
of the new file descriptor. A return value of -1 indicates an
error. The following errno error conditions are defined for
this command:
EBADF
EINVAL
EMFILE
ENFILE is not a possible error code for dup2 because
dup2 does not create a new opening of a file; duplicate
descriptors do not count toward the limit which ENFILE
indicates. EMFILE is possible because it refers to the limit on
distinct descriptor numbers in use in one process.
Here is an example showing how to use dup2 to do redirection.
Typically, redirection of the standard streams (like stdin) is
done by a shell or shell-like program before calling one of the
exec functions (see section Executing a File) to execute a new
program in a child process. When the new program is executed, it
creates and initializes the standard streams to point to the
corresponding file descriptors, before its main function is
invoked.
So, to redirect standard input to a file, the shell could do something like:
pid = fork ();
if (pid == 0)
{
char *filename;
char *program;
int file;
...
file = TEMP_FAILURE_RETRY (open (filename, O_RDONLY));
dup2 (file, STDIN_FILENO);
TEMP_FAILURE_RETRY (close (file));
execv (program, NULL);
}
There is also a more detailed example showing how to implement redirection in the context of a pipeline of processes in section Launching Jobs.
File descriptor flags are miscellaneous attributes of a file descriptor. These flags are associated with particular file descriptors, so that if you have created duplicate file descriptors from a single opening of a file, each descriptor has its own set of flags.
Currently there is just one file descriptor flag: FD_CLOEXEC,
which causes the descriptor to be closed if you use any of the
exec... functions (see section Executing a File).
The symbols in this section are defined in the header file `fcntl.h'.
This macro is used as the command argument to fcntl, to
specify that it should return the file descriptor flags associated
with the filedes argument.
The normal return value from fcntl with this command is a
nonnegative number which can be interpreted as the bitwise OR of the
individual flags (except that currently there is only one flag to use).
In case of an error, fcntl returns -1. The following
errno error conditions are defined for this command:
EBADF
This macro is used as the command argument to fcntl, to
specify that it should set the file descriptor flags associated with the
filedes argument. This requires a third int argument to
specify the new flags, so the form of the call is:
fcntl (filedes, F_SETFD, new_flags)
The normal return value from fcntl with this command is an
unspecified value other than -1, which indicates an error.
The flags and error conditions are the same as for the F_GETFD
command.
The following macro is defined for use as a file descriptor flag with
the fcntl function. The value is an integer constant usable
as a bit mask value.
This flag specifies that the file descriptor should be closed when
an exec function is invoked; see section Executing a File. When
a file descriptor is allocated (as with open or dup),
this bit is initially cleared on the new file descriptor, meaning that
descriptor will survive into the new program after exec.
If you want to modify the file descriptor flags, you should get the
current flags with F_GETFD and modify the value. Don't assume
that the flag listed here is the only ones that are implemented; your
program may be run years from now and more flags may exist then.
For example, here is a function to set or clear the flag FD_CLOEXEC
without altering any other flags:
/* Set theFD_CLOEXECflag of desc if value is nonzero, or clear the flag if value is 0. Return 0 on success, or -1 on error witherrnoset. */ int set_cloexec_flag (int desc, int value) { int oldflags = fcntl (desc, F_GETFD, 0); /* If reading the flags failed, return error indication now. if (oldflags < 0) return oldflags; /* Set just the flag we want to set. */ if (value != 0) oldflags |= FD_CLOEXEC; else oldflags &= ~FD_CLOEXEC; /* Store modified flag word in the descriptor. */ return fcntl (desc, F_SETFD, oldflags); }
File status flags are used to specify attributes of the opening of a file. Unlike the file descriptor flags discussed in section File Descriptor Flags, the file status flags are shared by duplicated file descriptors resulting from a single opening of the file.
The file status flags are initialized by the open function from
the flags argument of the open function. Some of the flags
are meaningful only in open and are not remembered subsequently;
many of the rest cannot subsequently be changed, though you can read
their values by examining the file status flags.
A few file status flags can be changed at any time using fcntl.
These include O_APPEND and O_NONBLOCK.
The symbols in this section are defined in the header file `fcntl.h'.
This macro is used as the command argument to fcntl, to
read the file status flags for the open file with descriptor
filedes.
The normal return value from fcntl with this command is a
nonnegative number which can be interpreted as the bitwise OR of the
individual flags. The flags are encoded like the flags argument
to open (see section Opening and Closing Files), but only the file
access modes and the O_APPEND and O_NONBLOCK flags are
meaningful here. Since the file access modes are not single-bit values,
you can mask off other bits in the returned flags with O_ACCMODE
to compare them.
In case of an error, fcntl returns -1. The following
errno error conditions are defined for this command:
EBADF
This macro is used as the command argument to fcntl, to set
the file status flags for the open file corresponding to the
filedes argument. This command requires a third int
argument to specify the new flags, so the call looks like this:
fcntl (filedes, F_SETFL, new_flags)
You can't change the access mode for the file in this way; that is,
whether the file descriptor was opened for reading or writing. You can
only change the O_APPEND and O_NONBLOCK flags.
The normal return value from fcntl with this command is an
unspecified value other than -1, which indicates an error. The
error conditions are the same as for the F_GETFL command.
The following macros are defined for use in analyzing and constructing file status flag values:
O_APPEND
write operations write the data at the end of the file, extending
it, regardless of the current file position.
O_NONBLOCK
read requests on the file can return immediately with a failure
status if there is no input immediately available, instead of blocking.
Likewise, write requests can also return immediately with a
failure status if the output can't be written immediately.
O_NDELAY
O_NONBLOCK, provided for compatibility with
BSD.
This macro stands for a mask that can be bitwise-ANDed with the file
status flag value to produce a value representing the file access mode.
The mode will be O_RDONLY, O_WRONLY, or O_RDWR.
O_RDONLY
O_WRONLY
O_RDWR
If you want to modify the file status flags, you should get the current
flags with F_GETFL and modify the value. Don't assume that the
flags listed here are the only ones that are implemented; your program
may be run years from now and more flags may exist then. For example,
here is a function to set or clear the flag O_NONBLOCK without
altering any other flags:
/* Set theO_NONBLOCKflag of desc if value is nonzero, or clear the flag if value is 0. Return 0 on success, or -1 on error witherrnoset. */ int set_nonblock_flag (int desc, int value) { int oldflags = fcntl (desc, F_GETFL, 0); /* If reading the flags failed, return error indication now. */ if (oldflags < 0) return oldflags; /* Set just the flag we want to set. */ if (value != 0) oldflags |= O_NONBLOCK; else oldflags &= ~O_NONBLOCK; /* Store modified flag word in the descriptor. */ return fcntl (desc, F_SETFL, oldflags); }
The remaining fcntl commands are used to support record
locking, which permits multiple cooperating programs to prevent each
other from simultaneously accessing parts of a file in error-prone
ways.
An exclusive or write lock gives a process exclusive access for writing to the specified part of the file. While a write lock is in place, no other process can lock that part of the file.
A shared or read lock prohibits any other process from requesting a write lock on the specified part of the file. However, other processes can request read locks.
The read and write functions do not actually check to see
whether there are any locks in place. If you want to implement a
locking protocol for a file shared by multiple processes, your application
must do explicit fcntl calls to request and clear locks at the
appropriate points.
Locks are associated with processes. A process can only have one kind
of lock set for each byte of a given file. When any file descriptor for
that file is closed by the process, all of the locks that process holds
on that file are released, even if the locks were made using other
descriptors that remain open. Likewise, locks are released when a
process exits, and are not inherited by child processes created using
fork (see section Creating a Process).
When making a lock, use a struct flock to specify what kind of
lock and where. This data type and the associated macros for the
fcntl function are declared in the header file `fcntl.h'.
This structure is used with the fcntl function to describe a file
lock. It has these members:
short int l_type
F_RDLCK, F_WRLCK, or
F_UNLCK.
short int l_whence
fseek or
lseek, and specifies what the offset is relative to. Its value
can be one of SEEK_SET, SEEK_CUR, or SEEK_END.
off_t l_start
l_whence member.
off_t l_len
0 is treated specially; it means the region extends to the end of
the file.
pid_t l_pid
fcntl with
the F_GETLK command, but is ignored when making a lock.
This macro is used as the command argument to fcntl, to
specify that it should get information about a lock. This command
requires a third argument of type struct flock * to be passed
to fcntl, so that the form of the call is:
fcntl (filedes, F_GETLK, lockp)
If there is a lock already in place that would block the lock described
by the lockp argument, information about that lock overwrites
*lockp. Existing locks are not reported if they are
compatible with making a new lock as specified. Thus, you should
specify a lock type of F_WRLCK if you want to find out about both
read and write locks, or F_RDLCK if you want to find out about
write locks only.
There might be more than one lock affecting the region specified by the
lockp argument, but fcntl only returns information about
one of them. The l_whence member of the lockp structure is
set to SEEK_SET and the l_start and l_len fields
set to identify the locked region.
If no lock applies, the only change to the lockp structure is to
update the l_type to a value of F_UNLCK.
The normal return value from fcntl with this command is an
unspecified value other than -1, which is reserved to indicate an
error. The following errno error conditions are defined for
this command:
EBADF
EINVAL
This macro is used as the command argument to fcntl, to
specify that it should set or clear a lock. This command requires a
third argument of type struct flock * to be passed to
fcntl, so that the form of the call is:
fcntl (filedes, F_SETLK, lockp)
If the process already has a lock on any part of the region, the old lock
on that part is replaced with the new lock. You can remove a lock
by specifying the a lock type of F_UNLCK.
If the lock cannot be set, fcntl returns immediately with a value
of -1. This function does not block waiting for other processes
to release locks. If fcntl succeeds, it return a value other
than -1.
The following errno error conditions are defined for this
function:
EACCES
EAGAIN
EAGAIN in this case, and other systems
use EACCES; your program should treat them alike, after
F_SETLK.
EBADF
EINVAL
ENOLCK
Well-designed file systems never report this error, because they have no limitation on the number of locks. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.
This macro is used as the command argument to fcntl, to
specify that it should set or clear a lock. It is just like the
F_SETLK command, but causes the process to block (or wait)
until the request can be specified.
This command requires a third argument of type struct flock *, as
for the F_SETLK command.
The fcntl return values and errors are the same as for the
F_SETLK command, but these additional errno error conditions
are defined for this command:
EINTR
EDEADLK
The following macros are defined for use as values for the l_type
member of the flock structure. The values are integer constants.
F_RDLCK
F_WRLCK
F_UNLCK
As an example of a situation where file locking is useful, consider a program that can be run simultaneously by several different users, that logs status information to a common file. One example of such a program might be a game that uses a file to keep track of high scores. Another example might be a program that records usage or accounting information for billing purposes.
Having multiple copies of the program simultaneously writing to the file could cause the contents of the file to become mixed up. But you can prevent this kind of problem by setting a write lock on the file before actually writing to the file.
If the program also needs to read the file and wants to make sure that the contents of the file are in a consistent state, then it can also use a read lock. While the read lock is set, no other process can lock that part of the file for writing.
Remember that file locks are only a voluntary protocol for controlling access to a file. There is still potential for access to the file by programs that don't use the lock protocol.
If you set the FASYNC status flag on a file descriptor
(see section File Status Flags), a SIGIO signal is sent whenever
input or output becomes possible on that file descriptor. The process
or process group to receive the signal can be selected by using the
F_SETOWN command to the fcntl function. If the file
descriptor is a socket, this also selects the recipient of SIGURG
signals that are delivered when out-of-band data arrives on that socket;
see section Out-of-Band Data.
If the file descriptor corresponds to a terminal device, then SIGIO
signals are sent to the foreground process group of the terminal.
See section Job Control.
The symbols in this section are defined in the header file `fcntl.h'.
This macro is used as the command argument to fcntl, to
specify that it should get information about the process or process
group to which SIGIO signals are sent. (For a terminal, this is
actually the foreground process group ID, which you can get using
tcgetpgrp; see section Functions for Controlling Terminal Access.)
The return value is interpreted as a process ID; if negative, its absolute value is the process group ID.
The following errno error condition is defined for this command:
EBADF
This macro is used as the command argument to fcntl, to
specify that it should set the process or process group to which
SIGIO signals are sent. This command requires a third argument
of type pid_t to be passed to fcntl, so that the form of
the call is:
fcntl (filedes, F_SETOWN, pid)
The pid argument should be a process ID. You can also pass a negative number whose absolute value is a process group ID.
The return value from fcntl with this command is -1
in case of error and some other value if successful. The following
errno error conditions are defined for this command:
EBADF
ESRCH
This chapter describes the GNU C library's functions for manipulating files. Unlike the input and output functions described in section Input/Output on Streams and section Low-Level Input/Output, these functions are concerned with operating on the files themselves, rather than on their contents.
Among the facilities described in this chapter are functions for examining or modifying directories, functions for renaming and deleting files, and functions for examining and setting file attributes such as access permissions and modification times.
Each process has associated with it a directory, called its current working directory or simply working directory, that is used in the resolution of relative file names (see section File Name Resolution).
When you log in and begin a new session, your working directory is
initially set to the home directory associated with your login account
in the system user database. You can find any user's home directory
using the getpwuid or getpwnam functions; see section User Database.
Users can change the working directory using shell commands like
cd. The functions described in this section are the primitives
used by those commands and by other programs for examining and changing
the working directory.
Prototypes for these functions are declared in the header file `unistd.h'.
Function: char * getcwd (char *buffer, size_t size)
The getcwd function returns an absolute file name representing
the current working directory, storing it in the character array
buffer that you provide. The size argument is how you tell
the system the allocation size of buffer.
The GNU library version of this function also permits you to specify a
null pointer for the buffer argument. Then getcwd
allocates a buffer automatically, as with malloc
(see section Unconstrained Allocation). If the size is greater than
zero, then the buffer is that large; otherwise, the buffer is as large
as necessary to hold the result.
The return value is buffer on success and a null pointer on failure.
The following errno error conditions are defined for this function:
EINVAL
ERANGE
EACCES
Here is an example showing how you could implement the behavior of GNU's
getcwd (NULL, 0) using only the standard behavior of
getcwd:
char *
gnu_getcwd ()
{
int size = 100;
char *buffer = (char *) xmalloc (size);
while (1)
{
char *value = getcwd (buffer, size);
if (value != 0)
return buffer;
size *= 2;
free (buffer);
buffer = (char *) xmalloc (size);
}
}
See section Examples of malloc, for information about xmalloc, which is
not a library function but is a customary name used in most GNU
software.
Function: char * getwd (char *buffer)
This is similar to getcwd. The GNU library provides getwd
for backwards compatibility with BSD. The buffer should be a
pointer to an array at least PATH_MAX bytes long.
Function: int chdir (const char *filename)
This function is used to set the process's working directory to filename.
The normal, successful return value from chdir is 0. A
value of -1 is returned to indicate an error. The errno
error conditions defined for this function are the usual file name
syntax errors (see section File Name Errors), plus ENOTDIR if the
file filename is not a directory.
The facilities described in this section let you read the contents of a directory file. This is useful if you want your program to list all the files in a directory, perhaps as part of a menu.
The opendir function opens a directory stream whose
elements are directory entries. You use the readdir function on
the directory stream to retrieve these entries, represented as
struct dirent objects. The name of the file for each entry is
stored in the d_name member of this structure. There are obvious
parallels here to the stream facilities for ordinary files, described in
section Input/Output on Streams.
This section describes what you find in a single directory entry, as you might obtain it from a directory stream. All the symbols are declared in the header file `dirent.h'.
This is a structure type used to return information about directory entries. It contains the following fields:
char *d_name
ino_t d_fileno
d_ino.
size_t d_namlen
This structure may contain additional members in the future.
When a file has multiple names, each name has its own directory entry.
The only way you can tell that the directory entries belong to a
single file is that they have the same value for the d_fileno
field.
File attributes such as size, modification times, and the like are part of the file itself, not any particular directory entry. See section File Attributes.
This section describes how to open a directory stream. All the symbols are declared in the header file `dirent.h'.
The DIR data type represents a directory stream.
You shouldn't ever allocate objects of the struct dirent or
DIR data types, since the directory access functions do that for
you. Instead, you refer to these objects using the pointers returned by
the following functions.
Function: DIR * opendir (const char *dirname)
The opendir function opens and returns a directory stream for
reading the directory whose file name is dirname. The stream has
type DIR *.
If unsuccessful, opendir returns a null pointer. In addition to
the usual file name syntax errors (see section File Name Errors), the
following errno error conditions are defined for this function:
EACCES
dirname.
EMFILE
ENFILE
The DIR type is typically implemented using a file descriptor,
and the opendir function in terms of the open function.
See section Low-Level Input/Output. Directory streams and the underlying
file descriptors are closed on exec (see section Executing a File).
This section describes how to read directory entries from a directory stream, and how to close the stream when you are done with it. All the symbols are declared in the header file `dirent.h'.
Function: struct dirent * readdir (DIR *dirstream)
This function reads the next entry from the directory. It normally returns a pointer to a structure containing information about the file. This structure is statically allocated and can be rewritten by a subsequent call.
Portability Note: On some systems, readdir may not
return entries for `.' and `..'. See section File Name Resolution.
If there are no more entries in the directory or an error is detected,
readdir returns a null pointer. The following errno error
conditions are defined for this function:
EBADF
Function: int closedir (DIR *dirstream)
This function closes the directory stream dirstream. It returns
0 on success and -1 on failure.
The following errno error conditions are defined for this
function:
EBADF
Here's a simple program that prints the names of the files in the current working directory:
#include <stddef.h>
#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>
int
main (void)
{
DIR *dp;
struct dirent *ep;
dp = opendir ("./");
if (dp != NULL)
{
while (ep = readdir (dp))
puts (ep->d_name);
(void) closedir (dp);
}
else
puts ("Couldn't open the directory.");
return 0;
}
The order in which files appear in a directory tends to be fairly random. A more useful program would sort the entries (perhaps by alphabetizing them) before printing them; see section Array Sort Function
This section describes how to reread parts of a directory that you have already read from an open directory stream. All the symbols are declared in the header file `dirent.h'.
Function: void rewinddir (DIR *dirstream)
The rewinddir function is used to reinitialize the directory
stream dirstream, so that if you call readdir it
returns information about the first entry in the directory again. This
function also notices if files have been added or removed to the
directory since it was opened with opendir. (Entries for these
files might or might not be returned by readdir if they were
added or removed since you last called opendir or
rewinddir.)
Function: off_t telldir (DIR *dirstream)
The telldir function returns the file position of the directory
stream dirstream. You can use this value with seekdir to
restore the directory stream to that position.
Function: void seekdir (DIR *dirstream, off_t pos)
The seekdir function sets the file position of the directory
stream dirstream to pos. The value pos must be the
result of a previous call to telldir on this particular stream;
closing and reopening the directory can invalidate values returned by
telldir.
In POSIX systems, one file can have many names at the same time. All of the names are equally real, and no one of them is preferred to the others.
To add a name to a file, use the link function. (The new name is
also called a hard link to the file.) Creating a new link to a
file does not copy the contents of the file; it simply makes a new name
by which the file can be known, in addition to the file's existing name
or names.
One file can have names in several directories, so the the organization of the file system is not a strict hierarchy or tree.
Since a particular file exists within a single file system, all its
names must be in directories in that file system. link reports
an error if you try to make a hard link to the file from another file
system.
The prototype for the link function is declared in the header
file `unistd.h'.
Function: int link (const char *oldname, const char *newname)
The link function makes a new link to the existing file named by
oldname, under the new name newname.
This function returns a value of 0 if it is successful and
-1 on failure. In addition to the usual file name syntax errors
(see section File Name Errors) for both oldname and newname, the
following errno error conditions are defined for this function:
EACCES
EEXIST
EMLINK
LINK_MAX; see
section Limits on File System Capacity.)
Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.
ENOENT
ENOSPC
EPERM
EROFS
EXDEV
The GNU system supports soft links or symbolic links. This is a kind of "file" that is essentially a pointer to another file name. Unlike hard links, symbolic links can be made to directories or across file systems with no restrictions. You can also make a symbolic link to a name which is not the name of any file. (Opening this link will fail until a file by that name is created.) Likewise, if the symbolic link points to an existing file which is later deleted, the symbolic link continues to point to the same file name even though the name no longer names any file.
The reason symbolic links work the way they do is that special things
happen when you try to open the link. The open function realizes
you have specified the name of a link, reads the file name contained in
the link, and opens that file name instead. The stat function
likewise operates on the file that the symbolic link points to, instead
of on the link itself. So does link, the function that makes a
hard link.
By contrast, other operations such as deleting or renaming the file
operate on the link itself. The functions readlink and
lstat also refrain from following symbolic links, because
their purpose is to obtain information about the link.
Prototypes for the functions listed in this section are in `unistd.h'.
Function: int symlink (const char *oldname, const char *newname)
The symlink function makes a symbolic link to oldname named
newname.
The normal return value from symlink is 0. A return value
of -1 indicates an error. In addition to the usual file name
syntax errors (see section File Name Errors), the following errno
error conditions are defined for this function:
EEXIST
EROFS
ENOSPC
EIO
Function: int readlink (const char *filename, char *buffer, size_t size)
The readlink function gets the value of the symbolic link
filename. The file name that the link points to is copied into
buffer. This file name string is not null-terminated;
readlink normally returns the number of characters copied. The
size argument specifies the maximum number of characters to copy,
usually the allocation size of buffer.
If the return value equals size, you cannot tell whether or not
there was room to return the entire name. So make a bigger buffer and
call readlink again. Here is an example:
char *
readlink_malloc (char *filename)
{
int size = 100;
while (1)
{
char *buffer = (char *) xmalloc (size);
int nchars = readlink (filename, buffer, size);
if (nchars < size)
return buffer;
free (buffer);
size *= 2;
}
}
A value of -1 is returned in case of error. In addition to the
usual file name syntax errors (see section File Name Errors), the following
errno error conditions are defined for this function:
EINVAL
EIO
You can delete a file with the functions unlink or remove.
(These names are synonymous.)
Deletion actually deletes a file name. If this is the file's only name, then the file is deleted as well. If the file has other names as well (see section Hard Links), it remains accessible under its other names.
Function: int unlink (const char *filename)
The unlink function deletes the file name filename. If
this is a file's sole name, the file itself is also deleted. (Actually,
if any process has the file open when this happens, deletion is
postponed until all processes have closed the file.)
The function unlink is declared in the header file `unistd.h'.
This function returns 0 on successful completion, and -1
on error. In addition to the usual file name syntax errors
(see section File Name Errors), the following errno error conditions are
defined for this function:
EACCESS
EBUSY
ENOENT
EPERM
unlink cannot be used to delete the name of a
directory, or can only be used this way by a privileged user.
To avoid such problems, use rmdir to delete directories.
EROFS
Function: int remove (const char *filename)
The remove function is another name for unlink.
remove is the ANSI C name, whereas unlink is the POSIX.1
name. The name remove is declared in `stdio.h'.
Function: int rmdir (const char *filename)
The rmdir function deletes a directory. The directory must be
empty before it can be removed; in other words, it can only contain
entries for `.' and `..'.
In most other respects, rmdir behaves like unlink. There
are two additional errno error conditions defined for
rmdir:
EEXIST
ENOTEMPTY
These two error codes are synonymous; some systems use one, and some use the other.
The prototype for this function is declared in the header file `unistd.h'.
The rename function is used to change a file's name.
Function: int rename (const char *oldname, const char *newname)
The rename function renames the file name oldname with
newname. The file formerly accessible under the name
oldname is afterward accessible as newname instead. (If the
file had any other names aside from oldname, it continues to have
those names.)
The directory containing the name newname must be on the same file system as the file (as indicated by the name oldname).
One special case for rename is when oldname and
newname are two names for the same file. The consistent way to
handle this case is to delete oldname. However, POSIX says that
in this case rename does nothing and reports success--which is
inconsistent. We don't know what your operating system will do. The
GNU system, when completed, will probably do the right thing (delete
oldname) unless you explicitly request strict POSIX compatibility
"even when it hurts".
If the oldname is not a directory, then any existing file named
newname is removed during the renaming operation. However, if
newname is the name of a directory, rename fails in this
case.
If the oldname is a directory, then either newname must not
exist or it must name a directory that is empty. In the latter case,
the existing directory named newname is deleted first. The name
newname must not specify a subdirectory of the directory
oldname which is being renamed.
One useful feature of rename is that the meaning of the name
newname changes "atomically" from any previously existing file
by that name to its new meaning (the file that was called
oldname). There is no instant at which newname is
nonexistent "in between" the old meaning and the new meaning.
If rename fails, it returns -1. In addition to the usual
file name syntax errors (see section File Name Errors), the following
errno error conditions are defined for this function:
EACCES
EBUSY
EEXIST
ENOTEMPTY
EINVAL
EISDIR
EMLINK
Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.
ENOENT
ENOSPC
EROFS
EXDEV
Directories are created with the mkdir function. (There is also
a shell command mkdir which does the same thing.)
Function: int mkdir (const char *filename, mode_t mode)
The mkdir function creates a new, empty directory whose name is
filename.
The argument mode specifies the file permissions for the new directory file. See section The Mode Bits for Access Permission, for more information about this.
A return value of 0 indicates successful completion, and
-1 indicates failure. In addition to the usual file name syntax
errors (see section File Name Errors), the following errno error
conditions are defined for this function:
EACCES
EEXIST
EMLINK
Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.
ENOSPC
EROFS
To use this function, your program should include the header file `sys/stat.h'.
When you issue an `ls -l' shell command on a file, it gives you information about the size of the file, who owns it, when it was last modified, and the like. This kind of information is called the file attributes; it is associated with the file itself and not a particular one of its names.
This section contains information about how you can inquire about and modify these attributes of files.
When you read the attributes of a file, they come back in a structure
called struct stat. This section describes the names of the
attributes, their data types, and what they mean. For the functions
to read the attributes of a file, see section Reading the Attributes of a File.
The header file `sys/stat.h' declares all the symbols defined in this section.
The stat structure type is used to return information about the
attributes of a file. It contains at least the following members:
mode_t st_mode
ino_t st_ino
dev_t st_dev
st_ino and
st_dev, taken together, uniquely identify the file.
nlink_t st_nlink
uid_t st_uid
gid_t st_gid
off_t st_size
time_t st_atime
unsigned long int st_atime_usec
time_t st_mtime
unsigned long int st_mtime_usec
time_t st_ctime
unsigned long int st_ctime_usec
unsigned int st_nblocks
The number of disk blocks is not strictly proportional to the size of the file, for two reasons: the file system may use some blocks for internal record keeping; and the file may be sparse--it may have "holes" which contain zeros but do not actually take up space on the disk.
You can tell (approximately) whether a file is sparse by comparing this
value with st_size, like this:
(st.st_blocks * 512 < st.st_size)
This test is not perfect because a file that is just slightly sparse might not be detected as sparse at all. For practical applications, this is not a problem.
unsigned int st_blksize
Some of the file attributes have special data type names which exist specifically for those attributes. (They are all aliases for well-known integer types that you know and love.) These typedef names are defined in the header file `sys/types.h' as well as in `sys/stat.h'. Here is a list of them.
This is an integer data type used to represent file modes. In the
GNU system, this is equivalent to unsigned int.
This is an arithmetic data type used to represent file serial numbers.
(In Unix jargon, these are sometimes called inode numbers.)
In the GNU system, this type is equivalent to unsigned long int.
This is an arithmetic data type used to represent file device numbers.
In the GNU system, this is equivalent to int.
This is an arithmetic data type used to represent file link counts.
In the GNU system, this is equivalent to unsigned short int.
To examine the attributes of files, use the functions stat,
fstat and lstat. They return the attribute information in
a struct stat object. All three functions are declared in the
header file `sys/stat.h'.
Function: int stat (const char *filename, struct stat *buf)
The stat function returns information about the attributes of the
file named by filename in the structure pointed at by buf.
If filename is the name of a symbolic link, the attributes you get
describe the file that the link points to. If the link points to a
nonexistent file name, then stat fails, reporting a nonexistent
file.
The return value is 0 if the operation is successful, and -1
on failure. In addition to the usual file name syntax errors
(see section File Name Errors, the following errno error conditions
are defined for this function:
ENOENT
Function: int fstat (int filedes, struct stat *buf)
The fstat function is like stat, except that it takes an
open file descriptor as an argument instead of a file name.
See section Low-Level Input/Output.
Like stat, fstat returns 0 on success and -1
on failure. The following errno error conditions are defined for
fstat:
EBADF
Function: int lstat (const char *filename, struct stat *buf)
The lstat function is like stat, except that it does not
follow symbolic links. If filename is the name of a symbolic
link, lstat returns information about the link itself; otherwise,
lstat works like stat. See section Symbolic Links.
The file mode, stored in the st_mode field of the file
attributes, contains two kinds of information: the file type code, and
the access permission bits. This section discusses only the type code,
which you can use to tell whether the file is a directory, whether it is
a socket, and so on. For information about the access permission,
section The Mode Bits for Access Permission.
There are two predefined ways you can access the file type portion of the file mode. First of all, for each type of file, there is a predicate macro which examines a file mode value and returns true or false--is the file of that type, or not. Secondly, you can mask out the rest of the file mode to get just a file type code. You can compare this against various constants for the supported file types.
All of the symbols listed in this section are defined in the header file `sys/stat.h'.
The following predicate macros test the type of a file, given the value
m which is the st_mode field returned by stat on
that file:
This macro returns nonzero if the file is a directory.
This macro returns nonzero if the file is a character special file (a device like a terminal).
This macro returns nonzero if the file is a block special file (a device like a disk).
This macro returns nonzero if the file is a regular file.
Macro: int S_ISFIFO (mode_t m)
This macro returns nonzero if the file is a FIFO special file, or a pipe. See section Pipes and FIFOs.
This macro returns nonzero if the file is a symbolic link. See section Symbolic Links.
Macro: int S_ISSOCK (mode_t m)
This macro returns nonzero if the file is a socket. See section Sockets.
An alterate non-POSIX method of testing the file type is supported for
compatibility with BSD. The mode can be bitwise ANDed with
S_IFMT to extract the file type code, and compared to the
appropriate type code constant. For example,
S_ISCHR (mode)
is equivalent to:
((mode & S_IFMT) == S_IFCHR)
This is a bit mask used to extract the file type code portion of a mode value.
These are the symbolic names for the different file type codes:
S_IFDIR
S_IFCHR
S_IFBLK
S_IFREG
S_IFLNK
S_IFSOCK
S_IFIFO
Every file has an owner which is one of the registered user names defined on the system. Each file also has a group, which is one of the defined groups. The file owner can often be useful for showing you who edited the file (especially when you edit with GNU Emacs), but its main purpose is for access control.
The file owner and group play a role in determining access because the file has one set of access permission bits for the user that is the owner, another set that apply to users who belong to the file's group, and a third set of bits that apply to everyone else. See section How Your Access to a File is Decided, for the details of how access is decided based on this data.
When a file is created, its owner is set from the effective user ID of the process that creates it (see section The Persona of a Process). The file's group ID may be set from either effective group ID of the process, or the group ID of the directory that contains the file, depending on the system where the file is stored. When you access a remote file system, it behaves according to its own rule, not according to the system your program is running on. Thus, your program must be prepared to encounter either kind of behavior, no matter what kind of system you run it on.
You can change the owner and/or group owner of an existing file using
the chown function. This is the primitive for the chown
and chgrp shell commands.
The prototype for this function is declared in `unistd.h'.
Function: int chown (const char *filename, uid_t owner, gid_t group)
The chown function changes the owner of the file filename to
owner, and its group owner to group.
Changing the owner of the file on certain systems clears the set-user-ID and set-group-ID bits of the file's permissions. (This is because those bits may not be appropriate for the new owner.) The other file permission bits are not changed.
The return value is 0 on success and -1 on failure.
In addition to the usual file name syntax errors (see section File Name Errors),
the following errno error conditions are defined for this function:
EPERM
Only privileged users or the file's owner can change the file's group. On most file systems, only privileged users can change the file owner; some file systems allow you to change the owner if you are currently the owner. When you access a remote file system, the behavior you encounter is determined by the system that actually holds the file, not by the system your program is running on.
See section Optional Features in File Support, for information about the
_POSIX_CHOWN_RESTRICTED macro.
EROFS
Function: int fchown (int filedes, int owner, int group)
This is like chown, except that it changes the owner of the file
with open file descriptor filedes.
The return value from fchown is 0 on success and -1
on failure. The following errno error codes are defined for this
function:
EBADF
EINVAL
EPERM
chmod, above.
EROFS
The file mode, stored in the st_mode field of the file
attributes, contains two kinds of information: the file type code, and
the access permission bits. This section discusses only the access
permission bits, which control who can read or write the file.
See section Testing the Type of a File, for information about the file type code.
All of the symbols listed in this section are defined in the header file `sys/stat.h'.
These symbolic constants are defined for the file mode bits that control access permission for the file:
S_IRUSR
S_IREAD
S_IREAD is an obsolete synonym provided for BSD
compatibility.
S_IWUSR
S_IWRITE
S_IWRITE is an obsolete synonym provided for BSD compatibility.
S_IXUSR
S_IEXEC
S_IEXEC is an obsolete
synonym provided for BSD compatibility.
S_IRWXU
S_IRGRP
S_IWGRP
S_IXGRP
S_IRWXG
S_IROTH
S_IWOTH
S_IXOTH
S_IRWXO
S_ISUID
S_ISGID
S_ISVTX
On an executable file, it modifies the swapping policies of the system. Normally, when a program terminates, its pages in core are immediately freed and reused. If the sticky bit is set on the executable file, the system keeps the pages in core for a while as if the program were still running. This is advantageous for a program that is likely to be run many times in succession.
On a directory, the sticky bit gives permission to delete a file in the directory if you can write the contents of that file. Ordinarily, a user either can delete all the files in the directory or cannot delete any of them (based on whether the user has write permission for the directory). The sticky bit makes it possible to control deletion for individual files.
The actual bit values of the symbols are listed in the table above so you can decode file mode values when debugging your programs. These bit values are correct for most systems, but they are not guaranteed.
Warning: Writing explicit numbers for file permissions is bad practice. It is not only nonportable, it also requires everyone who reads your program to remember what the bits mean. To make your program clean, use the symbolic names.
Recall that the operating system normally decides access permission for a file based on the effective user and group IDs of the process, and its supplementary group IDs, together with the file's owner, group and permission bits. These concepts are discussed in detail in section The Persona of a Process.
If the effective user ID of the process matches the owner user ID of the file, then permissions for read, write, and execute/search are controlled by the corresponding "user" (or "owner") bits. Likewise, if any of the effective group ID or supplementary group IDs of the process matches the group owner ID of the file, then permissions are controlled by the "group" bits. Otherwise, permissions are controlled by the "other" bits.
Privileged users, like `root', can access any file, regardless of its file permission bits. As a special case, for a file to be executable even for a privileged user, at least one of its execute bits must be set.
The primitive functions for creating files (for example, open or
mkdir) take a mode argument, which specifies the file
permissions for the newly created file. But the specified mode is
modified by the process's file creation mask, or umask,
before it is used.
The bits that are set in the file creation mask identify permissions that are always to be disabled for newly created files. For example, if you set all the "other" access bits in the mask, then newly created files are not accessible at all to processes in the "other" category, even if the mode argument specified to the creation function would permit such access. In other words, the file creation mask is the complement of the ordinary access permissions you want to grant.
Programs that create files typically specify a mode argument that includes all the permissions that make sense for the particular file. For an ordinary file, this is typically read and write permission for all classes of users. These permissions are then restricted as specified by the individual user's own file creation mask.
To change the permission of an existing file given its name, call
chmod. This function ignores the file creation mask; it uses
exactly the specified permission bits.
In normal use, the file creation mask is initialized in the user's login
shell (using the umask shell command), and inherited by all
subprocesses. Application programs normally don't need to worry about
the file creation mask. It will do automatically what it is supposed to
do.
When your program should create a file and bypass the umask for its
access permissions, the easiest way to do this is to use fchmod
after opening the file, rather than changing the umask.
In fact, changing the umask is usually done only by shells. They use
the umask function.
The functions in this section are declared in `sys/stat.h'.
Function: mode_t umask (mode_t mask)
The umask function sets the file creation mask of the current
process to mask, and returns the previous value of the file
creation mask.
Here is an example showing how to read the mask with umask
without changing it permanently:
mode_t
read_umask (void)
{
mask = umask (0);
umask (mask);
}
However, it is better to use getumask if you just want to read
the mask value, because that is reentrant (at least if you use the GNU
operating system).
Function: mode_t getumask (void)
Return the current value of the file creation mask for the current process. This function is a GNU extension.
Function: int chmod (const char *filename, mode_t mode)
The chmod function sets the access permission bits for the file
named by filename to mode.
If the filename names a symbolic link, chmod changes the
permission of the file pointed to by the link, not those of the link
itself. There is actually no way to set the mode of a link, which is
always -1.
This function returns 0 if successful and -1 if not. In
addition to the usual file name syntax errors (see section File Name Errors), the following errno error conditions are defined for
this function:
ENOENT
EPERM
EROFS
Function: int fchmod (int filedes, int mode)
This is like chmod, except that it changes the permissions of
the file currently open via descriptor filedes.
The return value from fchmod is 0 on success and -1
on failure. The following errno error codes are defined for this
function:
EBADF
EINVAL
EPERM
EROFS
When a program runs as a privileged user, this permits it to access
files off-limits to ordinary users--for example, to modify
`/etc/passwd'. Programs designed to be run by ordinary users but
access such files use the setuid bit feature so that they always run
with root as the effective user ID.
Such a program may also access files specified by the user, files which
conceptually are being accessed explicitly by the user. Since the
program runs as root, it has permission to access whatever file
the user specifies--but usually the desired behavior is to permit only
those files which the user could ordinarily access.
The program therefore must explicitly check whether the user would have the necessary access to a file, before it reads or writes the file.
To do this, use the function access, which checks for access
permission based on the process's real user ID rather than the
effective user ID. (The setuid feature does not alter the real user ID,
so it reflects the user who actually ran the program.)
There is another way you could check this access, which is easy to
describe, but very hard to use. This is to examine the file mode bits
and mimic the system's own access computation. This method is
undesirable because many systems have additional access control
features; your program cannot portably mimic them, and you would not
want to try to keep track of the diverse features that different systems
have. Using access is simple and automatically does whatever is
appropriate for the system you are using.
The symbols in this section are declared in `unistd.h'.
Function: int access (const char *filename, int how)
The access function checks to see whether the file named by
filename can be accessed in the way specified by the how
argument. The how argument either can be the bitwise OR of the
flags R_OK, W_OK, X_OK, or the existence test
F_OK.
This function uses the real user and group ID's of the calling
process, rather than the effective ID's, to check for access
permission. As a result, if you use the function from a setuid
or setgid program (see section How an Application Can Change Persona), it gives
information relative to the user who actually ran the program.
The return value is 0 if the access is permitted, and -1
otherwise. (In other words, treated as a predicate function,
access returns true if the requested access is denied.)
In addition to the usual file name syntax errors (see section File Name Errors), the following errno error conditions are defined for
this function:
EACCES
ENOENT
EROFS
These macros are defined in the header file `unistd.h' for use
as the how argument to the access function. The values
are integer constants.
Argument that means, test for read permission.
Argument that means, test for write permission.
Argument that means, test for execute/search permission.
Argument that means, test for existence of the file.
Each file has three timestamps associated with it: its access time,
its modification time, and its attribute modification time. These
correspond to the st_atime, st_mtime, and st_ctime
members of the stat structure; see section File Attributes.
All of these times are represented in calendar time format, as
time_t objects. This data type is defined in `time.h'.
For more information about representation and manipulation of time
values, see section Calendar Time.
When an existing file is opened, its attribute change time and modification time fields are updated. Reading from a file updates its access time attribute, and writing updates its modification time.
When a file is created, all three timestamps for that file are set to the current time. In addition, the attribute change time and modification time fields of the directory that contains the new entry are updated.
Adding a new name for a file with the link function updates the
attribute change time field of the file being linked, and both the
attribute change time and modification time fields of the directory
containing the new name. These same fields are affected if a file name
is deleted with unlink, remove, or rmdir. Renaming
a file with rename affects only the attribute change time and
modification time fields of the two parent directories involved, and not
the times for the file being renamed.
Changing attributes of a file (for example, with chmod) updates
its attribute change time field.
You can also change some of the timestamps of a file explicitly using
the utime function--all except the attribute change time. You
need to include the header file `utime.h' to use this facility.
The utimbuf structure is used with the utime function to
specify new access and modification times for a file. It contains the
following members:
time_t actime
time_t modtime
Function: int utime (const char *filename, const struct utimbuf *times)
This function is used to modify the file times associated with the file named filename.
If times is a null pointer, then the access and modification times
of the file are set to the current time. Otherwise, they are set to the
values from the actime and modtime members (respectively)
of the utimbuf structure pointed at by times.
The attribute modification time for the file is set to the current time in either case (since changing the timestamps is itself a modification of the file attributes).
The utime function returns 0 if successful and -1
on failure. In addition to the usual file name syntax errors
(see section File Name Errors), the following errno error conditions
are defined for this function:
EACCES
ENOENT
EPERM
EROFS
Each of the three time stamps has a corresponding microsecond part,
which extends its resolution. These fields are called
st_atime_usec, st_mtime_usec, and st_ctime_usec;
each has a value between 0 and 999,999, which indicates the time in
microseconds. They correspond to the tv_usec field of a
timeval structure; see section High-Resolution Calendar.
The utimes function is like utime, but also lets you specify
the fractional part of the file times. The prototype for this function is
in the header file `sys/time.h'.
Function: int utimes (const char *filename, struct timeval tvp[2])
This function sets the file access and modification times for the file
named by filename. The new file access time is specified by
tvp[0], and the new modification time by
tvp[1]. This function comes from BSD.
The return values and error conditions are the same as for the utime
function.
The mknod function is the primitive for making special files,
such as files that correspond to devices. The GNU library includes
this function for compatibility with BSD.
The prototype for mknod is declared in `sys/stat.h'.
Function: int mknod (const char *filename, int mode, int dev)
The mknod function makes a special file with name filename.
The mode specifies the mode of the file, and may include the various
special file bits, such as S_IFCHR (for a character special file)
or S_IFBLK (for a block special file). See section Testing the Type of a File.
The dev argument specifies which device the special file refers to. Its exact interpretation depends on the kind of special file being created.
The return value is 0 on success and -1 on error. In addition
to the usual file name syntax errors (see section File Name Errors), the
following errno error conditions are defined for this function:
EPERM
ENOSPC
EROFS
EEXIST
A pipe is a mechanism for interprocess communication; data written to the pipe by one process can be read by another process. The data is handled in a first-in, first-out (FIFO) order. The pipe has no name; it is created for one use and both ends must be inherited from the single process which created the pipe.
A FIFO special file is similar to a pipe, but instead of being an anonymous, temporary connection, a FIFO has a name or names like any other file. Processes open the FIFO by name in order to communicate through it.
A pipe or FIFO has to be open at both ends simultaneously. If you read
from a pipe or FIFO file that doesn't have any processes writing to it
(perhaps because they have all closed the file, or exited), the read
returns end-of-file. Writing to a pipe or FIFO that doesn't have a
reading process is treated as an error condition; it generates a
SIGPIPE signal, and fails with error code EPIPE if the
signal is handled or blocked.
Neither pipes nor FIFO special files allow file positioning. Both reading and writing operations happen sequentially; reading from the beginning of the file and writing at the end.
The primitive for creating a pipe is the pipe function. This
creates both the reading and writing ends of the pipe. It is not very
useful for a single process to use a pipe to talk to itself. In typical
use, a process creates a pipe just before it forks one or more child
processes (see section Creating a Process). The pipe is then used for
communication either between the parent or child processes, or between
two sibling processes.
The pipe function is declared in the header file
`unistd.h'.
Function: int pipe (int filedes[2])
The pipe function creates a pipe and puts the file descriptors
for the reading and writing ends of the pipe (respectively) into
filedes[0] and filedes[1].
An easy way to remember that the input end comes first is that file
descriptor 0 is standard input, and file descriptor 1 is
standard output.
If successful, pipe returns a value of 0. On failure,
-1 is returned. The following errno error conditions are
defined for this function:
EMFILE
ENFILE
ENFILE.
Here is an example of a simple program that creates a pipe. This program
uses the fork function (see section Creating a Process) to create
a child process. The parent process writes data to the pipe, which is
read by the child process.
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
/* Read characters from the pipe and echo them to stdout. */
void
read_from_pipe (int file)
{
FILE *stream;
int c;
stream = fdopen (file, "r");
while ((c = fgetc (stream)) != EOF)
putchar (c);
fclose (stream);
}
/* Write some random text to the pipe. */
void
write_to_pipe (int file)
{
FILE *stream;
stream = fdopen (file, "w");
fprintf (stream, "hello, world!\n");
fprintf (stream, "goodbye, world!\n");
fclose (stream);
}
int
main (void)
{
pid_t pid;
int mypipe[2];
/* Create the pipe. */
if (pipe (mypipe))
{
fprintf (stderr, "Pipe failed.\n");
return EXIT_FAILURE;
}
/* Create the child process. */
pid = fork ();
if (pid == (pid_t) 0)
{
/* This is the child process. */
read_from_pipe (mypipe[0]);
return EXIT_SUCCESS;
}
else if (pid < (pid_t) 0)
{
/* The fork failed. */
fprintf (stderr, "Fork failed.\n");
return EXIT_FAILURE;
}
else
{
/* This is the parent process. */
write_to_pipe (mypipe[1]);
return EXIT_SUCCESS;
}
}
A common use of pipes is to send data to or receive data from a program
being run as subprocess. One way of doing this is by using a combination of
pipe (to create the pipe), fork (to create the subprocess),
dup2 (to force the subprocess to use the pipe as its standard input
or output channel), and exec (to execute the new program). Or,
you can use popen and pclose.
The advantage of using popen and pclose is that the
interface is much simpler and easier to use. But it doesn't offer as
much flexibility as using the low-level functions directly.
Function: FILE * popen (const char *command, const char *mode)
The popen function is closely related to the system
function; see section Running a Command. It executes the shell command
command as a subprocess. However, instead of waiting for the
command to complete, it creates a pipe to the subprocess and returns a
stream that corresponds to that pipe.
If you specify a mode argument of "r", you can read from the
stream to retrieve data from the standard output channel of the subprocess.
The subprocess inherits its standard input channel from the parent process.
Similarly, if you specify a mode argument of "w", you can
write to the stream to send data to the standard input channel of the
subprocess. The subprocess inherits its standard output channel from
the parent process.
In the event of an error, popen returns a null pointer. This
might happen if the pipe or stream cannot be created, if the subprocess
cannot be forked, or if the program cannot be executed.
Function: int pclose (FILE *stream)
The pclose function is used to close a stream created by popen.
It waits for the child process to terminate and returns its status value,
as for the system function.
Here is an example showing how to use popen and pclose to
filter output through another program, in this case the paging program
more.
#include <stdio.h>
#include <stdlib.h>
void
write_data (FILE * stream)
{
int i;
for (i = 0; i < 100; i++)
fprintf (stream, "%d\n", i);
if (ferror (stream))
{
fprintf (stderr, "Output to stream failed.\n");
exit (EXIT_FAILURE);
}
}
int
main (void)
{
FILE *output;
output = popen ("more", "w");
if (!output)
{
fprintf (stderr, "Could not run more.\n");
return EXIT_FAILURE;
}
write_data (output);
pclose (output);
return EXIT_SUCCESS;
}
A FIFO special file is similar to a pipe, except that it is created in a
different way. Instead of being an anonymous communications channel, a
FIFO special file is entered into the file system by calling
mkfifo.
Once you have created a FIFO special file in this way, any process can open it for reading or writing, in the same way as an ordinary file. However, it has to be open at both ends simultaneously before you can proceed to do any input or output operations on it. Opening a FIFO for reading normally blocks until some other process opens the same FIFO for writing, and vice versa.
The mkfifo function is declared in the header file
`sys/stat.h'.
Function: int mkfifo (const char *filename, mode_t mode)
The mkfifo function makes a FIFO special file with name
filename. The mode argument is used to set the file's
permissions; see section Assigning File Permissions.
The normal, successful return value from mkfifo is 0. In
the case of an error, -1 is returned. In addition to the usual
file name syntax errors (see section File Name Errors), the following
errno error conditions are defined for this function:
EEXIST
ENOSPC
EROFS
Reading or writing pipe data is atomic if the size of data written
is less than PIPE_BUF. This means that the data transfer seems
to be an instantaneous unit, in that nothing else in the system can
observe a state in which it is partially complete. Atomic I/O may not
begin right away (it may need to wait for buffer space or for data), but
once it does begin, it finishes immediately.
Reading or writing a larger amount of data may not be atomic; for example, output data from other processes sharing the descriptor may be interspersed.
See section Limits on File System Capacity, for information about the PIPE_BUF
parameter.
This chapter describes the GNU facilities for interprocess communication using sockets.
A socket is a generalized interprocess communication channel.
Like a pipe, a socket is represented as a file descriptor. But,
unlike pipes, sockets support communication between unrelated
processes, and even between processes running on different machines
that communicate over a network. Sockets are the primary means of
communicating with other machines; telnet, rlogin,
ftp, talk, and the other familiar network programs use
sockets.
Not all operating systems support sockets. In the GNU library, the header file `sys/socket.h' exists regardless of the operating system, and the socket functions always exist, but if the system does not really support sockets, these functions always fail.
Incomplete: We do not currently document the facilities for broadcast messages or for configuring Internet interfaces.
When you create a socket, you must specify the style of communication you want to use and the type of protocol that should implement it. The communication style of a socket defines the user-level semantics of sending and receiving data on the socket. Choosing a communication style specifies the answers to questions such as these:
Designing a program to use unreliable communication styles usually involves taking precautions to detect lost or misordered packets and to retransmit data as needed.
You must also choose a namespace for naming the socket. A socket name ("address") is meaningful only in the context of a particular namespace. In fact, even the data type to use for a socket name may depend on the namespace. Namespaces are also called "domains", but we avoid that word as it can be confused with other usage of the same term. Each namespace has a symbolic name that starts with `PF_'. A corresponding symbolic name starting with `AF_' designates the address format for that namespace.
Finally you must next choose the protocol to carry out the communication. The protocol determines what low-level mechanism is used to transmit and receive data. Each protocol is valid for a particular namespace and communication style; a namespace is sometimes called a protocol family because of this, which is why the namespace names start with `PF_'.
The rules of a protocol apply to the data passing between two programs, perhaps on different computers; most of these rules are handled by the operating system, and you need not know about them. What you do need to know about protocols is this:
The GNU library includes support for several different kinds of sockets, each with different characteristics. This section describes the supported socket types. The symbolic constants listed here are defined in `sys/socket.h'.
The SOCK_STREAM style is like a pipe (see section Pipes and FIFOs);
it operates over a connection with a particular remote socket, and
transmits data reliably as a stream of bytes.
Use of this style is covered in detail in section Using Sockets with Connections.
The SOCK_DGRAM style is used for sending
individually-addressed packets, unreliably.
It is the diametrical opposite of SOCK_STREAM.
Each time you write data to a socket of this kind, that data becomes
one packet. Since SOCK_DGRAM sockets do not have connections,
you must specify the recipient address with each packet.
The only guarantee that the system makes about your requests to transmit data is that it will try its best to deliver each packet you send. It may succeed with the sixth packet after failing with the fourth and fifth packets; the seventh packet may arrive before the sixth, and may arrive a second time after the sixth.
The typical use for SOCK_DGRAM is in situations where it is
acceptible to simply resend a packet if no response is seen in a
reasonable amount of time.
See section Datagram Socket Operations, for detailed information about how to use datagram sockets.
This style provides access to low-level network protocols and interfaces. Ordinary user programs usually have no need to use this style.
The name of a socket is normally called an address. The functions and symbols for dealing with socket addresses were named inconsistently, sometimes using the term "name" and sometimes using "address". You can regard these terms as synonymous where sockets are concerned.
A socket newly created with the socket function has no
address. Other processes can find it for communication only if you
give it an address. We call this binding the address to the
socket, and the way to do it is with the bind function.
You need be concerned with the address of a socket if other processes are to find it and start communicating with it. You can specify an address for other sockets, but this is usually pointless; the first time you send data from a socket, or use it to initiate a connection, the system assigns an address automatically if you have not specified one.
Occasionally a client needs to specify an address because the server
discriminates based on addresses; for example, the rsh and rlogin
protocols look at the client's socket address and don't bypass password
checking unless it is less than IPPORT_RESERVED (see section Internet Ports).
The details of socket addresses vary depending on what namespace you are using. See section The File Namespace, or section The Internet Namespace, for specific information.
Regardless of the namespace, you use the same functions bind and
getsockname to set and examine a socket's address. These
functions use a phony data type, struct sockaddr *, to accept the
address. In practice, the address lives in a structure of some other
data type appropriate to the address format you are using, but you cast
its address to struct sockaddr * when you pass it to
bind.
The functions bind and getsockname use the generic data
type struct sockaddr * to represent a pointer to a socket
address. You can't use this data type effectively to interpret an
address or construct one; for that, you must use the proper data type
for the socket's namespace.
Thus, the usual practice is to construct an address in the proper
namespace-specific type, then cast a pointer to struct sockaddr *
when you call bind or getsockname.
The one piece of information that you can get from the struct
sockaddr data type is the address format designator which tells
you which data type to use to understand the address fully.
The symbols in this section are defined in the header file `sys/socket.h'.
The struct sockaddr type itself has the following members:
short int sa_family
char sa_data[14]
sa_data is essentially arbitrary.
Each address format has a symbolic name which starts with `AF_'. Each of them corresponds to a `PF_' symbol which designates the corresponding namespace. Here is a list of address format names:
AF_FILE
PF_FILE is the name of that namespace.) See section Details of File Namespace, for information about this address format.
AF_UNIX
AF_FILE, for compatibility.
(PF_UNIX is likewise a synonym for PF_FILE.)
AF_INET
PF_INET is the name of that namespace.)
See section Internet Socket Address Format.
AF_UNSPEC
The corresponding namespace designator symbol PF_UNSPEC exists
for completeness, but there is no reason to use it in a program.
`sys/socket.h' defines symbols starting with `AF_' for many different kinds of networks, all or most of which are not actually implemented. We will document those that really work, as we receive information about how to use them.
Use the bind function to assign an address to a socket. The
prototype for bind is in the header file `sys/socket.h'.
For examples of use, see section The File Namespace, or see section Internet Socket Example.
Function: int bind (int socket, struct sockaddr *addr, size_t length)
The bind function assigns an address to the socket
socket. The addr and length arguments specify the
address; the detailed format of the address depends on the namespace.
The first part of the address is always the format designator, which
specifies a namespace, and says that the address is in the format for
that namespace.
The return value is 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
EBADF
ENOTSOCK
EADDRNOTAVAIL
EADDRINUSE
EINVAL
EACCESS
IPPORT_RESERVED minus one; see
section Internet Ports.)
Additional conditions may be possible depending on the particular namespace of the socket.
Use the function getsockname to examine the address of an
Internet socket. The prototype for this function is in the header file
`sys/socket.h'.
Function: int getsockname (int socket, struct sockaddr *addr, size_t *length_ptr)
The getsockname function returns information about the
address of the socket socket in the locations specified by the
addr and length_ptr arguments. Note that the
length_ptr is a pointer; you should initialize it to be the
allocation size of addr, and on return it contains the actual
size of the address data.
The format of the address data depends on the socket namespace. The
length of the information is usually fixed for a given namespace, so
normally you can know exactly how much space is needed and can provide
that much. The usual practice is to allocate a place for the value
using the proper data type for the socket's namespace, then cast its
address to struct sockaddr * to pass it to getsockname.
The return value is 0 on success and -1 on error. The
following errno error conditions are defined for this function:
EBADF
ENOTSOCK
ENOBUFS
You can't read the address of a socket in the file namespace. This is consistent with the rest of the system; in general, there's no way to find a file's name from a descriptor for that file.
This section describes the details of the file namespace, whose
symbolic name (required when you create a socket) is PF_FILE.
In the file namespace, socket addresses are file names. You can specify any file name you want as the address of the socket, but you must have write permission on the directory containing it. In order to connect to a socket, you must have read permission for it. It's common to put these files in the `/tmp' directory.
One peculiarity of the file namespace is that the name is only used when opening the connection; once that is over with, the address is not meaningful and may not exist.
Another peculiarity is that you cannot connect to such a socket from another machine--not even if the other machine shares the file system which contains the name of the socket. You can see the socket in a directory listing, but connecting to it never succeeds. Some programs take advantage of this, such as by asking the client to send its own process ID, and using the process IDs to distinguish between clients. However, we recommend you not use this method in protocols you design, as we might someday permit connections from other machines that mount the same file systems. Instead, send each new client an identifying number if you want it to have one.
After you close a socket in the file namespace, you should delete the
file name from the file system. Use unlink or remove to
do this; see section Deleting Files.
The file namespace supports just one protocol for any communication
style; it is protocol number 0.
To create a socket in the file namespace, use the constant
PF_FILE as the namespace argument to socket or
socketpair. This constant is defined in `sys/socket.h'.
This designates the file namespace, in which socket addresses are file names, and its associated family of protocols.
This is a synonym for PF_FILE, for compatibility's sake.
The structure for specifying socket names in the file namespace is defined in the header file `sys/un.h':
This structure is used to specify file namespace socket addresses. It has the following members:
short int sun_family
AF_FILE to designate the file
namespace. See section Socket Addresses.
char sun_path[108]
Incomplete: Why is 108 a magic number? RMS suggests making
this a zero-length array and tweaking the example following to use
alloca to allocate an appropriate amount of storage based on
the length of the filename.
You should compute the length parameter for a socket address in
the file namespace as the sum of the size of the sun_family
component and the string length (not the allocation size!) of
the file name string.
Here is an example showing how to create and name a socket in the file namespace.
#include <stddef.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>
int
make_named_socket (const char *filename)
{
struct sockaddr_un name;
int sock;
size_t size;
/* Create the socket. */
sock = socket (PF_UNIX, SOCK_DGRAM, 0);
if (sock < 0)
{
perror ("socket");
exit (EXIT_FAILURE);
}
/* Bind a name to the socket. */
name.sun_family = AF_FILE;
strcpy (name.sun_path, filename);
/* The size of the address is
the offset of the start of the filename,
plus its length,
plus one for the terminating null byte. */
size = (offsetof (struct sockaddr_un, sun_path)
+ strlen (name.sun_path) + 1);
if (bind (sock, (struct sockaddr *) &name, size) < 0)
{
perror ("bind");
exit (EXIT_FAILURE);
}
return sock;
}
This section describes the details the protocols and socket naming conventions used in the Internet namespace.
To create a socket in the Internet namespace, use the symbolic name
PF_INET of this namespace as the namespace argument to
socket or socketpair. This macro is defined in
`sys/socket.h'.
This designates the Internet namespace and associated family of protocols.
A socket address for the Internet namespace includes the following components:
You must ensure that the address and port number are represented in a canonical format called network byte order. See section Byte Order Conversion, for information about this.
In the Internet namespace, a socket address consists of a host address and a port on that host. In addition, the protocol you choose serves effectively as a part of the address because local port numbers are meaningful only within a particular protocol.
The data type for representing socket addresses in the Internet namespace is defined in the header file `netinet/in.h'.
This is the data type used to represent socket addresses in the Internet namespace. It has the following members:
short int sin_family
AF_INET in this member.
See section Socket Addresses.
struct in_addr sin_addr
unsigned short int sin_port
When you call bind or getsockname, you should specify
sizeof (struct sockaddr_in) as the length parameter if
you are using an Internet namespace socket address.
Each computer on the Internet has one or more Internet addresses, numbers which identify that computer among all those on the Internet. Users typically write numeric host addresses as sequences of four numbers, separated by periods, as in `128.52.46.32'.
Each computer also has one or more host names, which are strings of words separated by periods, as in `churchy.gnu.ai.mit.edu'.
Programs that let the user specify a host typically accept both numeric addresses and host names. But the program needs a numeric address to open a connection; to use a host name, you must convert it to the numeric address it stands for.
An Internet host address is a number containing four bytes of data. These are divided into two parts, a network number and a local network address number within that network. The network number consists of the first one, two or three bytes; the rest of the bytes are the local address.
Network numbers are registered with the Network Information Center (NIC), and are divided into three classes--A, B, and C. The local network address numbers of individual machines are registered with the administrator of the particular network.
Class A networks have single-byte numbers in the range 0 to 127. There are only a small number of Class A networks, but they can each support a very large number of hosts. Medium-sized Class B networks have two-byte network numbers, with the first byte in the range 128 to 191. Class C networks are the smallest; they have three-byte network numbers, with the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes of an Internet address specifies a network. The remaining bytes of the Internet address specify the address within that network.
The Class A network 0 is reserved for broadcast to all networks. In addition, the host number 0 within each network is reserved for broadcast to all hosts in that network.
The Class A network 127 is reserved for loopback; you can always use the Internet address `127.0.0.1' to refer to the host machine.
Since a single machine can be a member of multiple networks, it can have multiple Internet host addresses. However, there is never supposed to be more than one machine with the same host address.
There are four forms of the standard numbers-and-dots notation for Internet addresses:
a.b.c.d
a.b.c
a.b.
a.b
a
Within each part of the address, the usual C conventions for specifying the radix apply. In other words, a leading `0x' or `0X' implies hexadecimal radix; a leading `0' implies octal; and otherwise decimal radix is assumed.
Internet host addresses are represented in some contexts as integers
(type unsigned long int). In other contexts, the integer is
packaged inside a structure of type struct in_addr. It would
be better if the usage were made consistent, but it is not hard to extract
the integer from the structure or put the integer into a structure.
The following basic definitions for Internet addresses appear in the header file `netinet/in.h':
This data type is used in certain contexts to contain an Internet host
address. It has just one field, named s_addr, which records the
host address number as an unsigned long int.
Macro: unsigned long int INADDR_ANY
You can use this constant to stand for "the address of this machine,"
instead of finding its actual address. This special constant saves you
the trouble of looking up the address of your own machine. Also, if
your machine has multiple network addresses on different networks (which
is not unusual), using INADDR_ANY permits the system to choose
whichever address makes communication most efficient.
These additional functions for manipulating Internet addresses are declared in `arpa/inet.h'. They represent Internet addresses in network byte order; they represent network numbers and local-address-within-network numbers in host byte order. See section Byte Order Conversion, for an explanation of network and host byte order.
Function: unsigned long int inet_addr (const char *name)
This function converts the Internet host address name
from the standard numbers-and-dots notation into binary data.
If the input is not valid, inet_addr returns -1.
Function: unsigned long int inet_network (const char *name)
This function extracts the network number from the address name,
given in the standard numbers-and-dots notation.
If the input is not valid, inet_network returns -1.
Function: char * inet_ntoa (struct in_addr addr)
This function converts the Internet host address addr to a string in the standard numbers-and-dots notation. The return value is a pointer into a statically-allocated buffer. Subsequent calls will overwrite the same buffer, so you should copy the string if you need to save it.
Function: struct in_addr inet_makeaddr (int net, int local)
This function makes an Internet host address by combining the network number net with the local-address-within-network number local.
Function: int inet_lnaof (struct in_addr addr)
This function returns the local-address-within-network part of the Internet host address addr.
Function: int inet_netof (struct in_addr addr)
This function returns the network number part of the Internet host address addr.
Besides the standard numbers-and-dots notation for Internet addresses, you can also refer to a host by a symbolic name. The advantage of a symbolic name is that it is usually easier to remember. For example, the machine with Internet address `128.52.46.32' is also known as `churchy.gnu.ai.mit.edu'; and other machines in the `gnu.ai.mit.edu' domain can refer to it simply as `churchy'.
Internally, the system uses a database to keep track of the mapping between host names and host numbers. This database is usually either the file `/etc/hosts' or an equivalent provided by a name server. The functions and other symbols for accessing this database are declared in `netdb.h'. They are BSD features, defined unconditionally if you include `netdb.h'.
This data type is used to represent an entry in the hosts database. It has the following members:
char *h_name
char **h_aliases
int h_addrtype
AF_INET. In principle other kinds of addresses could be
represented in the data base as well as Internet addresses; if this were
done, you might find a value in this field other than AF_INET.
See section Socket Addresses.
int h_length
char **h_addr_list
char *h_addr
h_addr_list[0]; in other words, it is the
first host address.
As far as the host database is concerned, each address is just a block
of memory h_length bytes long. But in other contexts there is an
implicit assumption that you can convert this to a struct in_addr or
an unsigned long int. Host addresses in a struct hostent
structure are always given in network byte order; see section Byte Order Conversion.
You can use gethostbyname or gethostbyaddr to search the
hosts database for information about a particular host. The information
is returned in a statically-allocated structure; you must copy the
information if you need to save it across calls.
Function: struct hostent * gethostbyname (const char *name)
The gethostbyname function returns information about the host
named name. If the lookup fails, it returns a null pointer.
Function: struct hostent * gethostbyaddr (const char *addr, int length, int format)
The gethostbyaddr function returns information about the host
with Internet address addr. The length argument is the
size (in bytes) of the address at addr. format specifies
the address format; for an Internet address, specify a value of
AF_INET.
If the lookup fails, gethostbyaddr returns a null pointer.
If the name lookup by gethostbyname or gethostbyaddr
fails, you can find out the reason by looking at the value of the
variable h_errno. (It would be cleaner design for these
functions to set errno, but use of h_errno is compatible
with other systems.) Before using h_errno, you must declare it
like this:
extern int h_errno;
Here are the error codes that you may find in h_errno:
HOST_NOT_FOUND
TRY_AGAIN
NO_RECOVERY
NO_ADDRESS
You can also scan the entire hosts database one entry at a time using
sethostent, gethostent, and endhostent. Be careful
in using these functions, because they are not reentrant.
Function: void sethostent (int stayopen)
This function opens the hosts database to begin scanning it. You can
then call gethostent to read the entries.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to gethostbyname or gethostbyaddr will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
Function: struct hostent * gethostent ()
This function returns the next entry in the hosts database. It returns a null pointer if there are no more entries.
This function closes the hosts database.
A socket address in the Internet namespace consists of a machine's Internet address plus a port number which distinguishes the sockets on a given machine (for a given protocol). Port numbers range from 0 to 65,535.
Port numbers less than IPPORT_RESERVED are reserved for standard
servers, such as finger and telnet. There is a database
that keeps track of these, and you can use the getservbyname
function to map a service name onto a port number; see section The Services Database.
If you write a server that is not one of the standard ones defined in
the database, you must choose a port number for it. Use a number
greater than IPPORT_USERRESERVED; such numbers are reserved for
servers and won't ever be generated automatically by the system.
Avoiding conflicts with servers being run by other users is up to you.
When you use a socket without specifying its address, the system
generates a port number for it. This number is between
IPPORT_RESERVED and IPPORT_USERRESERVED.
On the Internet, it is actually legitimate to have two different
sockets with the same port number, as long as they never both try to
communicate with the same socket address (host address plus port
number). You shouldn't duplicate a port number except in special
circumstances where a higher-level protocol requires it. Normally,
the system won't let you do it; bind normally insists on
distinct port numbers. To reuse a port number, you must set the
socket option SO_REUSEADDR. See section Socket-Level Options.
These macros are defined in the header file `netinet/in.h'.
Port numbers less than IPPORT_RESERVED are reserved for
superuser use.
Macro: int IPPORT_USERRESERVED
Port numbers greater than or equal to IPPORT_USERRESERVED are
reserved for explicit use; they will never be allocated automatically.
The database that keeps track of "well-known" services is usually either the file `/etc/services' or an equivalent from a name server. You can use these utilities, declared in `netdb.h', to access the services database.
This data type holds information about entries from the services database. It has the following members:
char *s_name
char **s_aliases
int s_port
char *s_proto
To get information about a particular service, use the
getservbyname or getservbyport functions. The information
is returned in a statically-allocated structure; you must copy the
information if you need to save it across calls.
Function: struct servent * getservbyname (const char *name, const char *proto)
The getservbyname function returns information about the
service named name using protocol proto. If it can't find
such a service, it returns a null pointer.
This function is useful for servers as well as for clients; servers use it to determine which port they should listen on (see section Listening for Connections).
Function: struct servent * getservbyport (int port, const char *proto)
The getservbyport function returns information about the
service at port port using protocol proto. If it can't
find such a service, it returns a null pointer.
You can also scan the services database using setservent,
getservent, and endservent. Be careful in using these
functions, because they are not reentrant.
Function: void setservent (int stayopen)
This function opens the services database to begin scanning it.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to getservbyname or getservbyport will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
Function: struct servent * getservent (void)
This function returns the next entry in the services database. If there are no more entries, it returns a null pointer.
Function: void endservent (void)
This function closes the services database.
Different kinds of computers use different conventions for the ordering of bytes within a word. Some computers put the most significant byte within a word first (this is called "big-endian" order), and others put it last ("little-endian" order).
So that machines with different byte order conventions can communicate, the Internet protocols specify a canonical byte order convention for data transmitted over the network. This is known as the network byte order.
When establishing an Internet socket connection, you must make sure that
the data in the sin_port and sin_addr members of the
sockaddr_in structure are represented in the network byte order.
If you are encoding integer data in the messages sent through the
socket, you should convert this to network byte order too. If you don't
do this, your program may fail when running on or talking to other kinds
of machines.
If you use getservbyname and gethostbyname or
inet_addr to get the port number and host address, the values are
already in the network byte order, and you can copy them directly into
the sockaddr_in structure.
Otherwise, you have to convert the values explicitly. Use
htons and ntohs to convert values for the sin_port
member. Use htonl and ntohl to convert values for the
sin_addr member. (Remember, struct in_addr is equivalent
to unsigned long int.) These functions are declared in
`netinet/in.h'.
Function: unsigned short int htons (unsigned short int hostshort)
This function converts the short integer hostshort from
host byte order to network byte order.
Function: unsigned short int ntohs (unsigned short int netshort)
This function converts the short integer netshort from
network byte order to host byte order.
Function: unsigned long int htonl (unsigned long int hostlong)
This function converts the long integer hostlong from
host byte order to network byte order.
Function: unsigned long int ntohl (unsigned long int netlong)
This function converts the long integer netlong from
network byte order to host byte order.
The communications protocol used with a socket controls low-level details of how data is exchanged. For example, the protocol implements things like checksums to detect errors in transmissions, and routing instructions for messages. Normal user programs have little reason to mess with these details directly.
The default communications protocol for the Internet namespace depends on the communication style. For stream communication, the default is TCP ("transmission control protocol"). For datagram communication, the default is UDP ("user datagram protocol"). For reliable datagram communication, the default is RDP ("reliable datagram protocol"). You should nearly always use the default.
Internet protocols are generally specified by a name instead of a
number. The network protocols that a host knows about are stored in a
database. This is usually either derived from the file
`/etc/protocols', or it may be an equivalent provided by a name
server. You look up the protocol number associated with a named
protocol in the database using the getprotobyname function.
Here are detailed descriptions of the utilities for accessing the protocols database. These are declared in `netdb.h'.
This data type is used to represent entries in the network protocols database. It has the following members:
char *p_name
char **p_aliases
int p_proto
socket.
You can use getprotobyname and getprotobynumber to search
the protocols database for a specific protocol. The information is
returned in a statically-allocated structure; you must copy the
information if you need to save it across calls.
Function: struct protoent * getprotobyname (const char *name)
The getprotobyname function returns information about the
network protocol named name. If there is no such protocol, it
returns a null pointer.
Function: struct protoent * getprotobynumber (int protocol)
The getprotobynumber function returns information about the
network protocol with number protocol. If there is no such
protocol, it returns a null pointer.
You can also scan the whole protocols database one protocol at a time by
using setprotoent, getprotoent, and endprotoent.
Be careful in using these functions, because they are not reentrant.
Function: void setprotoent (int stayopen)
This function opens the protocols database to begin scanning it.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to getprotobyname or getprotobynumber will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
Function: struct protoent * getprotoent (void)
This function returns the next entry in the protocols database. It returns a null pointer if there are no more entries.
Function: void endprotoent (void)
This function closes the protocols database.
Here is an example showing how to create and name a socket in the
Internet namespace. The newly created socket exists on the machine that
the program is running on. Rather than finding and using the machine's
Internet address, this example specifies INADDR_ANY as the host
address; the system replaces that with the machine's actual address.
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
make_socket (unsigned short int port)
{
int sock;
struct sockaddr_in name;
/* Create the socket. */
sock = socket (PF_INET, SOCK_STREAM, 0);
if (sock < 0)
{
perror ("socket");
exit (EXIT_FAILURE);
}
/* Give the socket a name. */
name.sin_family = AF_INET;
name.sin_port = htons (port);
name.sin_addr.s_addr = htonl (INADDR_ANY);
if (bind (sock, (struct sockaddr *) &name, sizeof (name)) < 0)
{
perror ("bind");
exit (EXIT_FAILURE);
}
return sock;
}
Here is another example, showing how you can fill in a sockaddr_in
structure, given a host name string and a port number:
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
void
init_sockaddr (struct sockaddr_in *name,
const char *hostname, unsigned short int port)
{
struct hostent *hostinfo;
name->sin_family = AF_INET;
name->sin_port = htons (port);
hostinfo = gethostbyname (serverhost);
if (hostinfo == NULL)
{
fprintf (stderr, "Unknown host %s.\n", hostname);
exit (EXIT_FAILURE);
}
name->sin_addr = *(struct in_addr *) hostinfo->h_addr;
}
Certain other namespaces and associated protocol families are supported
but not documented yet because they are not often used. PF_NS
refers to the Xerox Network Software protocols. PF_ISO stands
for Open Systems Interconnect. PF_CCITT refers to protocols from
CCITT. `socket.h' defines these symbols and others naming protocols
not actually implemented.
PF_IMPLINK is used for communicating between hosts and Internet
Message Processors. For information on this, and on PF_ROUTE, an
occasionally-used local area routing protocol, see the GNU Hurd Manual
(to appear in the future).
This section describes the actual library functions for opening and closing sockets. The same functions work for all namespaces and connection styles.
The primitive for creating a socket is the socket function,
declared in `sys/socket.h'.
Function: int socket (int namespace, int style, int protocol)
This function creates a socket and specifies communication style
style, which should be one of the socket styles listed in
section Communication Styles. The namespace argument specifies
the namespace; it must be PF_FILE (see section The File Namespace) or
PF_INET (see section The Internet Namespace). protocol
designates the specific protocol (see section Socket Concepts); zero is
usually right for protocol.
The return value from socket is the file descriptor for the new
socket, or -1 in case of error. The following errno error
conditions are defined for this function:
EPROTONOSUPPORT
EMFILE
ENFILE
EACCESS
ENOBUFS
The file descriptor returned by the socket function supports both
read and write operations. But, like pipes, sockets do not support file
positioning operations.
For examples of how to call the socket function,
see section The File Namespace, or section Internet Socket Example.
When you are finished using a socket, you can simply close its
file descriptor with close; see section Opening and Closing Files.
If there is still data waiting to be transmitted over the connection,
normally close tries to complete this transmission. You
can control this behavior using the SO_LINGER socket option to
specify a timeout period; see section Socket Options.
You can also shut down only reception or only transmission on a
connection by calling shutdown, which is declared in
`sys/socket.h'.
Function: int shutdown (int socket, int how)
The shutdown function shuts down the connection of socket
socket. The argument how specifies what action to
perform:
0
1
2
The return value is 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
EBADF
ENOTSOCK
ENOTCONN
A socket pair consists of a pair of connected (but unnamed)
sockets. It is very similar to a pipe and is used in much the same
way. Socket pairs are created with the socketpair function,
declared in `sys/socket.h'. A socket pair is much like a pipe; the
main difference is that the socket pair is bidirectional, whereas the
pipe has one input-only end and one output-only end (see section Pipes and FIFOs).
Function: int socketpair (int namespace, int style, int protocol, int filedes[2])
This function creates a socket pair, returning the file descriptors in
filedes[0] and filedes[1]. The socket pair
is a full-duplex communications channel, so that both reading and writing
may be performed at either end.
The namespace, style, and protocol arguments are
interpreted as for the socket function. style should be
one of the communication styles listed in section Communication Styles.
The namespace argument specifies the namespace, which must be
AF_FILE (see section The File Namespace); protocol specifies the
communications protocol, but zero is the only meaningful value.
If style specifies a connectionless communication style, then the two sockets you get are not connected, strictly speaking, but each of them knows the other as the default destination address, so they can send packets to each other.
The socketpair function returns 0 on success and -1
on failure. The following errno error conditions are defined
for this function:
EMFILE
EAFNOSUPPORT
EPROTONOSUPPORT
EOPNOTSUPP
The most common communication styles involve making a connection to a particular other socket, and then exchanging data with that socket over and over. Making a connection is asymmetric; one side (the client) acts to request a connection, while the other side (the server) makes a socket and waits for the connection request.
In making a connection, the client makes a connection while the server
waits for and accepts the connection. Here we discuss what the client
program must do, using the connect function.
Function: int connect (int socket, struct sockaddr *addr, size_t length)
The connect function initiates a connection from the socket
with file descriptor socket to the socket whose address is
specified by the addr and length arguments. (This socket
is typically on another machine, and it must be already set up as a
server.) See section Socket Addresses, for information about how these
arguments are interpreted.
Normally, connect waits until the server responds to the request
before it returns. You can set nonblocking mode on the socket
socket to make connect return immediately without waiting
for the response. See section File Status Flags, for information about
nonblocking mode.
The normal return value from connect is 0. If an error
occurs, connect returns -1. The following errno
error conditions are defined for this function:
EBADF
ENOTSOCK
EADDRNOTAVAIL
EAFNOSUPPORT
EISCONN
ETIMEDOUT
ECONNREFUSED
ENETUNREACH
EADDRINUSE
EINPROGRESS
EALREADY
Now let us consider what the server process must do to accept
connections on a socket. This involves the use of the listen
function to enable connection requests on the socket, and later using
the accept function (see section Accepting Connections) to act on a
request. The listen function is not allowed for sockets using
connectionless communication styles.
You can write a network server that does not even start running until a
connection to it is requested. See section inetd Servers.
In the Internet namespace, there are no special protection mechanisms for controlling access to connect to a port; any process on any machine can make a connection to your server. If you want to restrict access to your server, make it examine the addresses associated with connection requests or implement some other handshaking or identification protocol.
In the File namespace, the ordinary file protection bits control who has access to connect to the socket.
Function: int listen (int socket, unsigned int n)
The listen function enables the socket socket to
accept connections, thus making it a server socket.
The argument n specifies the length of the queue for pending connections.
The listen function returns 0 on success and -1
on failure. The following errno error conditions are defined
for this function:
EBADF
ENOTSOCK
EOPNOTSUPP
When a server receives a connection request, it can complete the
connection by accepting the request. Use the function accept
to do this.
A socket that has been established as a server can accept connection
requests from multiple clients. The server's original socket
does not become part of the connection; instead, accept
makes a new socket which participates in the connection.
accept returns the descriptor for this socket. The server's
original socket remains available for listening for further connection
requests.
The number of pending connection requests on a server socket is finite.
If connection requests arrive from clients faster than the server can
act upon them, the queue can fill up and additional requests are refused
with a ECONNREFUSED error. You can specify the maximum length of
this queue as an argument to the listen function, although the
system may also impose its own internal limit on the length of this
queue.
Function: int accept (int socket, struct sockaddr *addr, size_t *length_ptr)
This function is used to accept a connection request on the server socket socket.
The accept function waits if there are no connections pending,
unless the socket socket has nonblocking mode set. (You can use
select to wait for a pending connection, with a nonblocking
socket.) See section File Status Flags, for information about nonblocking
mode.
The addr and length_ptr arguments are used to return information about the name of the client socket that initiated the connection. See section Socket Addresses, for information about the format of the information.
Accepting a connection does not make socket part of the
connection. Instead, it creates a new socket which becomes
connected. The normal return value of accept is the file
descriptor for the new socket.
After accept, the original socket socket remains open and
unconnected, and continues listening until you close it. You can
accept further connections with socket by calling accept
again.
If an error occurs, accept returns -1. The following
errno error conditions are defined for this function:
EBADF
ENOTSOCK
EOPNOTSUPP
EWOULDBLOCK
The accept function is not allowed for sockets using
connectionless communication styles.
Function: int getpeername (int socket, struct sockaddr *addr, size_t *length_ptr)
The getpeername function returns the address of the socket that
socket is connected to; it stores the address in the memory space
specified by addr and length_ptr. It stores the length of
the address in *length_ptr.
See section Socket Addresses, for information about the format of the
address. In some operating systems, getpeername works only for
sockets in the Internet domain.
The return value is 0 on success and -1 on error. The
following errno error conditions are defined for this function:
EBADF
ENOTSOCK
ENOTCONN
ENOBUFS
Once a socket has been connected to a peer, you can use the ordinary
read and write operations (see section Input and Output Primitives) to
transfer data. A socket is a two-way communications channel, so read
and write operations can be performed at either end.
There are also some I/O modes that are specific to socket operations.
In order to specify these modes, you must use the recv and
send functions instead of the more generic read and
write functions. The recv and send functions take
an additional argument which you can use to specify various flags to
control the special I/O modes. For example, you can specify the
MSG_OOB flag to read or write out-of-band data, the
MSG_PEEK flag to peek at input, or the MSG_DONTROUTE flag
to control inclusion of routing information on output.
The send function is declared in the header file
`sys/socket.h'. If your flags argument is zero, you can just
as well use write instead of send; see section Input and Output Primitives. If the socket was connected but the connection has broken,
you get a SIGPIPE signal for any use of send or
write (see section Miscellaneous Signals).
Function: int send (int socket, void *buffer, size_t size, int flags)
The send function is like write, but with the additional
flags flags. The possible values of flags are described
in section Socket Data Options.
This function returns the number of bytes transmitted, or -1 on
failure. If the socket is nonblocking, then send (like
write) can return after sending just part of the data.
See section File Status Flags, for information about nonblocking mode.
Note, however, that a successful return value merely indicates that the message has been sent without error, not necessarily that it has been received without error.
The following errno error conditions are defined for this function:
EBADF
EINTR
ENOTSOCK
EMSGSIZE
EWOULDBLOCK
send blocks until the operation can be
completed.)
ENOBUFS
ENOTCONN
EPIPE
send generates a SIGPIPE signal first; if that
signal is ignored or blocked, or if its handler returns, then
send fails with EPIPE.
The recv function is declared in the header file
`sys/socket.h'. If your flags argument is zero, you can
just as well use read instead of recv; see section Input and Output Primitives.
Function: int recv (int socket, void *buffer, size_t size, int flags)
The recv function is like read, but with the additional
flags flags. The possible values of flags are described
In section Socket Data Options.
If nonblocking mode is set for socket, and no data is available to
be read, recv fails immediately rather than waiting. See section File Status Flags, for information about nonblocking mode.
This function returns the number of bytes received, or -1 on failure.
The following errno error conditions are defined for this function:
EBADF
ENOTSOCK
EWOULDBLOCK
recv blocks until there is input
available to be read.)
EINTR
ENOTCONN
The flags argument to send and recv is a bit
mask. You can bitwise-OR the values of the following macros together
to obtain a value for this argument. All are defined in the header
file `sys/socket.h'.
Send or receive out-of-band data. See section Out-of-Band Data.
Look at the data but don't remove it from the input queue. This is
only meaningful with input functions such as recv, not with
send.
Don't include routing information in the message. This is only meaningful with output operations, and is usually only of interest for diagnostic or routing programs. We don't try to explain it here.
Here is an example client program that makes a connection for a byte stream socket in the Internet namespace. It doesn't do anything particularly interesting once it has connected to the server; it just sends a text string to the server and exits.
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define PORT 5555
#define MESSAGE "Yow!!! Are we having fun yet?!?"
#define SERVERHOST "churchy.gnu.ai.mit.edu"
void
write_to_server (int filedes)
{
int nbytes;
nbytes = write (filedes, MESSAGE, strlen (MESSAGE) + 1);
if (nbytes < 0)
{
perror ("write");
exit (EXIT_FAILURE);
}
}
int
main (void)
{
extern void init_sockaddr (struct sockaddr_in *name,
const char *hostname, unsigned short int port);
int sock;
struct sockaddr_in servername;
/* Create the socket. */
sock = socket (PF_INET, SOCK_STREAM, 0);
if (sock < 0)
{
perror ("socket (client)");
exit (EXIT_FAILURE);
}
/* Connect to the server. */
init_sockaddr (&servername, SERVERHOST, PORT);
if (0 > connect (sock,
(struct sockaddr *) &servername,
sizeof (servername)))
{
perror ("connect (client)");
exit (EXIT_FAILURE);
}
/* Send data to the server. */
write_to_server (sock);
close (sock);
exit (EXIT_SUCCESS);
}
The server end is much more complicated. Since we want to allow
multiple clients to be connected to the server at the same time, it
would be incorrect to wait for input from a single client by simply
calling read or recv. Instead, the right thing to do is
to use select (see section Waiting for Input or Output) to wait for input on
all of the open sockets. This also allows the server to deal with
additional connection requests.
This particular server doesn't do anything interesting once it has gotten a message from a client. It does close the socket for that client when it detects an end-of-file condition (resulting from the client shutting down its end of the connection).
This program uses make_socket and init_sockaddr to set
up the socket address; see section Internet Socket Example.
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define PORT 5555
#define MAXMSG 512
int
read_from_client (int filedes)
{
char buffer[MAXMSG];
int nbytes;
nbytes = read (filedes, buffer, MAXMSG);
if (nbytes < 0)
{
/* Read error. */
perror ("read");
exit (EXIT_FAILURE);
}
else if (nbytes == 0)
/* End-of-file. */
return -1;
else
{
/* Data read. */
fprintf (stderr, "Server: got message: `%s'\n", buffer);
return 0;
}
}
int
main (void)
{
extern int make_socket (unsigned short int port);
int sock;
int status;
fd_set active_fd_set, read_fd_set;
int i;
struct sockaddr_in clientname;
size_t size;
/* Create the socket and set it up to accept connections. */
sock = make_socket (PORT);
if (listen (sock, 1) < 0)
{
perror ("listen");
exit (EXIT_FAILURE);
}
/* Initialize the set of active sockets. */
FD_ZERO (&active_fd_set);
FD_SET (sock, &active_fd_set);
while (1)
{
/* Block until input arrives on one or more active sockets. */
read_fd_set = active_fd_set;
if (select (FD_SETSIZE, &read_fd_set, NULL, NULL, NULL) < 0)
{
perror ("select");
exit (EXIT_FAILURE);
}
/* Service all the sockets with input pending. */
for (i = 0; i < FD_SETSIZE; ++i)
if (FD_ISSET (i, &read_fd_set))
{
if (i == sock)
{
/* Connection request on original socket. */
size = sizeof (clientname);
if (accept (sock,
(struct sockaddr *) &clientname, &size) < 0)
{
perror ("accept");
exit (EXIT_FAILURE);
}
fprintf (stderr, "Server: connect from host %s, port %hd.\n",
inet_ntoa (clientname.sin_addr),
ntohs (clientname.sin_port));
FD_SET (status, &active_fd_set);
}
else
{
/* Data arriving on an already-connected socket. */
if (read_from_client (i) < 0)
{
close (i);
FD_CLR (i, &active_fd_set);
}
}
}
}
}
Streams with connections permit out-of-band data that is
delivered with higher priority than ordinary data. Typically the
reason for sending out-of-band data is to send notice of an
exceptional condition. The way to send out-of-band data is using
send, specifying the flag MSG_OOB (see section Sending Data).
Out-of-band data is received with higher priority because the
receiving process need not read it in sequence; to read the next
available out-of-band data, use recv with the MSG_OOB
flag (see section Receiving Data). Ordinary read operations do not read
out-of-band data; they read only the ordinary data.
When a socket finds that out-of-band data is on its way, it sends a
SIGURG signal to the owner process or process group of the
socket. You can specify the owner using the F_SETOWN command
to the fcntl function; see section Interrupt-Driven Input. You must
also establish a handler for this signal, as described in section Signal Handling, in order to take appropriate action such as reading the
out-of-band data.
Alternatively, you can test for pending out-of-band data, or wait
until there is out-of-band data, using the select function; it
can wait for an exceptional condition on the socket. See section Waiting for Input or Output, for more information about select.
Notification of out-of-band data (whether with SIGURG or with
select) indicates that out-of-band data is on the way; the data
may not actually arrive until later. If you try to read the
out-of-band data before it arrives, recv fails with an
EWOULDBLOCK error.
Sending out-of-band data automatically places a "mark" in the stream of ordinary data, showing where in the sequence the out-of-band data "would have been". This is useful when the meaning of out-of-band data is "cancel everything sent so far". Here is how you can test, in the receiving process, whether any ordinary data was sent before the mark:
success = ioctl (socket, SIOCATMARK, &result);
Here's a function to discard any ordinary data preceding the out-of-band mark:
int
discard_until_mark (int socket)
{
while (1)
{
/* This is not an arbitrary limit; any size will do. */
char buffer[1024];
int result, success;
/* If we have reached the mark, return. */
success = ioctl (socket, SIOCATMARK, &result);
if (success < 0)
perror ("ioctl");
if (result)
return;
/* Otherwise, read a bunch of ordinary data and discard it.
This is guaranteed not to read past the mark
if it starts before the mark. */
success = read (socket, buffer, sizeof buffer);
if (success < 0)
perror ("read");
}
}
If you don't want to discard the ordinary data preceding the mark, you
may need to read some of it anyway, to make room in internal system
buffers for the out-of-band data. If you try to read out-of-band data
and get an EWOULDBLOCK error, try reading some ordinary data
(saving it so that you can use it when you want it) and see if that
makes room. Here is an example:
struct buffer
{
char *buffer;
int size;
struct buffer *next;
};
/* Read the out-of-band data from SOCKET and return it
as a `struct buffer', which records the address of the data
and its size.
It may be necessary to read some ordinary data
in order to make room for the out-of-band data.
If so, the ordinary data is saved as a chain of buffers
found in the `next' field of the value. */
struct buffer *
read_oob (int socket)
{
struct buffer *tail = 0;
struct buffer *list = 0;
while (1)
{
/* This is an arbitrary limit.
Does anyone know how to do this without a limit? */
char *buffer = (char *) xmalloc (1024);
struct buffer *link;
int success;
int result;
/* Try again to read the out-of-band data. */
success = recv (socket, buffer, sizeof buffer, MSG_OOB);
if (success >= 0)
{
/* We got it, so return it. */
struct buffer *link
= (struct buffer *) xmalloc (sizeof (struct buffer));
link->buffer = buffer;
link->size = success;
link->next = list;
return link;
}
/* If we fail, see if we are at the mark. */
success = ioctl (socket, SIOCATMARK, &result);
if (success < 0)
perror ("ioctl");
if (result)
{
/* At the mark; skipping past more ordinary data cannot help.
So just wait a while. */
sleep (1);
continue;
}
/* Otherwise, read a bunch of ordinary data and save it.
This is guaranteed not to read past the mark
if it starts before the mark. */
success = read (socket, buffer, sizeof buffer);
if (success < 0)
perror ("read");
/* Save this data in the buffer list. */
{
struct buffer *link
= (struct buffer *) xmalloc (sizeof (struct buffer));
link->buffer = buffer;
link->size = success;
/* Add the new link to the end of the list. */
if (tail)
tail->next = link;
else
list = link;
tail = link;
}
}
}
This section describes how to use communication styles that don't use
connections (styles SOCK_DGRAM and SOCK_RDM). Using
these styles, you group data into packets and each packet is an
independent communication. You specify the destination for each
packet individually.
Datagram packets are like letters: you send each one independently, with its own destination address, and they may arrive in the wrong order or not at all.
The listen and accept functions are not allowed for
sockets using connectionless communication styles.
The normal way of sending data on a datagram socket is by using the
sendto function, declared in `sys/socket.h'.
You can call connect on a datagram socket, but this only
specifies a default destination for further data transmission on the
socket. When a socket has a default destination, then you can use
send (see section Sending Data) or even write (see section Input and Output Primitives) to send a packet there. You can cancel the default
destination by calling connect using an address format of
AF_UNSPEC in the addr argument. See section Making a Connection, for
more information about the connect function.
Function: int sendto (int socket, void *buffer. size_t size, int flags, struct sockaddr *addr, size_t length)
The sendto function transmits the data in the buffer
through the socket socket to the destination address specified
by the addr and length arguments. The size argument
specifies the number of bytes to be transmitted.
The flags are interpreted the same way as for send; see
section Socket Data Options.
The return value and error conditions are also the same as for
send, but you cannot rely on the system to detect errors and
report them; the most common error is that the packet is lost or there
is no one at the specified address to receive it, and the operating
system on your machine usually does not know this.
It is also possible for one call to sendto to report an error
due to a problem related to a previous call.
The recvfrom function reads a packet from a datagram socket and
also tells you where it was sent from. This function is declared in
`sys/socket.h'.
Function: int recvfrom (int socket, void *buffer, size_t size, int flags, struct sockaddr *addr, size_t *length_ptr)
The recvfrom function reads one packet from the socket
socket into the buffer buffer. The size argument
specifies the maximum number of bytes to be read.
If the packet is longer than size bytes, then you get the first size bytes of the packet, and the rest of the packet is lost. There's no way to read the rest of the packet. Thus, when you use a packet protocol, you must always know how long a packet to expect.
The addr and length_ptr arguments are used to return the address where the packet came from. See section Socket Addresses. For a socket in the file domain, the address information won't be meaningful, since you can't read the address of such a socket (see section The File Namespace). You can specify a null pointer as the addr argument if you are not interested in this information.
The flags are interpreted the same way as for recv
(see section Socket Data Options). The return value and error conditions
are also the same as for recv.
You can use plain recv (see section Receiving Data) instead of
recvfrom if you know don't need to find out who sent the packet
(either because you know where it should come from or because you
treat all possible senders alike). Even read can be used if
you don't want to specify flags (see section Input and Output Primitives).
Here is a set of example programs that send messages over a datagram
stream in the file namespace. Both the client and server programs use the
make_named_socket function that was presented in section The File Namespace, to create and name their sockets.
First, here is the server program. It sits in a loop waiting for messages to arrive, bouncing each message back to the sender. Obviously, this isn't a particularly useful program, but it does show the general ideas involved.
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>
#define SERVER "/tmp/serversocket"
#define MAXMSG 512
int
main (void)
{
int sock;
char message[MAXMSG];
struct sockaddr_un name;
size_t size;
int nbytes;
/* Make the socket, then loop endlessly. */
sock = make_named_socket (SERVER);
while (1)
{
/* Wait for a datagram. */
size = sizeof (name);
nbytes = recvfrom (sock, message, MAXMSG, 0,
(struct sockaddr *) & name, &size);
if (nbytes < 0)
{
perror ("recfrom (server)");
exit (EXIT_FAILURE);
}
/* Give a diagnostic message. */
fprintf (stderr, "Server: got message: %s\n", message);
/* Bounce the message back to the sender. */
nbytes = sendto (sock, message, nbytes, 0,
(struct sockaddr *) & name, size);
if (nbytes < 0)
{
perror ("sendto (server)");
exit (EXIT_FAILURE);
}
}
}
Here is the client program corresponding to the server above.
It sends a datagram to the server and then waits for a reply. Notice that the socket for the client (as well as for the server) in this example has to be given a name. This is so that the server can direct a message back to the client. Since the socket has no associated connection state, the only way the server can do this is by referencing the name of the client.
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>
#define SERVER "/tmp/serversocket"
#define CLIENT "/tmp/mysocket"
#define MAXMSG 512
#define MESSAGE "Yow!!! Are we having fun yet?!?"
int
main (void)
{
extern int make_named_socket (const char *name);
int sock;
char message[MAXMSG];
struct sockaddr_un name;
size_t size;
int nbytes;
/* Make the socket. */
sock = make_named_socket (CLIENT);
/* Initialize the server socket address. */
name.sun_family = AF_UNIX;
strcpy (name.sun_path, SERVER);
size = strlen (name.sun_path) + sizeof (name.sun_family);
/* Send the datagram. */
nbytes = sendto (sock, MESSAGE, strlen (MESSAGE) + 1, 0,
(struct sockaddr *) & name, size);
if (nbytes < 0)
{
perror ("sendto (client)");
exit (EXIT_FAILURE);
}
/* Wait for a reply. */
nbytes = recvfrom (sock, message, MAXMSG, 0, NULL, 0);
if (nbytes < 0)
{
perror ("recfrom (client)");
exit (EXIT_FAILURE);
}
/* Print a diagnostic message. */
fprintf (stderr, "Client: got message: %s\n", message);
/* Clean up. */
remove (CLIENT);
close (sock);
}
Keep in mind that datagram socket communications are unreliable. In
this example, the client program waits indefinitely if the message
never reaches the server or if the server's response never comes
back. It's up to the user running the program to kill it and restart
it, if desired. A more automatic solution could be to use
select (see section Waiting for Input or Output) to establish a timeout period
for the reply, and in case of timeout either resend the message or
shut down the socket and exit.
inetd DaemonWe've explained above how to write a server program that does its own listening. Such a server must already be running in order for anyone to connect to it.
Another way to provide service for an Internet port is to let the daemon
program inetd do the listening. inetd is a program that
runs all the time and waits (using select) for messages on a
specified set of ports. When it receives a message, it accepts the
connection (if the socket style calls for connections) and then forks a
child process to run the corresponding server program. You specify the
ports and their programs in the file `/etc/inetd.conf'.
inetd Servers
Writing a server program to be run by inetd is very simple. Each time
someone requests a connection to the appropriate port, a new server
process starts. The connection already exists at this time; the
socket is available as the standard input descriptor and as the
standard output descriptor (descriptors 0 and 1) in the server
process. So the server program can begin reading and writing data
right away. Often the program needs only the ordinary I/O facilities;
in fact, a general-purpose filter program that knows nothing about
sockets can work as a byte stream server run by inetd.
You can also use inetd for servers that use connectionless
communication styles. For these servers, inetd does not try to accept
a connection, since no connection is possible. It just starts the
server program, which can read the incoming datagram packet from
descriptor 0. The server program can handle one request and then
exit, or you can choose to write it to keep reading more requests
until no more arrive, and then exit. You must specify which of these
two techniques the server uses, when you configure inetd.
inetd
The file `/etc/inetd.conf' tells inetd which ports to listen to
and what server programs to run for them. Normally each entry in the
file is one line, but you can split it onto multiple lines provided
all but the first line of the entry start with whitespace. Lines that
start with `#' are comments.
Here are two standard entries in `/etc/inetd.conf':
ftp stream tcp nowait root /libexec/ftpd ftpd talk dgram udp wait root /libexec/talkd talkd
An entry has this format:
service style protocol wait username program arguments
The service field says which service this program provides. It
should be the name of a service defined in `/etc/services'.
inetd uses service to decide which port to listen on for
this entry.
The fields style and protocol specify the communication style and the protocol to use for the listening socket. The style should be the name of a communication style, converted to lower case and with `SOCK_' deleted--for example, `stream' or `dgram'. protocol should be one of the protocols listed in `/etc/protocols'. The typical protocol names are `tcp' for byte stream connections and `udp' for unreliable datagrams.
The wait field should be either `wait' or `nowait'.
Use `wait' if style is a connectionless style and the
server, once started, handles multiple requests, as many as come in.
Use `nowait' if inetd should start a new process for each message
or request that comes in. If style uses connections, then
wait must be `nowait'.
user is the user name that the server should run as. inetd runs
as root, so it can set the user ID of its children arbitrarily. It's
best to avoid using `root' for user if you can; but some
servers, such as Telnet and FTP, read a username and password
themselves. These servers need to be root initially so they can log
in as commanded by the data coming over the network.
program together with arguments specifies the command to run to start the server. program should be an absolute file name specifying the executable file to run. arguments consists of any number of whitespace-separated words, which become the command-line arguments of program. The first word in arguments is argument zero, which should by convention be the program name itself (sans directories).
If you edit `/etc/inetd.conf', you can tell inetd to reread the
file and obey its new contents by sending the inetd process the
SIGHUP signal. You'll have to use ps to determine the
process ID of the inetd process, as it is not fixed.
This section describes how to read or set various options that modify the behavior of sockets and their underlying communications protocols.
When you are manipulating a socket option, you must specify which level the option pertains to. This describes whether the option applies to the socket interface, or to a lower-level communications protocol interface.
Here are the functions for examining and modifying socket options. They are declared in `sys/socket.h'.
Function: int getsockopt (int socket, int level, int optname, void *optval, size_t *optlen_ptr)
The getsockopt function gets information about the value of
option optname at level level for socket socket.
The option value is stored in a buffer that optval points to.
Before the call, you should supply in *optlen_ptr the
size of this buffer; on return, it contains the number of bytes of
information actually stored in the buffer.
Most options interpret the optval buffer as a single int
value.
The actual return value of getsockopt is 0 on success
and -1 on failure. The following errno error conditions
are defined:
EBADF
ENOTSOCK
ENOPROTOOPT
Function: int setsockopt (int socket, int level, int optname, void *optval, size_t optlen)
This function is used to set the socket option optname at level level for socket socket. The value of the option is passed in the buffer optval, which has size optlen.
The return value and error codes are the same as for getsockopt.
Use this constant as the level argument to getsockopt or
setsockopt to manipulate the socket-level options described in
this section.
Here is a table of socket-level option names; all are defined in the header file `sys/socket.h'.
SO_DEBUG
This option toggles recording of debugging information in the underlying
protocol modules. The value has type int; a nonzero value means
"yes".
SO_REUSEADDR
bind (see section Setting a Socket's Address)
should permit reuse of local addresses for this socket. If you enable
this option, you can actually have two sockets with the same Internet
port number; but the system won't allow you to use the two
identically-named sockets in a way that would confuse the Internet. The
reason for this option is that some higher-level Internet protocols,
including FTP, require you to keep reusing the same socket number.
The value has type int; a nonzero value means "yes".
SO_KEEPALIVE
int; a nonzero value means
"yes".
SO_DONTROUTE
int; a nonzero
value means "yes".
SO_LINGER
struct linger.
This structure type has the following members:
int l_onoff
close
blocks until the data is transmitted or the timeout period has expired.
int l_linger
int; a nonzero value means "yes".
read or recv without specifying the MSG_OOB
flag. See section Out-of-Band Data. The value has type int; a
nonzero value means "yes".
size_t, which is the size in bytes.
size_t, which is the size in bytes.
getsockopt only. It is used to
get the socket's communication style. SO_TYPE is the
historical name, and SO_STYLE is the preferred name in GNU.
The value has type int and its value designates a communication
style; see section Communication Styles.
This option can be used with getsockopt only. It is used to reset
the error status of the socket. The value is an int, which represents
the previous error status.
Many systems come with a database that records a list of networks known
to the system developer. This is usually kept either in the file
`/etc/networks' or in an equivalent from a name server. This data
base is useful for routing programs such as route, but it is not
useful for programs that simply communicate over the network. We
provide functions to access this data base, which are declared in
`netdb.h'.
This data type is used to represent information about entries in the networks database. It has the following members:
char *n_name
char **n_aliases
int n_addrtype
AF_INET for Internet networks.
unsigned long int n_net
Use the getnetbyname or getnetbyaddr functions to search
the networks database for information about a specific network. The
information is returned in a statically-allocated structure; you must
copy the information if you need to save it.
Function: struct netent * getnetbyname (const char *name)
The getnetbyname function returns information about the network
named name. It returns a null pointer if there is no such
network.
Function: struct netent * getnetbyaddr (long net, int type)
The getnetbyaddr function returns information about the network
of type type with number net. You should specify a value of
AF_INET for the type argument for Internet networks.
getnetbyaddr returns a null pointer if there is no such
network.
You can also scan the networks database using setnetent,
getnetent, and endnetent. Be careful in using these
functions, because they are not reentrant.
Function: void setnetent (int stayopen)
This function opens and rewinds the networks database.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to getnetbyname or getnetbyaddr will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
Function: struct netent * getnetent (void)
This function returns the next entry in the networks database. It returns a null pointer if there are no more entries.
Function: void endnetent (void)
This function closes the networks database.
This chapter describes functions that are specific to terminal devices. You can use these functions to do things like turn off input echoing; set serial line characteristics such as line speed and flow control; and change which characters are used for end-of-file, command-line editing, sending signals, and similar control functions.
Most of the functions in this chapter operate on file descriptors. See section Low-Level Input/Output, for more information about what a file descriptor is and how to open a file descriptor for a terminal device.
The functions described in this chapter only work on files that
correspond to terminal devices. You can find out whether a file
descriptor is associated with a terminal by using the isatty
function.
Prototypes for both isatty and ttyname are declared in
the header file `unistd.h'.
Function: int isatty (int filedes)
This function returns 1 if filedes is a file descriptor
associated with an open terminal device, and 0 otherwise.
If a file descriptor is associated with a terminal, you can get its
associated file name using the ttyname function. See also the
ctermid function, described in section Identifying the Controlling Terminal.
Function: char * ttyname (int filedes)
If the file descriptor filedes is associated with a terminal
device, the ttyname function returns a pointer to a
statically-allocated, null-terminated string containing the file name of
the terminal file. The value is a null pointer if the file descriptor
isn't associated with a terminal, or the file name cannot be determined.
Many of the remaining functions in this section refer to the input and output queues of a terminal device. These queues implement a form of buffering within the kernel independent of the buffering implemented by I/O streams (see section Input/Output on Streams).
The terminal input queue is also sometimes referred to as its typeahead buffer. It holds the characters that have been received from the terminal but not yet read by any process.
The size of the terminal's input queue is described by the
_POSIX_MAX_INPUT and MAX_INPUT parameters; see section Limits on File System Capacity. If input flow control is enabled by setting the
IXOFF input mode bit (see section Input Modes), the terminal driver
transmits STOP and START characters to the terminal when necessary to
prevent the queue from overflowing. Otherwise, input may be lost if it
comes in too fast from the terminal. (This is unlikely if you are
typing the input by hand!)
The terminal output queue is like the input queue, but for output;
it contains characters that have been written by processes, but not yet
transmitted to the terminal. If output flow control is enabled by
setting the IXON input mode bit (see section Input Modes), the
terminal driver obeys STOP and STOP characters sent by the terminal to
stop and restart transmission of output.
Clearing the terminal input queue means discarding any characters that have been received but not yet read. Similarly, clearing the terminal output queue means discarding any characters that have been written but not yet transmitted.
POSIX systems support two basic modes of input: canonical and noncanonical.
In canonical input processing mode, terminal input is processed in
lines terminated by newline ('\n'), EOF, or EOL characters. No
input can be read until an entire line has been typed by the user, and
the read function (see section Input and Output Primitives) returns at most a
single line of input, no matter how many bytes are requested.
In canonical input mode, the operating system provides input editing facilities: the ERASE and KILL characters are interpreted specially to perform editing operations within the current line of text. See section Characters for Input Editing.
The constants _POSIX_MAX_CANON and MAX_CANON parameterize
the maximum number of bytes which may appear in a single line of
canonical input. See section Limits on File System Capacity.
In noncanonical input processing mode, characters are not grouped into lines, and ERASE and KILL processing is not performed. The granularity with which bytes are read in noncanonical input mode is controlled by the MIN and TIME settings. See section Noncanonical Input.
Most programs use canonical input mode, because this gives the user a way to edit input line by line. The usual reason to use noncanonical mode is when the program accepts single-character commands or provides its own editing facilities.
The choice of canonical or noncanonical input is controlled by the
ICANON flag in the c_lflag member of struct termios.
See section Local Modes.
This section describes the various terminal attributes that control how input and output are done. The functions, data structures, and symbolic constants are all declared in the header file `termios.h'.
The entire collection of attributes of a terminal is stored in a
structure of type struct termios. This structure is used
with the functions tcgetattr and tcsetattr to read
and set the attributes.
Structure that records all the I/O attributes of a terminal. The structure includes at least the following members:
tcflag_t c_iflag
tcflag_t c_oflag
tcflag_t c_cflag
tcflag_t c_lflag
cc_t c_cc[NCCS]
The struct termios structure also contains members which
encode input and output transmission speeds, but the representation is
not specified. See section Line Speed, for how to examine and store the
speed values.
The following sections describe the details of the members of the
struct termios structure.
This is an unsigned integer type used to represent the various bit masks for terminal flags.
This is an unsigned integer type used to represent characters associated with various terminal control functions.
The value of this macro is the number of elements in the c_cc
array.
Function: int tcgetattr (int filedes, struct termios *termios_p)
This function is used to examine the attributes of the terminal device with file descriptor filedes. The attributes are returned in the structure that termios_p points to.
If successful, tcgetattr returns 0. A return value of -1
indicates an error. The following errno error conditions are
defined for this function:
EBADF
ENOTTY
Function: int tcsetattr (int filedes, int when, const struct termios *termios_p)
This function sets the attributes of the terminal device with file descriptor filedes. The new attributes are taken from the structure that termios_p points to.
The when argument specifies how to deal with input and output already queued. It can be one of the following values:
TCSANOW
TCSADRAIN
TCSAFLUSH
TCSADRAIN, but also discards any queued input.
TCSASOFT
If this function is called from a background process on its controlling
terminal, normally all processes in the process group are sent a
SIGTTOU signal, in the same way as if the process were trying to
write to the terminal. The exception is if the calling process itself
is ignoring or blocking SIGTTOU signals, in which case the
operation is performed and no signal is sent. See section Job Control.
If successful, tcsetattr returns 0. A return value of
-1 indicates an error. The following errno error
conditions are defined for this function:
EBADF
ENOTTY
EINVAL
when argument is not valid, or there is
something wrong with the data in the termios_p argument.
Although tcgetattr and tcsetattr specify the terminal
device with a file descriptor, the attributes are those of the terminal
device itself and not of the file descriptor. This means that the
effects of changing terminal attributes are persistent; if another
process opens the terminal file later on, it will see the changed
attributes even though it doesn't have anything to do with the open file
descriptor you originally specified in changing the attributes.
Similarly, if a single process has multiple or duplicated file descriptors for the same terminal device, changing the terminal attributes affects input and output to all of these file descriptors. This means, for example, that you can't open one file descriptor or stream to read from a terminal in the normal line-buffered, echoed mode; and simultaneously have another file descriptor for the same terminal that you use to read from it in single-character, non-echoed mode. Instead, you have to explicitly switch the terminal back and forth between the two modes.
When you set terminal modes, you should call tcgetattr first to
get the current modes of the particular terminal device, modify only
those modes that you are really interested in, and store the result with
tcsetattr.
It's a bad idea to simply initialize a struct termios structure
to a chosen set of attributes and pass it directly to tcsetattr.
Your program may be run years from now, on systems that support members
not documented in this manual. The way to avoid setting these members
to unreasonable values is to avoid changing them.
What's more, different terminal devices may require different mode settings in order to function properly. So you should avoid blindly copying attributes from one terminal device to another.
When a member contains a collection of independent flags, as the
c_iflag, c_oflag and c_cflag members do, even
setting the entire member is a bad idea, because particular operating
systems have their own flags. Instead, you should start with the
current value of the member and alter only the flags whose values matter
in your program, leaving any other flags unchanged.
Here is an example of how to set one flag (ISTRIP) in the
struct termios structure while properly preserving all the other
data in the structure:
int
set_istrip (int desc, int value)
{
struct termios settings;
int result;
result = tcgetattr (desc, &settings);
if (result < 0)
{
perror ("error in tcgetattr");
return 0;
}
settings.c_iflag &= ~ISTRIP;
if (value)
settings.c_iflag |= ISTRIP;
result = tcgetattr (desc, &settings);
if (result < 0)
{
perror ("error in tcgetattr");
return;
}
return 1;
}
This section describes the terminal attribute flags that control fairly low-level aspects of input processing: handling of parity errors, break signals, flow control, and RET and LFD characters.
All of these flags are bits in the c_iflag member of the
struct termios structure. The member is an integer, and you
change flags using the operators &, | and ^. Don't
try to specify the entire value for c_iflag---instead, change
only specific flags and leave the rest untouched (see section Setting Terminal Modes Properly).
INPCK
Parity checking on input processing is independent of whether parity
detection and generation on the underlying terminal hardware is enabled;
see section Control Modes. For example, you could clear the INPCK
input mode flag and set the PARENB control mode flag to ignore
parity errors on input, but still generate parity on output.
If this bit is set, what happens when a parity error is detected depends
on whether the IGNPAR or PARMRK bits are set. If neither
of these bits are set, a byte with a parity error is passed to the
application as a '\0' character.
IGNPAR
INPCK is also set.
PARMRK
INPCK is set and IGNPAR is not set.
The way erroneous bytes are marked is with two preceding bytes,
377 and 0. Thus, the program actually reads three bytes
for one erroneous byte received from the terminal.
If a valid byte has the value 0377, and ISTRIP (see below)
is not set, the program might confuse it with the prefix that marks a
parity error. So a valid byte 0377 is passed to the program as
two bytes, 0377 0377, in this case.
ISTRIP
IGNBRK
A break condition is defined in the context of asynchronous serial data transmission as a series of zero-value bits longer than a single byte.
BRKINT
IGNBRK is not set, a break condition
clears the terminal input and output queues and raises a SIGINT
signal for the foreground process group associated with the terminal.
If neither BRKINT nor IGNBRK are set, a break condition is
passed to the application as a single '\0' character if
PARMRK is not set, or otherwise as a three-character sequence
'\377', '\0', '\0'.
IGNCR
'\r') are
discarded on input. Discarding carriage return may be useful on
terminals that send both carriage return and linefeed when you type the
RET key.
ICRNL
IGNCR is not set, carriage return characters
('\r') received as input are passed to the application as newline
characters ('\n').
INLCR
'\n') received as input
are passed to the application as carriage return characters ('\r').
IXOFF
IXON
IXANY
IMAXBEL
007) to the terminal to ring the bell.
This section describes the terminal flags and fields that control how
output characters are translated and padded for display. All of these
are contained in the c_oflag member of the struct termios
structure.
The c_oflag member itself is an integer, and you change the flags
and fields using the operators &, |, and ^. Don't
try to specify the entire value for c_oflag---instead, change
only specific flags and leave the rest untouched (see section Setting Terminal Modes Properly).
If this bit is set, output data is processed in some unspecified way so
that it is displayed appropriately on the terminal device. This
typically includes mapping newline characters ('\n') onto
carriage return and linefeed pairs.
If this bit isn't set, the characters are transmitted as-is.
The following three bits are BSD features, and they have no effect on
non-BSD systems. On all systems, they are effective only if
OPOST is set.
If this bit is set, convert the newline character on output into a pair of characters, carriage return followed by linefeed.
If this bit is set, convert tab characters on output into the appropriate number of spaces to emulate a tab stop every eight columns.
If this bit is set, discard C-d characters (code 004) on
output. These characters cause many dial-up terminals to disconnect.
This section describes the terminal flags and fields that control
parameters usually associated with asynchronous serial data
transmission. These flags may not make sense for other kinds of
terminal ports (such as a network connection pseudo-terminal). All of
these are contained in the c_cflag member of the struct
termios structure.
The c_cflag member itself is an integer, and you change the flags
and fields using the operators &, |, and ^. Don't
try to specify the entire value for c_cflag---instead, change
only specific flags and leave the rest untouched (see section Setting Terminal Modes Properly).
CLOCAL
If this bit is not set and you call open without the
O_NONBLOCK flag set, open blocks until a modem
connection is established.
If this bit is not set and a modem disconnect is detected, a
SIGHUP signal is sent to the controlling process group for the
terminal (if it has one). Normally, this causes the process to exit;
see section Signal Handling. Reading from the terminal after a disconnect
causes an end-of-file condition, and writing causes an EIO error
to be returned. The terminal device must be closed and reopened to
clear the condition.
HUPCL
CREAD
CSTOPB
PARENB
If this bit is not set, no parity bit is added to output characters, and input characters are not checked for correct parity.
PARODD
PARENB is set. If PARODD is set,
odd parity is used, otherwise even parity is used.
The control mode flags also includes a field for the number of bits per
character. You can use the CSIZE macro as a mask to extract the
value, like this: settings.c_cflag & CSIZE.
CSIZE
CS5
CS6
CS7
CS8
CCTS_OFLOW
CRTS_IFLOW
MDMBUF
This section describes the flags for the c_lflag member of the
struct termios structure. These flags generally control
higher-level aspects of input processing than the input modes flags
described in section Input Modes, such as echoing, signals, and the choice
of canonical or noncanonical input.
The c_lflag member itself is an integer, and you change the flags
and fields using the operators &, |, and ^. Don't
try to specify the entire value for c_lflag---instead, change
only specific flags and leave the rest untouched (see section Setting Terminal Modes Properly).
ICANON
ECHO
ECHOE
This bit only controls the display behavior; the ICANON bit by
itself controls actual recognition of the ERASE character and erasure of
input, without which ECHOE is simply irrelevant.
ECHOK
If this bit is not set, the KILL character echoes just as it would if it were not the KILL character. Then it is up to the user to remember that the KILL character has erased the preceding input; there is no indication of this on the screen.
This bit only controls the display behavior; the ICANON bit by
itself controls actual recognition of the KILL character and erasure of
input, without which ECHOK is simply irrelevant.
ECHONL
ICANON bit is also set, then the
newline ('\n') character is echoed even if the ECHO bit
is not set.
ISIG
You should use caution when disabling recognition of these characters. Programs that cannot be interrupted interactively are very user-unfriendly. If you clear this bit, your program should provide some alternate interface that allows the user to interactively send the signals associated with these characters, or to escape from the program.
See section Characters that Cause Signals.
IEXTEN
ISIG, but controls implementation-defined
special characters. If it is set, it might override the default behavior
for the ICANON and ISIG local mode flags, and the IXON
and IXOFF input mode flags.
NOFLSH
TOSTOP
SIGTTOU signals are generated by background processes that
attempt to write to the terminal. See section Access to the Controlling Terminal.
The following bits are BSD extensions; the GNU library defines these symbols on any system if you ask for them, but the settings of the bits have no effect except on BSD systems.
ECHOKE
ECHOK is set. If
ECHOKE is set, then the KILL character erases the whole screen
line; otherwise, the KILL character moves to the next screen line.
The setting of ECHOKE has no effect when ECHOK is clear.
ECHOPRT
ECHOCTL
ALTWERASE
If this bit is clear, then the beginning of a word is a nonwhitespace character following a whitespace character. If the bit is set, then the beginning of a word is an alphanumeric character or underscore following a character which is none of those.
FLUSHO
NOKERNINFO
PENDIN
The terminal line speed tells the computer how fast to read and write data on the terminal.
If the terminal is connected to a real serial line, the terminal speed you specify actually controls the line--if it doesn't match the terminal's own idea of the speed, communication does not work. Real serial ports accept only certain standard speeds. Also, particular hardware may not support even all the standard speeds. Specifying a speed of zero hangs up a dialup connection and turns off modem control signals.
If the terminal is not a real serial line (for example, if it is a network connection), then the line speed won't really affect data transmission speed, but some programs will use it to determine the amount of padding needed. It's best to specify a line speed value that matches the actual speed of the actual terminal, but you can safely experiment with different values to vary the amount of padding.
There are actually two line speeds for each terminal, one for input and one for output. You can set them independently, but most often terminals use the same speed for both directions.
The speed values are stored in the struct termios structure, but
don't try to access them in the struct termios structure
directly. Instead, you should use the following functions to read and
store them:
Function: speed_t cfgetospeed (const struct termios *termios_p)
This function returns the output line speed stored in the structure
*termios_p.
Function: speed_t cfgetispeed (const struct termios *termios_p)
This function returns the input line speed stored in the structure
*termios_p.
Function: int cfsetospeed (struct termios *termios_p, speed_t speed)
This function stores speed in *termios_p as the output
speed. The normal return value is 0; a value of -1
indicates an error. If speed is not a speed, cfsetospeed
returns -1.
Function: int cfsetispeed (struct termios *termios_p, speed_t speed)
This function stores speed in *termios_p as the input
speed. The normal return value is 0; a value of -1
indicates an error. If speed is not a speed, cfsetospeed
returns -1.
Function: int cfsetspeed (struct termios *termios_p, speed_t speed)
This function stores speed in *termios_p as both the
input and output speeds. The normal return value is 0; a value
of -1 indicates an error. If speed is not a speed,
cfsetspeed returns -1. This function is an extension in
4.4 BSD.
The speed_t type is an unsigned integer data type used to
represent line speeds.
The functions cfsetospeed and cfsetispeed report errors
only for speed values that the system simply cannot handle. If you
specify a speed value that is basically acceptable, then those functions
will succeed. But they do not check that a particular hardware device
can actually support the specified speeds--in fact, they don't know
which device you plan to set the speed for. If you use tcsetattr
to set the speed of a particular device to a value that it cannot
handle, tcsetattr returns -1.
Portability note: In the GNU library, the functions above
accept speeds measured in bits per second as input, and return speed
values measured in bits per second. Other libraries require speeds to
be indicated by special codes. For POSIX.1 portability, you must use
one of the following symbols to represent the speed; their precise
numeric values are system-dependent, but each name has a fixed meaning:
B110 stands for 110 bps, B300 for 300 bps, and so on.
There is no portable way to represent any speed but these, but these are
the only speeds that typical serial lines can support.
B0 B50 B75 B110 B134 B150 B200 B300 B600 B1200 B1800 B2400 B4800 B9600 B19200 B38400
BSD defines two additional speed symbols as aliases: EXTA is an
alias for B19200 and EXTB is an alias for B38400.
These aliases are obsolete.
Function: int cfmakeraw (struct termios *termios_p)
t->c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP
|INLCR|IGNCR|ICRNL|IXON);
t->c_oflag &= ~OPOST;
t->c_lflag &= ~(ECHO|ECHONL|ICANON|ISIG|IEXTEN);
t->c_cflag &= ~(CSIZE|PARENB);
t->c_cflag |= CS8;
In canonical input, the terminal driver recognizes a number of special
characters which perform various control functions. These include the
ERASE character (usually DEL) for editing input, and other editing
characters. The INTR character (normally C-c) for sending a
SIGINT signal, and other signal-raising characters, may be
available in either canonical or noncanonical input mode. All these
characters are described in this section.
The particular characters used are specified in the c_cc member
of the struct termios structure. This member is an array; each
element specifies the character for a particular role. Each element has
a symbolic constant that stands for the index of that element--for
example, INTR is the index of the element that specifies the INTR
character, so storing '=' in termios.c_cc[INTR]
specifies `=' as the INTR character.
On some systems, you can disable a particular special character function
by specifying the value _POSIX_VDISABLE for that role. This
value is unequal to any possible character code. See section Optional Features in File Support, for more information about how to tell whether the operating
system you are using supports _POSIX_VDISABLE.
These special characters are active only in canonical input mode. See section Two Styles of Input: Canonical or Not.
This is the subscript for the EOF character in the special control
character array. termios.c_cc[VEOF] holds the character
itself.
The EOF character is recognized only in canonical input mode. It acts
as a line terminator in the same way as a newline character, but if the
EOF character is typed at the beginning of a line it causes read
to return a byte count of zero, indicating end-of-file. The EOF
character itself is discarded.
Usually, the EOF character is C-d.
This is the subscript for the EOL character in the special control
character array. termios.c_cc[VEOL] holds the character
itself.
The EOL character is recognized only in canonical input mode. It acts as a line terminator, just like a newline character. The EOL character is not discarded; it is read as the last character in the input line.
You don't need to use the EOL character to make RET end a line. Just set the ICRNL flag. In fact, this is the default state of affairs.
This is the subscript for the ERASE character in the special control
character array. termios.c_cc[VERASE] holds the
character itself.
The ERASE character is recognized only in canonical input mode. When the user types the erase character, the previous character typed is discarded. (If the terminal generates multibyte character sequences, this may cause more than one byte of input to be discarded.) This cannot be used to erase past the beginning of the current line of text. The ERASE character itself is discarded.
Usually, the ERASE character is DEL.
This is the subscript for the KILL character in the special control
character array. termios.c_cc[VKILL] holds the character
itself.
The KILL character is recognized only in canonical input mode. When the user types the kill character, the entire contents of the current line of input are discarded. The kill character itself is discarded too.
The KILL character is usually C-u.
These special characters are active only in canonical input mode. See section Two Styles of Input: Canonical or Not. They are BSD extensions; the GNU library defines the symbols on any system if you ask for them, but the characters you specify don't actually do anything except on a BSD system.
This is the subscript for the EOL2 character in the special control
character array. termios.c_cc[VEOL2] holds the character
itself.
The EOL2 character works just like the EOL character (see above), but it can be a different character. Thus, you can specify two characters to terminate an input line, but setting EOL to one of them and EOL2 to the other.
This is the subscript for the WERASE character in the special control
character array. termios.c_cc[VWERASE] holds the character
itself.
The WERASE character is recognized only in canonical input mode. It erases an entire word of prior input.
This is the subscript for the REPRINT character in the special control
character array. termios.c_cc[VREPRINT] holds the character
itself.
The REPRINT character is recognized only in canonical input mode. It reprints the current input line.
This is the subscript for the LNEXT character in the special control
character array. termios.c_cc[VLNEXT] holds the character
itself.
The LNEXT character is recognized only when IEXTEN is set. It
disables the editing significance of the next character the user types.
It is the analogue of the C-q command in Emacs. "LNEXT" stands
for "literal next."
The LNEXT character is usually C-v.
These special characters may be active in either canonical or noncanonical
input mode, but only when the ISIG flag is set (see section Local Modes).
This is the subscript for the INTR character in the special control
character array. termios.c_cc[VINTR] holds the character
itself.
The INTR (interrupt) character raises a SIGINT signal for all
processes in the foreground job associated with the terminal. The INTR
character itself is then discarded. See section Signal Handling, for more
information about signals.
Typically, the INTR character is C-c.
This is the subscript for the QUIT character in the special control
character array. termios.c_cc[VQUIT] holds the character
itself.
The QUIT character raises a SIGQUIT signal for all processes in
the foreground job associated with the terminal. The QUIT character
itself is then discarded. See section Signal Handling, for more information
about signals.
Typically, the QUIT character is C-\.
This is the subscript for the SUSP character in the special control
character array. termios.c_cc[VSUSP] holds the character
itself.
The SUSP (suspend) character is recognized only if the implementation
supports job control (see section Job Control). It causes a SIGTSTP
signal to be sent to all processes in the foreground job associated with
the terminal. The SUSP character itself is then discarded.
See section Signal Handling, for more information about signals.
Typically, the SUSP character is C-z.
Few applications disable the normal interpretation of the SUSP
character. If your program does this, it should provide some other
mechanism for the user to stop the job. When the user invokes this
mechanism, the program should send a SIGTSTP signal to the
process group of the process, not just to the process itself.
See section Signaling Another Process.
This is the subscript for the DSUSP character in the special control
character array. termios.c_cc[VDSUSP] holds the character
itself.
The DSUSP (suspend) character is recognized only if the implementation
supports job control (see section Job Control). It sends a SIGTSTP
signal, like the SUSP character, but not right away--only when the
program tries to read it as input. Not all systems with job control
support DSUSP; only BSD systems.
See section Signal Handling, for more information about signals.
Typically, the DSUSP character is C-y.
These special characters may be active in either canonical or noncanonical
input mode, but their use is controlled by the flags IXON and
IXOFF (see section Input Modes).
This is the subscript for the START character in the special control
character array. termios.c_cc[VSTART] holds the
character itself.
The START character is used to support the IXON and IXOFF
input modes. If IXON is set, receiving a START character resumes
suspended output; the START character itself is discarded. If
IXOFF is set, the system may also transmit START characters to
the terminal.
The usual value for the START character is C-q. You may not be able to change this value--the hardware may insist on using C-q regardless of what you specify.
This is the subscript for the STOP character in the special control
character array. termios.c_cc[VSTOP] holds the character
itself.
The STOP character is used to support the IXON and IXOFF
input modes. If IXON is set, receiving a STOP character causes
output to be suspended; the STOP character itself is discarded. If
IXOFF is set, the system may also transmit STOP characters to the
terminal, to prevent the input queue from overflowing.
The usual value for the STOP character is C-s. You may not be able to change this value--the hardware may insist on using C-s regardless of what you specify.
Here are two additional special characters that are meaningful on BSD systems.
This is the subscript for the DISCARD character in the special control
character array. termios.c_cc[VDISCARD] holds the character
itself.
The DISCARD character is recognized only when IEXTEN is set. Its
effect is to toggle the discard-output flag. When this flag is set, all
program output is discarded. Setting the flag also discards all output
currently in the output buffer.
This is the subscript for the STATUS character in the special control
character array. termios.c_cc[VSTATUS] holds the character
itself.
The STATUS character's effect is to print out a status message about how the current process is running.
The STATUS character is recognized only when canonical mode. This is a peculiar design decision, since the STATUS character's meaning has nothing to do with input, but that's the way it was done.
In noncanonical input mode, the special editing characters such as ERASE and KILL are ignored. The system facilities for the user to edit input are disabled in noncanonical mode, so that all input characters (unless they are special for signal or flow-control purposes) are passed to the application program exactly as typed. It is up to the application program to give the user ways to edit the input, if appropriate.
Noncanonical mode offers special parameters called MIN and TIME for controlling whether and how long to wait for input to be available. You can even use them to avoid ever waiting--to return immediately with whatever input is available, or with no input.
The MIN and TIME are stored in elements of the c_cc array, which
is a member of the struct termios structure. Each element of
this array has a particular role, and each element has a symbolic
constant that stands for the index of that element. VMIN and
VMAX are the names for the indices in the array of the MIN and
TIME slots.
This is the subscript for the MIN slot in the c_cc array. Thus,
termios.c_cc[VMIN] is the value itself.
The MIN slot is only meaningful in noncanonical input mode; it
specifies the minimum number of bytes that must be available in the
input queue in order for read to return.
This is the subscript for the TIME slot in the c_cc array. Thus,
termios.c_cc[VTIME] is the value itself.
The TIME slot is only meaningful in noncanonical input mode; it specifies how long to wait for input before returning, in units of 0.1 seconds.
The MIN and TIME values interact to determine the criterion for when
read should return; their precise meanings depend on which of
them are nonzero. There are four possible cases:
In this case, read always returns immediately with as many
characters as are available in the queue, up to the number requested.
If no input is immediately available, read returns a value of
zero.
In this case, read waits for time TIME for input to become
available; the availability of a single byte is enough to satisfy the
read request and cause read to return. When it returns, it
returns as many characters as are available, up to the number requested.
If no input is available before the timer expires, read returns a
value of zero.
In this case, read waits until at least MIN bytes are available
in the queue. At that time, read returns as many characters as
are available, up to the number requested. read can return more
than MIN characters if more than MIN happen to be in the queue.
In this case, TIME specifies how long to wait after each input character
to see if more input arrives. read keeps waiting until either
MIN bytes have arrived, or TIME elapses with no further input.
read can return no input if TIME elapses before the first input
character arrives. read can return more than MIN characters if
more than MIN happen to be in the queue.
What happens if MIN is 50 and you ask to read just 10 bytes?
Normally, read waits until there are 50 bytes in the buffer (or,
more generally, the wait condition described above is satisfied), and
then reads 10 of them, leaving the other 40 buffered in the operating
system for a subsequent call to read.
Portability note: On some systems, the MIN and TIME slots are actually the same as the EOF and EOL slots. This causes no serious problem because the MIN and TIME slots are used only in noncanonical input and the EOF and EOL slots are used only in canonical input, but it isn't very clean. The GNU library allocates separate slots for these uses.
These functions perform miscellaneous control actions on terminal
devices. As regards terminal access, they are treated like doing
output: if any of these functions is used by a background process on its
controlling terminal, normally all processes in the process group are
sent a SIGTTOU signal. The exception is if the calling process
itself is ignoring or blocking SIGTTOU signals, in which case the
operation is performed and no signal is sent. See section Job Control.
Function: int tcsendbreak (int filedes, int duration)
This function generates a break condition by transmitting a stream of zero bits on the terminal associated with the file descriptor filedes. The duration of the break is controlled by the duration argument. If zero, the duration is between 0.25 and 0.5 seconds. The meaning of a nonzero value depends on the operating system.
This function does nothing if the terminal is not an asynchronous serial data port.
The return value is normally zero. In the event of an error, a value
of -1 is returned. The following errno error conditions
are defined for this function:
EBADF
ENOTTY
Function: int tcdrain (int filedes)
The tcdrain function waits until all queued
output to the terminal filedes has been transmitted.
The return value is normally zero. In the event of an error, a value
of -1 is returned. The following errno error conditions
are defined for this function:
EBADF
ENOTTY
EINTR
Function: int tcflush (int filedes, int queue)
The tcflush function is used to clear the input and/or output
queues associated with the terminal file filedes. The queue
argument specifies which queue(s) to clear, and can be one of the
following values:
TCIFLUSH
Clear any input data received, but not yet read.
TCOFLUSH
Clear any output data written, but not yet transmitted.
TCIOFLUSH
Clear both queued input and output.
The return value is normally zero. In the event of an error, a value
of -1 is returned. The following errno error conditions
are defined for this function:
EBADF
ENOTTY
EINVAL
It is unfortunate that this function is named tcflush, because
the term "flush" is normally used for quite another operation--waiting
until all output is transmitted--and using it for discarding input or
output would be confusing. Unfortunately, the name tcflush comes
from POSIX and we cannot change it.
Function: int tcflow (int filedes, int action)
The tcflow function is used to perform operations relating to
XON/XOFF flow control on the terminal file specified by filedes.
The action argument specifies what operation to perform, and can be one of the following values:
TCOOFF
TCOON
TCIOFF
TCION
For more information about the STOP and START characters, see section Special Characters.
The return value is normally zero. In the event of an error, a value
of -1 is returned. The following errno error conditions
are defined for this function:
EBADF
ENOTTY
EINVAL
Here is an example program that shows how you can set up a terminal device to read single characters in noncanonical input mode, without echo.
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
/* Use this variable to remember original terminal attributes. */
struct termios saved_attributes;
void
reset_input_mode (void)
{
tcsetattr (STDIN_FILENO, TCSANOW, &saved_attributes);
}
void
set_input_mode (void)
{
struct termios tattr;
char *name;
/* Make sure stdin is a terminal. */
if (!isatty (STDIN_FILENO))
{
fprintf (stderr, "Not a terminal.\n");
exit (EXIT_FAILURE);
}
/* Save the terminal attributes so we can restore them later. */
tcgetattr (STDIN_FILENO, &saved_attributes);
atexit (reset_input_mode);
/* Set the funny terminal modes. */
tcgetattr (STDIN_FILENO, &tattr);
tattr.c_lflag &= ~(ICANON|ECHO); /* Clear ICANON and ECHO. */
tattr.c_cc[VMIN] = 1;
tattr.c_cc[VTIME] = 0;
tcsetattr (STDIN_FILENO, TCSAFLUSH, &tattr);
}
int
main (void)
{
char c;
set_input_mode ();
while (1)
{
read (STDIN_FILENO, &c, 1);
if (c == '\004') /* C-d */
break;
else
putchar (c);
}
return EXIT_SUCCESS;
}
This program is careful to restore the original terminal modes before
exiting or terminating with a signal. It uses the atexit
function (see section Cleanups on Exit) to make sure this is done
by exit.
The signals handled in the example are the ones that typically occur due to actions of the user. It might be desirable to handle other signals such as SIGSEGV that can result from bugs in the program.
The shell is supposed to take care of resetting the terminal modes when a process is stopped or continued; see section Job Control. But some existing shells do not actually do this, so you may wish to establish handlers for job control signals that reset terminal modes. The above example does so.
This chapter contains information about functions for performing mathematical computations, such as trigonometric functions. Most of these functions have prototypes declared in the header file `math.h'.
All of the functions that operate on floating-point numbers accept
arguments and return results of type double. In the future,
there may be additional functions that operate on float and
long double values. For example, cosf and cosl
would be versions of the cos function that operate on
float and long double arguments, respectively. In the
meantime, you should avoid using these names yourself. See section Reserved Names.
Many of the functions listed in this chapter are defined mathematically
over a domain that is only a subset of real numbers. For example, the
acos function is defined over the domain between -1 and
1. If you pass an argument to one of these functions that is
outside the domain over which it is defined, the function sets
errno to EDOM to indicate a domain error. On
machines that support IEEE floating point, functions reporting error
EDOM also return a NaN.
Some of these functions are defined mathematically to result in a complex value over parts of their domains. The most familiar example of this is taking the square root of a negative number. The functions in this chapter take only real arguments and return only real values; therefore, if the value ought to be nonreal, this is treated as a domain error.
A related problem is that the mathematical result of a function may not
be representable as a floating point number. If magnitude of the
correct result is too large to be represented, the function sets
errno to ERANGE to indicate a range error, and
returns a particular very large value (named by the macro
HUGE_VAL) or its negation (- HUGE_VAL).
If the magnitude of the result is too small, a value of zero is returned
instead. In this case, errno might or might not be
set to ERANGE.
The only completely reliable way to check for domain and range errors is
to set errno to 0 before you call the mathematical function
and test errno afterward. As a consequence of this use of
errno, use of the mathematical functions is not reentrant if you
check for errors.
None of the mathematical functions ever generates signals as a result of
domain or range errors. In particular, this means that you won't see
SIGFPE signals generated within these functions. (See section Signal Handling, for more information about signals.)
An expression representing a particular very large number. On machines that use IEEE floating point format, the value is "infinity". On other machines, it's typically the largest positive number that can be represented.
The value of this macro is used as the return value from various mathematical functions in overflow situations.
For more information about floating-point representations and limits,
see section Floating Point Parameters. In particular, the macro
DBL_MAX might be more appropriate than HUGE_VAL for many
uses other than testing for an error in a mathematical function.
These are the familiar sin, cos, and tan functions.
The arguments to all of these functions are in units of radians; recall
that pi radians equals 180 degrees.
The math library doesn't define a symbolic constant for pi, but you can define your own if you need one:
#define PI 3.14159265358979323846264338327
You can also compute the value of pi with the expression acos
(-1.0).
Function: double sin (double x)
This function returns the sine of x, where x is given in
radians. The return value is in the range -1 to 1.
Function: double cos (double x)
This function returns the cosine of x, where x is given in
radians. The return value is in the range -1 to 1.
Function: double tan (double x)
This function returns the tangent of x, where x is given in radians.
The following errno error conditions are defined for this function:
ERANGE
tan sets errno to ERANGE and returns
either positive or negative HUGE_VAL.
These are the usual arc sine, arc cosine and arc tangent functions, which are the inverses of the sine, cosine and tangent functions, respectively.
Function: double asin (double x)
This function computes the arc sine of x---that is, the value whose
sine is x. The value is in units of radians. Mathematically,
there are infinitely many such values; the one actually returned is the
one between -pi/2 and pi/2 (inclusive).
asin fails, and sets errno to EDOM, if x is
out of range. The arc sine function is defined mathematically only
over the domain -1 to 1.
Function: double acos (double x)
This function computes the arc cosine of x---that is, the value
whose cosine is x. The value is in units of radians.
Mathematically, there are infinitely many such values; the one actually
returned is the one between 0 and pi (inclusive).
acos fails, and sets errno to EDOM, if x is
out of range. The arc cosine function is defined mathematically only
over the domain -1 to 1.
Function: double atan (double x)
This function computes the arc tangent of x---that is, the value
whose tangent is x. The value is in units of radians.
Mathematically, there are infinitely many such values; the one actually
returned is the one between -pi/2 and pi/2
(inclusive).
Function: double atan2 (double y, double x)
This is the two argument arc tangent function. It is similar to computing
the arc tangent of y/x, except that the signs of both arguments
are used to determine the quadrant of the result, and x is
permitted to be zero. The return value is given in radians and is in
the range -pi to pi, inclusive.
If x and y are coordinates of a point in the plane,
atan2 returns the signed angle between the line from the origin
to that point and the x-axis. Thus, atan2 is useful for
converting Cartesian coordinates to polar coordinates. (To compute the
radial coordinate, use hypot; see section Exponentiation and Logarithms.)
The function atan2 sets errno to EDOM if both
x and y are zero; the return value is not defined in this
case.
Function: double exp (double x)
The exp function returns the value of e (the base of natural
logarithms) raised to power x.
The function fails, and sets errno to ERANGE, if the
magnitude of the result is too large to be representable.
Function: double log (double x)
This function returns the natural logarithm of x. exp (log
(x)) equals x, exactly in mathematics and approximately in
C.
The following errno error conditions are defined for this function:
EDOM
ERANGE
Function: double log10 (double x)
This function returns the base-10 logarithm of x. Except for the
different base, it is similar to the log function. In fact,
log10 (x) equals log (x) / log (10).
Function: double pow (double base, double power)
This is a general exponentiation function, returning base raised to power.
The following errno error conditions are defined for this function:
EDOM
ERANGE
Function: double sqrt (double x)
This function returns the nonnegative square root of x.
The sqrt function fails, and sets errno to EDOM, if
x is negative. Mathematically, the square root would be a complex
number.
Function: double cbrt (double x)
This function returns the cube root of x. This function cannot fail; every representable real value has a represetable real cube root.
Function: double hypot (double x, double y)
The hypot function returns sqrt (x*x +
y*y). (This is the length of the hypotenuse of a right
triangle with sides of length x and y, or the distance
of the point (x, y) from the origin.) See also the function
cabs in section Absolute Value.
Function: double expm1 (double x)
This function returns a value equivalent to exp (x) - 1.
It is computed in a way that is accurate even if the value of x is
near zero--a case where exp (x) - 1 would be inaccurate due
to subtraction of two numbers that are nearly equal.
Function: double log1p (double x)
This function returns a value equivalent to log (1 + x).
It is computed in a way that is accurate even if the value of x is
near zero.
The functions in this section are related to the exponential functions; see section Exponentiation and Logarithms.
Function: double sinh (double x)
The sinh function returns the hyperbolic sine of x, defined
mathematically as exp (x) - exp (-x) / 2. The
function fails, and sets errno to ERANGE, if the value of
x is too large; that is, if overflow occurs.
Function: double cosh (double x)
The cosh function returns the hyperbolic cosine of x,
defined mathematically as exp (x) + exp (-x) / 2.
The function fails, and sets errno to ERANGE, if the value
of x is too large; that is, if overflow occurs.
Function: double tanh (double x)
This function returns the hyperbolic tangent of x, whose
mathematical definition is sinh (x) / cosh (x).
Function: double asinh (double x)
This function returns the inverse hyperbolic sine of x---the value whose hyperbolic sine is x.
Function: double acosh (double x)
This function returns the inverse hyperbolic cosine of x---the
value whose hyperbolic cosine is x. If x is less than
1, acosh returns HUGE_VAL.
Function: double atanh (double x)
This function returns the inverse hyperbolic tangent of x---the
value whose hyperbolic tangent is x. If the absolute value of
x is greater than or equal to 1, atanh returns
HUGE_VAL.
This section describes the GNU facilities for generating a series of pseudo-random numbers. The numbers generated are not truly random; typically, they form a sequence that repeats periodically, with a period so large that you can ignore it for ordinary purposes. The random number generator works by remembering at all times a seed value which it uses to compute the next random number and also to compute a new seed.
Although the generated numbers look unpredictable within one run of a program, the sequence of numbers is exactly the same from one run to the next. This is because the initial seed is always the same. This is convenient when you are debugging a program, but it is unhelpful if you want the program to behave unpredictably. If you want truly random numbers, not just pseudo-random, specify a seed based on the current time.
You can get repeatable sequences of numbers on a particular machine type by specifying the same initial seed value for the random number generator. There is no standard meaning for a particular seed value; the same seed, used in different C libraries or on different CPU types, will give you different random numbers.
The GNU library supports the standard ANSI C random number functions
plus another set derived from BSD. We recommend you use the standard
ones, rand and srand.
This section describes the random number functions that are part of the ANSI C standard.
To use these facilities, you should include the header file `stdlib.h' in your program.
The value of this macro is an integer constant expression that
represents the maximum possible value returned by the rand
function. In the GNU library, it is 037777777, which is the
largest signed integer representable in 32 bits. In other libraries, it
may be as low as 32767.
The rand function returns the next pseudo-random number in the
series. The value is in the range from 0 to RAND_MAX.
Function: void srand (unsigned int seed)
This function establishes seed as the seed for a new series of
pseudo-random numbers. If you call rand before a seed has been
established with srand, it uses the value 1 as a default
seed.
To produce truly random numbers (not just pseudo-random), do srand
(time (0)).
This section describes a set of random number generation functions that are derived from BSD. There is no advantage to using these functions with the GNU C library; we support them for BSD compatibility only.
The prototypes for these functions are in `stdlib.h'.
This function returns the next pseudo-random number in the sequence.
The range of values returned is from 0 to RAND_MAX.
Function: void srandom (unsigned int seed)
The srandom function sets the seed for the current random number
state based on the integer seed. If you supply a seed value
of 1, this will cause random to reproduce the default set
of random numbers.
To produce truly random numbers (not just pseudo-random), do
srandom (time (0)).
Function: void * initstate (unsigned int seed, void *state, size_t size)
The initstate function is used to initialize the random number
generator state. The argument state is an array of size
bytes, used to hold the state information. The size must be at least 8
bytes, and optimal sizes are 8, 16, 32, 64, 128, and 256. The bigger
the state array, the better.
The return value is the previous value of the state information array.
You can use this value later as an argument to setstate to
restore that state.
Function: void * setstate (void *state)
The setstate function restores the random number state
information state. The argument must have been the result of
a previous call to initstate or setstate.
The return value is the previous value of the state information array.
You can use thise value later as an argument to setstate to
restore that state.
This chapter contains information about functions for doing basic arithmetic operations, such as splitting a float into its integer and fractional parts. These functions are declared in the header file `math.h'.
The IEEE floating point format used by most modern computers supports values that are "not a number". These values are called NaNs. "Not a number" values result from certain operations which have no meaningful numeric result, such as zero divided by zero or infinity divided by infinity.
One noteworthy property of NaNs is that they are not equal to
themselves. Thus, x == x can be 0 if the value of x is a
NaN. You can use this to test whether a value is a NaN or not: if it is
not equal to itself, then it is a NaN. But the recommended way to test
for a NaN is with the isnan function (see section Predicates on Floats).
Almost any arithmetic operation in which one argument is a NaN returns a NaN.
An expression representing a value which is "not a number". This macro is a GNU extension, available only on machines that support "not a number" values--that is to say, on all machines that support IEEE floating point.
You can use `#ifdef NAN' to test whether the machine supports
NaNs. (Of course, you must arrange for GNU extensions to be visible,
such as by defining _GNU_SOURCE, and then you must include
`math.h'.)
This section describes some miscellaneous test functions on doubles.
Prototypes for these functions appear in `math.h'. These are BSD
functions, and thus are available if you define _BSD_SOURCE or
_GNU_SOURCE.
Function: int isinf (double x)
This function returns -1 if x represents negative infinity,
1 if x represents positive infinity, and 0 otherwise.
Function: int isnan (double x)
This function returns a nonzero value if x is a "not a number"
value, and zero otherwise. (You can just as well use x !=
x to get the same result).
Function: int finite (double x)
This function returns a nonzero value if x is finite or a "not a number" value, and zero otherwise.
Function: double infnan (int error)
This function is provided for compatibility with BSD. The other
mathematical functions use infnan to decide what to return on
occasion of an error. Its argument is an error code, EDOM or
ERANGE; infnan returns a suitable value to indicate this
with. -ERANGE is also acceptable as an argument, and corresponds
to -HUGE_VAL as a value.
In the BSD library, on certain machines, infnan raises a fatal
signal in all cases. The GNU library does not do likewise, because that
does not fit the ANSI C specification.
Portability Note: The functions listed in this section are BSD extensions.
These functions are provided for obtaining the absolute value (or
magnitude) of a number. The absolute value of a real number
x is x is x is positive, -x if x is
negative. For a complex number z, whose real part is x and
whose imaginary part is y, the absolute value is sqrt
(x*x + y*y).
Prototypes for abs and labs are in `stdlib.h';
fabs and cabs are declared in `math.h'.
Function: int abs (int number)
This function returns the absolute value of number.
Most computers use a two's complement integer representation, in which
the absolute value of INT_MIN (the smallest possible int)
cannot be represented; thus, abs (INT_MIN) is not defined.
Function: long int labs (long int number)
This is similar to abs, except that both the argument and result
are of type long int rather than int.
Function: double fabs (double number)
This function returns the absolute value of the floating-point number number.
Function: double cabs (struct { double real, imag; } z)
The cabs function returns the absolute value of the complex
number z, whose real part is z.real and whose
imaginary part is z.imag. (See also the function
hypot in section Exponentiation and Logarithms.) The value is:
sqrt (z.real*z.real + z.imag*z.imag)
The functions described in this section are primarily provided as a way to efficiently perform certain low-level manipulations on floating point numbers that are represented internally using a binary radix; see section Floating Point Representation Concepts. These functions are required to have equivalent behavior even if the representation does not use a radix of 2, but of course they are unlikely to be particularly efficient in those cases.
All these functions are declared in `math.h'.
Function: double frexp (double value, int *exponent)
The frexp function is used to split the number value
into a normalized fraction and an exponent.
If the argument value is not zero, the return value is value
times a power of two, and is always in the range 1/2 (inclusive) to 1
(exclusive). The corresponding exponent is stored in
*exponent; the return value multiplied by 2 raised to this
exponent equals the original number value.
For example, frexp (12.8, &exponent) returns 0.8 and
stores 4 in exponent.
If value is zero, then the return value is zero and
zero is stored in *exponent.
Function: double ldexp (double value, int exponent)
This function returns the result of multiplying the floating-point
number value by 2 raised to the power exponent. (It can
be used to reassemble floating-point numbers that were taken apart
by frexp.)
For example, ldexp (0.8, 4) returns 12.8.
The following functions which come from BSD provide facilities
equivalent to those of ldexp and frexp:
Function: double scalb (double value, int exponent)
The scalb function is the BSD name for ldexp.
Function: double logb (double x)
This BSD function returns the integer part of the base-2 logarithm of
x, an integer value represented in type double. This is
the highest integer power of 2 contained in x. The sign of
x is ignored. For example, logb (3.5) is 1.0 and
logb (4.0) is 2.0.
When 2 raised to this power is divided into x, it gives a
quotient between 1 (inclusive) and 2 (exclusive).
If x is zero, the value is minus infinity (if the machine supports such a value), or else a very small number. If x is infinity, the value is infinity.
The value returned by logb is one less than the value that
frexp would store into *exponent.
Function: double copysign (double value, double sign)
The copysign function returns a value whose absolute value is the
same as that of value, and whose sign matches that of sign.
This is a BSD function.
The functions listed here perform operations such as rounding, truncation, and remainder in division of floating point numbers. Some of these functions convert floating point numbers to integer values. They are all declared in `math.h'.
You can also convert floating-point numbers to integers simply by
casting them to int. This discards the fractional part,
effectively rounding towards zero. However, this only works if the
result can actually be represented as an int---for very large
numbers, this is impossible. The functions listed here return the
result as a double instead to get around this problem.
Function: double ceil (double x)
The ceil function rounds x upwards to the nearest integer,
returning that value as a double. Thus, ceil (1.5)
is 2.0.
Function: double floor (double x)
The ceil function rounds x downwards to the nearest
integer, returning that value as a double. Thus, floor
(1.5) is 1.0 and floor (-1.5) is -2.0.
Function: double rint (double x)
This function rounds x to an integer value according to the current rounding mode. See section Floating Point Parameters, for information about the various rounding modes. The default rounding mode is to round to the nearest integer; some machines support other modes, but round-to-nearest is always used unless you explicit select another.
Function: double modf (double value, double *integer_part)
This function breaks the argument value into an integer part and a
fractional part (between -1 and 1, exclusive). Their sum
equals value. Each of the parts has the same sign as value,
so the rounding of the integer part is towards zero.
modf stores the integer part in *integer_part, and
returns the fractional part. For example, modf (2.5, &intpart)
returns 0.5 and stores 2.0 into intpart.
Function: double fmod (double numerator, double denominator)
This function computes the remainder of dividing numerator by
denominator. Specifically, the return value is
numerator - n * denominator, where n
is the quotient of numerator divided by denominator, rounded
towards zero to an integer. Thus, fmod (6.5, 2.3) returns
1.9, which is 6.5 minus 4.6.
The result has the same sign as the numerator and has magnitude less than the magnitude of the denominator.
If denominator is zero, fmod fails and sets errno to
EDOM.
Function: double drem (double numerator, double denominator)
The function drem is like fmod except that it rounds the
internal quotient n to the nearest integer instead of towards zero
to an integer. For example, drem (6.5, 2.3) returns -0.4,
which is 6.5 minus 6.9.
The absolute value of the result is less than or equal to half the
absolute value of the denominator. The difference between
fmod (numerator, denominator) and drem
(numerator, denominator) is always either
denominator, minus denominator, or zero.
If denominator is zero, drem fails and sets errno to
EDOM.
This section describes functions for performing integer division. These
functions are redundant in the GNU C library, since in GNU C the `/'
operator always rounds towards zero. But in other C implementations,
`/' may round differently with negative arguments. div and
ldiv are useful because they specify how to round the quotient:
towards zero. The remainder has the same sign as the numerator.
These functions are specified to return a result r such that
r.quot*denominator + r.rem equals
numerator.
To use these facilities, you should include the header file `stdlib.h' in your program.
This is a structure type used to hold the result returned by the div
function. It has the following members:
int quot
int rem
Function: div_t div (int numerator, int denominator)
This function div computes the quotient and remainder from
the division of numerator by denominator, returning the
result in a structure of type div_t.
If the result cannot be represented (as in a division by zero), the behavior is undefined.
Here is an example, albeit not a very useful one.
div_t result; result = div (20, -6);
Now result.quot is -3 and result.rem is 2.
This is a structure type used to hold the result returned by the ldiv
function. It has the following members:
long int quot
long int rem
(This is identical to div_t except that the components are of
type long int rather than int.)
Function: ldiv_t ldiv (long int numerator, long int denominator)
The ldiv function is similar to div, except that the
arguments are of type long int and the result is returned as a
structure of type ldiv.
This section describes functions for "reading" integer and
floating-point numbers from a string. It may be more convenient in some
cases to use sscanf or one of the related functions; see
section Formatted Input. But often you can make a program more robust by
finding the tokens in the string by hand, then converting the numbers
one by one.
These functions are declared in `stdlib.h'.
Function: long int strtol (const char *string, char **tailptr, int base)
The strtol ("string-to-long") function converts the initial
part of string to a signed integer, which is returned as a value
of type long int.
This function attempts to decompose string as follows:
isspace function
(see section Classification of Characters). These are discarded.
If base is zero, decimal radix is assumed unless the series of digits begins with `0' (specifying octal radix), or `0x' or `0X' (specifying hexadecimal radix); in other words, the same syntax used for integer constants in C.
Otherwise base must have a value between 2 and 35.
If base is 16, the digits may optionally be preceeded by
`0x' or `0X'.
strtol stores a pointer to this tail in
*tailptr.
If the string is empty, contains only whitespace, or does not contain an
initial substring that has the expected syntax for an integer in the
specified base, no conversion is performed. In this case,
strtol returns a value of zero and the value stored in
*tailptr is the value of string.
In a locale other than the standard "C" locale, this function
may recognize additional implementation-dependent syntax.
If the string has valid syntax for an integer but the value is not
representable because of overflow, strtol returns either
LONG_MAX or LONG_MIN (see section Range of an Integer Type), as
appropriate for the sign of the value. It also sets errno
to ERANGE to indicate there was overflow.
There is an example at the end of this section.
Function: unsigned long int strtoul (const char *string, char **tailptr, int base)
The strtoul ("string-to-unsigned-long") function is like
strtol except that it returns its value with type unsigned
long int. The value returned in case of overflow is ULONG_MAX
(see section Range of an Integer Type).
Function: long int atol (const char *string)
This function is similar to the strtol function with a base
argument of 10, except that it need not detect overflow errors.
The atol function is provided mostly for compatibility with
existing code; using strtol is more robust.
Function: int atoi (const char *string)
This function is like atol, except that it returns an int
value rather than long int. The atoi function is also
considered obsolete; use strtol instead.
Here is a function which parses a string as a sequence of integers and returns the sum of them:
sum_ints_from_string (char *string)
{
int sum = 0;
while (1) {
char *tail;
int next;
/* Skip whitespace by hand, to detect the end. */
while (isspace (*string)) string++;
if (*string == 0)
break;
/* There is more nonwhitespace, */
/* so it ought to be another number. */
errno = 0;
/* Parse it. */
next = strtol (string, &tail, 0);
/* Add it in, if not overflow. */
if (errno)
printf ("Overflow\n");
else
sum += next;
/* Advance past it. */
string = tail;
}
return sum;
}
These functions are declared in `stdlib.h'.
Function: double strtod (const char *string, char **tailptr)
The strtod ("string-to-double") function converts the initial
part of string to a floating-point number, which is returned as a
value of type double.
This function attempts to decompose string as follows:
isspace function
(see section Classification of Characters). These are discarded.
*tailptr.
If the string is empty, contains only whitespace, or does not contain an
initial substring that has the expected syntax for a floating-point
number, no conversion is performed. In this case, strtod returns
a value of zero and the value returned in *tailptr is the
value of string.
In a locale other than the standard "C" locale, this function may
recognize additional locale-dependent syntax.
If the string has valid syntax for a floating-point number but the value
is not representable because of overflow, strtod returns either
positive or negative HUGE_VAL (see section Mathematics), depending on
the sign of the value. Similarly, if the value is not representable
because of underflow, strtod returns zero. It also sets errno
to ERANGE if there was overflow or underflow.
Function: double atof (const char *string)
This function is similar to the strtod function, except that it
need not detect overflow and underflow errors. The atof function
is provided mostly for compatibility with existing code; using
strtod is more robust.
This chapter describes functions for manipulating dates and times, including functions for determining what the current time is and conversion between different time representations.
The time functions fall into three main categories:
If you're trying to optimize your program or measure its efficiency, it's
very useful to be able to know how much processor time or CPU
time it has used at any given point. Processor time is different from
actual wall clock time because it doesn't include any time spent waiting
for I/O or when some other process is running. Processor time is
represented by the data type clock_t, and is given as a number of
clock ticks relative to an arbitrary base time marking the beginning
of a single program invocation.
To get the elapsed CPU time used by a process, you can use the
clock function. This facility is declared in the header file
`time.h'.
In typical usage, you call the clock function at the beginning and
end of the interval you want to time, subtract the values, and then divide
by CLOCKS_PER_SEC (the number of clock ticks per second), like this:
#include <time.h> clock_t start, end; double elapsed; start = clock(); ... /* Do the work. */ end = clock(); elapsed = ((double) (end - start)) / CLOCKS_PER_SEC;
Different computers and operating systems vary wildly in how they keep track of processor time. It's common for the internal processor clock to have a resolution somewhere between hundredths and millionths of a second.
In the GNU system, clock_t is equivalent to long int and
CLOCKS_PER_SEC is an integer value. But in other systems, both
clock_t and the type of the macro CLOCKS_PER_SEC can be
either integer or floating-point types. Casting processor time values
to double, as in the example above, makes sure that operations
such as arithmetic and printing work properly and consistently no matter
what the underlying representation is.
The value of this macro is the number of clock ticks per second measured
by the clock function.
This is an obsolete name for CLOCKS_PER_SEC.
This is the type of the value returned by the clock function.
Values of type clock_t are in units of clock ticks.
Function: clock_t clock (void)
This function returns the elapsed processor time. The base time is
arbitrary but doesn't change within a single process. If the processor
time is not available or cannot be represented, clock returns the
value (clock_t)(-1).
The times function returns more detailed information about
elapsed processor time in a struct tms object. You should
include the header file `sys/times.h' to use this facility.
The tms structure is used to return information about process
times. It contains at least the following members:
clock_t tms_utime
clock_t tms_stime
clock_t tms_cutime
tms_utime values and the tms_cutime
values of all terminated child processes of the calling process, whose
status has been reported to the parent process by wait or
waitpid; see section Process Completion. In other words, it represents
the total CPU time used in executing the instructions of all the terminated
child processes of the calling process.
clock_t tms_cstime
tms_cutime, but represents the total CPU time
used by the system on behalf of all the terminated child processes of the
calling process.
All of the times are given in clock ticks. These are absolute values; in a newly created process, they are all zero. See section Creating a Process.
Function: clock_t times (struct tms *buffer)
The times function stores the processor time information for
the calling process in buffer.
The return value is the same as the value of clock(): the elapsed
real time relative to an arbitrary base. The base is a constant within a
particular process, and typically represents the time since system
start-up. A value of (clock_t)(-1) is returned to indicate failure.
Portability Note: The clock function described in
section Basic CPU Time Inquiry, is specified by the ANSI C standard. The
times function is a feature of POSIX.1. In the GNU system, the
value returned by the clock function is equivalent to the sum of
the tms_utime and tms_stime fields returned by
times.
This section describes facilities for keeping track of dates and times according to the Gregorian calendar.
There are three representations for date and time information:
time_t data type) is a compact
representation, typically giving the number of seconds elapsed since
some implementation-specific base time.
struct
timeval data type) that includes fractions of a second. Use this time
representation instead of ordinary calendar time when you need greater
precision.
struct
tm data type) represents the date and time as a set of components
specifying the year, month, and so on, for a specific time zone.
This time representation is usually used in conjunction with formatting
date and time values.
This section describes the time_t data type for representing
calendar time, and the functions which operate on calendar time objects.
These facilities are declared in the header file `time.h'.
This is the data type used to represent calendar time. In the GNU C
library and other POSIX-compliant implementations, time_t is
equivalent to long int. When interpreted as an absolute time
value, it represents the number of seconds elapsed since 00:00:00 on
January 1, 1970, Coordinated Universal Time. (This date is sometimes
referred to as the epoch.)
In other systems, time_t might be either an integer or
floating-point type.
Function: double difftime (time_t time1, time_t time0)
The difftime function returns the number of seconds elapsed
between time time1 and time time0, as a value of type
double.
In the GNU system, you can simply subtract time_t values. But on
other systems, the time_t data type might use some other encoding
where subtraction doesn't work directly.
Function: time_t time (time_t *result)
The time function returns the current time as a value of type
time_t. If the argument result is not a null pointer, the
time value is also stored in *result. If the calendar
time is not available, the value (time_t)(-1) is returned.
The time_t data type used to represent calendar times has a
resolution of only one second. Some applications need more precision.
So, the GNU C library also contains functions which are capable of representing calendar times to a higher resolution than one second. The functions and the associated data types described in this section are declared in `sys/time.h'.
The struct timeval structure represents a calendar time. It
has the following members:
long int tv_sec
time_t value.
long int tv_usec
Some times struct timeval values are user for time intervals. Then the
tv_sec member is the number of seconds in the interval, and
tv_usec is the number of addictional microseconds.
The struct timezone structure is used to hold minimal information
about the local time zone. It has the following members:
int tz_minuteswest
int tz_dsttime
It is often necessary to subtract two values of type struct
timeval. Here is the best way to do this. It works even on some
peculiar operating systems where the tv_sec member has an
unsigned type.
/* Subtract the `struct timeval' values X and Y,
storing the result in RESULT.
Return 1 if the difference is negative, otherwise 0. */
int
timeval_subtract (result, x, y)
struct timeval *result, *x, *y;
{
/* Perform the carry for the later subtraction by updating y. */
if (x->tv_usec < y->tv_usec) {
int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
y->tv_usec -= 1000000 * nsec;
y->tv_sec += nsec;
}
if (x->tv_usec - y->tv_usec > 1000000) {
int nsec = (y->tv_usec - x->tv_usec) / 1000000;
y->tv_usec += 1000000 * nsec;
y->tv_sec -= nsec;
}
/* Compute the time remaining to wait.
tv_usec is certainly positive. */
result->tv_sec = x->tv_sec - y->tv_sec;
result->tv_usec = x->tv_usec - y->tv_usec;
/* Return 1 if result is negative. */
return x->tv_sec < y->tv_sec;
}
Function: int gettimeofday (struct timeval *tp, struct timezone *tzp)
The gettimeofday function returns the current date and time in the
struct timeval structure indicated by tp. Information about the
time zone is returned in the structure pointed at tzp. If the tzp
argument is a null pointer, time zone information is ignored.
The return value is 0 on success and -1 on failure. The
following errno error condition is defined for this function:
ENOSYS
struct timezone to represent time zone
information. Use tzname et al instead. Say something
more helpful here.
Function: int settimeofday (const struct timeval *tp, const struct timezone *tzp)
The settimeofday function sets the current date and time
according to the arguments. As for gettimeofday, time zone
information is ignored if tzp is a null pointer.
You must be a privileged user in order to use settimeofday.
The return value is 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
EPERM
ENOSYS
Function: int adjtime (const struct timeval *delta, struct timeval *olddelta)
This function speeds up or slows down the system clock in order to make gradual adjustments in the current time. This ensures that the time reported by the system clock is always monotonically increasing, which might not happen if you simply set the current time.
The delta argument specifies a relative adjustment to be made to the current time. If negative, the system clock is slowed down for a while until it has lost this much time. If positive, the system clock is speeded up for a while.
If the olddelta argument is not a null pointer, the adjtime
function returns information about any previous time adjustment that
has not yet completed.
This function is typically used to synchronize the clocks of computers
in a local network. You must be a privileged user to use it.
The return value is 0 on success and -1 on failure. The
following errno error condition is defined for this function:
EPERM
Portability Note: The gettimeofday, settimeofday,
and adjtime functions are derived from BSD.
Calender time is represented as a number of seconds. This is convenient for calculation, but has no resemblance to the way people normally represent dates and times. By contrast, broken-down time is a binary representation separated into year, month, day, and so on. Broken down time values are not useful for calculations, but they are useful for printing human readable time.
A broken-down time value is always relative to a choice of local time zone, and it also indicates which time zone was used.
The symbols in this section are declared in the header file `time.h'.
This is the data type used to represent a broken-down time. The structure contains at least the following members, which can appear in any order:
int tm_sec
0 to 59. (The actual upper limit is 61, to allow
for "leap seconds".)
int tm_min
0 to
59.
int tm_hour
0 to
23.
int tm_mday
1 to 31.
int tm_mon
0 to
11.
int tm_year
1900.
int tm_wday
0 to 6.
int tm_yday
0 to
365.
int tm_isdst
long int tm_gmtoff
timezone (see section Functions and Variables for Time Zones). You can
also think of this as the "number of seconds west" of GMT. The
tm_gmtoff field is a GNU library extension.
const char *tm_zone
Function: struct tm * localtime (const time_t *time)
The localtime function converts the calendar time pointed to by
time to broken-down time representation, expressed relative to the
user's specified time zone.
The return value is a pointer to a static broken-down time structure, which might be overwritten by subsequent calls to any of the date and time functions. (But no other library function overwrites the contents of this object.)
Calling localtime has one other effect: it sets the variable
tzname with information about the current time zone. See section Functions and Variables for Time Zones.
Function: struct tm * gmtime (const time_t *time)
This function is similar to localtime, except that the broken-down
time is expressed as Coordinated Universal Time (UTC)---that is, as
Greenwich Mean Time (GMT) rather than relative to the local time zone.
Recall that calendar times are always expressed in coordinated universal time.
Function: time_t mktime (struct tm *brokentime)
The mktime function is used to convert a broken-down time structure
to a calendar time representation. It also "normalizes" the contents of
the broken-down time structure, by filling in the day of week and day of
year based on the other date and time components.
The mktime function ignores the specified contents of the
tm_wday and tm_yday members of the broken-down time
structure. It uses the values of the other components to compute the
calendar time; it's permissible for these components to have
unnormalized values outside of their normal ranges. The last thing that
mktime does is adjust the components of the brokentime
structure (including the tm_wday and tm_yday).
If the specified broken-down time cannot be represented as a calendar time,
mktime returns a value of (time_t)(-1) and does not modify
the contents of brokentime.
Calling mktime also sets the variable tzname with
information about the current time zone. See section Functions and Variables for Time Zones.
The functions described in this section format time values as strings. These functions are declared in the header file `time.h'.
Function: char * asctime (const struct tm *brokentime)
The asctime function writes the broken-down time value pointed at by
brokentime into a string in a standard format:
"Tue May 21 13:46:22 1991\n"
The abbreviations for the days of week are: `Sun', `Mon', `Tue', `Wed', `Thu', `Fri', and `Sat'.
The abbreviations for the months are: `Jan', `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and `Dec'.
The return value points to a statically allocated string, which might be overwritten by subsequent calls to any of the date and time functions. (But no other library function overwrites the contents of this string.)
Function: char * ctime (const time_t *time)
The ctime function is similar to asctime, except that
the time value is specified in calendar time (rather than local time)
format. It is equivalent to
asctime (localtime (time))
ctime sets the variable tzname, because localtime
does so. See section Functions and Variables for Time Zones.
Function: size_t strftime (char *s, size_t size, const char *template, const struct tm *brokentime)
This function is similar to the sprintf function (see section Formatted Input), but the conversion specifications that can appear in the format
template template are specialized for printing components of the date
and time brokentime according to the locale currently specified for
time conversion (see section Locales and Internationalization).
Ordinary characters appearing in the template are copied to the output string s; this can include multibyte character sequences. Conversion specifiers are introduced by a `%' character, and are replaced in the output string as follows:
%a
%A
%b
%B
%c
%d
01 to 31).
%H
00 to
23).
%I
01 to
12).
%j
001 to 366).
%m
01 to 12).
%M
%p
%S
%U
%W
%w
0.
%x
%X
%y
00 to
99).
%Y
%Z
%%
The size parameter can be used to specify the maximum number of
characters to be stored in the array s, including the terminating
null character. If the formatted time requires more than size
characters, the excess characters are discarded. The return value from
strftime is the number of characters placed in the array s,
not including the terminating null character. If the value equals
size, it means that the array s was too small; you should
repeat the call, providing a bigger array.
For an example of strftime, see section Time Functions Example.
TZ
In the GNU system, a user can specify the time zone by means of the
TZ environment variable. For information about how to set
environment variables, see section Environment Variables. The functions for
accessing the time zone are declared in `time.h'.
The value of the TZ variable can be of one of three formats. The
first format is used when there is no Daylight Saving Time (or summer
time) in the local time zone:
std offset
The std string specifies the name of the time zone. It must be three or more characters long and must not contain a leading colon or embedded digits, commas, or plus or minus signs. There is no space character separating the time zone name from the offset, so these restrictions are necessary to parse the specification correctly.
The offset specifies the time value one must add to the local time
to get a Coordinated Universal Time value. It has syntax like
[+|-]hh[:mm[:ss]]. This
is positive if the local time zone is west of the Prime Meridian and
negative if it is east. The hour must be between 0 and
24, and the minute and seconds between 0 and 59.
For example, here is how we would specify Eastern Standard Time, but without any daylight savings time alternative:
EST+5
The second format is used when there is Daylight Saving Time:
std offset dst [offset],start[/time],end[/time]
The initial std and offset specify the standard time zone, as described above. The dst string and offset specify the name and offset for the corresponding daylight savings time time zone; if the offset is omitted, it defaults to one hour ahead of standard time.
The remainder of the specification describes when daylight savings time is in effect. The start field is when daylight savings time goes into effect and the end field is when the change is made back to standard time. The following formats are recognized for these fields:
Jn
1 and 365.
February 29 is never counted, even in leap years.
n
0 and 365.
February 29 is counted in leap years.
Mm.w.d
0 (Sunday) and 6. The week
w must be between 1 and 5; week 1 is the
first week in which day d occurs, and week 5 specifies the
last d day in the month. The month m should be
between 1 and 12.
The time fields specify when, in the local time currently in
effect, the change to the other time occurs. If omitted, the default is
02:00:00.
For example, here is how one would specify the Eastern time zone in the United States, including the appropriate daylight saving time and its dates of applicability. The normal offset from GMT is 5 hours; since this is west of the prime meridian, the sign is positive. Summer time begins on the first Sunday in April at 2:00am, and ends on the last Sunday in October at 2:00am.
EST+5EDT,M4.1.0/M10.5.0
The schedule of daylight savings time in any particular jurisdiction has changed over the years. To be strictly correct, the conversion of dates and times in the past should be based on the schedule that was in effect then. However, the system has no facilities to let you specify how the schedule has changed from year to year. The most you can do is specify one particular schedule--usually the present day schedule--and this is used to convert any date, no matter when.
The third format looks like this:
:characters
Each operating system interprets this format differently; in the GNU C library, characters is the name of a file which describes the time zone.
If the TZ environment variable does not have a value, the
operation chooses a time zone by default. Each operating system has its
own rules for choosing the default time zone, so there is little we can
say about them.
The array tzname contains two strings, which are the standard
three-letter names of the pair of time zones (standard and daylight
savings) that the user has selected. tzname[0] is the name of
the standard time zone (for example, "EST"), and tzname[1]
is the name for the time zone when daylight savings time is in use (for
example, "EDT"). These correspond to the std and dst
strings (respectively) from the TZ environment variable.
The tzname array is initialized from the TZ environment
variable whenever tzset, ctime, strftime,
mktime, or localtime is called.
The tzset function initializes the tzname variable from
the value of the TZ environment variable. It is not usually
necessary for your program to call this function, because it is called
automatically when you use the other time conversion functions that
depend on the time zone.
The following variables are defined for compatibility with System V
Unix. These variables are set by calling localtime.
This contains the difference between GMT and local standard time, in
seconds. For example, in the U.S. Eastern time zone, the value is
5*60*60.
This variable has a nonzero value if the standard U.S. daylight savings time rules apply.
Here is an example program showing the use of some of the local time and calendar time functions.
#include <time.h>
#include <stdio.h>
#define SIZE 256
int
main (void)
{
char buffer[SIZE];
time_t curtime;
struct tm *loctime;
/* Get the current time. */
curtime = time (NULL);
/* Convert it to local time representation. */
loctime = localtime (&curtime);
/* Print out the date and time in the standard format. */
fputs (asctime (loctime), stdout);
/* Print it out in a nice format. */
strftime (buffer, SIZE, "Today is %A, %B %d.\n", loctime);
fputs (buffer, stdout);
strftime (buffer, SIZE, "The time is %I:%M %p.\n", loctime);
fputs (buffer, stdout);
return 0;
}
It produces output like this:
Wed Jul 31 13:02:36 1991 Today is Wednesday, July 31. The time is 01:02 PM.
The alarm and setitimer functions provide a mechanism for a
process to interrupt itself at some future time. They do this by setting a
timer; when the timer expires, the process recieves a signal.
Each process has three independent interval timers available:
SIGALRM signal to the process when it expires.
SIGVTALRM signal to the process when it expires.
SIGPROF signal to the process when it expires.
You can only have one timer of each kind set at any given time. If you set a timer that has not yet expired, that timer is simply reset to the new value.
You should establish a handler for the appropriate alarm signal using
signal or sigaction before issuing a call to setitimer
or alarm. Otherwise, an unusual chain of events could cause the
timer to expire before your program establishes the handler, and in that
case it would be terminated, since that is the default action for the alarm
signals. See section Signal Handling.
The setitimer function is the primary means for setting an alarm.
This facility is declared in the header file `sys/time.h'. The
alarm function, declared in `unistd.h', provides a somewhat
simpler interface for setting the real-time timer.
This structure is used to specify when a timer should expire. It contains the following members:
struct timeval it_interval
struct timeval it_value
The struct timeval data type is described in section High-Resolution Calendar.
Function: int setitimer (int which, struct itimerval *old, struct itimerval *new)
The setitimer function sets the timer specified by which
according to new. The which argument can have a value of
ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF.
If old is not a null pointer, setitimer returns information
about any previous unexpired timer of the same kind in the structure it
points to.
The return value is 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
EINVAL
Function: int getitimer (int which, struct itimerval *old)
The getitimer function stores information about the timer specified
by which in the structure pointed at by old.
The return value and error conditions are the same as for setitimer.
ITIMER_REAL
setitimer and getitimer functions to specify the real-time
timer.
ITIMER_VIRTUAL
setitimer and getitimer functions to specify the virtual
timer.
ITIMER_PROF
setitimer and getitimer functions to specify the profiling
timer.
Function: unsigned int alarm (unsigned int seconds)
The alarm function sets the real-time timer to expire in
seconds seconds. If you want to cancel any existing alarm, you
can do this by calling alarm with a seconds argument of
zero.
The return value indicates how many seconds remain before the previous
alarm would have been sent. If there is no previous alarm, alarm
returns zero.
The alarm function could be defined in terms of setitimer
like this:
unsigned int
alarm (unsigned int seconds)
{
struct itimerval old, new;
new.it_interval.tv_usec = 0;
new.it_interval.tv_sec = 0;
new.it_value.tv_usec = 0;
new.it_value.tv_sec = (long int) seconds;
if (setitimer (ITIMER_REAL, &new, &old) < 0)
return 0;
else
return old.it_value.tv_sec;
}
There is an example showing the use of the alarm function in
section Signal Handlers That Return.
If you simply want your process to wait for a given number of seconds,
you should use the sleep function. See section Sleeping.
You shouldn't count on the signal arriving precisely when the timer expires. In a multiprocessing environment there is typically some amount of delay involved.
Portability Note: The setitimer and getitimer
functions are derived from BSD Unix, while the alarm function is
specified by the POSIX.1 standard. setitimer is more powerful than
alarm, but alarm is more widely used.
The function sleep gives a simple way to make the program wait
for short periods of time. If your program doesn't use signals (except
to terminate), then you can expect sleep to wait reliably for
the specified amount of time. Otherwise, sleep can return sooner
if a signal arrives; if you want to wait for a given period regardless
of signals, use select (see section Waiting for Input or Output) and don't
specify any descriptors to wait for.
Function: unsigned int sleep (unsigned int seconds)
The sleep function waits for seconds or until a signal
is delivered, whichever happens first.
If sleep function returns because the requested time has
elapsed, it returns a value of zero. If it returns because of delivery
of a signal, its return value is the remaining time in the sleep period.
The sleep function is declared in `unistd.h'.
Resist the temptation to implement a sleep for a fixed amount of time by
using the return value of sleep, when nonzero, to call
sleep again. This will work with a certain amount of accuracy as
long as signals arrive infrequently. But each signal can cause the
eventual wakeup time to be off by an additional second or so. Suppose a
few signals happen to arrive in rapid succession by bad luck--there is
no limit on how much this could shorten or lengthen the wait.
Instead, compute the time at which the program should stop waiting, and
keep trying to wait until that time. This won't be off by more than a
second. With just a little more work, you can use select and
make the waiting period quite accurate. (Of course, heavy system load
can cause unavoidable additional delays--unless the machine is
dedicated to one application, there is no way you can avoid this.)
On some systems, sleep can do strange things if your program uses
SIGALRM explicitly. Even if SIGALRM signals are being
ignored or blocked when sleep is called, sleep might
return prematurely on delivery of a SIGALRM signal. If you have
established a handler for SIGALRM signals and a SIGALRM
signal is delivered while the process is sleeping, the action taken
might be just to cause sleep to return instead of invoking your
handler. And, if sleep is interrupted by delivery of a signal
whose handler requests an alarm or alters the handling of SIGALRM,
this handler and sleep will interfere.
On the GNU system, it is safe to use sleep and SIGALRM in
the same program, because sleep does not work by means of
SIGALRM.
The function getrusage and the data type struct rusage
are used for examining the usage figures of a process. They are declared
in `sys/resource.h'.
Function: int getrusage (int processes, struct rusage *rusage)
This function reports the usage totals for processes specified by
processes, storing the information in *rusage.
In most systems, processes has only two valid values:
RUSAGE_SELF
RUSAGE_CHILDREN
In the GNU system, you can also inquire about a particular child process by specifying its process ID.
The return value of getrusage is zero for success, and -1
for failure.
EINVAL
One way of getting usage figures for a particular child process is with
the function wait4, which returns totals for a child when it
terminates. See section BSD Process Wait Functions.
This data type records a collection usage amounts for various sorts of resources. It has the following members, and possibly others:
struct timeval ru_utime
struct timeval ru_stime
long ru_majflt
long ru_inblock
long ru_oublock
long ru_msgsnd
long ru_msgrcv
long ru_nsignals
An additional historical function for examining usage figures,
vtimes, is supported but not documented here. It is declared in
`sys/vtimes.h'.
You can specify limits for the resource usage of a process. When the process tries to exceed a limit, it may get a signal, or the system call by which it tried to do so may fail, depending on the limit. Each process initially inherits its limit values from its parent, but it can subsequently change them.
The symbols in this section are defined in `sys/resource.h'.
Function: int getrlimit (int resource, struct rlimit *rlp)
Read the current value and the maximum value of resource resource
and store them in *rlp.
The return value is 0 on success and -1 on failure. The
only possible errno error condition is EFAULT.
Function: int setrlimit (int resource, struct rlimit *rlp)
Store the current value and the maximum value of resource resource
in *rlp.
The return value is 0 on success and -1 on failure. The
following errno error condition is possible:
EPERM
This structure is used with getrlimit to receive limit values,
and with setrlimit to specify limit values. It has two fields:
rlim_cur
rlim_max
In getrlimit, the structure is an output; it receives the current
values. In setrlimit, it specifies the new values.
Here is a list of resources that you can specify a limit for. Those that are sizes are measured in bytes.
RLIMIT_CPU
SIGXCPU. The value is
measured in seconds. See section Nonstandard Signals.
RLIMIT_FSIZE
SIGXFSZ. See section Nonstandard Signals.
RLIMIT_DATA
RLIMIT_STACK
SIGSEGV signal.
See section Program Error Signals.
RLIMIT_CORE
RLIMIT_RSS
RLIMIT_OPEN_FILES
EMFILE. See section Error Codes.
RLIM_NLIMITS
RLIM_NLIMITS.
This constant stands for a value of "infinity" when supplied as
the limit value in setrlimit.
Two historical functions for setting resource limits, ulimit and
vlimit, are not documented here. The latter is declared in
`sys/vlimit.h' and comes from BSD.
When several processes try to run, their respective priorities determine what share of the CPU each process gets. This section describes how you can read and set the priority of a process. All these functions and macros are declared in `sys/resource.h'.
The range of valid priority values depends on the operating system, but
typically it runs from -20 to 20. A lower priority value
means the process runs more often. These constants describe the range of
priority values:
Function: int getpriority (int class, int id)
Read the priority of a class of processes; class and id specify which ones (see below).
The return value is the priority value on success, and -1 on
failure. The following errno error condition are possible for
this function:
ESRCH
EINVAL
When the return value is -1, it could indicate failure, or it
could be the priority value. The only way to make certain is to set
errno = 0 before calling getpriority, then use errno
!= 0 afterward as the criterion for failure.
Function: int setpriority (int class, int id, int priority)
Read the priority of a class of processes; class and id specify which ones (see below).
The return value is 0 on success and -1 on failure. The
following errno error condition are defined for this function:
ESRCH
EINVAL
EPERM
EACCES
The arguments class and id together specify a set of processes you are interested in. These are the possible values for class:
PRIO_PROCESS
PRIO_PGRP
PRIO_USER
If the argument id is 0, it stands for the current process, current process group, or the current user, according to class.
Function: int nice (int increment)
Increment the priority of the current process by increment. The return value is not meaningful.
Here is an equivalent definition for nice:
int
nice (int increment)
{
int old = getpriority (PRIO_PROCESS, 0);
setpriority (PRIO_PROCESS, 0, old + increment);
}
Sometimes when your program detects an unusual situation inside a deeply
nested set of function calls, you would like to be able to immediately
return to an outer level of control. This section describes how you can
do such non-local exits using the setjmp and longjmp
functions.
As an example of a situation where a non-local exit can be useful, suppose you have an interactive program that has a "main loop" that prompts for and executes commands. Suppose the "read" command reads input from a file, doing some lexical analysis and parsing of the input while processing it. If a low-level input error is detected, it would be useful to be able to return immediately to the "main loop" instead of having to make each of the lexical analysis, parsing, and processing phases all have to explicitly deal with error situations initially detected by nested calls.
(On the other hand, if each of these phases has to do a substantial amount of cleanup when it exits--such as closing files, deallocating buffers or other data structures, and the like--then it can be more appropriate to do a normal return and have each phase do its own cleanup, because a non-local exit would bypass the intervening phases and their associated cleanup code entirely. Alternatively, you could use a non-local exit but do the cleanup explicitly either before or after returning to the "main loop".)
In some ways, a non-local exit is similar to using the `return' statement to return from a function. But while `return' abandons only a single function call, transferring control back to the point at which it was called, a non-local exit can potentially abandon many levels of nested function calls.
You identify return points for non-local exits calling the function
setjmp. This function saves information about the execution
environment in which the call to setjmp appears in an object of
type jmp_buf. Execution of the program continues normally after
the call to setjmp, but if a exit is later made to this return
point by calling longjmp with the corresponding jmp_buf
object, control is transferred back to the point where setjmp was
called. The return value from setjmp is used to distinguish
between an ordinary return and a return made by a call to
longjmp, so calls to setjmp usually appear in an `if'
statement.
Here is how the example program described above might be set up:
#include <setjmp.h>
#include <stdlib.h>
#include <stdio.h>
jmp_buf main_loop;
void
abort_to_main_loop (int status)
{
longjmp (main_loop, status);
}
int
main (void)
{
while (1)
if (setjmp (main_loop))
puts ("Back at main loop....");
else
do_command ();
}
void
do_command (void)
{
char buffer[128];
if (fgets (buffer, 128, stdin) == NULL)
abort_to_main_loop (-1);
else
exit (EXIT_SUCCESS);
}
The function abort_to_main_loop causes an immediate transfer of
control back to the main loop of the program, no matter where it is
called from.
The flow of control inside the main function may appear a little
mysterious at first, but it is actually a common idiom with
setjmp. A normal call to setjmp returns zero, so the
"else" clause of the conditional is executed. If
abort_to_main_loop is called somewhere within the execution of
do_command, then it actually appears as if the same call
to setjmp in main were returning a second time with a value
of -1.
So, the general pattern for using setjmp looks something like:
if (setjmp (buffer)) /* Code to clean up after premature return. */ ... else /* Code to be executed normally after setting up the return point. */ ...
Here are the details on the functions and data structures used for performing non-local exits. These facilities are declared in `setjmp.h'.
Objects of type jmp_buf hold the state information to
be restored by a non-local exit. The contents of a jmp_buf
identify a specific place to return to.
Macro: int setjmp (jmp_buf state)
When called normally, setjmp stores information about the
execution state of the program in state and returns zero. If
longjmp is later used to perform a non-local exit to this
state, setjmp returns a nonzero value.
Function: void longjmp (jmp_buf state, int value)
This function restores current execution to the state saved in
state, and continues execution from the call to setjmp that
established that return point. Returning from setjmp by means of
longjmp returns the value argument that was passed to
longjmp, rather than 0. (But if value is given as
0, setjmp returns 1).
There are a lot of obscure but important restrictions on the use of
setjmp and longjmp. Most of these restrictions are
present because non-local exits require a fair amount of magic on the
part of the C compiler and can interact with other parts of the language
in strange ways.
The setjmp function is actually a macro without an actual
function definition, so you shouldn't try to `#undef' it or take
its address. In addition, calls to setjmp are safe in only the
following contexts:
Return points are valid only during the dynamic extent of the function
that called setjmp to establish them. If you longjmp to
a return point that was established in a function that has already
returned, unpredictable and disastrous things are likely to happen.
You should use a nonzero value argument to longjmp. While
longjmp refuses to pass back a zero argument as the return value
from setjmp, this is intended as a safety net against accidental
misuse and is not really good programming style.
When you perform a non-local exit, accessible objects generally retain
whatever values they had at the time longjmp was called. The
exception is that the values of automatic variables local to the
function containing the setjmp call that have been changed since
the call to setjmp are indeterminate, unless you have declared
them volatile.
In BSD Unix systems, setjmp and longjmp also save and
restore the set of blocked signals; see section Blocking Signals. However,
the POSIX.1 standard requires setjmp and longjmp not to
change the set of blocked signals, and provides an additional pair of
functions (sigsetjmp and sigsetjmp) to get the BSD
behavior.
The behavior of setjmp and longjmp in the GNU library is
controlled by feature test macros; see section Feature Test Macros. The
default in the GNU system is the POSIX.1 behavior rather than the BSD
behavior.
The facilities in this section are declared in the header file `setjmp.h'.
This is similar to jmp_buf, except that it can also store state
information about the set of blocked signals.
Function: int sigsetjmp (sigjmp_buf state, int savesigs)
This is similar to setjmp. If savesigs is nonzero, the set
of blocked signals is saved in state and will be restored if a
siglongjmp is later performed with this state.
Function: void siglongjmp (sigjmp_buf state, int value)
This is similar to longjmp except for the type of its state
argument. If the sigsetjmp call that set this state used a
nonzero savesigs flag, siglongjmp also restores the set of
blocked signals.
A signal is a software interrupt delivered to a process. The operating system uses signals to report exceptional situations to an executing program. Some signals report errors such as references to invalid memory addresses; others report asynchronous events, such as disconnection of a phone line.
The GNU C library defines a variety of signal types, each for a particular kind of event. Some kinds of events make it inadvisable or impossible for the program to proceed as usual, and the corresponding signals normally abort the program. Other kinds of signals that report harmless events are ignored by default.
If you anticipate an event that causes signals, you can define a handler function and tell the operating system to run it when that particular type of signal arrives.
Finally, one process can send a signal to another process; this allows a parent process to abort a child, or two related processes to communicate and synchronize.
This section explains basic concepts of how signals are generated, what happens after a signal is delivered, and how programs can handle signals.
A signal reports the occurrence of an exceptional event. These are some of the events that can cause (or generate, or raise) a signal:
kill or raise by the same process.
kill from another process. Signals are a limited but
useful form of interprocess communication.
Each of these kinds of events (excepting explicit calls to kill
and raise) generates its own particular kind of signal. The
various kinds of signals are listed and described in detail in
section Standard Signals.
In general, the events that generate signals fall into three major categories: errors, external events, and explicit requests.
An error means that a program has done something invalid and cannot
continue execution. But not all kinds of errors generate signals--in
fact, most do not. For example, opening a nonexistant file is an error,
but it does not raise a signal; instead, open returns -1.
In general, errors that are necessarily associated with certain library
functions are reported by returning a value that indicates an error.
The errors which raise signals are those which can happen anywhere in
the program, not just in library calls. These include division by zero
and invalid memory addresses.
An external event generally has to do with I/O or other processes. These include the arrival of input, the expiration of a timer, and the termination of a child process.
An explicit request means the use of a library function such as
kill whose purpose is specifically to generate a signal.
Signals may be generated synchronously or asynchronously. A synchronous signal pertains to a specific action in the program, and is delivered (unless blocked) during that action. Errors generate signals synchronously, and so do explicit requests by a process to generate a signal for that same process.
Asynchronous signals are generated by events outside the control of the process that receives them. These signals arrive at unpredictable times during execution. External events generate signals asynchronously, and so do explicit requests that apply to some other process.
A given type of signal is either typically synchrous or typically asynchronous. For example, signals for errors are typically synchronous because errors generate signals synchronously. But any type of signal can be generated synchronously or asynchronously with an explicit request.
When a signal is generated, it becomes pending. Normally it remains pending for just a short period of time and then is delivered to the process that was signaled. However, if that kind of signal is currently blocked, it may remain pending indefinitely--until signals of that kind are unblocked. Once unblocked, it will be delivered immediately. See section Blocking Signals.
When the signal is delivered, whether right away or after a long delay,
the specified action for that signal is taken. For certain
signals, such as SIGKILL and SIGSTOP, the action is fixed,
but for most signals, the program has a choice: ignore the signal,
specify a handler function, or accept the default action for
that kind of signal. The program specifies its choice using functions
such as signal or sigaction (see section Specifying Signal Actions). We
sometimes say that a handler catches the signal. While the
handler is running, that particular signal is normally blocked.
If the specified action for a kind of signal is to ignore it, then any such signal which is generated is discarded immediately. This happens even if the signal is also blocked at the time. A signal discarded in this way will never be delivered, not even if the program subsequently specifies a different action for that kind of signal and then unblocks it.
If a signal arrives which the program has neither handled nor ignored, its default action takes place. Each kind of signal has its own default action, documented below (see section Standard Signals). For most kinds of signals, the default action is to terminate the process. For certain kinds of signals that represent "harmless" events, the default action is to do nothing.
When a signal terminates a process, its parent process can determine the
cause of termination by examining the termination status code reported
by the wait or waitpid functions. (This is discussed in
more detail in section Process Completion.) The information it can get
includes the fact that termination was due to a signal, and the kind of
signal involved. If a program you run from a shell is terminated by a
signal, the shell typically prints some kind of error message.
The signals that normally represent program errors have a special property: when one of these signals terminates the process, it also writes a core dump file which records the state of the process at the time of termination. You can examine the core dump with a debugger to investigate what caused the error.
If you raise a "program error" signal by explicit request, and this terminates the process, it makes a core dump file just as if the signal had been due directly to an error.
This section lists the names for various standard kinds of signals and describes what kind of event they mean. Each signal name is a macro which stands for a positive integer--the signal number for that kind of signal. Your programs should never make assumptions about the numeric code for a particular kind of signal, but rather refer to them always by the names defined here. This is because the number for a given kind of signal can vary from system to system, but the meanings of the names are standardized and fairly uniform.
The signal names are defined in the header file `signal.h'.
The value of this symbolic constant is the total number of signals
defined. Since the signal numbers are allocated consecutively,
NSIG is also one greater than the largest defined signal number.
The following signals are generated when a serious program error is detected by the operating system or the computer itself. In general, all of these signals are indications that your program is seriously broken in some way, and there's usually no way to continue the computation which encountered the error.
Some programs handle program error signals in order to tidy up before terminating; for example, programs that turn off echoing of terminal input should handle program error signals in order to turn echoing back on. The handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (See section Handlers That Terminate the Process.)
Termination is the sensible ultimate outcome from a program error in
most programs. However, programming systems such as Lisp that can load
compiled user programs might need to keep executing even if a user
program incurs an error. These programs have handlers which use
longjmp to return control to the command level.
The default action for all of these signals is to cause the process to
terminate. If you block or ignore these signals or establish handlers
for them that return normally, your program will probably break horribly
when such signals happen, unless they are generated by raise or
kill instead of a real error.
When one of these program error signals terminates a process, it also
writes a core dump file which records the state of the process at
the time of termination. The core dump file is named `core' and is
written in whichever directory is current in the process at the time.
(On the GNU system, you can specify the file name for core dumps with
the environment variable COREFILE.) The purpose of core dump
files is so that you can examine them with a debugger to investigate
what caused the error.
The SIGFPE signal reports a fatal arithmetic error. Although the
name is derived from "floating-point exception", this signal actually
covers all arithmetic errors, including division by zero and overflow.
If a program stores integer data in a location which is then used in a
floating-point operation, this often causes an "invalid operation"
exception, because the processor cannot recognize the data as a
floating-point number.
Actual floating-point exceptions are a complicated subject because there
are many types of exceptions with subtly different meanings, and the
SIGFPE signal doesn't distinguish between them. The IEEE
Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985)
defines various floating-point exceptions and requires conforming
computer systems to report their occurrences. However, this standard
does not specify how the exceptions are reported, or what kinds of
handling and control the operating system can offer to the programmer.
BSD systems provide the SIGFPE handler with an extra argument
that distinguishes various causes of the exception. In order to access
this argument, you must define the handler to accept two arguments,
which means you must cast it to a one-argument function type in order to
establish the handler. The GNU library does provide this extra
argument, but the value is meaningful only on operating systems that
provide the information (BSD systems and GNU systems).
FPE_INTOVF_TRAP
FPE_INTDIV_TRAP
FPE_SUBRNG_TRAP
FPE_FLTOVF_TRAP
FPE_FLTDIV_TRAP
FPE_FLTUND_TRAP
FPE_DECOVF_TRAP
The name of this signal is derived from "illegal instruction"; it
means your program is trying to execute garbage or a privileged
instruction. Since the C compiler generates only valid instructions,
SIGILL typically indicates that the executable file is corrupted,
or that you are trying to execute data. Some common ways of getting
into the latter situation are by passing an invalid object where a
pointer to a function was expected, or by writing past the end of an
automatic array (or similar problems with pointers to automatic
variables) and corrupting other data on the stack such as the return
address of a stack frame.
This signal is generated when a program tries to read or write outside the memory that is allocated for it. (Actually, the signals only occur when the program goes far enough outside to be detected by the system's memory protection mechanism.) The name is an abbreviation for "segmentation violation".
The most common way of getting a SIGSEGV condition is by
dereferencing a null or uninitialized pointer. A null pointer refers to
the address 0, and most operating systems make sure this address is
always invalid precisely so that dereferencing a null pointer will cause
SIGSEGV. (Some operating systems place valid memory at address
0, and dereferencing a null pointer does not cause a signal on these
systems.) As for uninitialized pointer variables, they contain random
addresses which may or may not be valid.
Another common way of getting into a SIGSEGV situation is when
you use a pointer to step through an array, but fail to check for the
end of the array.
This signal is generated when an invalid pointer is dereferenced. Like
SIGSEGV, this signal is typically the result of dereferencing an
uninitialized pointer. The difference between the two is that
SIGSEGV indicates an invalid access to valid memory, while
SIGBUS indicates an access to an invalid address. In particular,
SIGBUS signals often result from dereferencing a misaligned
pointer, such as referring to a four-word integer at an address not
divisible by four. (Each kind of computer has its own requirements for
address alignment.)
The name of this signal is an abbreviation for "bus error".
This signal indicates an error detected by the program itself and
reported by calling abort. See section Aborting a Program.
These signals are all used to tell a process to terminate, in one way or another. They have different names because they're used for slightly different purposes, and programs might want to handle them differently.
The reason for handling these signals is usually so your program can tidy up as appropriate before actually terminating. For example, you might want to save state information, delete temporary files, or restore the previous terminal modes. Such a handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (See section Handlers That Terminate the Process.)
The (obvious) default action for all of these signals is to cause the process to terminate.
The SIGHUP ("hang-up") signal is used to report that the user's
terminal is disconnected, perhaps because a network or telephone
connection was broken. For more information about this, see section Control Modes.
This signal is also used to report the termination of the controlling process on a terminal to jobs associated with that session; this termination effectively disconnects all processes in the session from the controlling terminal. For more information, see section Termination Internals.
The SIGINT ("program interrupt") signal is sent when the user
types the INTR character (normally C-c). See section Special Characters, for information about terminal driver support for
C-c.
The SIGQUIT signal is similar to SIGINT, except that it's
controlled by a different key--the QUIT character, usually
C-\---and produces a core dump when it terminates the process,
just like a program error signal. You can think of this as a
program error condition "detected" by the user.
See section Program Error Signals, for information about core dumps. See section Special Characters, for information about terminal driver support.
Certain kinds of cleanups are best omitted in handling SIGQUIT.
For example, if the program creates temporary files, it should handle
the other termination requests by deleting the temporary files. But it
is better for SIGQUIT not to delete them, so that the user can
examine them in conjunction with the core dump.
The SIGTERM signal is a generic signal used to cause program
termination. Unlike SIGKILL, this signal can be blocked,
handled, and ignored.
The shell command kill generates SIGTERM by default.
The SIGKILL signal is used to cause immediate program termination.
It cannot be handled or ignored, and is therefore always fatal. It is
also not possible to block this signal.
This signal is generated only by explicit request. Since it cannot be
handled, you should generate it only as a last resort, after first
trying a less drastic method such as C-c or SIGTERM. If a
process does not respond to any other termination signals, sending it a
SIGKILL signal will almost always cause it to go away.
In fact, if SIGKILL fails to terminate a process, that by itself
constitutes an operating system bug which you should report.
These signals are used to indicate the expiration of timers. See section Setting an Alarm, for information about functions that cause these signals to be sent.
The default behavior for these signals is to cause program termination. This default is rarely useful, but no other default would be useful; most of the ways of using these signals would require handler functions in any case.
This signal typically indicates expiration of a timer that measures real
or clock time. It is used by the alarm function, for example.
This signal typically indicates expiration of a timer that measures CPU time used by the current process. The name is an abbreviation for "virtual time alarm".
This signal is typically indicates expiration of a timer that measures both CPU time used by the current process, and CPU time expended on behalf of the process by the system. Such a timer is used to implement code profiling facilities, hence the name of this signal.
The signals listed in this section are used in conjunction with
asynchronous I/O facilities. You have to take explicit action by
calling fcntl to enable a particular file descriptior to generate
these signals (see section Interrupt-Driven Input). The default action for these
signals is to ignore them.
This signal is sent when a file descriptor is ready to perform input or output.
On most operating systems, terminals and sockets are the only kinds of
files that can generate SIGIO; other kinds, including ordinary
files, never generate SIGIO even if you ask them to.
This signal is sent when "urgent" or out-of-band data arrives on a socket. See section Out-of-Band Data.
These signals are used to support job control. If your system doesn't support job control, then these macros are defined but the signals themselves can't be raised or handled.
You should generally leave these signals alone unless you really understand how job control works. See section Job Control.
This signal is sent to a parent process whenever one of its child processes terminates or stops.
The default action for this signal is to ignore it. If you establish a
handler for this signal while there are child processes that have
terminated but not reported their status via wait or
waitpid (see section Process Completion), whether your new handler
applies to those processes or not depends on the particular operating
system.
You can send a SIGCONT signal to a process to make it continue.
The default behavior for this signal is to make the process continue if
it is stopped, and to ignore it otherwise.
Most programs have no reason to handle SIGCONT; they simply
resume execution without realizing they were ever stopped. You can use
a handler for SIGCONT to make a program do something special when
it is stopped and continued--for example, to reprint a prompt when it
is suspended while waiting for input.
The SIGSTOP signal stops the process. It cannot be handled,
ignored, or blocked.
The SIGTSTP signal is an interactive stop signal. Unlike
SIGSTOP, this signal can be handled and ignored.
Your program should handle this signal if you have a special need to
leave files or system tables in a secure state when a process is
stopped. For example, programs that turn off echoing should handle
SIGTSTP so they can turn echoing back on before stopping.
This signal is generated when the user types the SUSP character (normally C-z). For more information about terminal driver support, see section Special Characters.
A process cannot read from the the user's terminal while it is running
as a background job. When any process in a background job tries to
read from the terminal, all of the processes in the job are sent a
SIGTTIN signal. The default action for this signal is to
stop the process. For more information about how this interacts with
the terminal driver, see section Access to the Controlling Terminal.
This is similar to SIGTTIN, but is generated when a process in a
background job attempts to write to the terminal or set its modes.
Again, the default action is to stop the process.
While a process is stopped, no more signals can be delivered to it until
it is continued, except SIGKILL signals and (obviously)
SIGCONT signals. The SIGKILL signal always causes
termination of the process and can't be blocked or ignored. You can
block or ignore SIGCONT, but it always causes the process to
be continued anyway if it is stopped. Sending a SIGCONT signal
to a process causes any pending stop signals for that process to be
discarded. Likewise, any pending SIGCONT signals for a process
are discarded when it receives a stop signal.
When a process in an orphaned process group (see section Orphaned Process Groups) receives a SIGTSTP, SIGTTIN, or SIGTTOU
signal and does not handle it, the process does not stop. Stopping the
process would be unreasonable since there would be no way to continue
it. What happens instead depends on the operating system you are
using. Some systems may do nothing; others may deliver another signal
instead, such as SIGKILL or SIGHUP.
These signals are used to report various other conditions. The default action for all of them is to cause the process to terminate.
If you use pipes or FIFOs, you have to design your application so that
one process opens the pipe for reading before another starts writing.
If the reading process never starts, or terminates unexpectedly, writing
to the pipe or FIFO raises a SIGPIPE signal. If SIGPIPE
is blocked, handled or ignored, the offending call fails with
EPIPE instead.
Pipes and FIFO special files are discussed in more detail in section Pipes and FIFOs.
Another cause of SIGPIPE is when you try to output to a socket
that isn't connected. See section Sending Data.
The SIGUSR1 and SIGUSR2 signals are set aside for you to
use any way you want. They're useful for interprocess communication.
Since these signals are normally fatal, you should write a signal handler
for them in the program that receives the signal.
There is an example showing the use of SIGUSR1 and SIGUSR2
in section Signaling Another Process.
Particular operating systems support additional signals not listed above. The ANSI C standard reserves all identifiers beginning with `SIG' followed by an uppercase letter for the names of signals. You should consult the documentation or header files for your particular operating system and processor type to find out about the specific signals it supports.
For example, some systems support extra signals which correspond to hardware traps. Some other kinds of signals commonly supported are used to implement limits on CPU time or file system usage, asynchronous changes to terminal configuration, and the like. Systems may also define signal names that are aliases for standard signal names.
You can generally assume that the default action (or the action set up by the shell) for implementation-defined signals is reasonable, and not worry about them yourself. In fact, it's usually a bad idea to ignore or block signals you don't know anything about, or try to establish a handler for signals whose meanings you don't know.
Here are some of the other signals found on commonly used operating systems:
SIGCLD
SIGCHLD.
SIGTRAP
SIGIOT
SIGABRT.
Default action is to dump core.
SIGEMT
SIGSYS
SIGPOLL
SIGIO.
SIGXCPU
SIGXFSZ
SIGWINCH
We mentioned above that the shell prints a message describing the signal
that terminated a child process. The clean way to print a message
describing a signal is to use the functions strsignal and
psignal. These functions use a signal number to specify which
kind of signal to describe. The signal number may come from the
termination status of a child process (see section Process Completion) or it
may come from a signal handler in the same process.
Function: char * strsignal (int signum)
This function returns a pointer to a statically-allocated string containing a message describing the signal signum. You should not modify the contents of this string; and, since it can be rewritten on subsequent calls, you should save a copy of it if you need to reference it later.
This function is a GNU extension, declared in the header file `string.h'.
Function: void psignal (int signum, const char *message)
This function prints a message describing the signal signum to the
standard error output stream stderr; see section Standard Streams.
If you call psignal with a message that is either a null
pointer or an empty string, psignal just prints the message
corresponding to signum, adding a trailing newline.
If you supply a non-null message argument, then psignal
prefixes its output with this string. It adds a colon and a space
character to separate the message from the string corresponding
to signum.
This function is a BSD feature, declared in the header file `stdio.h'.
There is also an array sys_siglist which contains the messages
for the various signal codes. This array exists on BSD systems, unlike
strsignal.
The simplest way to change the action for a signal is to use the
signal function. You can specify a built-in action (such as to
ignore the signal), or you can establish a handler.
The GNU library also implements the more versatile sigaction
facility. This section describes both facilities and gives suggestions
on which to use when.
The signal function provides a simple interface for establishing
an action for a particular signal. The function and associated macros
are declared in the header file `signal.h'.
This is the type of signal handler functions. Signal handlers take one
integer argument specifying the signal number, and have return type
void. So, you should define handler functions like this:
void handler (int signum) { ... }
The name sighandler_t for this data type is a GNU extension.
Function: sighandler_t signal (int signum, sighandler_t action)
The signal function establishes action as the action for
the signal signum.
The first argument, signum, identifies the signal whose behavior you want to control, and should be a signal number. The proper way to specify a signal number is with one of the symbolic signal names described in section Standard Signals---don't use an explicit number, because the numerical code for a given kind of signal may vary from operating system to operating system.
The second argument, action, specifies the action to use for the signal signum. This can be one of the following:
SIG_DFL
SIG_DFL specifies the default action for the particular signal.
The default actions for various kinds of signals are stated in
section Standard Signals.
SIG_IGN
SIG_IGN specifies that the signal should be ignored.
Your program generally should not ignore signals that represent serious
events or that are normally used to request termination. You cannot
ignore the SIGKILL or SIGSTOP signals at all. You can
ignore program error signals like SIGSEGV, but ignoring the error
won't enable the program to continue executing meaningfully. Ignoring
user requests such as SIGINT, SIGQUIT, and SIGTSTP
is unfriendly.
When you do not wish signals to be delivered during a certain part of the program, the thing to do is to block them, not ignore them. See section Blocking Signals.
handler
For more information about defining signal handler functions, see section Defining Signal Handlers.
If you set the action for a signal to SIG_IGN, or if you set it
to SIG_DFL and the default action is to ignore that signal, then
any pending signals of that type are discarded (even if they are
blocked). Discarding the pending signals means that they will never be
delivered, not even if you subsequently specify another action and
unblock this kind of signal.
The signal function returns the action that was previously in
effect for the specified signum. You can save this value and
restore it later by calling signal again.
If signal can't honor the request, it returns SIG_ERR
instead. The following errno error conditions are defined for
this function:
EINVAL
SIGKILL or SIGSTOP.
Here is a simple example of setting up a handler to delete temporary files when certain fatal signals happen:
#include <signal.h>
void
termination_handler (int signum)
{
struct temp_file *p;
for (p = temp_file_list; p; p = p->next)
unlink (p->name);
}
int
main (void)
{
...
if (signal (SIGINT, termination_handler) == SIG_IGN)
signal (SIGINT, SIG_IGN);
if (signal (SIGHUP, termination_handler) == SIG_IGN)
signal (SIGHUP, SIG_IGN);
if (signal (SIGTERM, termination_handler) == SIG_IGN)
signal (SIGTERM, SIG_IGN);
...
}
Note how if a given signal was previously set to be ignored, this code avoids altering that setting. This is because non-job-control shells often ignore certain signals when starting children, and it is important for the children to respect this.
We do not handle SIGQUIT or the program error signals in this
example because these are designed to provide information for debugging
(a core dump), and the temporary files may give useful information.
Function: sighandler_t ssignal (int signum, sighandler_t action)
The ssignal function does the same thing as signal; it is
provided only for compatibility with SVID.
The value of this macro is used as the return value from signal
to indicate an error.
The sigaction function has the same basic effect as
signal: to specify how a signal should be handled by the process.
However, sigaction offers more control, at the expense of more
complexity. In particular, sigaction allows you to specify
additional flags to control when the signal is generated and how the
handler is invoked.
The sigaction function is declared in `signal.h'.
Structures of type struct sigaction are used in the
sigaction function to specify all the information about how to
handle a particular signal. This structure contains at least the
following members:
sighandler_t sa_handler
signal function. The value can be SIG_DFL,
SIG_IGN, or a function pointer. See section Basic Signal Handling.
sigset_t sa_mask
sa_mask. If you want that signal not to be blocked within its
handler, you must write code in the handler to unblock it.
int sa_flags
sigaction.
Function: int sigaction (int signum, const struct sigaction *action, struct sigaction *old_action)
The action argument is used to set up a new action for the signal
signum, while the old_action argument is used to return
information about the action previously associated with this symbol.
(In other words, old_action has the same purpose as the
signal function's return value--you can check to see what the
old action in effect for the signal was, and restore it later if you
want.)
Either action or old_action can be a null pointer. If old_action is a null pointer, this simply suppresses the return of information about the old action. If action is a null pointer, the action associated with the signal signum is unchanged; this allows you to inquire about how a signal is being handled without changing that handling.
The return value from sigaction is zero if it succeeds, and
-1 on failure. The following errno error conditions are
defined for this function:
EINVAL
SIGKILL or SIGSTOP.
signal and sigaction
It's possible to use both the signal and sigaction
functions within a single program, but you have to be careful because
they can interact in slightly strange ways.
The sigaction function specifies more information than the
signal function, so the return value from signal cannot
express the full range of sigaction possibilities. Therefore, if
you use signal to save and later reestablish an action, it may
not be able to reestablish properly a handler that was established with
sigaction.
To avoid having problems as a result, always use sigaction to
save and restore a handler if your program uses sigaction at all.
Since sigaction is more general, it can properly save and
reestablish any action, regardless of whether it was established
originally with signal or sigaction.
If you establish an action with signal and then examine it with
sigaction, the handler address that you get may not be the same
as what you specified with signal. It may not even be suitable
for use as an action argument with signal. But you can rely on
using it as an argument to sigaction.
So, you're better off using one or the other of the mechanisms consistently within a single program.
Portability Note: The basic signal function is a feature
of ANSI C, while sigaction is part of the POSIX.1 standard. If
you are concerned about portability to non-POSIX systems, then you
should use the signal function instead.
sigaction Function Example
In section Basic Signal Handling, we gave an example of establishing a
simple handler for termination signals using signal. Here is an
equivalent example using sigaction:
#include <signal.h>
void
termination_handler (int signum)
{
struct temp_file *p;
for (p = temp_file_list; p; p = p->next)
unlink (p->name);
}
int
main (void)
{
...
struct sigaction new_action, old_action;
/* Set up the structure to specify the new action. */
new_action.sa_handler = termination_handler;
sigemptyset (&new_action.sa_mask);
new_action.sa_flags = 0;
sigaction (SIGINT, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGINT, &new_action, NULL);
sigaction (SIGHUP, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGHUP, &new_action, NULL);
sigaction (SIGTERM, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGTERM, &new_action, NULL);
...
}
The program just loads the new_action structure with the desired
parameters and passes it in the sigaction call. The usage of
sigemptyset is described later; see section Blocking Signals.
As in the example using signal, we avoid handling signals
previously set to be ignored. Here we can avoid altering the signal
handler even momentarily, by using the feature of sigaction that
lets us examine the current action without specifying a new one.
Here is another example. It retrieves information about the current
action for SIGINT without changing that action.
struct sigaction query_action; if (sigaction (SIGINT, NULL, &query_action) < 0) /*sigactionreturns -1 in case of error. */ else if (query_action.sa_handler == SIG_DFL) /*SIGINTis handled in the default, fatal manner. */ else if (query_action.sa_handler == SIG_IGN) /*SIGINTis ignored. */ else /* A programmer-defined signal handler is in effect. */
sigaction
The sa_flags member of the sigaction structure is a
catch-all for special features. Most of the time, SA_RESTART is
a good value to use for this field.
The value of sa_flags is interpreted as a bit mask. Thus, you
should choose the flags you want to set, OR those flags together,
and store the result in the sa_flags member of your
sigaction structure.
Each signal number has its own set of flags. Each call to
sigaction affects one particular signal number, and the flags
that you specify apply only to that particular signal.
In the GNU C library, establishing a handler with signal sets all
the flags to zero except for SA_RESTART, whose value depends on
the settings you have made with siginterrupt. See section Primitives Interrupted by Signals, to see what this is about.
These macros are defined in the header file `signal.h'.
This flag is meaningful only for the SIGCHLD signal. When the
flag is set, the system delivers the signal for a terminated child
process but not for one that is stopped. By default, SIGCHLD is
delivered for both terminated children and stopped children.
Setting this flag for a signal other than SIGCHLD has no effect.
If this flag is set for a particular signal number, the system uses the signal stack when delivering that kind of signal. See section BSD Signal Handling.
This flag controls what happens when a signal is delivered during
certain primitives (such as open, read or write),
and the signal handler returns normally. There are two alternatives:
the library function can resume, or it can return failure with error
code EINTR.
The choice is controlled by the SA_RESTART flag for the
particular kind of signal that was delivered. If the flag is set,
returning from a handler resumes the library function. If the flag is
clear, returning from a handler makes the function fail.
See section Primitives Interrupted by Signals.
When a new process is created (see section Creating a Process), it inherits
handling of signals from its parent process. However, when you load a
new process image using the exec function (see section Executing a File), any signals that you've defined your own handlers for revert to
their SIG_DFL handling. (If you think about it a little, this
makes sense; the handler functions from the old program are specific to
that program, and aren't even present in the address space of the new
program image.) Of course, the new program can establish its own
handlers.
When a program is run by a shell, the shell normally sets the initial
actions for the child process to SIG_DFL or SIG_IGN, as
appropriate. It's a good idea to check to make sure that the shell has
not set up an initial action of SIG_IGN before you establish your
own signal handlers.
Here is an example of how to establish a handler for SIGHUP, but
not if SIGHUP is currently ignored:
...
struct sigaction temp;
sigaction (SIGHUP, NULL, &temp);
if (temp.sa_handler != SIG_IGN)
{
temp.sa_handler = handle_sighup;
sigemptyset (&temp.sa_mask);
sigaction (SIGHUP, &temp, NULL);
}
This section describes how to write a signal handler function that can
be established with the signal or sigaction functions.
A signal handler is just a function that you compile together with the
rest of the program. Instead of directly invoking the function, you use
signal or sigaction to tell the operating system to call
it when a signal arrives. This is known as establishing the
handler. See section Specifying Signal Actions.
There are two basic strategies you can use in signal handler functions:
You need to take special care in writing handler functions because they can be called asynchronously. That is, a handler might be called at any point in the program, unpredictably. If two signals arrive during a very short interval, one handler can run within another. This section describes what your handler should do, and what you should avoid.
Handlers which return normally are usually used for signals such as
SIGALRM and the I/O and interprocess communication signals. But
a handler for SIGINT might also return normally after setting a
flag that tells the program to exit at a convenient time.
It is not safe to return normally from the handler for a program error signal, because the behavior of the program when the handler function returns is not defined after a program error. See section Program Error Signals.
Handlers that return normally must modify some global variable in order
to have any effect. Typically, the variable is one that is examined
periodically by the program during normal operation. Its data type
should be sig_atomic_t for reasons described in section Atomic Data Access and Signal Handling.
Here is a simple example of such a program. It executes the body of
the loop until it has noticed that a SIGALRM signal has arrived.
This technique is useful because it allows the iteration in progress
when the signal arrives to complete before the loop exits.
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
/* This flag controls termination of the main loop. */
volatile sig_atomic_t keep_going = 1;
/* The signal handler just clears the flag and re-enables itself. */
void
catch_alarm (int sig)
{
keep_going = 0;
signal (sig, catch_alarm);
}
void
do_stuff (void)
{
puts ("Doing stuff while waiting for alarm....");
}
int
main (void)
{
/* Establish a handler for SIGALRM signals. */
signal (SIGALRM, catch_alarm);
/* Set an alarm to go off in a little while. */
alarm (2);
/* Check the flag once in a while to see when to quit. */
while (keep_going)
do_stuff ();
return EXIT_SUCCESS;
}
Handler functions that terminate the program are typically used to cause orderly cleanup or recovery from program error signals and interactive interrupts.
The cleanest way for a handler to terminate the process is to raise the same signal that ran the handler in the first place. Here is how to do this:
volatile sig_atomic_t fatal_error_in_progress = 0;
void
fatal_error_signal (int sig)
{
/* Since this handler is established for more than one kind of signal,
it might still get invoked recursively by delivery of some other kind
of signal. Use a static variable to keep track of that. */
if (fatal_error_in_progress)
raise (sig);
fatal_error_in_progress = 1;
/* Now do the clean up actions:
- reset terminal modes
- kill child processes
- remove lock files */
...
/* Now reraise the signal. Since the signal is blocked,
it will receive its default handling, which is
to terminate the process. We could just call
exit or abort, but reraising the signal
sets the return status from the process correctly. */
raise (sig);
}
You can do a nonlocal transfer of control out of a signal handler using
the setjmp and longjmp facilities (see section Non-Local Exits).
When the handler does a nonlocal control transfer, the part of the program that was running will not continue. If this part of the program was in the middle of updating an important data structure, the data structure will remain inconsistent. Since the program does not terminate, the inconsistency is likely to be noticed later on.
There are two ways to avoid this problem. One is to block the signal for the parts of the program that update important data structures. Blocking the signal delays its delivery until it is unblocked, once the critical updating is finished. See section Blocking Signals.
The other way to re-initialize the crucial data structures in the signal handler, or make their values consistent.
Here is a rather schematic example showing the reinitialization of one global variable.
#include <signal.h>
#include <setjmp.h>
jmp_buf return_to_top_level;
volatile sig_atomic_t waiting_for_input;
void
handle_sigint (int signum)
{
/* We may have been waiting for input when the signal arrived,
but we are no longer waiting once we transfer control. */
waiting_for_input = 0;
longjmp (return_to_top_level, 1);
}
int
main (void)
{
...
signal (SIGINT, sigint_handler);
...
while (1) {
prepare_for_command ();
if (setjmp (return_to_top_level) == 0)
read_and_execute_command ();
}
}
/* Imagine this is a subroutine used by various commands. */
char *
read_data ()
{
if (input_from_terminal) {
waiting_for_input = 1;
...
waiting_for_input = 0;
} else {
...
}
}
What happens if another signal arrives when your signal handler function is running?
When the handler for a particular signal is invoked, that signal is
normally blocked until the handler returns. That means that if two
signals of the same kind arrive close together, the second one will be
held until the first has been handled. (The handler can explicitly
unblock the signal using sigprocmask, if you want to allow more
signals of this type to arrive; see section Process Signal Mask.)
However, your handler can still be interrupted by delivery of another
kind of signal. To avoid this, you can use the sa_mask member of
the action structure passed to sigaction to explicitly specify
which signals should be blocked while the signal handler runs. These
signals are in addition to the signal for which the handler was invoked,
and any other signals that are normally blocked by the process.
See section Blocking Signals for a Handler.
Portability Note: Always use sigaction to establish a
handler for a signal that you expect to receive asynchronously, if you
want your program to work properly on System V Unix. On this system,
the handling of a signal whose handler was established with
signal automatically sets the signal's action back to
SIG_DFL, and the handler must re-establish itself each time it
runs. This practice, while inconvenient, does work when signals cannot
arrive in succession. However, if another signal can arrive right away,
it may arrive before the handler can re-establish itself. Then the
second signal would receive the default handling, which could terminate
the process.
If multiple signals of the same type are delivered to your process before your signal handler has a chance to be invoked at all, the handler may only be invoked once, as if only a single signal had arrived. In effect, the signals merge into one. This situation can arise when the signal is blocked, or in a multiprocessing environment where the system is busy running some other processes while the signals are delivered. This means, for example, that you cannot reliably use a signal handler to count signals. The only distinction you can reliably make is whether at least one signal has arrived since a given time in the past.
Here is an example of a handler for SIGCHLD that compensates for
the fact that the number of signals recieved may not equal the number of
child processes generate them. It assumes that the program keeps track
of all the child processes with a chain of structures as follows:
struct process
{
struct process *next;
/* The process ID of this child. */
int pid;
/* The descriptor of the pipe or pseudo terminal
on which output comes from this child. */
int input_descriptor;
/* Nonzero if this process has stopped or terminated. */
sig_atomic_t have_status;
/* The status of this child; 0 if running,
otherwise a status value from waitpid. */
int status;
};
struct process *process_list;
This example also uses a flag to indicate whether signals have arrived since some time in the past--whenever the program last cleared it to zero.
/* Nonzero means some child's status has changed
so look at process_list for the details. */
int process_status_change;
Here is the handler itself:
void
sigchld_handler (int signo)
{
int old_errno = errno;
while (1) {
register int pid;
int w;
struct process *p;
/* Keep asking for a status until we get a definitive result. */
do
{
errno = 0;
pid = waitpid (WAIT_ANY, &w, WNOHANG | WUNTRACED);
}
while (pid <= 0 && errno == EINTR);
if (pid <= 0) {
/* A real failure means there are no more
stopped or terminated child processes, so return. */
errno = old_errno;
return;
}
/* Find the process that signaled us, and record its status. */
for (p = process_list; p; p = p->next)
if (p->pid == pid) {
p->status = w;
/* Indicate that the status field
has data to look at. We do this only after storing it. */
p->have_status = 1;
/* If process has terminated, stop waiting for its output. */
if (WIFSIGNALED (w) || WIFEXITED (w))
if (p->input_descriptor)
FD_CLR (p->input_descriptor, &input_wait_mask);
/* The program should check this flag from time to time
to see if there is any news in process_list. */
++process_status_change;
}
/* Loop around to handle all the processes
that have something to tell us. */
}
}
Here is the proper way to check the flag process_status_change:
if (process_status_change) {
struct process *p;
process_status_change = 0;
for (p = process_list; p; p = p->next)
if (p->have_status) {
... Examine p->status ...
}
}
It is vital to clear the flag before examining the list; otherwise, if a signal were delivered just before the clearing of the flag, and after the appropriate element of the process list had been checked, the status change would go unnoticed until the next signal arrived to set the flag again. You could, of course, avoid this problem by blocking the signal while scanning the list, but it is much more elegant to guarantee correctness by doing things in the right order.
The loop which checks process status avoids examining p->status
until it sees that status has been validly stored. This is to make sure
that the status cannot change in the middle of accessing it. Once
p->have_status is set, it means that the child process is stopped
or terminated, and in either case, it cannot stop or terminate again
until the program has taken notice. See section Atomic Usage Patterns, for more
information about coping with interruptions during accessings of a
variable.
Here is another way you can test whether the handler has run since the last time you checked. This technique uses a counter which is never changed outside the handler. Instead of clearing the count, the program remembers the previous value and sees whether it has changed since the previous check. The advantage of this method is that different parts of the program can check independently, each part checking whether there has been a signal since that part last checked.
sig_atomic_t process_status_change;
sig_atomic_t last_process_status_change;
...
{
sig_atomic_t prev = last_process_status_change;
last_process_status_change = process_status_change;
if (last_process_status_change != prev) {
struct process *p;
for (p = process_list; p; p = p->next)
if (p->have_status) {
... Examine p->status ...
}
}
}
Handler functions usually don't do very much. The best practice is to
write a handler that does nothing but set an external variable that the
program checks regularly, and leave all serious work to the program.
This is best because the handler can be called at asynchronously, at
unpredictable times--perhaps in the middle of a system call, or even
between the beginning and the end of a C operator that requires multiple
instructions. The data structures being manipulated might therefore be
in an inconsistent state when the handler function is invoked. Even
copying one int variable into another can take two instructions
on most machines.
This means you have to be very careful about what you do in a signal handler.
volatile. This tells the compiler that
the value of the variable might change asynchronously, and inhibits
certain optimizations that would be invalidated by such modifications.
A function can be non-reentrant if it uses memory that is not on the stack.
For example, suppose that the signal handler uses gethostbyname.
This function returns its value in a static object, reusing the same
object each time. If the signal happens to arrive during a call to
gethostbyname, or even after one (while the program is still
using the value), it will clobber the value that the program asked for.
However, if the program does not use gethostbyname or any other
function that returns information in the same object, or if it always
blocks signals around each use, then you are safe.
There are a large number of library functions that return values in a fixed object, always reusing the same object in this fashion, and all of them cause the same problem. The description of a function in this manual always mentions this behavior.
This case arises when you do I/O using streams. Suppose that the
signal handler prints a message with fprintf. Suppose that the
program was in the middle of an fprintf call using the same
stream when the signal was delivered. Both the signal handler's message
and the program's data could be corrupted, because both calls operate on
the same data structure--the stream itself.
However, if you know that the stream that the handler uses cannot possibly be used by the program at a time when signals can arrive, then you are safe. It is no problem if the program uses some other stream.
malloc and free are not reentrant,
because they use a static data structure which records what memory
blocks are free. As a result, no library functions that allocate or
free memory are reentrant. This includes functions that allocate space
to store a result.
The best way to avoid the need to allocate memory in a handler is to allocate in advance space for signal handlers to use.
The best way to avoid freeing memory in a handler is to flag or record the objects to be freed, and have the program check from time to time whether anything is waiting to be freed. But this must be done with care, because placing an object on a chain is not atomic, and if it is interrupted by another signal handler that does the same thing, you could "lose" one of the objects.
On the GNU system, malloc and free are safe to use in
signal handlers because it blocks signals. As a result, the library
functions that allocate space for a result are also safe in signal
handlers. The obstack allocation functions are safe as long as you
don't use the same obstack both inside and outside of a signal handler.
The relocating allocation functions (see section Relocating Allocator) are certainly not safe to use in a signal handler.
errno is non-reentrant, but you can
correct for this: in the handler, save the original value of
errno and restore it before returning normally. This prevents
errors that occur within the signal handler from being confused with
errors from system calls at the point the program is interrupted to run
the handler.
This technique is generally applicable; if you want to call in a handler a function that modifies a particular object in memory, you can make this safe by saving and restoring that object.
Whether the data in your application concerns atoms, or mere text, you have to be careful about the fact that access to a single datum is not necessarily atomic. This means that it can take more than one instruction to read or write a single object. In such cases, a signal handler can run in the middle of reading or writing the object.
There are three ways you can cope with this problem. You can use data types that are always accessed atomically; you can carefully arrange that nothing untoward happens if an access is interrupted, or you can block all signals around any access that had better not be interrupted (see section Blocking Signals).
Here is an example which shows what can happen if a signal handler runs in the middle of modifying a variable. (Interrupting the reading of a variable can also lead to paradoxical results, but here we only show writing.)
#include <signal.h>
#include <stdio.h>
struct two_words { int a, b; } memory;
void
handler(int signum)
{
printf ("%d,%d\n", memory.a, memory.b);
alarm (1);
}
int
main (void)
{
static struct two_words zeros = { 0, 0 }, ones = { 1, 1 };
signal (SIGALRM, handler);
memory = zeros;
alarm (1);
while (1)
{
memory = zeros;
memory = ones;
}
}
This program fills memory with zeros, ones, zeros, ones,
alternating forever; meanwhile, once per second, the alarm signal handler
prints the current contents. (Calling printf in the handler is
safe in this program because it is certainly not being called outside
the handler when the signal happens.)
Clearly, this program can print a pair of zeros or a pair of ones. But
that's not all it can do! On most machines, it takes several
instructions to store a new value in memory, and the value is
stored one word at a time. If the signal is delivered in between these
instructions, the handler might find that memory.a is zero and
memory.b is one (or vice versa).
On some machines it may be possible to store a new value in
memory with just one instruction that cannot be interrupted. On
these machines, the handler will always print two zeros or two ones.
To avoid uncertainty about interrupting access to a variable, you can
use a particular data type for which access is always atomic:
sig_atomic_t. Reading and writing this data type is guaranteed
to happen in a single instruction, so there's no way for a handler to
run "in the middle" of an access.
The type sig_atomic_t is always an integer data type, but which
one it is, and how many bits it contains, may vary from machine to
machine.
This is an integer data type. Objects of this type are always accessed atomically.
In practice, you can assume that int and other integer types no
longer than int are atomic. You can also assume that pointer
types are atomic; that is very convenient. Both of these are true on
all of the machines that the GNU C library supports, and on all POSIX
systems we know of.
Certain patterns of access avoid any problem even if an access is interrupted. For example, a flag which is set by the handler, and tested and cleared by the main program from time to time, is always safe even if access actually requires two instructions. To show that this is so, we must consider each access that could be interrupted, and show that there is no problem if it is interrupted.
An interrupt in the middle of testing the flag is safe because either it's recognized to be nonzero, in which case the precise value doesn't matter, or it will be seen to be nonzero the next time it's tested.
An interrupt in the middle of clearing the flag is no problem because either the value ends up zero, which is what happens if a signal comes in just before the flag is cleared, or the value ends up nonzero, and subsequent events occur as if the signal had come in just after the flag was cleared. As long as the code handles both of these cases properly, it can also handle a signal in the middle of clearing the flag. (This is an example of the sort of reasoning you need to do to figure out whether non-atomic usage is safe.)
Sometimes you can insure uninterrupted access to one object by protecting its use with another object, perhaps one whose type guarantees atomicity. See section Signals Close Together Merge into One, for an example.
A signal can arrive and be handled while an I/O primitive such as
open or read is waiting for an I/O device. If the signal
handler returns, the system faces the question: what should happen next?
POSIX specifies one approach: make the primitive fail right away. The
error code for this kind of failure is EINTR. This is flexible,
but usually inconvenient. Typically, POSIX applications that use signal
handlers must check for EINTR after each library function that
can return it, in order to try the call again. Often programmers forget
to check, which is a common source of error.
The GNU library provides a convenient way to retry a call after a
temporary failure, with the macro TEMP_FAILURE_RETRY:
Macro: TEMP_FAILURE_RETRY (expression)
This macro evaluates expression once. If it fails and reports
error code EINTR, TEMP_FAILURE_RETRY evaluates it again,
and over and over until the result is not a temporary failure.
The value returned by TEMP_FAILURE_RETRY is whatever value
expression produced.
BSD avoids EINTR entirely and provides a more convenient
approach: to restart the interrupted primitive, instead of making it
fail. If you choose this approach, you need not be concerned with
EINTR.
You can choose either approach with the GNU library. If you use
sigaction to establish a signal handler, you can specify how that
handler should behave. If you specify the SA_RESTART flag,
return from that handler will resume a primitive; otherwise, return from
that handler will cause EINTR. See section Flags for sigaction.
Another way to specify the choice is with the siginterrupt
function. See section POSIX and BSD Signal Facilities.
When you don't specify with sigaction or siginterrupt what
a particular handler should do, it uses a default choice. The default
choice in the GNU library depends on the feature test macros you have
defined. If you define _BSD_SOURCE or _GNU_SOURCE before
calling signal, the default is to resume primitives; otherwise,
the default is to make them fail with EINTR. (The library
contains alternate versions of the signal function, and the
feature test macros determine which one you really call.) See section Feature Test Macros.
The primitives affected by this issue are close, fcntl
(operation F_SETLK), open, read, recv,
recvfrom, select, send, sendto,
tcdrain, waitpid, wait, and write.
There is one situation where resumption never happens no matter which
choice you make: when a data-transfer function such as read or
write is interrupted by a signal after transferring part of the
data. In this case, the function returns the number of bytes already
transferred, indicating partial success.
This might at first appear to cause unreliable behavior on
record-oriented devices (including datagram sockets; see section Datagram Socket Operations),
where splitting one read or write into two would read or
write two records. Actually, there is no problem, because interruption
after a partial transfer cannot happen on such devices; they always
transfer an entire record in one burst, with no waiting once data
transfer has started.
Besides signals that are generated as a result of a hardware trap or interrupt, your program can explicitly send signals to itself or to another process.
A process can send itself a signal with the raise function. This
function is declared in `signal.h'.
Function: int raise (int signum)
The raise function sends the signal signum to the calling
process. It returns zero if successful and a nonzero value if it fails.
About the only reason for failure would be if the value of signum
is invalid.
Function: int gsignal (int signum)
The gsignal function does the same thing as raise; it is
provided only for compatibility with SVID.
One convenient use for raise is to reproduce the default behavior
of a signal that you have trapped. For instance, suppose a user of your
program types the SUSP character (usually C-z; see section Special Characters) to send it an interactive stop stop signal
(SIGTSTP), and you want to clean up some internal data buffers
before stopping. You might set this up like this:
#include <signal.h>
/* When a stop signal arrives, set the action back to the default
and then resend the signal after doing cleanup actions. */
void
tstp_handler (int sig)
{
signal (SIGTSTP, SIG_DFL);
/* Do cleanup actions here. */
...
raise (SIGTSTP);
}
/* When the process is continued again, restore the signal handler. */
void
cont_handler (int sig)
{
signal (SIGCONT, cont_handler);
signal (SIGTSTP, tstp_handler);
}
/* Enable both handlers during program initialization. */
int
main (void)
{
signal (SIGCONT, cont_handler);
signal (SIGTSTP, tstp_handler);
...
}
Portability note: raise was invented by the ANSI C
committee. Older systems may not support it, so using kill may
be more portable. See section Signaling Another Process.
The kill function can be used to send a signal to another process.
In spite of its name, it can be used for a lot of things other than
causing a process to terminate. Some examples of situations where you
might want to send signals between processes are:
This section assumes that you know a little bit about how processes work. For more information on this subject, see section Child Processes.
The kill function is declared in `signal.h'.
Function: int kill (pid_t pid, int signum)
The kill function sends the signal signum to the process
or process group specified by pid. Besides the signals listed in
section Standard Signals, signum can also have a value of zero to
check the validity of the pid.
The pid specifies the process or process group to receive the signal:
pid > 0
pid == 0
pid < -1
pid == -1
A process can send a signal to itself with kill (getpid(),
signum);. If kill is used by a process to send a signal to
itself, and the signal is not blocked, then kill delivers at
least one signal (which might be some other pending unblocked signal
instead of the signal signum) to that process before it returns.
The return value from kill is zero if the signal can be sent
successfully. Otherwise, no signal is sent, and a value of -1 is
returned. If pid specifies sending a signal to several processes,
kill succeeds if it can send the signal to at least one of them.
There's no way you can tell which of the processes got the signal
or whether all of them did.
The following errno error conditions are defined for this function:
EINVAL
EPERM
ESCRH
Function: int killpg (int pgid, int signum)
This is similar to kill, but sends signal signum to the
process group pgid. This function is provided for compatibility
with BSD; using kill to do this is more portable.
As a simple example of kill, the call kill (getpid (),
sig) has the same effect as raise (sig).
kill
There are restrictions that prevent you from using kill to send
signals to any random process. These are intended to prevent antisocial
behavior such as arbitrarily killing off processes belonging to another
user. In typical use, kill is used to pass signals between
parent, child, and sibling processes, and in these situations you
normally do have permission to send signals. The only common execption
is when you run a setuid program in a child process; if the program
changes its real UID as well as its effective UID, you may not have
permission to send a signal. The su program does this.
Whether a process has permission to send a signal to another process is determined by the user IDs of the two processes. This concept is discussed in detail in section The Persona of a Process.
Generally, for a process to be able to send a signal to another process, either the sending process must belong to a privileged user (like `root'), or the real or effective user ID of the sending process must match the real or effective user ID of the receiving process. If the receiving process has changed its effective user ID from the set-user-ID mode bit on its process image file, then the owner of the process image file is used in place of its current effective user ID. In some implementations, a parent process might be able to send signals to a child process even if the user ID's don't match, and other implementations might enforce other restrictions.
The SIGCONT signal is a special case. It can be sent if the
sender is part of the same session as the receiver, regardless of
user IDs.
kill for CommunicationSIGUSR1 and
SIGUSR2 signals are provided for. Since these signals are fatal
by default, the process that is supposed to receive them must trap them
through signal or sigaction.
In this example, a parent process forks a child process and then waits
for the child to complete its initialization. The child process tells
the parent when it is ready by sending it a SIGUSR1 signal, using
the kill function.
#include <signal.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
/* When a SIGUSR1 signal arrives, set this variable. */
volatile sig_atomic_t usr_interrupt = 0;
void
synch_signal (int sig)
{
usr_interrupt = 1;
}
/* The child process executes this function. */
void
child_function (void)
{
/* Perform initialization. */
printf ("I'm here!!! My pid is %d.\n", (int) getpid ());
/* Let parent know you're done. */
kill (getppid (), SIGUSR1);
/* Continue with execution. */
puts ("Bye, now....");
exit (0);
}
int
main (void)
{
struct sigaction usr_action;
sigset_t block_mask;
pid_t child_id;
/* Establish the signal handler. */
sigfillset (&block_mask);
usr_action.sa_handler = synch_signal;
usr_action.sa_mask = block_mask;
usr_action.sa_flags = 0;
sigaction (SIGUSR1, &usr_action, NULL);
/* Create the child process. */
child_id = fork ();
if (child_id == 0)
child_function (); /* Does not return. */
/* Busy wait for the child to send a signal. */
while (!usr_interrupt)
;
/* Now continue execution. */
puts ("That's all, folks!");
return 0;
}
This example uses a busy wait, which is bad, because it wastes CPU cycles that other programs could otherwise use. It is better to ask the system to wait until the signal arrives. See the example in section Waiting for a Signal.
Blocking a signal means telling the operating system to hold it and
deliver it later. Generally, a program does not block signals
indefinitely--it might as well ignore them by setting their actions to
SIG_IGN. But it is useful to block signals briefly, to prevent
them from interrupting sensitive operations. For instance:
sigprocmask function to block signals while you
modify global variables that are also modified by the handlers for these
signals.
sa_mask in your sigaction call to block
certain signals while a particular signal handler runs. This way, the
signal handler can run without being interrupted itself by signals.
Temporary blocking of signals with sigprocmask gives you a way to
prevent interrupts during critical parts of your code. If signals
arrive in that part of the program, they are delivered later, after you
unblock them.
One example where this is useful is for sharing data between a signal
handler and the rest of the program. If the type of the data is not
sig_atomic_t (see section Atomic Data Access and Signal Handling), then the signal
handler could run when the rest of the program has only half finished
reading or writing the data. This would lead to confusing consequences.
To make the program reliable, you can prevent the signal handler from running while the rest of the program is examining or modifying that data--by blocking the appropriate signal around the parts of the program that touch the data.
Blocking signals is also necessary when you want to perform a certain
action only if a signal has not arrived. Suppose that the handler for
the signal sets a flag of type sig_atomic_t; you would like to
test the flag and perform the action if the flag is not set. This is
unreliable. Suppose the signal is delivered immediately after you test
the flag, but before the consequent action: then the program will
perform the action even though the signal has arrived.
The only way to test reliably for whether a signal has yet arrived is to test while the signal is blocked.
All of the signal blocking functions use a data structure called a signal set to specify what signals are affected. Thus, every activity involves two stages: creating the signal set, and then passing it as an argument to a library function.
These facilities are declared in the header file `signal.h'.
The sigset_t data type is used to represent a signal set.
Internally, it may be implemented as either an integer or structure
type.
For portability, use only the functions described in this section to
initialize, change, and retrieve information from sigset_t
objects--don't try to manipulate them directly.
There are two ways to initialize a signal set. You can initially
specify it to be empty with sigemptyset and then add specified
signals individually. Or you can specify it to be full with
sigfillset and then delete specified signals individually.
You must always initialize the signal set with one of these two
functions before using it in any other way. Don't try to set all the
signals explicitly because the sigset_t object might include some
other information (like a version field) that needs to be initialized as
well. (In addition, it's not wise to put into your program an
assumption that the system has no signals aside from the ones you know
about.)
Function: int sigemptyset (sigset_t *set)
This function initializes the signal set set to exclude all of the
defined signals. It always returns 0.
Function: int sigfillset (sigset_t *set)
This function initializes the signal set set to include
all of the defined signals. Again, the return value is 0.
Function: int sigaddset (sigset_t *set, int signum)
This function adds the signal signum to the signal set set.
All sigaddset does is modify set; it does not block or
unblock any signals.
The return value is 0 on success and -1 on failure.
The following errno error condition is defined for this function:
EINVAL
Function: int sigdelset (sigset_t *set, int signum)
This function removes the signal signum from the signal set
set. All sigdelset does is modify set; it does not
block or unblock any signals. The return value and error conditions are
the same as for sigaddset.
Finally, there is a function to test what signals are in a signal set:
Function: int sigismember (const sigset_t *set, int signum)
The sigismember function tests whether the signal signum is
a member of the signal set set. It returns 1 if the signal
is in the set, 0 if not, and -1 if there is an error.
The following errno error condition is defined for this function:
EINVAL
The collection of signals that are currently blocked is called the signal mask. Each process has its own signal mask. When you create a new process (see section Creating a Process), it inherits its parent's mask. You can block or unblock signals with total flexibility by modifying the signal mask.
The prototype for the sigprocmask function is in `signal.h'.
Function: int sigprocmask (int how, const sigset_t *set, sigset_t *oldset)
The sigprocmask function is used to examine or change the calling
process's signal mask. The how argument determines how the signal
mask is changed, and must be one of the following values:
SIG_BLOCK
set---add them to the existing mask. In
other words, the new mask is the union of the existing mask and
set.
SIG_UNBLOCK
SIG_SETMASK
The last argument, oldset, is used to return information about the
old process signal mask. If you just want to change the mask without
looking at it, pass a null pointer as the oldset argument.
Similarly, if you want to know what's in the mask without changing it,
pass a null pointer for set (in this case the how argument
is not significant). The oldset argument is often used to
remember the previous signal mask in order to restore it later. (Since
the signal mask is inherited over fork and exec calls, you
can't predict what its contents are when your program starts running.)
If invoking sigprocmask causes any pending signals to be
unblocked, at least one of those signals is delivered to the process
before sigprocmask returns. The order in which pending signals
are delivered is not specified, but you can control the order explicitly
by making multiple sigprockmask calls to unblock various signals
one at a time.
The sigprocmask function returns 0 if successful, and -1
to indicate an error. The following errno error conditions are
defined for this function:
EINVAL
You can't block the SIGKILL and SIGSTOP signals, but
if the signal set includes these, sigprocmask just ignores
them instead of returning an error status.
Remember, too, that blocking program error signals such as SIGFPE
leads to undesirable results for signals generated by an actual program
error (as opposed to signals sent with raise or kill).
This is because your program may be too broken to be able to continue
executing to a point where the signal is unblocked again.
See section Program Error Signals.
Now for a simple example. Suppose you establish a handler for
SIGALRM signals that sets a flag whenever a signal arrives, and
your main program checks this flag from time to time and then resets it.
You can prevent additional SIGALRM signals from arriving in the
meantime by wrapping the critical part of the code with calls to
sigprocmask, like this:
/* This variable is set by the SIGALRM signal handler. */
volatile sig_atomic_t flag = 0;
int
main (void)
{
sigset_t block_alarm;
...
/* Initialize the signal mask. */
sigemptyset (&block_alarm);
sigaddset (&block_alarm, SIGALRM);
while (1)
{
/* Check if a signal has arrived; if so, reset the flag. */
sigprocmask (SIG_BLOCK, &block_alarm, NULL);
if (flag)
{
actions-if-not-arrived
flag = 0;
}
sigprocmask (SIG_UNBLOCK, &block_alarm, NULL);
...
}
}
When a signal handler is invoked, you usually want it to be able to finish without being interrupted by another signal. From the moment the handler starts until the moment it finishes, you must block signals that might confuse it or corrupt its data.
When a handler function is invoked on a signal, that signal is
automatically blocked (in addition to any other signals that are already
in the process's signal mask) during the time the handler is running.
If you set up a handler for SIGTSTP, for instance, then the
arrival of that signal forces further SIGTSTP signals to wait
during the execution of the handler.
However, by default, other kinds of signals are not blocked; they can arrive during handler execution.
The reliable way to block other kinds of signals during the execution of
the handler is to use the sa_mask member of the sigaction
structure.
Here is an example:
#include <signal.h>
#include <stddef.h>
void catch_stop ();
void
install_handler (void)
{
struct sigaction setup_action;
sigset_t block_mask;
sigemptyset (&block_mask);
/* Block other terminal-generated signals while handler runs. */
sigaddset (&block_mask, SIGINT);
sigaddset (&block_mask, SIGQUIT);
setup_action.sa_handler = catch_stop;
setup_action.sa_mask = block_mask;
setup_action.sa_flags = 0;
sigaction (SIGTSTP, &setup_action, NULL);
}
This is more reliable than blocking the other signals explicitly in the code for the handler. If you block signals explicity in the handler, you can't avoid at least a short interval at the beginning of the handler where they are not yet blocked.
You cannot remove signals from the process's current mask using this
mechanism. However, you can make calls to sigprocmask within
your handler to block or unblock signals as you wish.
In any case, when the handler returns, the system restores the mask that was in place before the handler was entered.
You can find out which signals are pending at any time by calling
sigpending. This function is declared in `signal.h'.
Function: int sigpending (sigset_t *set)
The sigpending function stores information about pending signals
in set. If there is a pending signal that is blocked from
delivery, then that signal is a member of the returned set. (You can
test whether a particular signal is a member of this set using
sigismember; see section Signal Sets.)
The return value is 0 if successful, and -1 on failure.
Testing whether a signal is pending is not often useful. Testing when that signal is not blocked is almost certainly bad design.
Here is an example.
#include <signal.h>
#include <stddef.h>
sigset_t base_mask, waiting_mask;
sigemptyset (&base_mask);
sigaddset (&base_mask, SIGINT);
sigaddset (&base_mask, SIGTSTP);
/* Block user interrupts while doing other processing. */
sigprocmask (SIG_SETMASK, &base_mask, NULL);
...
/* After a while, check to see whether any signals are pending. */
sigpending (&waiting_mask);
if (sigismember (&waiting_mask, SIGINT)) {
/* User has tried to kill the process. */
}
else if (sigismember (&waiting_mask, SIGTSTP)) {
/* User has tried to stop the process. */
}
Remember that if there is a particular signal pending for your process,
additional signals of that same type that arrive in the meantime might
be discarded. For example, if a SIGINT signal is pending when
another SIGINT signal arrives, your program will probably only
see one of them when you unblock this signal.
Portability Note: The sigpending function is new in
POSIX.1. Older systems have no equivalent facility.
Instead of blocking a signal using the library facilities, you can get almost the same results by making the handler set a flag to be tested later, when you "unblock". Here is an example:
/* If this flag is nonzero, don't handle the signal right away. */
volatile sig_atomic_t signal_pending;
/* This is nonzero if a signal arrived and was not handled. */
volatile sig_atomic_t defer_signal;
void
handler (int signum)
{
if (defer_signal)
signal_pending = signum;
else
... /* "Really" handle the signal. */
}
...
void
update_mumble (int frob)
{
/* Prevent signals from having immediate effect. */
defer_signal++;
/* Now update mumble, without worrying about interruption. */
mumble.a = 1;
mumble.b = hack ();
mumble.c = frob;
/* We have updated mumble. Handle any signal that came in. */
defer_signal--;
if (defer_signal == 0 && signal_pending != 0)
raise (signal_pending);
}
Note how the particular signal that arrives is stored in
signal_pending. That way, we can handle several types of
inconvenient signals with the same mechanism.
We increment and decrement defer_signal so that nested critical
sections will work properly; thus, if update_mumble were called
with signal_pending already nonzero, signals would be deferred
not only within update_mumble, but also within the caller. This
is also why we do not check signal_pending if defer_signal
is still nonzero.
The incrementing and decrementing of defer_signal require more
than one instruction; it is possible for a signal to happen in the
middle. But that does not cause any problem. If the signal happens
early enough to see the value from before the increment or decrement,
that is equivalent to a signal which came before the beginning of the
increment or decrement, which is a case that works properly.
It is absolutely vital to decrement defer_signal before testing
signal_pending, because this avoids a subtle bug. If we did
these things in the other order, like this,
if (defer_signal == 1 && signal_pending != 0)
raise (signal_pending);
defer_signal--;
then a signal arriving in between the if statement and the decrement
would be effetively "lost" for an indefinite amount of time. The
handler would merely set defer_signal, but the program having
already tested this variable, it would not test the variable again.
Bugs like these are called timing errors. They are especially bad because they happen only rarely and are nearly impossible to reproduce. You can't expect to find them with a debugger as you would find a reproducible bug. So it is worth being especially careful to avoid them.
(You would not be tempted to write the code in this order, given the use
of defer_signal as a counter which must be tested along with
signal_pending. After all, testing for zero is cleaner than
testing for one. But if you did not use defer_signal as a
counter, and gave it values of zero and one only, then either order
might seem equally simple. This is a further advantage of using a
counter for defer_signal: it will reduce the chance you will
write the code in the wrong order and create a subtle bug.)
If your program is driven by external events, or uses signals for synchronization, then when it has nothing to do it should probably wait until a signal arrives.
pause
The simple way to wait until a signal arrives is to call pause.
Please read about its disadvantages, in the following section, before
you use it.
The pause function suspends program execution until a signal
arrives whose action is either to execute a handler function, or to
terminate the process.
If the signal causes a handler function to be executed, then
pause returns. This is considered an unsuccessful return (since
"successful" behavior would be to suspend the program forever), so the
return value is -1. Even if you specify that other primitives
should resume when a system handler returns (see section Primitives Interrupted by Signals), this has no effect on pause; it always fails when a
signal is handled.
The following errno error conditions are defined for this function:
EINTR
If the signal causes program termination, pause doesn't return
(obviously).
The pause function is declared in `unistd.h'.
pause
The simplicity of pause can conceal serious timing errors that
can make a program hang mysteriously.
It is safe to use pause if the real work of your program is done
by the signal handlers themselves, and the "main program" does nothing
but call pause. Each time a signal is delivered, the handler
will do the next batch of work that is to be done, and then return, so
that the main loop of the program can call pause again.
You can't safely use pause to wait until one more signal arrives,
and then resume real work. Even if you arrange for the signal handler
to cooperate by setting a flag, you still can't use pause
reliably. Here is an example of this problem:
/* usr_interrupt is set by the signal handler. */
if (!usr_interrupt)
pause ();
/* Do work once the signal arrives. */
...
This has a bug: the signal could arrive after the variable
usr_interrupt is checked, but before the call to pause.
If no further signals arrive, the process would never wake up again.
You can put an upper limit on the excess waiting by using sleep
in a loop, instead of using pause. (See section Sleeping, for more
about sleep.) Here is what this looks like:
/* usr_interrupt is set by the signal handler.
while (!usr_interrupt)
sleep (1);
/* Do work once the signal arrives. */
...
For some purposes, that is good enough. But with a little more
complexity, you can wait reliably until a particular signal handler is
run, using sigsuspend.
sigsuspend
The clean and reliable way to wait for a signal to arrive is to block it
and then use sigsuspend. By using sigsuspend in a loop,
you can wait for certain kinds of signals, while letting other kinds of
signals be handled by their handlers.
Function: int sigsuspend (const sigset_t *set)
This function replaces the process's signal mask with set and then suspends the process until a signal is delivered whose action is either to terminate the process or invoke a signal handling function. In other words, the program is effectively suspended until one of the signals that is not a member of set arrives.
If the process is woken up by deliver of a signal that invokes a handler
function, and the handler function returns, then sigsuspend also
returns.
The mask remains set only as long as sigsuspend is waiting.
The function sigsuspend always restores the previous signal mask
when it returns.
The return value and error conditions are the same as for pause.
With sigsuspend, you can replace the pause or sleep
loop in the previous section with something completely reliable:
sigset_t mask, oldmask; ... /* Set up the mask of signals to temporarily block. */ sigemptyset (&mask); sigaddset (&mask, SIGUSR1); ... /* Wait for a signal to arrive. */ sigprocmask (SIG_BLOCK, &mask, &oldmask); while (!usr_interrupt) sigsuspend (&oldmask); sigprocmask (SIG_UNBLOCK, &mask, NULL);
This last piece of code is a little tricky. The key point to remember
here is that when sigsuspend returns, it resets the process's
signal mask to the original value, the value from before the call to
sigsuspend---in this case, the SIGUSR1 signal is once
again blocked. The second call to sigprocmask is
necessary to explicitly unblock this signal.
One other point: you may be wondering why the while loop is
necessary at all, since the program is apparently only waiting for one
SIGUSR1 signal. The answer is that the mask passed to
sigsuspend permits the process to be woken up by the delivery of
other kinds of signals, as well--for example, job control signals. If
the process is woken up by a signal that doesn't set
usr_interrupt, it just suspends itself again until the "right"
kind of signal eventually arrives.
This technique takes a few more lines of preparation, but that is needed just once for each kind of wait criterion you want to use. The code that actually waits is just four lines.
This section describes alternative signal handling functions derived from BSD Unix. These facilities were an advance, in their time; today, they are mostly obsolete, and supported mainly for compatibility with BSD Unix.
They do provide one feature that is not available through the POSIX functions: You can specify a separate stack for use in certain signal handlers. Using a signal stack is the only way you can handle a signal caused by stack overflow.
There are many similarities between the BSD and POSIX signal handling facilities, because the POSIX facilities were inspired by the BSD facilities. Besides having different names for all the functions to avoid conflicts, the main differences between the two are:
int bit mask, rather than
as a sigset_t object.
The BSD facilities are declared in `signal.h'.
This data type is the BSD equivalent of struct sigaction
(see section Advanced Signal Handling); it is used to specify signal actions
to the sigvec function. It contains the following members:
sighandler_t sv_handler
int sv_mask
int sv_flags
sv_onstack.
These symbolic constants can be used to provide values for the
sv_flags field of a sigvec structure. This field is a bit
mask value, so you bitwise-OR the flags of interest to you together.
If this bit is set in the sv_flags field of a sigvec
structure, it means to use the signal stack when delivering the signal.
If this bit is set in the sv_flags field of a sigvec
structure, it means that system calls interrupted by this kind of signal
should not be restarted if the handler returns; instead, the system
calls should return with a EINTR error status. See section Primitives Interrupted by Signals.
If this bit is set in the sv_flags field of a sigvec
structure, it means to reset the action for the signal back to
SIG_DFL when the signal is received.
Function: int sigvec (int signum, const struct sigvec *action,struct sigvec *old_action)
This function is the equivalent of sigaction (see section Advanced Signal Handling); it installs the action action for the signal signum,
returning information about the previous action in effect for that signal
in old_action.
Function: int siginterrupt (int signum, int failflag)
This function specifies which approach to use when certain primitives
are interrupted by handling signal signum. If failflag is
false, signal signum restarts primitives. If failflag is
true, handling signum causes these primitives to fail with error
code EINTR. See section Primitives Interrupted by Signals.
Macro: int sigmask (int signum)
This macro returns a signal mask that has the bit for signal signum
set. You can bitwise-OR the results of several calls to sigmask
together to specify more than one signal. For example,
(sigmask (SIGTSTP) | sigmask (SIGSTOP) | sigmask (SIGTTIN) | sigmask (SIGTTOU))
specifies a mask that includes all the job-control stop signals.
Function: int sigblock (int mask)
This function is the equivalent of sigprocmask (see section Process Signal Mask) with a how argument of SIG_BLOCK: it adds the
signals specified by mask to the calling process's signal mask.
The return value is the previous set of blocked signals.
Function: int sigsetmask (int mask)
This function is the equivalent of sigprocmask (see section Process Signal Mask) with a how argument of SIG_SETMASK: it sets
the calling process's signal mask to mask. The return value is
the previous set of blocked signals.
Function: int sigpause (int mask)
This function is the equivalent of sigsuspend (see section Waiting for a Signal): it sets the calling process's signal mask to mask,
and waits for a signal to arrive. On return the previous set of blocked
signals is restored.
A signal stack is a special area of memory to be used as the execution
stack during signal handlers. It should be fairly large, to avoid any
danger that it will overflow in turn--we recommend at least 16,000
bytes. You can use malloc to allocate the space for the stack.
Then call sigstack to tell the system to use that space for the
signal stack.
You don't need to write signal handlers differently in order to use a signal stack. Switching from one stack to the other happens automatically. However, some debuggers on some machines may get confused if you examine a stack trace while a handler that uses the signal stack is running.
This structure describes a signal stack. It contains the following members:
void *ss_sp
int ss_onstack
Function: int sigstack (const struct sigstack *stack, struct sigstack *oldstack)
The sigstack function specifies an alternate stack for use during
signal handling. When a signal is received by the process and its
action indicates that the signal stack is used, the system arranges a
switch to the currently installed signal stack while the handler for
that signal is executed.
If oldstack is not a null pointer, information about the currently installed signal stack is returned in the location it points to. If stack is not a null pointer, then this is installed as the new stack for use by signal handlers.
The return value is 0 on success and 1 on failure.
Processes are the primitive units for allocation of system resources. Each process has its own address space and (usually) one thread of control. A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies.
This chapter explains what your program should do to handle the startup of a process, to terminate its process, and to receive information (arguments and the environment) from the parent process.
The system starts a C program by calling the function main. It
is up to you to write a function named main---otherwise, you
won't even be able to link your program without errors.
You can define main either to take no arguments, or to take two
arguments that represent the command line arguments to the program, like
this:
int main (int argc, char *argv[])
The command line arguments are the whitespace-separated tokens given in
the shell command used to invoke the program; thus, in `cat foo
bar', the arguments are `foo' and `bar'. The only way a
program can look at its command line arguments is via the arguments of
main. If main doesn't take arguments, then you cannot get
at the command line.
The value of the argc argument is the number of command line
arguments. The argv argument is a vector of C strings; its
elements are the individual command line argument strings. The file
name of the program being run is also included in the vector as the
first element; the value of argc counts this element. A null
pointer always follows the last element: argv[argc]
is this null pointer.
For the command `cat foo bar', argc is 3 and argv has
three elements, "cat", "foo" and "bar".
If the syntax for the command line arguments to your program is simple
enough, you can simply pick the arguments off from argv by hand.
But unless your program takes a fixed number of arguments, or all of the
arguments are interpreted in the same way (as file names, for example),
you are usually better off using getopt to do the parsing.
POSIX recommends these conventions for command line arguments.
getopt (see section Parsing Program Options) makes it easy to implement them.
isalnum;
see section Classification of Characters).
ld command requires an argument--an output file name.
The implementation of getopt in the GNU C library normally makes
it appear as if all the option arguments were specified before all the
non-option arguments for the purposes of parsing, even if the user of
your program intermixed option and non-option arguments. It does this
by reordering the elements of the argv array. This behavior is
nonstandard; if you want to suppress it, define the
_POSIX_OPTION_ORDER environment variable. See section Standard Environment Variables.
GNU adds long options to these conventions. Long options consist of `--' followed by a name made of alphanumeric characters and dashes. Option names are typically one to three words long, with hyphens to separate words. Users can abbreviate the option names as long as the abbreviations are unique.
To specify an argument for a long option, write `--name=value'. This syntax enables a long option to accept an argument that is itself optional.
Eventually, the GNU system will provide completion for long option names in the shell.
Here are the details about how to call the getopt function. To
use this facility, your program must include the header file
`unistd.h'.
If the value of this variable is nonzero, then getopt prints an
error message to the standard error stream if it encounters an unknown
option character or an option with a missing required argument. This is
the default behavior. If you set this variable to zero, getopt
does not print any messages, but it still returns the character ?
to indicate an error.
When getopt encounters an unknown option character or an option
with a missing required argument, it stores that option character in
this variable. You can use this for providing your own diagnostic
messages.
This variable is set by getopt to the index of the next element
of the argv array to be processed. Once getopt has found
all of the option arguments, you can use this variable to determine
where the remaining non-option arguments begin. The initial value of
this variable is 1.
This variable is set by getopt to point at the value of the
option argument, for those options that accept arguments.
Function: int getopt (int argc, char **argv, const char *options)
The getopt function gets the next option argument from the
argument list specified by the argv and argc arguments.
Normally these values come directly from the arguments received by
main.
The options argument is a string that specifies the option characters that are valid for this program. An option character in this string can be followed by a colon (`:') to indicate that it takes a required argument.
If the options argument string begins with a hyphen (`-'), this is treated specially. It permits arguments that are not options to be returned as if they were associated with option character `\0'.
The getopt function returns the option character for the next
command line option. When no more option arguments are available, it
returns -1. There may still be more non-option arguments; you
must compare the external variable optind against the argc
parameter to check this.
If the option has an argument, getopt returns the argument by
storing it in the varables optarg. You don't ordinarily need to
copy the optarg string, since it is a pointer into the original
argv array, not into a static area that might be overwritten.
If getopt finds an option character in argv that was not
included in options, or a missing option argument, it returns
`?' and sets the external variable optopt to the actual
option character. If the first character of options is a colon
(`:'), then getopt returns `:' instead of `?' to
indicate a missing option argument. In addition, if the external
variable opterr is nonzero (which is the default), getopt
prints an error message.
getopt
Here is an example showing how getopt is typically used. The
key points to notice are:
getopt is called in a loop. When getopt returns
-1, indicating no more options are present, the loop terminates.
switch statement is used to dispatch on the return value from
getopt. In typical use, each case just sets a variable that
is used later in the program.
#include <unistd.h>
#include <stdio.h>
int
main (int argc, char **argv)
{
int aflag = 0;
int bflag = 0;
char *cvalue = NULL;
int index;
int c;
opterr = 0;
while ((c = getopt (argc, argv, "abc:")) != -1)
switch (c)
{
case 'a':
aflag = 1;
break;
case 'b':
bflag = 1;
break;
case 'c':
cvalue = optarg;
break;
case '?':
if (isprint (optopt))
fprintf (stderr, "Unknown option `-%c'.\n", optopt);
else
fprintf (stderr,
"Unknown option character `\\x%x'.\n",
optopt);
return 1;
default:
abort ();
}
printf ("aflag = %d, bflag = %d, cvalue = %s\n", aflag, bflag, cvalue);
for (index = optind; index < argc; index++)
printf ("Non-option argument %s\n", argv[index]);
return 0;
}
Here are some examples showing what this program prints with different combinations of arguments:
% testopt aflag = 0, bflag = 0, cvalue = (null) % testopt -a -b aflag = 1, bflag = 1, cvalue = (null) % testopt -ab aflag = 1, bflag = 1, cvalue = (null) % testopt -c foo aflag = 0, bflag = 0, cvalue = foo % testopt -cfoo aflag = 0, bflag = 0, cvalue = foo % testopt arg1 aflag = 0, bflag = 0, cvalue = (null) Non-option argument arg1 % testopt -a arg1 aflag = 1, bflag = 0, cvalue = (null) Non-option argument arg1 % testopt -c foo arg1 aflag = 0, bflag = 0, cvalue = foo Non-option argument arg1 % testopt -a -- -b aflag = 1, bflag = 0, cvalue = (null) Non-option argument -b % testopt -a - aflag = 1, bflag = 0, cvalue = (null) Non-option argument -
To accept GNU-style long options as well as single-character options,
use getopt_long instead of getopt. You should make every
program accept long options if it uses any options, for this takes
little extra work and helps beginners remember how to use the program.
This structure describes a single long option name for the sake of
getopt_long. The argument longopts must be an array of
these structures, one for each long option. Terminate the array with an
element containing all zeros.
The struct option structure has these fields:
const char *name
int has_arg
no_argument,
required_argument and optional_argument.
int *flag
int val
If flag is a null pointer, then the val is a value which
identifies this option. Often these values are chosen to uniquely
identify particular long options.
If flag is not a null pointer, it should be the address of an
int variable which is the flag for this option. The value in
val is the value to store in the flag to indicate that the option
was seen.
Function: int getopt_long (int argc, char **argv, const char *shortopts, struct option *longopts, int *indexptr)
Decode options from the vector argv (whose length is argc).
The argument shortopts describes the short options to accept, just as
it does in getopt. The argument longopts describes the long
options to accept (see above).
When getopt_long encounters a short option, it does the same
thing that getopt would do: it returns the character code for the
option, and stores the options argument (if it has one) in optarg.
When getopt_long encounters a long option, it takes actions based
on the flag and val fields of the definition of that
option.
If flag is a null pointer, then getopt_long returns the
contents of val to indicate which option it found. You should
arrange distinct values in the val field for options with
different meanings, so you can decode these values after
getopt_long returns. If the long option is equivalent to a short
option, you can use the short option's character code in val.
If flag is not a null pointer, that means this option should just
set a flag in the program. The flag is a variable of type int
that you define. Put the address of the flag in the flag field.
Put in the val field the value you would like this option to
store in the flag. In this case, getopt_long returns 0.
For any long option, getopt_long tells you the index in the array
longopts of the options definition, by storing it into
*indexptr. You can get the name of the option with
longopts[*indexptr].name. So you can distinguish among
long options either by the values in their val fields or by their
indices. You can also distinguish in this way among long options that
set flags.
When a long option has an argument, getopt_long puts the argument
value in the variable optarg before returning. When the option
has no argument, the value in optarg is a null pointer. This is
how you can tell whether an optional argument was supplied.
When getopt_long has no more options to handle, it returns
-1, and leaves in the variable optind the index in
argv of the next remaining argument.
#include <stdio.h>
/* Flag set by `--verbose'. */
static int verbose_flag;
int
main (argc, argv)
int argc;
char **argv;
{
int c;
while (1)
{
static struct option long_options[] =
{
/* These options set a flag. */
{"verbose", 0, &verbose_flag, 1},
{"brief", 0, &verbose_flag, 0},
/* These options don't set a flag.
We distinguish them by their indices. */
{"add", 1, 0, 0},
{"append", 0, 0, 0},
{"delete", 1, 0, 0},
{"create", 0, 0, 0},
{"file", 1, 0, 0},
{0, 0, 0, 0}
};
/* getopt_long stores the option index here. */
int option_index = 0;
c = getopt_long (argc, argv, "abc:d:",
long_options, &option_index);
/* Detect the end of the options. */
if (c == -1)
break;
switch (c)
{
case 0:
/* If this option set a flag, do nothing else now. */
if (long_options[option_index].flag != 0)
break;
printf ("option %s", long_options[option_index].name);
if (optarg)
printf (" with arg %s", optarg);
printf ("\n");
break;
case 'a':
puts ("option -a\n");
break;
case 'b':
puts ("option -b\n");
break;
case 'c':
printf ("option -c with value `%s'\n", optarg);
break;
case 'd':
printf ("option -d with value `%s'\n", optarg);
break;
case '?':
/* getopt_long already printed an error message. */
break;
default:
abort ();
}
}
/* Instead of reporting `--verbose'
and `--brief' as they are encountered,
we report the final status resulting from them. */
if (verbose_flag)
puts ("verbose flag is set");
/* Print any remaining command line arguments (not options). */
if (optind < argc)
{
printf ("non-option ARGV-elements: ");
while (optind < argc)
printf ("%s ", argv[optind++]);
putchar ('\n');
}
exit (0);
}
When a program is executed, it receives information about the context in
which it was invoked in two ways. The first mechanism uses the
argv and argc arguments to its main function, and is
discussed in section Program Arguments. The second mechanism uses
environment variables and is discussed in this section.
The argv mechanism is typically used to pass command-line arguments specific to the particular program being invoked. The environment, on the other hand, keeps track of information that is shared by many programs, changes infrequently, and that is less frequently accessed.
The environment variables discussed in this section are the same
environment variables that you set using assignments and the
export command in the shell. Programs executed from the shell
inherit all of the environment variables from the shell.
Standard environment variables are used for information about the user's home directory, terminal type, current locale, and so on; you can define additional variables for other purposes. The set of all environment variables that have values is collectively known as the environment.
Names of environment variables are case-sensitive and must not contain the character `='. System-defined environment variables are invariably uppercase.
The values of environment variables can be anything that can be represented as a string. A value must not contain an embedded null character, since this is assumed to terminate the string.
The value of an environment variable can be accessed with the
getenv function. This is declared in the header file
`stdlib.h'.
Function: char * getenv (const char *name)
This function returns a string that is the value of the environment
variable name. You must not modify this string. In some systems
not using the GNU library, it might be overwritten by subsequent calls
to getenv (but not by any other library function).
If the environment variable name is not defined, the value is a
null pointer.
Function: int putenv (const char *string)
The putenv function adds or removes definitions from the environment.
If the string is of the form `name=value', the
definition is added to the environment. Otherwise, the string is
interpreted as the name of an environment variable, and any definition
for this variable in the environment is removed.
The GNU library provides this function for compatibility with SVID; it may not be available in other systems.
You can deal directly with the underlying representation of environment objects to add more variables to the environment (for example, to communicate with another program you are about to execute; see section Executing a File).
The environment is represented as an array of strings. Each string is of the format `name=value'. The order in which strings appear in the environment is not significant, but the same name must not appear more than once. The last element of the array is a null pointer.
This variable is declared in the header file `unistd.h'.
If you just want to get the value of an environment variable, use
getenv.
These environment variables have standard meanings. This doesn't mean that they are always present in the environment; but if these variables are present, they have these meanings, and that you shouldn't try to use these environment variable names for some other purpose.
HOME
This is a string representing the user's home directory, or initial default working directory.
The user can set HOME to any value.
If you need to make sure to obtain the proper home directory
for a particular user, you should not use HOME; instead,
look up the user's name in the user database (see section User Database).
For most purposes, it is better to use HOME, precisely because
this lets the user specify the value.
LOGNAME
This is the name that the user used to log in. Since the value in the
environment can be tweaked arbitrarily, this is not a reliable way to
identify the user who is running a process; a function like
getlogin (see section Identifying Who Logged In) is better for that purpose.
For most purposes, it is better to use LOGNAME, precisely because
this lets the user specify the value.
PATH
A path is a sequence of directory names which is used for
searching for a file. The variable PATH holds a path used
for searching for programs to be run.
The execlp and execvp functions (see section Executing a File)
use this environment variable, as do many shells and other utilities
which are implemented in terms of those functions.
The syntax of a path is a sequence of directory names separated by colons. An empty string instead of a directory name stands for the current directory (see section Working Directory).
A typical value for this environment variable might be a string like:
:/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local:/usr/local/bin
This means that if the user tries to execute a program named foo,
the system will look for files named `foo', `/bin/foo',
`/etc/foo', and so on. The first of these files that exists is
the one that is executed.
TERM
This specifies the kind of terminal that is receiving program output.
Some programs can make use of this information to take advantage of
special escape sequences or terminal modes supported by particular kinds
of terminals. Many programs which use the termcap library
(see section 'Finding a Terminal Description' in The Termcap Library Manual) use the TERM environment variable, for example.
TZ
This specifies the time zone. See section Specifying the Time Zone with TZ, for information about
the format of this string and how it is used.
LANG
This specifies the default locale to use for attribute categories where
neither LC_ALL nor the specific environment variable for that
category is set. See section Locales and Internationalization, for more information about
locales.
LC_COLLATE
This specifies what locale to use for string sorting.
LC_CTYPE
This specifies what locale to use for character sets and character classification.
LC_MONETARY
This specifies what locale to use for formatting monetary values.
LC_NUMERIC
This specifies what locale to use for formatting numbers.
LC_TIME
This specifies what locale to use for formatting date/time values.
_POSIX_OPTION_ORDER
If this environment variable is defined, it suppresses the usual
reordering of command line arguments by getopt. See section Program Argument Syntax Conventions.
The usual way for a program to terminate is simply for its main
function to return. The exit status value returned from the
main function is used to report information back to the process's
parent process or shell.
A program can also terminate normally by calling the exit
function.
In addition, programs can be terminated by signals; this is discussed in
more detail in section Signal Handling. The abort function causes
a signal that kills the program.
A process terminates normally when the program calls exit.
Returning from main is equivalent to calling exit, and
the value that main returns is used as the argument to exit.
Function: void exit (int status)
The exit function terminates the process with status
status. This function does not return.
Normal termination causes the following actions:
atexit or on_exit
functions are called in the reverse order of their registration. This
mechanism allows your application to specify its own "cleanup" actions
to be performed at program termination. Typically, this is used to do
things like saving program state information in a file, or unlocking
locks in shared data bases.
tmpfile function are removed; see section Temporary Files.
_exit is called, terminating the program. See section Termination Internals.
When a program exits, it can return to the parent process a small
amount of information about the cause of termination, using the
exit status. This is a value between 0 and 255 that the exiting
process passes as an argument to exit.
Normally you should use the exit status to report very broad information about success or failure. You can't provide a lot of detail about the reasons for the failure, and most parent processes would not want much detail anyway.
There are conventions for what sorts of status values certain programs should return. The most common convention is simply 0 for success and 1 for failure. Programs that perform comparison use a different convention: they use status 1 to indicate a mismatch, and status 2 to indicate an inability to compare. Your program should follow an existing convention if an existing convention makes sense for it.
A general convention reserves status values 128 and up for special purposes. In particular, the value 128 is used to indicate failure to execute another program in a subprocess. This convention is not universally obeyed, but it is a good idea to follow it in your programs.
Warning: Don't try to use the number of errors as the exit status. This is actually not very useful; a parent process would generally not care how many errors occurred. Worse than that, it does not work, because the status value is truncated to eight bits. Thus, if the program tried to report 256 errors, the parent would receive a report of 0 errors--that is, success.
For the same reason, it does not work to use the value of errno
as the exit status--these can exceed 255.
Portability note: Some non-POSIX systems use different
conventions for exit status values. For greater portability, you can
use the macros EXIT_SUCCESS and EXIT_FAILURE for the
conventional status value for success and failure, respectively. They
are declared in the file `stdlib.h'.
This macro can be used with the exit function to indicate
successful program completion.
On POSIX systems, the value of this macro is 0. On other
systems, the value might be some other (possibly non-constant) integer
expression.
This macro can be used with the exit function to indicate
unsuccessful program completion in a general sense.
On POSIX systems, the value of this macro is 1. On other
systems, the value might be some other (possibly non-constant) integer
expression. Other nonzero status values also indicate future. Certain
programs use different nonzero status values to indicate particular
kinds of "non-success". For example, diff uses status value
1 to mean that the files are different, and 2 or more to
mean that there was difficulty in opening the files.
Your program can arrange to run its own cleanup functions if normal
termination happens. If you are writing a library for use in various
application programs, then it is unreliable to insist that all
applications call the library's cleanup functions explicitly before
exiting. It is much more robust to make the cleanup invisible to the
application, by setting up a cleanup function in the library itself
using atexit or on_exit.
Function: int atexit (void (*function) (void))
The atexit function registers the function function to be
called at normal program termination. The function is called with
no arguments.
The return value from atexit is zero on success and nonzero if
the function cannot be registered.
Function: int on_exit (void (*function)(int status, void *arg), void *arg)
This function is a somewhat more powerful variant of atexit. It
accepts two arguments, a function function and an arbitrary
pointer arg. At normal program termination, the function is
called with two arguments: the status value passed to exit,
and the arg.
This function is included in the GNU C library only for compatibility for SunOS, and may not be supported by other implementations.
Here's a trivial program that illustrates the use of exit and
atexit:
#include <stdio.h>
#include <stdlib.h>
void
bye (void)
{
puts ("Goodbye, cruel world....");
}
int
main (void)
{
atexit (bye);
exit (EXIT_SUCCESS);
}
When this program is executed, it just prints the message and exits.
You can abort your program using the abort function. The prototype
for this function is in `stdlib.h'.
The abort function causes abnormal program termination. This
does not execute cleanup functions registered with atexit or
on_exit.
This function actually terminates the process by raising a
SIGABRT signal, and your program can include a handler to
intercept this signal; see section Signal Handling.
Future Change Warning: Proposed Federal censorship regulations may prohibit us from giving you information about the possibility of calling this function. We would be required to say that this is not an acceptable way of terminating a program.
The _exit function is the primitive used for process termination
by exit. It is declared in the header file `unistd.h'.
Function: void _exit (int status)
The _exit function is the primitive for causing a process to
terminate with status status. Calling this function does not
execute cleanup functions registered with atexit or
on_exit.
When a process terminates for any reason--either by an explicit termination call, or termination as a result of a signal--the following things happen:
wait or waitpid; see
section Process Completion.
init process, with process ID 1.)
SIGCHLD signal is sent to the parent process.
SIGHUP signal is sent to each process in the foreground job,
and the controlling terminal is disassociated from that session.
See section Job Control.
SIGHUP
signal and a SIGCONT signal are sent to each process in the
group. See section Job Control.
Processes are the primitive units for allocation of system resources. Each process has its own address space and (usually) one thread of control. A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies.
Processes are organized hierarchically. Each process has a parent process which explicitly arranged to create it. The processes created by a given parent are called its child processes. A child inherits many of its attributes from the parent process.
This chapter describes how a program can create, terminate, and control child processes. Actually, there are three distinct operations involved: creating a new child process, causing the new process to execute a program, and coordinating the completion of the child process with the original program.
The system function provides a simple, portable mechanism for
running another program; it does all three steps automatically. If you
need more control over the details of how this is done, you can use the
primitive functions to do each step individually instead.
The easy way to run another program is to use the system
function. This function does all the work of running a subprogram, but
it doesn't give you much control over the details: you have to wait
until the subprogram terminates before you can do anything else.
Function: int system (const char *command)
This function executes command as a shell command. In the GNU C
library, it always uses the default shell sh to run the command.
In particular, it searches the directories in PATH to find
programs to execute. The return value is -1 if it wasn't
possible to create the shell process, and otherwise is the status of the
shell process. See section Process Completion, for details on how this
status code can be interpreted.
The system function is declared in the header file
`stdlib.h'.
Portability Note: Some C implementations may not have any
notion of a command processor that can execute other programs. You can
determine whether a command processor exists by executing
system (NULL); if the return value is nonzero, a command
processor is available.
The popen and pclose functions (see section Pipe to a Subprocess) are closely related to the system function. They
allow the parent process to communicate with the standard input and
output channels of the command being executed.
This section gives an overview of processes and of the steps involved in creating a process and making it run another program.
Each process is named by a process ID number. A unique process ID is allocated to each process when it is created. The lifetime of a process ends when its termination is reported to its parent process; at that time, all of the process resources, including its process ID, are freed.
Processes are created with the fork system call (so the operation
of creating a new process is sometimes called forking a process).
The child process created by fork is an exact clone of the
original parent process, except that it has its own process ID.
After forking a child process, both the parent and child processes
continue to execute normally. If you want your program to wait for a
child process to finish executing before continuing, you must do this
explicitly after the fork operation, by calling wait or
waitpid (see section Process Completion). These functions give you
limited information about why the child terminated--for example, its
exit status code.
A newly forked child process continues to execute the same program as
its parent process, at the point where the fork call returns.
You can use the return value from fork to tell whether the program
is running in the parent process or the child.
Having several processes run the same program is only occasionally
useful. But the child can execute another program using one of the
exec functions; see section Executing a File. The program that the
process is executing is called its process image. Starting
execution of a new program causes the process to forget all about its
previous process image; when the new program exits, the process exits
too, instead of returning to the previous process image.
The pid_t data type represents process IDs. You can get the
process ID of a process by calling getpid. The function
getppid returns the process ID of the parent of the current
process (this is also known as the parent process ID). Your
program should include the header files `unistd.h' and
`sys/types.h' to use these functions.
The pid_t data type is a signed integer type which is capable
of representing a process ID. In the GNU library, this is an int.
The getpid function returns the process ID of the current process.
Function: pid_t getppid (void)
The getppid function returns the process ID of the parent of the
current process.
The fork function is the primitive for creating a process.
It is declared in the header file `unistd.h'.
The fork function creates a new process.
If the operation is successful, there are then both parent and child
processes and both see fork return, but with different values: it
returns a value of 0 in the child process and returns the child's
process ID in the parent process.
If process creation failed, fork returns a value of -1 in
the parent process. The following errno error conditions are
defined for fork:
EAGAIN
ENOMEM
The specific attributes of the child process that differ from the parent process are:
The vfork function is similar to fork but more efficient;
however, there are restrictions you must follow to use it safely.
While fork makes a complete copy of the calling process's address
space and allows both the parent and child to execute independently,
vfork does not make this copy. Instead, the child process
created with vfork shares its parent's address space until it calls
one of the exec functions. In the meantime, the parent process
suspends execution.
You must be very careful not to allow the child process created with
vfork to modify any global data or even local variables shared
with the parent. Furthermore, the child process cannot return from (or
do a long jump out of) the function that called vfork! This
would leave the parent process's control information very confused. If
in doubt, use fork instead.
Some operating systems don't really implement vfork. The GNU C
library permits you to use vfork on all systems, but actually
executes fork if vfork isn't available. If you follow
the proper precautions for using vfork, your program will still
work even if the system uses fork instead.
This section describes the exec family of functions, for executing
a file as a process image. You can use these functions to make a child
process execute a new program after it has been forked.
The functions in this family differ in how you specify the arguments, but otherwise they all do the same thing. They are declared in the header file `unistd.h'.
Function: int execv (const char *filename, char *const argv[])
The execv function executes the file named by filename as a
new process image.
The argv argument is an array of null-terminated strings that is
used to provide a value for the argv argument to the main
function of the program to be executed. The last element of this array
must be a null pointer. See section Program Arguments, for information on
how programs can access these arguments.
The environment for the new process image is taken from the
environ variable of the current process image; see section Environment Variables, for information about environments.
Function: int execl (const char *filename, const char *arg0, ...)
This is similar to execv, but the argv strings are
specified individually instead of as an array. A null pointer must be
passed as the last such argument.
Function: int execve (const char *filename, char *const argv[], char *const env[])
This is similar to execv, but permits you to specify the environment
for the new program explicitly as the env argument. This should
be an array of strings in the same format as for the environ
variable; see section Environment Access.
Function: int execle (const char *filename, const char *arg0, char *const env[], ...)
This is similar to execl, but permits you to specify the
environment for the new program explicitly. The environment argument is
passed following the null pointer that marks the last argv
argument, and should be an array of strings in the same format as for
the environ variable.
Function: int execvp (const char *filename, char *const argv[])
The execvp function is similar to execv, except that it
searches the directories listed in the PATH environment variable
(see section Standard Environment Variables) to find the full file name of a
file from filename if filename does not contain a slash.
This function is useful for executing system utility programs, because it looks for them in the places that the user has chosen. Shells use it to run the commands that users type.
Function: int execlp (const char *filename, const char *arg0, ...)
This function is like execl, except that it performs the same
file name searching as the execvp function.
The size of the argument list and environment list taken together must
not be greater than ARG_MAX bytes. See section General Capacity Limits. In
the GNU system, the size (which compares against ARG_MAX)
includes, for each string, the number of characters in the string, plus
the size of a char *, plus one, rounded up to a multiple of the
size of a char *. Other systems may have somewhat different
rules for counting.
These functions normally don't return, since execution of a new program
causes the currently executing program to go away completely. A value
of -1 is returned in the event of a failure. In addition to the
usual file name syntax errors (see section File Name Errors), the following
errno error conditions are defined for these functions:
E2BIG
ARG_MAX bytes. The GNU system has no
specific limit on the argument list size, so this error code cannot
result, but you may get ENOMEM instead if the arguments are too
big for available memory.
ENOEXEC
ENOMEM
If execution of the new file succeeds, it updates the access time field of the file as if the file had been read. See section File Times, for more details about access times of files.
The point at which the file is closed again is not specified, but is at some point before the process exits or before another process image is executed.
Executing a new process image completely changes the contents of memory, copying only the argument and environment strings to new locations. But many other attributes of the process are unchanged:
If the set-user-ID and set-group-ID mode bits of the process image file are set, this affects the effective user ID and effective group ID (respectively) of the process. These concepts are discussed in detail in section The Persona of a Process.
Signals that are set to be ignored in the existing process image are also set to be ignored in the new process image. All other signals are set to the default action in the new process image. For more information about signals, see section Signal Handling.
File descriptors open in the existing process image remain open in the
new process image, unless they have the FD_CLOEXEC
(close-on-exec) flag set. The files that remain open inherit all
attributes of the open file description from the existing process image,
including file locks. File descriptors are discussed in section Low-Level Input/Output.
Streams, by contrast, cannot survive through exec functions,
because they are located in the memory of the process itself. The new
process image has no streams except those it creates afresh. Each of
the streams in the pre-exec process image has a descriptor inside
it, and these descriptors do survive through exec (provided that
they do not have FD_CLOEXEC set. The new process image can
reconnect these to new streams using fdopen (see section Descriptors and Streams).
The functions described in this section are used to wait for a child process to terminate or stop, and determine its status. These functions are declared in the header file `sys/wait.h'.
Function: pid_t waitpid (pid_t pid, int *status_ptr, int options)
The waitpid function is used to request status information from a
child process whose process ID is pid. Normally, the calling
process is suspended until the child process makes status information
available by terminating.
Other values for the pid argument have special interpretations. A
value of -1 or WAIT_ANY requests status information for
any child process; a value of 0 or WAIT_MYPGRP requests
information for any child process in the same process group as the
calling process; and any other negative value - pgid
requests information for any child process whose process group ID is
pgid.
If status information for a child process is available immediately, this
function returns immediately without waiting. If more than one eligible
child process has status information available, one of them is chosen
randomly, and its status is returned immediately. To get the status
from the other eligible child processes, you need to call waitpid
again.
The options argument is a bit mask. Its value should be the
bitwise OR (that is, the `|' operator) of zero or more of the
WNOHANG and WUNTRACED flags. You can use the
WNOHANG flag to indicate that the parent process shouldn't wait;
and the WUNTRACED flag to request status information from stopped
processes as well as processes that have terminated.
The status information from the child process is stored in the object that status_ptr points to, unless status_ptr is a null pointer.
The return value is normally the process ID of the child process whose
status is reported. If the WNOHANG option was specified and no
child process is waiting to be noticed, the value is zero. A value of
-1 is returned in case of error. The following errno
error conditions are defined for this function:
EINTR
ECHILD
EINVAL
These symbolic constants are defined as values for the pid argument
to the waitpid function.
WAIT_ANY
This constant macro (whose value is -1) specifies that
waitpid should return status information about any child process.
WAIT_MYPGRP
0) specifies that waitpid should
return status information about any child process in the same process
group as the calling process.
These symbolic constants are defined as flags for the options
argument to the waitpid function. You can bitwise-OR the flags
together to obtain a value to use as the argument.
WNOHANG
This flag specifies that waitpid should return immediately
instead of waiting, if there is no child process ready to be noticed.
WUNTRACED
This flag specifies that waitpid should report the status of any
child processes that have been stopped as well as those that have
terminated.
Function: pid_t wait (int *status_ptr)
This is a simplified version of waitpid, and is used to wait
until any one child process terminates. The call:
wait (&status)
is exactly equivalent to:
waitpid (-1, &status, 0)
Here's an example of how to use waitpid to get the status from
all child processes that have terminated, without ever waiting. This
function is designed to be a handler for SIGCHLD, the signal that
indicates that at least one child process has terminated.
void
sigchld_handler (int signum)
{
int pid;
int status;
while (1)
{
pid = waitpid (WAIT_ANY, &status, WNOHANG);
if (pid < 0)
{
perror ("waitpid");
break;
}
if (pid == 0)
break;
notice_termination (pid, status);
}
}
If the exit status value (see section Program Termination) of the child
process is zero, then the status value reported by waitpid or
wait is also zero. You can test for other kinds of information
encoded in the returned status value using the following macros.
These macros are defined in the header file `sys/wait.h'.
Macro: int WIFEXITED (int status)
This macro returns a nonzero value if the child process terminated
normally with exit or _exit.
Macro: int WEXITSTATUS (int status)
If WIFEXITED is true of status, this macro returns the
low-order 8 bits of the exit status value from the child process.
See section Exit Status.
Macro: int WIFSIGNALED (int status)
This macro returns a nonzero value if the child process terminated because it received a signal that was not handled. See section Signal Handling.
Macro: int WTERMSIG (int status)
If WIFSIGNALED is true of status, this macro returns the
signal number of the signal that terminated the child process.
Macro: int WCOREDUMP (int status)
This macro returns a nonzero value if the child process terminated and produced a core dump.
Macro: int WIFSTOPPED (int status)
This macro returns a nonzero value if the child process is stopped.
Macro: int WSTOPSIG (int status)
If WIFSTOPPED is true of status, this macro returns the
signal number of the signal that caused the child process to stop.
The GNU library also provides these related facilities for compatibility
with BSD Unix. BSD uses the union wait data type to represent
status values rather than an int. The two representations are
actually interchangeable; they describe the same bit patterns. The GNU
C Library defines macros such as WEXITSTATUS so that they will
work on either kind of object, and the wait function is defined
to accept either type of pointer as its status_ptr argument.
These functions are declared in `sys/wait.h'.
This data type represents program termination status values. It has the following members:
int w_termsig
WTERMSIG macro.
int w_coredump
WCOREDUMP macro.
int w_retcode
WEXISTATUS macro.
int w_stopsig
WSTOPSIG macro.
Instead of accessing these members directly, you should use the equivalent macros.
Function: pid_t wait3 (union wait *status_ptr, int options, struct rusage *usage)
If usage is a null pointer, wait3 is equivalent to
waitpid (-1, status_ptr, options).
If usage is not null, wait3 stores usage figures for the
child process in *rusage (but only if the child has
terminated, not if it has stopped). See section Resource Usage.
Function: pid_t wait4 (pid_t pid, union wait *status_ptr, int options, struct rusage *usage)
If usage is a null pointer, wait4 is equivalent to
waitpid (pid, status_ptr, options).
If usage is not null, wait4 stores usage figures for the
child process in *rusage (but only if the child has
terminated, not if it has stopped). See section Resource Usage.
Here is an example program showing how you might write a function
similar to the built-in system. It executes its command
argument using the equivalent of `sh -c command'.
#include <stddef.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
/* Execute the command using this shell program. */
#define SHELL "/bin/sh"
int
my_system (const char *command)
{
int status;
pid_t pid;
pid = fork ();
if (pid == 0)
{
/* This is the child process. Execute the shell command. */
execl (SHELL, SHELL, "-c", command, NULL);
_exit (EXIT_FAILURE);
}
else if (pid < 0)
/* The fork failed. Report failure. */
status = -1;
else
/* This is the parent process. Wait for the child to complete. */
if (waitpid (pid, &status, 0) != pid)
status = -1;
return status;
}
There are a couple of things you should pay attention to in this example.
Remember that the first argv argument supplied to the program
represents the name of the program being executed. That is why, in the
call to execl, SHELL is supplied once to name the program
to execute and a second time to supply a value for argv[0].
The execl call in the child process doesn't return if it is
successful. If it fails, you must do something to make the child
process terminate. Just returning a bad status code with return
would leave two processes running the original program. Instead, the
right behavior is for the child process to report failure to its parent
process.
Call _exit to accomplish this. The reason for using _exit
instead of exit is to avoid flushing fully buffered streams such
as stdout. The buffers of these streams probably contain data
that was copied from the parent process by the fork, data that
will be output eventually by the parent process. Calling exit in
the child would output the data twice. See section Termination Internals.
Job control refers to the protocol for allowing a user to move between multiple process groups (or jobs) within a single login session. The job control facilities are set up so that appropriate behavior for most programs happens automatically and they need not do anything special about job control. So you can probably ignore the material in this chapter unless you are writing a shell or login program.
You need to be familiar with concepts relating to process creation (see section Process Creation Concepts) and signal handling (see section Signal Handling) in order to understand this material presented in this chapter.
The fundamental purpose of an interactive shell is to read
commands from the user's terminal and create processes to execute the
programs specified by those commands. It can do this using the
fork (see section Creating a Process) and exec
(see section Executing a File) functions.
A single command may run just one process--but often one command uses
several processes. If you use the `|' operator in a shell command,
you explicitly request several programs in their own processes. But
even if you run just one program, it can use multiple processes
internally. For example, a single compilation command such as `cc
-c foo.c' typically uses four processes (though normally only two at any
given time). If you run make, its job is to run other programs
in separate processes.
The processes belonging to a single command are called a process
group or job. This is so that you can operate on all of them at
once. For example, typing C-c sends the signal SIGINT to
terminate all the processes in the foreground process group.
A session is a larger group of processes. Normally all the proccesses that stem from a single login belong to the same session.
Every process belongs to a process group. When a process is created, it
becomes a member of the same process group and session as its parent
process. You can put it in another process group using the
setpgid function, provided the process group belongs to the same
session.
The only way to put a process in a different session is to make it the
initial process of a new session, or a session leader, using the
setsid function. This also puts the session leader into a new
process group, and you can't move it out of that process group again.
Usually, new sessions are created by the system login program, and the session leader is the process running the user's login shell.
A shell that supports job control must arrange to control which job can use the terminal at any time. Otherwise there might be multiple jobs trying to read from the terminal at once, and confusion about which process should receive the input typed by the user. To prevent this, the shell must cooperate with the terminal driver using the protocol described in this chapter.
The shell can give unlimited access to the controlling terminal to only one process group at a time. This is called the foreground job on that controlling terminal. Other process groups managed by the shell that are executing without such access to the terminal are called background jobs.
If a background job needs to read from or write to its controlling
terminal, it is stopped by the terminal driver. The user can stop
a foreground job by typing the SUSP character (see section Special Characters) and a program can stop any job by sending it a
SIGSTOP signal. It's the responsibility of the shell to notice
when jobs stop, to notify the user about them, and to provide mechanisms
for allowing the user to interactively continue stopped jobs and switch
jobs between foreground and background.
See section Access to the Controlling Terminal, for more information about I/O to the controlling terminal,
Not all operating systems support job control. The GNU system does support job control, but if you are using the GNU library on some other system, that system may not support job control itself.
You can use the _POSIX_JOB_CONTROL macro to test at compile-time
whether the system supports job control. See section Overall System Options.
If job control is not supported, then there can be only one process
group per session, which behaves as if it were always in the foreground.
The functions for creating additional process groups simply fail with
the error code ENOSYS.
The macros naming the various job control signals (see section Job Control Signals) are defined even if job control is not supported. However, the system never generates these signals, and attempts to send a job control signal or examine or specify their actions report errors or do nothing.
One of the attributes of a process is its controlling terminal. Child
processes created with fork inherit the controlling terminal from
their parent process. In this way, all the processes in a session
inherit the controlling terminal from the session leader. A session
leader that has control of a terminal is called the controlling
process of that terminal.
You generally do not need to worry about the exact mechanism used to allocate a controlling terminal to a session, since it is done for you by the system when you log in.
An individual process disconnects from its controlling terminal when it
calls setsid to become the leader of a new session.
See section Process Group Functions.
Processes in the foreground job of a controlling terminal have unrestricted access to that terminal; background proesses do not. This section describes in more detail what happens when a process in a background job tries to access its controlling terminal.
When a process in a background job tries to read from its controlling
terminal, the process group is usually sent a SIGTTIN signal.
This normally causes all of the processes in that group to stop (unless
they handle the signal and don't stop themselves). However, if the
reading process is ignoring or blocking this signal, then read
fails with an EIO error instead.
Similarly, when a process in a background job tries to write to its
controlling terminal, the default behavior is to send a SIGTTOU
signal to the process group. However, the behavior is modified by the
TOSTOP bit of the local modes flags (see section Local Modes). If
this bit is not set (which is the default), then writing to the
controlling terminal is always permitted without sending a signal.
Writing is also permitted if the SIGTTOU signal is being ignored
or blocked by the writing process.
Most other terminal operations that a program can do are treated as reading or as writing. (The description of each operation should say which.)
For more information about the primitive read and write
functions, see section Input and Output Primitives.
When a controlling process terminates, its terminal becomes free and a new session can be established on it. (In fact, another user could log in on the terminal.) This could cause a problem if any processes from the old session are still trying to use that terminal.
To prevent problems, process groups that continue running even after the
session leader has terminated are marked as orphaned process
groups. Processes in an orphaned process group cannot read from or
write to the controlling terminal. Attempts to do so will fail with an
EIO error.
When a process group becomes an orphan, its processes are sent a
SIGHUP signal. Ordinarily, this causes the processes to
terminate. However, if a program ignores this signal or establishes a
handler for it (see section Signal Handling), it can continue running as in
the orphan process group even after its controlling process terminates;
but it still cannot access the terminal any more.
This section describes what a shell must do to implement job control, by presenting an extensive sample program to illustrate the concepts involved.
All of the program examples included in this chapter are part of a simple shell program. This section presents data structures and utility functions which are used throughout the example.
The sample shell deals mainly with two data structures. The
job type contains information about a job, which is a
set of subprocesses linked together with pipes. The process type
holds information about a single subprocess. Here are the relevant
data structure declarations:
/* A process is a single process. */
typedef struct process
{
struct process *next; /* next process in pipeline */
char **argv; /* for exec */
pid_t pid; /* process ID */
char completed; /* true if process has completed */
char stopped; /* true if process has stopped */
int status; /* reported status value */
} process;
/* A job is a pipeline of processes. */
typedef struct job
{
struct job *next; /* next active job */
char *command; /* command line, used for messages */
process *first_process; /* list of processes in this job */
pid_t pgid; /* process group ID */
char notified; /* true if user told about stopped job */
struct termios tmodes; /* saved terminal modes */
int stdin, stdout, stderr; /* standard i/o channels */
} job;
/* The active jobs are linked into a list. This is its head. */
job *first_job = NULL;
Here are some utility functions that are used for operating on job
objects.
/* Find the active job with the indicated pgid. */
job *
find_job (pid_t pgid)
{
job *j;
for (j = first_job; j; j = j->next)
if (j->pgid == pgid)
return j;
return NULL;
}
/* Return true if all processes in the job have stopped or completed. */
int
job_is_stopped (job *j)
{
process *p;
for (p = j->first_process; p; p = p->next)
if (!p->completed && !p->stopped)
return 0;
return 1;
}
/* Return true if all processes in the job have completed. */
int
job_is_completed (job *j)
{
process *p;
for (p = j->first_process; p; p = p->next)
if (!p->completed)
return 0;
return 1;
}
When a shell program that normally performs job control is started, it has to be careful in case it has been invoked from another shell that is already doing its own job control.
A subshell that runs interactively has to ensure that it has been placed
in the foreground by its parent shell before it can enable job control
itself. It does this by getting its initial process group ID with the
getpgrp function, and comparing it to the process group ID of the
current foreground job associated with its controlling terminal (which
can be retrieved using the tcgetpgrp function).
If the subshell is not running as a foreground job, it must stop itself
by sending a SIGTTIN signal to its own process group. It may not
arbitrarily put itself into the foreground; it must wait for the user to
tell the parent shell to do this. If the subshell is continued again,
it should repeat the check and stop itself again if it is still not in
the foreground.
Once the subshell has been placed into the foreground by its parent
shell, it can enable its own job control. It does this by calling
setpgid to put itself into its own process group, and then
calling tcsetpgrp to place this process group into the
foreground.
When a shell enables job control, it should set itself to ignore all the
job control stop signals so that it doesn't accidentally stop itself.
You can do this by setting the action for all the stop signals to
SIG_IGN.
A subshell that runs non-interactively cannot and should not support job control. It must leave all processes it creates in the same process group as the shell itself; this allows the non-interactive shell and its child processes to be treated as a single job by the parent shell. This is easy to do--just don't use any of the job control primitives--but you must remember to make the shell do it.
Here is the initialization code for the sample shell that shows how to do all of this.
/* Keep track of attributes of the shell. */
#include <sys/types.h>
#include <termios.h>
#include <unistd.h>
pid_t shell_pgid;
struct termios shell_tmodes;
int shell_terminal;
int shell_is_interactive;
/* Make sure the shell is running interactively as the foreground job
before proceeding. */
void
init_shell ()
{
/* See if we are running interactively. */
shell_terminal = STDIN_FILENO;
shell_is_interactive = isatty (shell_terminal);
if (shell_is_interactive)
{
/* Loop until we are in the foreground. */
while (tcgetpgrp (shell_terminal) != (shell_pgid = getpgrp ()))
kill (- shell_pgid, SIGTTIN);
/* Ignore interactive and job-control signals. */
signal (SIGINT, SIG_IGN);
signal (SIGQUIT, SIG_IGN);
signal (SIGTSTP, SIG_IGN);
signal (SIGTTIN, SIG_IGN);
signal (SIGTTOU, SIG_IGN);
signal (SIGCHLD, SIG_IGN);
/* Put ourselves in our own process group. */
shell_pgid = getpid ();
if (setpgid (shell_pgid, shell_pgid) < 0)
{
perror ("Couldn't put the shell in its own process group");
exit (1);
}
/* Grab control of the terminal. */
tcsetpgrp (shell_terminal, shell_pgid);
/* Save default terminal attributes for shell. */
tcgetattr (shell_terminal, &shell_tmodes);
}
}
Once the shell has taken responsibility for performing job control on its controlling terminal, it can launch jobs in response to commands typed by the user.
To create the processes in a process group, you use the same fork
and exec functions described in section Process Creation Concepts.
Since there are multiple child processes involved, though, things are a
little more complicated and you must be careful to do things in the
right order. Otherwise, nasty race conditions can result.
You have two choices for how to structure the tree of parent-child relationships among the processes. You can either make all the processes in the process group be children of the shell process, or you can make one process in group be the ancestor of all the other processes in that group. The sample shell program presented in this chapter uses the first approach because it makes bookkeeping somewhat simpler.
As each process is forked, it should put itself in the new process group
by calling setpgid; see section Process Group Functions. The first
process in the new group becomes its process group leader, and its
process ID becomes the process group ID for the group.
The shell should also call setpgid to put each of its child
processes into the new process group. This is because there is a
potential timing problem: each child process must be put in the process
group before it begins executing a new program, and the shell depends on
having all the child processes in the group before it continues
executing. If both the child processes and the shell call
setpgid, this ensures that the right things happen no matter which
process gets to it first.
If the job is being launched as a foreground job, the new process group
also needs to be put into the foreground on the controlling terminal
using tcsetpgrp. Again, this should be done by the shell as well
as by each of its child processes, to avoid race conditions.
The next thing each child process should do is to reset its signal actions.
During initialization, the shell process set itself to ignore job
control signals; see section Initializing the Shell. As a result, any child
processes it creates also ignore these signals by inheritance. This is
definitely undesirable, so each child process should explicitly set the
actions for these signals back to SIG_DFL just after it is forked.
Since shells follow this convention, applications can assume that they
inherit the correct handling of these signals from the parent process.
But every application has a responsibility not to mess up the handling
of stop signals. Applications that disable the normal interpretation of
the SUSP character should provide some other mechanism for the user to
stop the job. When the user invokes this mechanism, the program should
send a SIGTSTP signal to the process group of the process, not
just to the process itself. See section Signaling Another Process.
Finally, each child process should call exec in the normal way.
This is also the point at which redirection of the standard input and
output channels should be handled. See section Duplicating Descriptors,
for an explanation of how to do this.
Here is the function from the sample shell program that is responsible for launching a program. The function is executed by each child process immediately after it has been forked by the shell, and never returns.
void
launch_process (process *p, pid_t pgid,
int infile, int outfile, int errfile,
int foreground)
{
pid_t pid;
if (shell_is_interactive)
{
/* Put the process into the process group and give the process group
the terminal, if appropriate.
This has to be done both by the shell and in the individual
child processes because of potential race conditions. */
pid = getpid ();
if (pgid == 0) pgid = pid;
setpgid (pid, pgid);
if (foreground)
tcsetpgrp (shell_terminal, pgid);
/* Set the handling for job control signals back to the default. */
signal (SIGINT, SIG_DFL);
signal (SIGQUIT, SIG_DFL);
signal (SIGTSTP, SIG_DFL);
signal (SIGTTIN, SIG_DFL);
signal (SIGTTOU, SIG_DFL);
signal (SIGCHLD, SIG_DFL);
}
/* Set the standard input/output channels of the new process. */
if (infile != STDIN_FILENO)
{
dup2 (infile, STDIN_FILENO);
close (infile);
}
if (outfile != STDOUT_FILENO)
{
dup2 (outfile, STDOUT_FILENO);
close (outfile);
}
if (errfile != STDERR_FILENO)
{
dup2 (errfile, STDERR_FILENO);
close (errfile);
}
/* Exec the new process. Make sure we exit. */
execvp (p->argv[0], p->argv);
perror ("execvp");
exit (1);
}
If the shell is not running interactively, this function does not do anything with process groups or signals. Remember that a shell not performing job control must keep all of its subprocesses in the same process group as the shell itself.
Next, here is the function that actually launches a complete job. After creating the child processes, this function calls some other functions to put the newly created job into the foreground or background; these are discussed in section Foreground and Background.
void
launch_job (job *j, int foreground)
{
process *p;
pid_t pid;
int mypipe[2], infile, outfile;
infile = j->stdin;
for (p = j->first_process; p; p = p->next)
{
/* Set up pipes, if necessary. */
if (p->next)
{
if (pipe (mypipe) < 0)
{
perror ("pipe");
exit (1);
}
outfile = mypipe[1];
}
else
outfile = j->stdout;
/* Fork the child processes. */
pid = fork ();
if (pid == 0)
/* This is the child process. */
launch_process (p, j->pgid, infile, outfile, j->stderr, foreground);
else if (pid < 0)
{
/* The fork failed. */
perror ("fork");
exit (1);
}
else
{
/* This is the parent process. */
p->pid = pid;
if (shell_is_interactive)
{
if (!j->pgid)
j->pgid = pid;
setpgid (pid, j->pgid);
}
}
/* Clean up after pipes. */
if (infile != j->stdin)
close (infile);
if (outfile != j->stdout)
close (outfile);
infile = mypipe[0];
}
format_job_info (j, "launched");
if (!shell_is_interactive)
wait_for_job (j);
else if (foreground)
put_job_in_foreground (j, 0);
else
put_job_in_background (j, 0);
}
Now let's consider what actions must be taken by the shell when it launches a job into the foreground, and how this differs from what must be done when a background job is launched.
When a foreground job is launched, the shell must first give it access
to the controlling terminal by calling tcsetpgrp. Then, the
shell should wait for processes in that process group to terminate or
stop. This is discussed in more detail in section Stopped and Terminated Jobs.
When all of the processes in the group have either completed or stopped,
the shell should regain control of the terminal for its own process
group by calling tcsetpgrp again. Since stop signals caused by
I/O from a background process or a SUSP character typed by the user
are sent to the process group, normally all the processes in the job
stop together.
The foreground job may have left the terminal in a strange state, so the
shell should restore its own saved terminal modes before continuing. In
case the job is merely been stopped, the shell should first save the
current terminal modes so that it can restore them later if the job is
continued. The functions for dealing with terminal modes are
tcgetattr and tcsetattr; these are described in
section Terminal Modes.
Here is the sample shell's function for doing all of this.
/* Put job j in the foreground. If cont is nonzero,
restore the saved terminal modes and send the process group a
SIGCONT signal to wake it up before we block. */
void
put_job_in_foreground (job *j, int cont)
{
/* Put the job into the foreground. */
tcsetpgrp (shell_terminal, j->pgid);
/* Send the job a continue signal, if necessary. */
if (cont)
{
tcsetattr (shell_terminal, TCSADRAIN, &j->tmodes);
if (kill (- j->pgid, SIGCONT) < 0)
perror ("kill (SIGCONT)");
}
/* Wait for it to report. */
wait_for_job (j);
/* Put the shell back in the foreground. */
tcsetpgrp (shell_terminal, shell_pgid);
/* Restore the shell's terminal modes. */
tcgetattr (shell_terminal, &j->tmodes);
tcsetattr (shell_terminal, TCSADRAIN, &shell_tmodes);
}
If the process group is launched as a background job, the shell should remain in the foreground itself and continue to read commands from the terminal.
In the sample shell, there is not much that needs to be done to put a job into the background. Here is the function it uses:
/* Put a job in the background. If the cont argument is true, send
the process group a SIGCONT signal to wake it up. */
void
put_job_in_background (job *j, int cont)
{
/* Send the job a continue signal, if necessary. */
if (cont)
if (kill (-j->pgid, SIGCONT) < 0)
perror ("kill (SIGCONT)");
}
When a foreground process is launched, the shell must block until all of
the processes in that job have either terminated or stopped. It can do
this by calling the waitpid function; see section Process Completion. Use the WUNTRACED option so that status is reported
for processes that stop as well as processes that terminate.
The shell must also check on the status of background jobs so that it
can report terminated and stopped jobs to the user; this can be done by
calling waitpid with the WNOHANG option. A good place to
put a such a check for terminated and stopped jobs is just before
prompting for a new command.
The shell can also receive asynchronous notification that there is
status information available for a child process by establishing a
handler for SIGCHLD signals. See section Signal Handling.
In the sample shell program, the SIGCHLD signal is normally
ignored. This is to avoid reentrancy problems involving the global data
structures the shell manipulates. But at specific times when the shell
is not using these data structures--such as when it is waiting for
input on the terminal--it makes sense to enable a handler for
SIGCHLD. The same function that is used to do the synchronous
status checks (do_job_notification, in this case) can also be
called from within this handler.
Here are the parts of the sample shell program that deal with checking the status of jobs and reporting the information to the user.
/* Store the status of the process pid that was returned by waitpid.
Return 0 if all went well, nonzero otherwise. */
int
mark_process_status (pid_t pid, int status)
{
job *j;
process *p;
if (pid > 0)
{
/* Update the record for the process. */
for (j = first_job; j; j = j->next)
for (p = j->first_process; p; p = p->next)
if (p->pid == pid)
{
p->status = status;
if (WIFSTOPPED (status))
p->stopped = 1;
else
{
p->completed = 1;
if (WIFSIGNALED (status))
fprintf (stderr, "%d: Terminated by signal %d.\n",
(int) pid, WTERMSIG (p->status));
}
return 0;
}
fprintf (stderr, "No child process %d.\n", pid);
return -1;
}
else if (pid == 0 || errno == ECHILD)
/* No processes ready to report. */
return -1;
else {
/* Other weird errors. */
perror ("waitpid");
return -1;
}
}
/* Check for processes that have status information available,
without blocking. */
void
update_status (void)
{
int status;
pid_t pid;
do
pid = waitpid (WAIT_ANY, &status, WUNTRACED|WNOHANG);
while (!mark_process_status (pid, status));
}
/* Check for processes that have status information available,
blocking until all processes in the given job have reported. */
void
wait_for_job (job *j)
{
int status;
pid_t pid;
do
pid = waitpid (WAIT_ANY, &status, WUNTRACED);
while (!mark_process_status (pid, status)
&& !job_is_stopped (j)
&& !job_is_completed (j));
}
/* Format information about job status for the user to look at. */
void
format_job_info (job *j, const char *status)
{
fprintf (stderr, "%ld (%s): %s\n", (long)j->pgid, status, j->command);
}
/* Notify the user about stopped or terminated jobs.
Delete terminated jobs from the active job list. */
void
do_job_notification (void)
{
job *j, *jlast, *jnext;
process *p;
/* Update status information for child processes. */
update_status ();
jlast = NULL;
for (j = first_job; j; j = jnext)
{
jnext = j->next;
/* If all processes have completed, tell the user the job has
completed and delete it from the list of active jobs. */
if (job_is_completed (j)) {
format_job_info (j, "completed");
if (jlast)
jlast->next = jnext;
else
first_job = jnext;
free_job (j);
}
/* Notify the user about stopped jobs,
marking them so that we won't do this more than once. */
else if (job_is_stopped (j) && !j->notified) {
format_job_info (j, "stopped");
j->notified = 1;
jlast = j;
}
/* Don't say anything about jobs that are still running. */
else
jlast = j;
}
}
The shell can continue a stopped job by sending a SIGCONT signal
to its process group. If the job is being continued in the foreground,
the shell should first invoke tcsetpgrp to give the job access to
the terminal, and restore the saved terminal settings. After continuing
a job in the foreground, the shell should wait for the job to stop or
complete, as if the job had just been launched in the foreground.
The sample shell program uses the same set of
functions---put_job_in_foreground and
put_job_in_background---to handle both newly created and
continued jobs. The definitions of these functions were given in
section Foreground and Background. When continuing a stopped job, a
nonzero value is passed as the cont argument to ensure that the
SIGCONT signal is sent and the terminal modes reset, as
appropriate.
This leaves only a function for updating the shell's internal bookkeeping about the job being continued:
/* Mark a stopped job J as being running again. */
void
mark_job_as_running (job *j)
{
Process *p;
for (p = j->first_process; p; p = p->next)
p->stopped = 0;
j->notified = 0;
}
/* Continue the job J. */
void
continue_job (job *j, int foreground)
{
mark_job_as_running (j);
if (foreground)
put_job_in_foreground (j, 1);
else
put_job_in_background (j, 1);
}
The code extracts for the sample shell included in this chapter are only
a part of the entire shell program. In particular, nothing at all has
been said about how job and program data structures are
allocated and initialized.
Most real shells provide a complex user interface that has support for a command language; variables; abbreviations, substitutions, and pattern matching on file names; and the like. All of this is far too complicated to explain here! Instead, we have concentrated on showing how to implement the core process creation and job control functions that can be called from such a shell.
Here is a table summarizing the major entry points we have presented:
void init_shell (void)
void launch_job (job *j, int foreground)
void do_job_notification (void)
SIGCHLD signals.
See section Stopped and Terminated Jobs.
void continue_job (job *j, int foreground)
Of course, a real shell would also want to provide other functions for
managing jobs. For example, it would be useful to have commands to list
all active jobs or to send a signal (such as SIGKILL) to a job.
This section contains detailed descriptions of the functions relating to job control.
You can use the ctermid function to get a file name that you can
use to open the controlling terminal. In the GNU library, it returns
the same string all the time: "/dev/tty". That is a special
"magic" file name that refers to the controlling terminal of the
current process (if it has one). The function ctermid is
declared in the header file `stdio.h'.
Function: char * ctermid (char *string)
The ctermid function returns a string containing the file name of
the controlling terminal for the current process. If string is
not a null pointer, it should be an array that can hold at least
L_ctermid characters; the string is returned in this array.
Otherwise, a pointer to a string in a static area is returned, which
might get overwritten on subsequent calls to this function.
An empty string is returned if the file name cannot be determined for any reason. Even if a file name is returned, access to the file it represents is not guaranteed.
The value of this macro is an integer constant expression that
represents the size of a string large enough to hold the file name
returned by ctermid.
See also the isatty and ttyname functions, in
section Identifying Terminals.
Here are descriptions of the functions for manipulating process groups. Your program should include the header files `sys/types.h' and `unistd.h' to use these functions.
The setsid function creates a new session. The calling process
becomes the session leader, and is put in a new process group whose
process group ID is the same as the process ID of that process. There
are initially no other processes in the new process group, and no other
process groups in the new session.
This function also makes the calling process have no controlling terminal.
The setsid function returns the new process group ID of the
calling process if successful. A return value of -1 indicates an
error. The following errno error conditions are defined for this
function:
EPERM
The getpgrp function has two definitions: one derived from BSD
Unix, and one from the POSIX.1 standard. The feature test macros you
have selected (see section Feature Test Macros) determine which definition
you get. Specifically, you get the BSD version if you define
_BSD_SOURCE; otherwise, you get the POSIX version if you define
_POSIX_SOURCE or _GNU_SOURCE. Programs written for old
BSD systems will not include `unistd.h', which defines
getpgrp specially under _BSD_SOURCE. You must link such
programs with the -lbsd-compat option to get the BSD definition.
POSIX.1 Function: pid_t getpgrp (void)
The POSIX.1 definition of getpgrp returns the process group ID of
the calling process.
BSD Function: pid_t getpgrp (pid_t pid)
The BSD definition of getpgrp returns the process group ID of the
process pid. You can supply a value of 0 for the pid
argument to get information about the calling process.
Function: int setpgid (pid_t pid, pid_t pgid)
The setpgid function puts the process pid into the process
group pgid. As a special case, either pid or pgid can
be zero to indicate the process ID of the calling process.
This function fails on a system that does not support job control. See section Job Control is Optional, for more information.
If the operation is successful, setpgid returns zero. Otherwise
it returns -1. The following errno error conditions are
defined for this function:
EACCES
exec
function since it was forked.
EINVAL
ENOSYS
EPERM
ESRCH
Function: int setpgrp (pid_t pid, pid_t pgid)
This is the BSD Unix name for setpgid. Both functions do exactly
the same thing.
These are the functions for reading or setting the foreground process group of a terminal. You should include the header files `sys/types.h' and `unistd.h' in your application to use these functions.
Although these functions take a file descriptor argument to specify the terminal device, the foreground job is associated with the terminal file itself and not a particular open file descriptor.
Function: pid_t tcgetpgrp (int filedes)
This function returns the process group ID of the foreground process group associated with the terminal open on descriptor filedes.
If there is no foreground process group, the return value is a number
greater than 1 that does not match the process group ID of any
existing process group. This can happen if all of the processes in the
job that was formerly the foreground job have terminated, and no other
job has yet been moved into the foreground.
In case of an error, a value of -1 is returned. The
following errno error conditions are defined for this function:
EBADF
ENOSYS
ENOTTY
Function: int tcsetpgrp (int filedes, pid_t pgid)
This function is used to set a terminal's foreground process group ID. The argument filedes is a descriptor which specifies the terminal; pgid specifies the process group. The calling process must be a member of the same session as pgid and must have the same controlling terminal.
For terminal access purposes, this function is treated as output. If it
is called from a background process on its controlling terminal,
normally all processes in the process group are sent a SIGTTOU
signal. The exception is if the calling process itself is ignoring or
blocking SIGTTOU signals, in which case the operation is
performed and no signal is sent.
If successful, tcsetpgrp returns 0. A return value of
-1 indicates an error. The following errno error
conditions are defined for this function:
EBADF
EINVAL
ENOSYS
ENOTTY
EPERM
Every user who can log in on the system is identified by a unique number called the user ID. Each process has an effective user ID which says which user's access permissions it has.
Users are classified into groups for access control purposes. Each process has one or more group ID values which say which groups the process can use for access to files.
The effective user and group IDs of a process collectively form its persona. This determines which files the process can access. Normally, a process inherits its persona from the parent process, but under special circumstances a process can change its persona and thus change its access permissions.
Each file in the system also has a user ID and a group ID. Access control works by comparing the user and group IDs of the file with those of the running process.
The system keeps a database of all the registered users, and another database of all the defined groups. There are library functions you can use to examine these databases.
Each user account on a computer system is identified by a user name (or login name) and user ID. Normally, each user name has a unique user ID, but it is possible for several login names to have the same user ID. The user names and corresponding user IDs are stored in a data base which you can access as described in section User Database.
Users are classified in groups. Each user name also belongs to one or more groups, and has one default group. Users who are members of the same group can share resources (such as files) that are not accessible to users who are not a member of that group. Each group has a group name and group ID. See section Group Database, for how to find information about a group ID or group name.
At any time, each process has a single user ID and a group ID which determine the privileges of the process. These are collectively called the persona of the process, because they determine "who it is" for purposes of access control. These IDs are also called the effective user ID and effective group ID of the process.
Your login shell starts out with a persona which consists of your user ID and your default group ID. In normal circumstances, all your other processes inherit these values.
A process also has a real user ID which identifies the user who created the process, and a real group ID which identifies that user's default group. These values do not play a role in access control, so we do not consider them part of the persona. But they are also important.
Both the real and effective user ID can be changed during the lifetime of a process. See section Why Change the Persona of a Process?.
In addition, a user can belong to multiple groups, so the persona includes supplementary group IDs that also contribute to access permission.
For details on how a process's effective user IDs and group IDs affect its permission to access files, see section How Your Access to a File is Decided.
The user ID of a process also controls permissions for sending signals
using the kill function. See section Signaling Another Process.
The most obvious situation where it is necessary for a process to change
its user and/or group IDs is the login program. When
login starts running, its user ID is root. Its job is to
start a shell whose user and group IDs are those of the user who is
logging in. (To accomplish this fully, login must set the real
user and group IDs as well as its persona. But this is a special case.)
The more common case of changing persona is when an ordinary user program needs access to a resource that wouldn't ordinarily be accessible to the user actually running it.
For example, you may have a file that is controlled by your program but that shouldn't be read or modified directly by other users, either because it implements some kind of locking protocol, or because you want to preserve the integrity or privacy of the information it contains. This kind of restricted access can be implemented by having the program change its effective user or group ID to match that of the resource.
Thus, imagine a game program that saves scores in a file. The game
program itself needs to be able to update this file no matter who is
running it, but if users can write the file without going through the
game, they can give themselves any scores they like. Some people
consider this undesirable, or even reprehensible. It can be prevented
by creating a new user ID and login name (say, games) to own the
scores file, and make the file writable only by this user. Then, when
the game program wants to update this file, it can change its effective
user ID to be that for games. In effect, the program must
adopt the persona of games so it can write the scores file.
The ability to change the persona of a process can be a source of unintentional privacy violations, or even intentional abuse. Because of the potential for problems, changing persona is restricted to special circumstances.
You can't arbitrarily set your user ID or group ID to anything you want; only privileged processes can do that. Instead, the normal way for a program to change its persona is that it has been set up in advance to change to a particular user or group. This is the function of the setuid and setgid bits of a file's access mode. See section The Mode Bits for Access Permission.
When the setuid bit of an executable file is set, executing that file automatically changes the effective user ID to the user that owns the file. Likewise, executing a file whose setgid bit is set changes the effective group ID to the group of the file. See section Executing a File. Creating a file that changes to a particular user or group ID thus requires full access to that user or group ID.
See section File Attributes, for a more general discussion of file modes and accessibility.
A process can always change its effective user (or group) ID back to its real ID. Programs do this so as to turn off their special privileges when they are not needed, which makes for more robustness.
Here are detailed descriptions of the functions for reading the user and group IDs of a process, both real and effective. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'.
This is an integer data type used to represent user IDs. In the GNU
library, this is an alias for unsigned int.
This is an integer data type used to represent group IDs. In the GNU
library, this is an alias for unsigned int.
The getuid function returns the real user ID of the process.
The getgid function returns the real group ID of the process.
Function: uid_t geteuid (void)
The geteuid function returns the effective user ID of the process.
Function: gid_t getegid (void)
The getegid function returns the effective group ID of the process.
Function: int getgroups (int count, gid_t *groups)
The getgroups function is used to inquire about the supplementary
group IDs of the process. Up to count of these group IDs are
stored in the array groups; the return value from the function is
the number of group IDs actually stored. If count is smaller than
the total number of supplementary group IDs, then getgroups
returns a value of -1 and errno is set to EINVAL.
If count is zero, then getgroups just returns the total
number of supplementary group IDs. On systems that do not support
supplementary groups, this will always be zero.
Here's how to use getgroups to read all the supplementary group
IDs:
gid_t *
read_all_groups (void)
{
int ngroups = getgroups (NULL, 0);
gid_t *groups = (gid_t *) xmalloc (ngroups * sizeof (gid_t));
int val = getgroups (ngroups, groups);
if (val < 0)
{
free (groups);
return NULL;
}
return groups;
}
This section describes the functions for altering the user ID (real and/or effective) of a process. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'.
Function: int setuid (uid_t newuid)
This function sets both the real and effective user ID of the process to newuid, provided that the process has appropriate privileges.
If the process is not privileged, then newuid must either be equal
to the real user ID or the saved user ID (if the system supports the
_POSIX_SAVED_IDS feature). In this case, setuid sets only
the effective user ID and not the real user ID.
The setuid function returns a value of 0 to indicate
successful completion, and a value of -1 to indicate an error.
The following errno error conditions are defined for this
function:
EINVAL
EPERM
Function: int setreuid (uid_t ruid, uid_t euid)
This function sets the real user ID of the process to ruid and the effective user ID to euid.
The setreuid function exists for compatibility with 4.3 BSD Unix,
which does not support saved IDs. You can use this function to swap the
effective and real user IDs of the process. (Privileged processes are
not limited to this particular usage.) If saved IDs are supported, you
should use that feature instead of this function. See section Enabling and Disabling Setuid Access.
The return value is 0 on success and -1 on failure.
The following errno error conditions are defined for this
function:
EPERM
This section describes the functions for altering the group IDs (real and effective) of a process. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'.
Function: int setgid (gid_t newgid)
This function sets both the real and effective group ID of the process to newgid, provided that the process has appropriate privileges.
If the process is not privileged, then newgid must either be equal
to the real group ID or the saved group ID. In this case, setgid
sets only the effective group ID and not the real group ID.
The return values and error conditions for setgid are the same
as those for setuid.
Function: int setregid (gid_t rgid, fid_t egid)
This function sets the real group ID of the process to rgid and the effective group ID to egid.
The setregid function is provided for compatibility with 4.3 BSD
Unix, which does not support saved IDs. You can use this function to
swap the effective and real group IDs of the process. (Privileged
processes are not limited to this usage.) If saved IDs are supported,
you should use that feature instead of using this function.
See section Enabling and Disabling Setuid Access.
The return values and error conditions for setregid are the same
as those for setreuid.
The GNU system also lets privileged processes change their supplementary
group IDs. To use setgroups or initgroups, your programs
should include the header file `grp.h'.
Function: int setgroups (size_t count, gid_t *groups)
This function sets the process's supplementary group IDs. It can only be called from privileged processes. The count argument specifies the number of group IDs in the array groups.
This function returns 0 if successful and -1 on error.
The following errno error conditions are defined for this
function:
EPERM
Function: int initgroups (const char *user, gid_t gid)
The initgroups function effectively calls setgroups to
set the process's supplementary group IDs to be the normal default for
the user name user. The group ID gid is also included.
A typical setuid program does not need its special access all of the time. It's a good idea to turn off this access when it isn't needed, so it can't possibly give unintended access.
If the system supports the saved user ID feature, you can accomplish
this with setuid. When the game program starts, its real user ID
is jdoe, its effective user ID is games, and its saved
user ID is also games. The program should record both user ID
values once at the beginning, like this:
user_user_id = getuid (); game_user_id = geteuid ();
Then it can turn off game file access with
setuid (user_user_id);
and turn it on with
setuid (game_user_id);
Throughout this process, the real user ID remains jdoe and the
saved user ID remains games, so the program can always set its
effective user ID to either one.
On other systems that don't support the saved user ID feature, you can
turn setuid access on and off by using setreuid to swap the real
and effective user IDs of the process, as follows:
setreuid (geteuid (), getuid ());
This special case is always allowed--it cannot fail.
Why does this have the effect of toggling the setuid access? Suppose a
game program has just started, and its real user ID is jdoe while
its effective user ID is games. In this state, the game can
write the scores file. If it swaps the two uids, the real becomes
games and the effective becomes jdoe; now the program has
only jdoe access. Another swap brings games back to
the effective user ID and restores access to the scores file.
In order to handle both kinds of systems, test for the saved user ID feature with a preprocessor conditional, like this:
#ifdef _POSIX_SAVED_IDS setuid (user_user_id); #else setreuid (geteuid (), getuid ()); #endif
Here's an example showing how to set up a program that changes its effective user ID.
This is part of a game program called caber-toss that
manipulates a file `scores' that should be writable only by the game
program itself. The program assumes that its executable
file will be installed with the set-user-ID bit set and owned by the
same user as the `scores' file. Typically, a system
administrator will set up an account like games for this purpose.
The executable file is given mode 4755, so that doing an
`ls -l' on it produces output like:
-rwsr-xr-x 1 games 184422 Jul 30 15:17 caber-toss
The set-user-ID bit shows up in the file modes as the `s'.
The scores file is given mode 644, and doing an `ls -l' on
it shows:
-rw-r--r-- 1 games 0 Jul 31 15:33 scores
Here are the parts of the program that show how to set up the changed
user ID. This program is conditionalized so that it makes use of the
saved IDs feature if it is supported, and otherwise uses setreuid
to swap the effective and real user IDs.
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
/* Save the effective and real UIDs. */
static uid_t euid, ruid;
/* Restore the effective UID to its original value. */
void
do_setuid (void)
{
int status;
#ifdef _POSIX_SAVED_IDS
status = setuid (euid);
#else
status = setreuid (ruid, euid);
#endif
if (status < 0) {
fprintf (stderr, "Couldn't set uid.\n");
exit (status);
}
}
/* Set the effective UID to the real UID. */
void
undo_setuid (void)
{
int status;
#ifdef _POSIX_SAVED_IDS
status = setuid (ruid);
#else
status = setreuid (euid, ruid);
#endif
if (status < 0) {
fprintf (stderr, "Couldn't set uid.\n");
exit (status);
}
}
/* Main program. */
int
main (void)
{
/* Save the real and effective user IDs. */
ruid = getuid ();
euid = geteuid ();
undo_setuid ();
/* Do the game and record the score. */
...
}
Notice how the first thing the main function does is to set the
effective user ID back to the real user ID. This is so that any other
file accesses that are performed while the user is playing the game use
the real user ID for determining permissions. Only when the program
needs to open the scores file does it switch back to the original
effective user ID, like this:
/* Record the score. */
int
record_score (int score)
{
FILE *stream;
char *myname;
/* Open the scores file. */
do_setuid ();
stream = fopen (SCORES_FILE, "a");
undo_setuid ();
/* Write the score to the file. */
if (stream)
{
myname = cuserid (NULL);
if (score < 0)
fprintf (stream, "%10s: Couldn't lift the caber.\n", myname);
else
fprintf (stream, "%10s: %d feet.\n", myname, score);
fclose (stream);
return 0;
}
else
return -1;
}
It is easy for setuid programs to give the user access that isn't intended--in fact, if you want to avoid this, you need to be careful. Here are some guidelines for preventing unintended access and minimizing its consequences when it does occur:
setuid programs with privileged user IDs such as
root unless it is absolutely necessary. If the resource is
specific to your particular program, it's better to define a new,
nonprivileged user ID or group ID just to manage that resource.
system and exec functions in
combination with changing the effective user ID. Don't let users of
your program execute arbitrary programs under a changed user ID.
Executing a shell is especially bad news. Less obviously, the
execlp and execvp functions are a potential risk (since
the program they execute depends on the user's PATH environment
variable).
If you must exec another program under a changed ID, specify an
absolute file name (see section File Name Resolution) for the executable,
and make sure that the protections on that executable and all
containing directories are such that ordinary users cannot replace it
with some other program.
setuid part of your program needs to access other files
besides the controlled resource, it should verify that the real user
would ordinarily have permission to access those files. You can use the
access function (see section How Your Access to a File is Decided) to check this; it
uses the real user and group IDs, rather than the effective IDs.
You can use the functions listed in this section to determine the login
name of the user who is running a process, and the name of the user who
logged in the current session. See also the function getuid and
friends (see section Reading the Persona of a Process).
The getlogin function is declared in `unistd.h', while
cuserid and L_cuserid are declared in `stdio.h'.
Function: char * getlogin (void)
The getlogin function returns a pointer to a string containing the
name of the user logged in on the controlling terminal of the process,
or a null pointer if this information cannot be determined. The string
is statically allocated and might be overwritten on subsequent calls to
this function or to cuserid.
Function: char * cuserid (char *string)
The cuserid function returns a pointer to a string containing a
user name associated with the effective ID of the process. If
string is not a null pointer, it should be an array that can hold
at least L_cuserid characters; the string is returned in this
array. Otherwise, a pointer to a string in a static area is returned.
This string is statically allocated and might be overwritten on
subsequent calls to this function or to getlogin.
An integer constant that indicates how long an array you might need to store a user name.
These functions let your program identify positively the user who is running or the user who logged in this session. (These can differ when setuid programs are involved; See section The Persona of a Process.) The user cannot do anything to fool these functions.
For most purposes, it is more useful to use the environment variable
LOGNAME to find out who the user is. This is more flexible
precisely because the user can set LOGNAME arbitrarily.
See section Standard Environment Variables.
This section describes all about now to search and scan the database of registered users. The database itself is kept in the file `/etc/passwd' on most systems, but on some systems a special network server gives access to it.
The functions and data structures for accessing the system user database are declared in the header file `pwd.h'.
The passwd data structure is used to hold information about
entries in the system user data base. It has at least the following members:
char *pw_name
char *pw_passwd.
uid_t pw_uid
gid_t pw_gid
char *pw_gecos
char *pw_dir
char *pw_shell
You can search the system user database for information about a
specific user using getpwuid or getpwnam. These
functions are declared in `pwd.h'.
Function: struct passwd * getpwuid (uid_t uid)
This function returns a pointer to a statically-allocated structure
containing information about the user whose user ID is uid. This
structure may be overwritten on subsequent calls to getpwuid.
A null pointer value indicates there is no user in the data base with user ID uid.
Function: struct passwd * getpwnam (const char *name)
This function returns a pointer to a statically-allocated structure
containing information about the user whose user name is name.
This structure may be overwritten on subsequent calls to
getpwnam.
A null pointer value indicates there is no user named name.
This section explains how a program can read the list of all users in the system, one user at a time. The functions described here are declared in `pwd.h'.
The recommended way to scan the users is to open the user file and
then call fgetpwent for each successive user:
Function: struct passwd * fgetpwent (FILE *stream)
This function reads the next user entry from stream and returns a
pointer to the entry. The structure is statically allocated and is
rewritten on subsequent calls to getpwent. You must copy the
contents of the structure if you wish to save the information.
This stream must correspond to a file in the same format as the standard password database file. This function comes from System V.
Another way to scan all the entries in the group database is with
setpwent, getpwent, and endpwent. But this method
is less robust than fgetpwent, so we provide it only for
compatibility with SVID. In particular, these functions are not
reentrant and are not suitable for use in programs with multiple threads
of control.
Function: void setpwent (void)
This function initializes a stream which getpwent uses to read
the user database.
Function: struct passwd * getpwent (void)
The getpwent function reads the next entry from the stream
initialized by setpwent. It returns a pointer to the entry. The
structure is statically allocated and is rewritten on subsequent calls
to getpwent. You must copy the contents of the structure if you
wish to save the information.
Function: void endpwent (void)
This function closes the internal stream used by getpwent.
Function: int putpwent (const struct passwd *p, FILE *stream)
This function writes the user entry *p to the stream
stream, in the format used for the standard user database
file. The return value is zero on success and nonzero on failure.
This function exists for compatibility with SVID. We recommend that you
avoid using it, because it makes sense only on the assumption that the
struct passwd structure has no members except the standard ones;
on a system which merges the traditional Unix data base with other
extended information about users, adding an entry using this function
would inevitably leave out much of the important information.
The function putpwent is declared in `pwd.h'.
This section describes all about how to search and scan the database of registered groups. The database itself is kept in the file `/etc/group' on most systems, but on some systems a special network service provides access to it.
The functions and data structures for accessing the system group database are declared in the header file `grp.h'.
The group structure is used to hold information about an entry in
the system group database. It has at least the following members:
char *gr_name
gid_t gr_gid
char **gr_mem
You can search the group database for information about a specific
group using getgrgid or getgrnam. These functions are
declared in `grp.h'.
Function: struct group * getgrgid (gid_t gid)
This function returns a pointer to a statically-allocated structure
containing information about the group whose group ID is gid.
This structure may be overwritten by subsequent calls to
getgrgid.
A null pointer indicates there is no group with ID gid.
Function: struct group * getgrnam (const char *name)
This function returns a pointer to a statically-allocated structure
containing information about the group whose group name is name.
This structure may be overwritten by subsequent calls to
getgrnam.
A null pointer indicates there is no group named name.
This section explains how a program can read the list of all groups in the system, one group at a time. The functions described here are declared in `grp.h'.
The recommended way to scan the groups is to open the group file and
then call fgetgrent for each successive group:
Function: struct group * fgetgrent (FILE *stream)
The fgetgrent function reads the next entry from stream.
It returns a pointer to the entry. The structure is statically
allocated and is rewritten on subsequent calls to getgrent. You
must copy the contents of the structure if you wish to save the
information.
The stream must correspond to a file in the same format as the standard group database file.
Another way to scan all the entries in the group database is with
setgrent, getgrent, and endgrent. But this method
is less robust than fgetgrent, so we provide it only for
compatibility with SVID. In particular, these functions are not
reentrant and are not suitable for use in programs with multiple threads
of control.
Function: void setgrent (void)
This function initializes a stream for reading from the group data base.
You use this stream by calling getgrent.
Function: struct group * getgrent (void)
The getgrent function reads the next entry from the stream
initialized by setgrent. It returns a pointer to the entry. The
structure is statically allocated and is rewritten on subsequent calls
to getgrent. You must copy the contents of the structure if you
wish to save the information.
Function: void endgrent (void)
This function closes the internal stream used by getgrent.
Here is an example program showing the use of the system database inquiry functions. The program prints some information about the user running the program.
#include <grp.h>
#include <pwd.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
int
main (void)
{
uid_t me;
struct passwd *my_passwd;
struct group *my_group;
char **members;
/* Get information about the user ID. */
me = getuid ();
my_passwd = getpwuid (me);
if (!my_passwd)
{
printf ("Couldn't find out about user %d.\n", (int) me);
exit (EXIT_FAILURE);
}
/* Print the information. */
printf ("I am %s.\n", my_passwd->pw_gecos);
printf ("My login name is %s.\n", my_passwd->pw_name);
printf ("My uid is %d.\n", (int) (my_passwd->pw_uid));
printf ("My home directory is %s.\n", my_passwd->pw_dir);
printf ("My default shell is %s.\n", my_passwd->pw_shell);
/* Get information about the default group ID. */
my_group = getgrgid (my_passwd->pw_gid);
if (!my_group)
{
printf ("Couldn't find out about group %d.\n", (int) my_passwd->pw_gid);
exit (EXIT_FAILURE);
}
/* Print the information. */
printf ("My default group is %s (%d).\n",
my_group->gr_name, (int) (my_passwd->pw_gid));
printf ("The members of this group are:\n");
members = my_group->gr_mem;
while (*members)
{
printf (" %s\n", *(members));
members++;
}
return EXIT_SUCCESS;
}
Here is some output from this program:
I am Throckmorton Snurd. My login name is snurd. My uid is 31093. My home directory is /home/fsg/snurd. My default shell is /bin/sh. My default group is guest (12). The members of this group are: friedman tami
This chapter describes functions that return information about the particular machine that is in use--the type of hardware, the type of software, and the individual machine's name.
This section explains how to identify the particular machine that your program is running on. The identification of a machine consists of its Internet host name and Internet address; see section The Internet Namespace.
Prototypes for these functions appear in `unistd.h'. The shell
commands hostname and hostid work by calling them.
Function: int gethostname (char *name, size_t size)
This function returns the name of the host machine in the array name. The size argument specifies the size of this array, in bytes.
The return value is 0 on success and -1 on failure. In
the GNU C library, gethostname fails if size is not large
enough; then you can try again with a larger array. The following
errno error condition is defined for this function:
ENAMETOOLONG
On some systems, there is a symbol for the maximum possible host name
length: MAXHOSTNAMELEN. It is defined in `sys/param.h'.
But you can't count on this to exist, so it is cleaner to handle
failure and try again.
gethostname stores the beginning of the host name in name
even if the host name won't entirely fit. For some purposes, a
truncated host name is good enough. If it is, you can ignore the
error code.
Function: int sethostname (const char *name, size_t length)
The sethostname function sets the name of the host machine to
name, a string with length length. Only privileged
processes are allowed to do this. Usually it happens just once, at
system boot time.
The return value is 0 on success and -1 on failure.
The following errno error condition is defined for this function:
EPERM
Function: long int gethostid (void)
This function returns the Internet address of the machine the program is running on.
Function: int sethostid (long int id)
The sethostid function sets the address of the host machine to
id. Only privileged processes are allowed to do this. Usually it
happens just once, at system boot time.
The return value is 0 on success and -1 on failure.
The following errno error condition is defined for this function:
EPERM
You can use the uname function to find out some information about
the type of computer your program is running on. This function and the
associated data type are declared in the header file
`sys/utsname.h'.
The utsname structure is used to hold information returned
by the uname function. It has the following members:
char sysname[]
char nodename[]
gethostname;
see section Host Identification.
char release[]
char version[]
char machine[]
The GNU C Library fills in this field based on the configuration name that was specified when building and installing the library. GNU uses a three-part name to describe a system configuration; the three parts are cpu, manufacturer and system-type, and they are separated with dashes. Any possible combination of three names is potentially meaningful, but most such combinations are meaningless in practice and even the meaningful ones are not necessarily supported by any particular GNU program.
Since the value in machine is supposed to describe just the
hardware, it consists of the first two parts of the configuration name:
`cpu-manufacturer'.
Here is a list of all the possible alternatives:
"i386-anything","m68k-hp","sparc-sun","m68k-sun","m68k-sony","mips-dec"
Function: int uname (struct utsname *info)
The uname function fills in the structure pointed to by
info with information about the operating system and host machine.
A non-negative value indicates that the data was successfully stored.
-1 as the value indicates an error. The only error possible is
EFAULT, which we normally don't mention as it is always a
possibility.
The functions and macros listed in this chapter give information about configuration parameters of the operating system--for example, capacity limits, presence of optional POSIX features, and the default path for executable files (see section String-Valued Parameters).
The POSIX.1 and POSIX.2 standards specify a number of parameters that describe capacity limitations of the system. These limits can be fixed constants for a given operating system, or they can vary from machine to machine. For example, some limit values may be configurable by the system administrator, either at run time or by rebuilding the kernel, and this should not require recompiling application programs.
Each of the following limit parameters has a macro that is defined in
`limits.h' only if the system has a fixed, uniform limit for the
parameter in question. If the system allows different file systems or
files to have different limits, then the macro is undefined; use
sysconf to find out the limit that applies at a particular time
on a particular machine. See section Using sysconf.
Each of these parameters also has another macro, with a name starting with `_POSIX', which gives the lowest value that the limit is allowed to have on any POSIX system. See section Minimum Values for General Capacity Limits.
If defined, the unvarying maximum combined length of the argv and
environ arguments that can be passed to the exec functions.
If defined, the unvarying maximum number of processes that can exist with the same real user ID at any one time.
If defined, the unvarying maximum number of files that a single process can have open simultaneously.
If defined, the unvarying maximum number of streams that a single process can have open simultaneously. See section Opening Streams.
If defined, the unvarying maximum length of a time zone name. See section Functions and Variables for Time Zones.
These limit macros are always defined in `limits.h'.
The maximum number of supplementary group IDs that one process can have.
The value of this macro is actually a lower bound for the maximum. That
is, you can count on being able to have that many supplementary group
IDs, but a particular machine might let you have even more. You can use
sysconf to see whether a particular machine will let you have
more (see section Using sysconf).
The largest value that can fit in an object of type ssize_t.
Effectively, this is the limit on the number of bytes that can be read
or written in a single operation.
This macro is defined in all POSIX systems because this limit is never configurable.
The largest number of repetitions you are guaranteed is allowed in the construct `\{min,max\}' in a regular expression.
The value of this macro is actually a lower bound for the maximum. That
is, you can count on being able to have that many supplementary group
IDs, but a particular machine might let you have even more. You can use
sysconf to see whether a particular machine will let you have
more (see section Using sysconf). And even the value that sysconf tells
you is just a lower bound--larger values might work.
This macro is defined in all POSIX.2 systems, because POSIX.2 says it should always be defined even if there is no specific imposed limit.
POSIX defines certain system-specific options that not all POSIX systems support. Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee any of these features is supported; it depends on the system you are using.
You can test for the availability of a given option using the macros in
this section, together with the function sysconf. The macros are
defined only if you include `unistd.h'.
For the following macros, if the macro is defined in `unistd.h',
then the option is supported. Otherwise, the option may or may not be
supported; use sysconf to find out. See section Using sysconf.
If this symbol is defined, it indicates that the system supports job control. Otherwise, the implementation behaves as if all processes within a session belong to a single process group. See section Job Control.
If this symbol is defined, it indicates that the system remembers the effective user and group IDs of a process before it executes an executable file with the set-user-ID or set-group-ID bits set, and that explicitly changing the effective user or group IDs back to these values is permitted. If this option is not defined, then if a nonprivileged process changes its effective user or group ID to the real user or group ID of the process, it can't change it back again. See section Enabling and Disabling Setuid Access.
For the following macros, if the macro is defined in `unistd.h',
then its value indicates whether the option is supported. A value of
-1 means no, and any other value means yes. If the macro is not
defined, then the option may or may not be supported; use sysconf
to find out. See section Using sysconf.
If this symbol is defined, it indicates that the system has the POSIX.2
C compiler command, c89. The GNU C library always defines this
as 1, on the assumption that you would not have installed it if
you didn't have a C compiler.
If this symbol is defined, it indicates that the system has the POSIX.2
Fortran compiler command, fort77. The GNU C library never
defines this, because we don't know what the system has.
If this symbol is defined, it indicates that the system has the POSIX.2
asa command to interpret Fortran carriage control. The GNU C
library never defines this, because we don't know what the system has.
If this symbol is defined, it indicates that the system has the POSIX.2
localedef command. The GNU C library never defines this, because
we don't know what the system has.
If this symbol is defined, it indicates that the system has the POSIX.2
commands ar, make, and strip. The GNU C library
always defines this as 1, on the assumption that you had to have
ar and make to install the library, and it's unlikely that
strip would be absent when those are present.
Macro: long int _POSIX_VERSION
This constant represents the version of the POSIX.1 standard to which
the implementation conforms. For an implementation conforming to the
1990 POSIX.1 standard, the value is the integer 199009L.
_POSIX_VERSION is always defined (in `unistd.h') in any
POSIX system.
Usage Note: Don't try to test whether the system supports POSIX
by including `unistd.h' and then checking whether
_POSIX_VERSION is defined. On a non-POSIX system, this will
probably fail because there is no `unistd.h'. We do not know of
any way you can reliably test at compilation time whether your
target system supports POSIX or whether `unistd.h' exists.
The GNU C compiler predefines the symbol __POSIX__ if the target
system is a POSIX system. Provided you do not use any other compilers
on POSIX systems, testing defined (__POSIX__) will reliably
detect such systems.
Macro: long int _POSIX2_C_VERSION
This constant represents the version of the POSIX.2 standard which the library and system kernel support. We don't know what value this will be for the first version of the POSIX.2 standard, because the value is based on the year and month in which the standard is officially adopted.
The value of this symbol says nothing about the utilities installed on the system.
Usage Note: You can use this macro to tell whether a POSIX.1
system library supports POSIX.2 as well. Any POSIX.1 system contains
`unistd.h', so include that file and then test defined
(_POSIX2_C_VERSION).
sysconf
When your system has configurable system limits, you can use the
sysconf function to find out the value that applies to any
particular machine. The function and the associated parameter
constants are declared in the header file `unistd.h'.
sysconfFunction: long int sysconf (int parameter)
This function is used to inquire about runtime system parameters. The parameter argument should be one of the `_SC_' symbols listed below.
The normal return value from sysconf is the value you requested.
A value of -1 is returned both if the implementation does not
impose a limit, and in case of an error.
The following errno error conditions are defined for this function:
EINVAL
sysconf Parameters
Here are the symbolic constants for use as the parameter argument
to sysconf. The values are all integer constants (more
specifically, enumeration type values).
_SC_ARG_MAX
ARG_MAX.
_SC_CHILD_MAX
CHILD_MAX.
_SC_OPEN_MAX
OPEN_MAX.
_SC_STREAM_MAX
STREAM_MAX.
_SC_TZNAME_MAX
TZNAME_MAX.
_SC_NGROUPS_MAX
NGROUPS_MAX.
_SC_JOB_CONTROL
_POSIX_JOB_CONTROL.
_SC_SAVED_IDS
_POSIX_SAVED_IDS.
_SC_VERSION
_POSIX_VERSION.
_SC_CLK_TCK
CLOCKS_PER_SEC;
see section Basic CPU Time Inquiry.
_SC_2_C_DEV
c89.
_SC_2_FORT_DEV
fort77.
_SC_2_FORT_RUN
asa command to
interpret Fortran carriage control.
_SC_2_LOCALEDEF
localedef
command.
_SC_2_SW_DEV
ar,
make, and strip.
_SC_BC_BASE_MAX
obase in the bc
utility.
_SC_BC_DIM_MAX
bc
utility.
_SC_BC_SCALE_MAX
scale in the bc
utility.
_SC_BC_STRING_MAX
bc utility.
_SC_COLL_WEIGHTS_MAX
_SC_EXPR_NEST_MAX
expr utility.
_SC_LINE_MAX
_SC_VERSION
_SC_2_VERSION
sysconf
We recommend that you first test for a macro definition for the
parameter you are interested in, and call sysconf only if the
macro is not defined. For example, here is how to test whether job
control is supported:
int
have_job_control (void)
{
#ifdef _POSIX_JOB_CONTROL
return 1;
#else
int value = sysconf (_SC_JOB_CONTROL);
if (value < 0)
/* If the system is that badly wedged,
there's no use trying to go on. */
fatal (strerror (errno));
return value;
#endif
}
Here is how to get the value of a numeric limit:
int
get_child_max ()
{
#ifdef CHILD_MAX
return CHILD_MAX;
#else
int value = sysconf (_SC_CHILD_MAX);
if (value < 0)
fatal (strerror (errno));
return value;
#endif
}
Here are the names for the POSIX minimum upper bounds for the system limit parameters. The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far.
_POSIX_ARG_MAX
exec functions.
Its value is 4096.
_POSIX_CHILD_MAX
6.
_POSIX_NGROUPS_MAX
0.
_POSIX_OPEN_MAX
16.
_POSIX_SSIZE_MAX
ssize_t. Its value is 32767.
_POSIX_STREAM_MAX
8.
_POSIX_TZNAME_MAX
3.
_POSIX2_RE_DUP_MAX
255.
The POSIX.1 standard specifies a number of parameters that describe the limitations of the file system. It's possible for the system to have a fixed, uniform limit for a parameter, but this isn't the usual case. On most systems, it's possible for different file systems (and, for some parameters, even different files) to have different maximum limits. For example, this is very likely if you use NFS to mount some of the file systems from other machines.
Each of the following macros is defined in `limits.h' only if the
system has a fixed, uniform limit for the parameter in question. If the
system allows different file systems or files to have different limits,
then the macro is undefined; use pathconf or fpathconf to
find out the limit that applies to a particular file. See section Using pathconf.
Each parameter also has another macro, with a name starting with `_POSIX', which gives the lowest value that the limit is allowed to have on any POSIX system. See section Minimum Values for File System Limits.
The uniform system limit (if any) for the number of names for a given file. See section Hard Links.
The uniform system limit (if any) for the amount of text in a line of input when input editing is enabled. See section Two Styles of Input: Canonical or Not.
The uniform system limit (if any) for the total number of characters typed ahead as input. See section I/O Queues.
The uniform system limit (if any) for the length of a file name component.
The uniform system limit (if any) for the length of an entire file name (that
is, the argument given to system calls such as open).
The uniform system limit (if any) for the number of bytes that can be written atomically to a pipe. If multiple processes are writing to the same pipe simultaneously, output from different processes might be interleaved in chunks of this size. See section Pipes and FIFOs.
These are alternative macro names for some of the same information.
This is the BSD name for NAME_MAX. It is defined in
`dirent.h'.
The value of this macro is an integer constant expression that represents the maximum length of a file name string. It is defined in `stdio.h'.
Unlike PATH_MAX, this macro is defined even if there is no actual
limit imposed. In such a case, its value is typically a very large
number. This is always the case on the GNU system.
Usage Note: Don't use FILENAME_MAX as the size of an
array in which to store a file name! You can't possibly make an array
that big! Use dynamic allocation (see section Memory Allocation) instead.
POSIX defines certain system-specific options in the system calls for operating on files. Some systems support these options and others do not. Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee any of these features is supported; it depends on the system you are using. They can also vary between file systems on a single machine.
This section describes the macros you can test to determine whether a
particular option is supported on your machine. If a given macro is
defined in `unistd.h', then its value says whether the
corresponding feature is supported. (A value of -1 indicates no;
any other value indicates yes.) If the macro is undefined, it means
particular files may or may not support the feature.
Since all the machines that support the GNU C library also support NFS,
one can never make a general statement about whether all file systems
support the _POSIX_CHOWN_RESTRICTED and _POSIX_NO_TRUNC
features. So these names are never defined as macros in the GNU C
library.
Macro: int _POSIX_CHOWN_RESTRICTED
If this option is in effect, the chown function is restricted so
that the only changes permitted to nonprivileged processes is to change
the group owner of a file to either be the effective group ID of the
process, or one of its supplementary group IDs. See section File Owner.
If this option is in effect, file name components longer than
NAME_MAX generate an ENAMETOOLONG error. Otherwise, file
name components that are too long are silently truncated.
Macro: unsigned char _POSIX_VDISABLE
This option is only meaningful for files that are terminal devices. If it is enabled, then handling for special control characters can be disabled individually. See section Special Characters.
If one of these macros is undefined, that means that the option might be
in effect for some files and not for others. To inquire about a
particular file, call pathconf or fpathconf.
See section Using pathconf.
Here are the names for the POSIX minimum upper bounds for some of the above parameters. The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far.
_POSIX_LINK_MAX
8; thus, you
can always make up to eight names for a file without running into a
system limit.
_POSIX_MAX_CANON
255.
_POSIX_MAX_INPUT
255.
_POSIX_NAME_MAX
14.
_POSIX_PATH_MAX
255.
_POSIX_PIPE_BUF
512.
pathconfWhen your machine allows different files to have different values for a file system parameter, you can use the functions in this section to find out the value that applies to any particular file.
These functions and the associated constants for the parameter argument are declared in the header file `unistd.h'.
Function: long int pathconf (const char *filename, int parameter)
This function is used to inquire about the limits that apply to the file named filename.
The parameter argument should be one of the `_PC_' constants listed below.
The normal return value from pathconf is the value you requested.
A value of -1 is returned both if the implementation does not
impose a limit, and in case of an error. In the former case,
errno is not set, while in the latter case, errno is set
to indicate the cause of the problem. So the only way to use this
function robustly is to store 0 into errno just before
calling it.
Besides the usual file name syntax errors (see section File Name Errors), the following error condition is defined for this function:
EINVAL
Function: long int fpathconf (int filedes, int parameter)
This is just like pathconf except that an open file descriptor
is used to specify the file for which information is requested, instead
of a file name.
The following errno error conditions are defined for this function:
EBADF
EINVAL
Here are the symbolic constants that you can use as the parameter
argument to pathconf and fpathconf. The values are all
integer constants.
_PC_LINK_MAX
LINK_MAX.
_PC_MAX_CANON
MAX_CANON.
_PC_MAX_INPUT
MAX_INPUT.
_PC_NAME_MAX
NAME_MAX.
_PC_PATH_MAX
PATH_MAX.
_PC_PIPE_BUF
PIPE_BUF.
_PC_CHOWN_RESTRICTED
_POSIX_CHOWN_RESTRICTED.
_PC_NO_TRUNC
_POSIX_NO_TRUNC.
_PC_VDISABLE
_POSIX_VDISABLE.
The POSIX.2 standard specifies certain system limits that you can access
through sysconf that apply to utility behavior rather than the
behavior of the library or the operating system.
The GNU C library defines macros for these limits, and sysconf
returns values for them if you ask; but these values convey no
meaningful information. They are simply the smallest values that
POSIX.2 permits.
The largest value of obase that the bc utility is
guaranteed to support.
The largest value of scale that the bc utility is
guaranteed to support.
The largest number of elements in one array that the bc utility
is guaranteed to support.
The largest number of characters in one string constant that the
bc utility is guaranteed to support.
The largest number of elements in one array that the bc utility
is guaranteed to support.
The largest number of weights that can necessarily be used in defining the collating sequence for a locale.
The maximum number of expressions that can be nested within parenthesis
by the expr utility.
The largest text line that the text-oriented POSIX.2 utilities can support. (If you are using the GNU versions of these utilities, then there is no actual limit except that imposed by the available virtual memory, but there is no way that the library can tell you this.)
_POSIX2_BC_BASE_MAX
obase in the bc utility. Its value is 99.
_POSIX2_BC_DIM_MAX
bc utility. Its value is 2048.
_POSIX2_BC_SCALE_MAX
scale in the bc utility. Its value is 99.
_POSIX2_BC_STRING_MAX
bc utility. Its value is 1000.
_POSIX2_COLL_WEIGHTS_MAX
2.
_POSIX2_EXPR_NEST_MAX
expr utility.
Its value is 32.
_POSIX2_LINE_MAX
2048.
POSIX.2 defines a way to get string-valued parameters from the operating
system with the function confstr:
Function: size_t confstr (int parameter, char *buf, size_t len)
This function reads the value of a string-valued system parameter, storing the string into len bytes of memory space starting at buf. The parameter argument should be one of the `_CS_' symbols listed below.
The normal return value from confstr is the length of the string
value that you asked for. If you supply a null pointer for buf,
then confstr does not try to store the string; it just returns
its length. A value of 0 indicates an error.
If the string you asked for is too long for the buffer (that is, longer
than len - 1), then confstr stores just that much
(leaving room for the terminating null character). You can tell that
this has happened because confstr returns a value greater than or
equal to len.
The following errno error conditions are defined for this function:
EINVAL
Currently there is just one parameter you can read with confstr:
_CS_PATH
The way to use confstr without any arbitrary limit on string size
is to call it twice: first call it to get the length, allocate the
buffer accordingly, and then call confstr again to fill the
buffer, like this:
char *
get_default_path (void)
{
size_t len = confstr (_CS_PATH, NULL, 0);
char *buffer = (char *) xmalloc (len);
if (confstr (_CS_PATH, buf, len + 1) == 0)
{
free (buffer);
return NULL;
}
return buffer;
}
Some of the facilities implemented by the C library really should be thought of as parts of the C language itself. These facilities ought to be documented in the C Language Manual, not in the library manual; but since we don't have the language manual yet, and documentation for these features has been written, we are publishing it here.
When you're writing a program, it's often a good idea to put in checks at strategic places for "impossible" errors or violations of basic assumptions. These checks are helpful in debugging problems due to misunderstandings between different parts of the program.
The assert macro, defined in the header file `assert.h',
provides a convenient way to abort the program while printing a message
about where in the program the error was detected.
Once you think your program is debugged, you can disable the error
checks performed by the assert macro by recompiling with the
macro NDEBUG defined. This means you don't actually have to
change the program source code to disable these checks.
But disabling these consistency checks is undesirable unless they make the program significantly slower. All else being equal, more error checking is good no matter who is running the program. A wise user would rather have a program crash, visibly, than have it return nonsense without indicating anything might be wrong.
Macro: void assert (int expression)
Verify the programmer's belief that expression should be nonzero at this point in the program.
If NDEBUG is not defined, assert tests the value of
expression. If it is false (zero), assert aborts the
program (see section Aborting a Program) after printing a message of the
form:
`file':linenum: Assertion `expression' failed.
on the standard error stream stderr (see section Standard Streams).
The filename and line number are taken from the C preprocessor macros
__FILE__ and __LINE__ and specify where the call to
assert was written.
If the preprocessor macro NDEBUG is defined at the point where
`assert.h' is included, the assert macro is defined to do
absolutely nothing.
Warning: Even the argument expression expression is not
evaluated if NDEBUG is in effect. So never use assert
with arguments that involve side effects. For example, assert
(++i > 0); is a bad idea, because i will not be incremented if
NDEBUG is defined.
Usage note: The assert facility is designed for
detecting internal inconsistency; it is not suitable for
reporting invalid input or improper usage by the user of the
program.
The information in the diagnostic messages printed by the assert
macro is intended to help you, the programmer, track down the cause of a
bug, but is not really useful for telling a user of your program why his
or her input was invalid or why a command could not be carried out. So
you can't use assert to print the error messages for these
eventualities.
What's more, your program should not abort when given invalid input, as
assert would do--it should exit with nonzero status (see section Exit Status) after printing its error messages, or perhaps read another
command or move on to the next input file.
See section Error Messages, for information on printing error messages for problems that do not represent bugs in the program.
ANSI C defines a syntax for declaring a function to take a variable number or type of arguments. (Such functions are referred to as varargs functions or variadic functions.) However, the language itself provides no mechanism for such functions to access their non-required arguments; instead, you use the variable arguments macros defined in `stdarg.h'.
This section describes how to declare variadic functions, how to write them, and how to call them properly.
Compatibility Note: Many older C dialects provide a similar, but incompatible, mechanism for defining functions with variable numbers of arguments, using `varargs.h'.
Ordinary C functions take a fixed number of arguments. When you define
a function, you specify the data type for each argument. Every call to
the function should supply the expected number of arguments, with types
that can be converted to the specified ones. Thus, if the function
`foo' is declared with int foo (int, char *); then you must
call it with two arguments, a number (any kind will do) and a string
pointer.
But some functions perform operations that can meaningfully accept an unlimited number of arguments.
In some cases a function can handle any number of values by operating on
all of them as a block. For example, consider a function that allocates
a one-dimensional array with malloc to hold a specified set of
values. This operation makes sense for any number of values, as long as
the length of the array corresponds to that number. Without facilities
for variable arguments, you would have to define a separate function for
each possible array size.
The library function printf (see section Formatted Output) is an
example of another class of function where variable arguments are
useful. This function prints its arguments (which can vary in type as
well as number) under the control of a format template string.
These are good reasons to define a variadic function which can handle as many arguments as the caller chooses to pass.
Some functions such as open take a fixed set of arguments, but
occasionally ignore the last few. Strict adherence to ANSI C requires
these functions to be defined as variadic; in practice, however, the GNU
C compiler and most other C compilers let you define such a function to
take a fixed set of arguments--the most it can ever use--and then only
declare the function as variadic (or not declare its arguments
at all!).
Defining and using a variadic function involves three steps:
A function that accepts a variable number of arguments must be declared with a prototype that says so. You write the fixed arguments as usual, and then tack on `...' to indicate the possibility of additional arguments. The syntax of ANSI C requires at least one fixed argument before the `...'. For example,
int
func (const char *a, int b, ...)
{
...
}
outlines a definition of a function func which returns an
int and takes two required arguments, a const char * and
an int. These are followed by any number of anonymous
arguments.
Portability note: For some C compilers, the last required
argument must not be declared register in the function
definition. Furthermore, this argument's type must be
self-promoting: that is, the default promotions must not change
its type. This rules out array and function types, as well as
float, char (whether signed or not) and short int
(whether signed or not). This is actually an ANSI C requirement.
Ordinary fixed arguments have individual names, and you can use these names to access their values. But optional arguments have no names--nothing but `...'. How can you access them?
The only way to access them is sequentially, in the order they were written, and you must use special macros from `stdarg.h' in the following three step process:
va_list using
va_start. The argument pointer when initialized points to the
first optional argument.
va_arg.
The first call to va_arg gives you the first optional argument,
the next call gives you the second, and so on.
You can stop at any time if you wish to ignore any remaining optional arguments. It is perfectly all right for a function to access fewer arguments than were supplied in the call, but you will get garbage values if you try to access too many arguments.
va_end.
(In practice, with most C compilers, calling va_end does nothing
and you do not really need to call it. This is always true in the GNU C
compiler. But you might as well call va_end just in case your
program is someday compiled with a peculiar compiler.)
See section Argument Access Macros, for the full definitions of va_start,
va_arg and va_end.
Steps 1 and 3 must be performed in the function that accepts the
optional arguments. However, you can pass the va_list variable
as an argument to another function and perform all or part of step 2
there.
You can perform the entire sequence of the three steps multiple times within a single function invocation. If you want to ignore the optional arguments, you can do these steps zero times.
You can have more than one argument pointer variable if you like. You
can initialize each variable with va_start when you wish, and
then you can fetch arguments with each argument pointer as you wish.
Each argument pointer variable will sequence through the same set of
argument values, but at its own pace.
Portability note: With some compilers, once you pass an
argument pointer value to a subroutine, you must not keep using the same
argument pointer value after that subroutine returns. For full
portability, you should just pass it to va_end. This is actually
an ANSI C requirement, but most ANSI C compilers work happily
regardless.
There is no general way for a function to determine the number and type of the optional arguments it was called with. So whoever designs the function typically designs a convention for the caller to tell it how many arguments it has, and what kind. It is up to you to define an appropriate calling convention for each variadic function, and write all calls accordingly.
One kind of calling convention is to pass the number of optional arguments as one of the fixed arguments. This convention works provided all of the optional arguments are of the same type.
A similar alternative is to have one of the required arguments be a bit mask, with a bit for each possible purpose for which an optional argument might be supplied. You would test the bits in a predefined sequence; if the bit is set, fetch the value of the next argument, otherwise use a default value.
A required argument can be used as a pattern to specify both the number
and types of the optional arguments. The format string argument to
printf is one example of this (see section Formatted Output Functions).
Another possibility is to pass an "end marker" value as the last
optional argument. For example, for a function that manipulates an
arbitrary number of pointer arguments, a null pointer might indicate the
end of the argument list. (This assumes that a null pointer isn't
otherwise meaningful to the function.) The execl function works
in just this way; see section Executing a File.
You don't have to write anything special when you call a variadic function. Just write the arguments (required arguments, followed by optional ones) inside parentheses, separated by commas, as usual. But you should prepare by declaring the function with a prototype, and you must know how the argument values are converted.
In principle, functions that are defined to be variadic must also be declared to be variadic using a function prototype whenever you call them. (See section Syntax for Variable Arguments, for how.) This is because some C compilers use a different calling convention to pass the same set of argument values to a function depending on whether that function takes variable arguments or fixed arguments.
In practice, the GNU C compiler always passes a given set of argument
types in the same way regardless of whether they are optional or
required. So, as long as the argument types are self-promoting, you can
safely omit declaring them. Usually it is a good idea to declare the
argument types for variadic functions, and indeed for all functions.
But there are a few functions which it is extremely convenient not to
have to declare as variadic--for example, open and
printf.
Since the prototype doesn't specify types for optional arguments, in a
call to a variadic function the default argument promotions are
performed on the optional argument values. This means the objects of
type char or short int (whether signed or not) are
promoted to either int or unsigned int, as
appropriate; and that objects of type float are promoted to type
double. So, if the caller passes a char as an optional
argument, it is promoted to an int, and the function should get
it with va_arg (ap, int).
Conversion of the required arguments is controlled by the function prototype in the usual way: the argument expression is converted to the declared argument type as if it were being assigned to a variable of that type.
Here are descriptions of the macros used to retrieve variable arguments. These macros are defined in the header file `stdarg.h'.
The type va_list is used for argument pointer variables.
Macro: void va_start (va_list ap, last_required)
This macro initializes the argument pointer variable ap to point to the first of the optional arguments of the current function; last_required must be the last required argument to the function.
See section Old-Style Variadic Functions, for an alternate definition of va_start
found in the header file `varargs.h'.
Macro: type va_arg (va_list ap, type)
The va_arg macro returns the value of the next optional argument,
and modifies the value of ap to point to the subsequent argument.
Thus, successive uses of va_arg return successive optional
arguments.
The type of the value returned by va_arg is type as
specified in the call. type must be a self-promoting type (not
char or short int or float) that matches the type
of the actual argument.
Macro: void va_end (va_list ap)
This ends the use of ap. After a va_end call, further
va_arg calls with the same ap may not work. You should invoke
va_end before returning from the function in which va_start
was invoked with the same ap argument.
In the GNU C library, va_end does nothing, and you need not ever
use it except for reasons of portability.
Here is a complete sample function that accepts a variable number of arguments. The first argument to the function is the count of remaining arguments, which are added up and the result returned. While trivial, this function is sufficient to illustrate how to use the variable arguments facility.
#include <stdarg.h>
#include <stdio.h>
int
add_em_up (int count,...)
{
va_list ap;
int i, sum;
va_start (ap, count); /* Initialize the argument list. */
sum = 0;
for (i = 0; i < count; i++)
sum += va_arg (ap, int); /* Get the next argument value. */
va_end (ap); /* Clean up. */
return sum;
}
int
main (void)
{
/* This call prints 16. */
printf ("%d\n", add_em_up (3, 5, 5, 6));
/* This call prints 55. */
printf ("%d\n", add_em_up (10, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10));
return 0;
}
Before ANSI C, programmers used a slightly different facility for writing variadic functions. The GNU C compiler still supports it; currently, it is more portable than the ANSI C facility, since support for ANSI C is still not universal. The header file which defines the old-fashioned variadic facility is called `varargs.h'.
Using `varargs.h' is almost the same as using `stdarg.h'. There is no difference in how you call a variadic function; See section Calling Variadic Functions. The only difference is in how you define them. First of all, you must use old-style non-prototype syntax, like this:
tree
build (va_alist)
va_dcl
{
Secondly, you must give va_start just one argument, like this:
va_list p; va_start (p);
These are the special macros used for defining old-style variadic functions:
This macro stands for the argument name list required in a variadic function.
This macro declares the implicit argument or arguments for a variadic function.
Macro: void va_start (va_list ap)
This macro, as defined in `varargs.h', initializes the argument pointer variable ap to point to the first argument of the current function.
The other argument macros, va_arg and va_end, are the same
in `varargs.h' as in `stdarg.h'; see section Argument Access Macros for
details.
It does not work to include both `varargs.h' and `stdarg.h' in
the same compilation; they define va_start in conflicting ways.
The null pointer constant is guaranteed not to point to any real object.
You can assign it to any pointer variable since it has type void
*. The preferred way to write a null pointer constant is with
NULL.
This is a null pointer constant.
You can also use 0 or (void *)0 as a null pointer
constant, but using NULL is cleaner because it makes the purpose
of the constant more evident.
If you use the null pointer constant as a function argument, then for complete portability you should make sure that the function has a prototype declaration. Otherwise, if the target machine has two different pointer representations, the compiler won't know which representation to use for that argument. You can avoid the problem by explicitly casting the constant to the proper pointer type, but we recommend instead adding a prototype for the function you are calling.
The result of subtracting two pointers in C is always an integer, but the
precise data type varies from C compiler to C compiler. Likewise, the
data type of the result of sizeof also varies between compilers.
ANSI defines standard aliases for these two types, so you can refer to
them in a portable fashion. They are defined in the header file
`stddef.h'.
This is the signed integer type of the result of subtracting two
pointers. For example, with the declaration char *p1, *p2;, the
expression p2 - p1 is of type ptrdiff_t. This will
probably be one of the standard signed integer types (short
int, int or long int), but might be a nonstandard
type that exists only for this purpose.
This is an unsigned integer type used to represent the sizes of objects.
The result of the sizeof operator is of this type, and functions
such as malloc (see section Unconstrained Allocation) and
memcpy (see section Copying and Concatenation) accept arguments of
this type to specify object sizes.
Usage Note: size_t is the preferred way to declare any
arguments or variables that hold the size of an object.
In the GNU system size_t is equivalent to either
unsigned int or unsigned long int. These types
have identical properties on the GNU system, and for most purposes, you
can use them interchangeably. However, they are distinct as data types,
which makes a difference in certain contexts.
For example, when you specify the type of a function argument in a
function prototype, it makes a difference which one you use. If the
system header files declare malloc with an argument of type
size_t and you declare malloc with an argument of type
unsigned int, you will get a compilation error if size_t
happens to be unsigned long int on your system. To avoid any
possibility of error, when a function argument or value is supposed to
have type size_t, never declare its type in any other way.
Compatibility Note: Pre-ANSI C implementations generally used
unsigned int for representing object sizes and int for
pointer subtraction results. They did not necessarily define either
size_t or ptrdiff_t. Unix systems did define
size_t, in `sys/types.h', but the definition was usually a
signed type.
Most of the time, if you choose the proper C data type for each object in your program, you need not be concerned with just how it is represented or how many bits it uses. When you do need such information, the C language itself does not provide a way to get it. The header files `limits.h' and `float.h' contain macros which give you this information in full detail.
The most common reason that a program needs to know how many bits are in
an integer type is for using an array of long int as a bit vector.
You can access the bit at index n with
vector[n / LONGBITS] & (1 << (n % LONGBITS))
provided you define LONGBITS as the number of bits in a
long int.
There is no operator in the C language that can give you the number of
bits in an integer data type. But you can compute it from the macro
CHAR_BIT, defined in the header file `limits.h'.
CHAR_BIT
char---eight, on most systems.
The value has type int.
You can compute the number of bits in any data type type like this:
sizeof (type) * CHAR_BIT
Suppose you need to store an integer value which can range from zero to one million. Which is the smallest type you can use? There is no general rule; it depends on the C compiler and target machine. You can use the `MIN' and `MAX' macros in `limits.h' to determine which type will work.
Each signed integer type has a pair of macros which give the smallest and largest values that it can hold. Each unsigned integer type has one such macro, for the maximum value; the minimum value is, of course, zero.
The values of these macros are all integer constant expressions. The
`MAX' and `MIN' macros for char and short
int types have values of type int. The `MAX' and
`MIN' macros for the other types have values of the same type
described by the macro--thus, ULONG_MAX has type
unsigned long int.
SCHAR_MIN
This is the minimum value that can be represented by a signed char.
SCHAR_MAX
UCHAR_MAX
These are the maximum values that can be represented by a
signed char and unsigned char, respectively.
CHAR_MIN
This is the minimum value that can be represented by a char.
It's equal to SCHAR_MIN if char is signed, or zero
otherwise.
CHAR_MAX
This is the maximum value that can be represented by a char.
It's equal to SCHAR_MAX if char is signed, or
UCHAR_MAX otherwise.
SHRT_MIN
This is the minimum value that can be represented by a signed
short int. On most machines that the GNU C library runs on,
short integers are 16-bit quantities.
SHRT_MAX
USHRT_MAX
These are the maximum values that can be represented by a
signed short int and unsigned short int,
respectively.
INT_MIN
This is the minimum value that can be represented by a signed
int. On most machines that the GNU C system runs on, an int is
a 32-bit quantity.
INT_MAX
UINT_MAX
These are the maximum values that can be represented by, respectively,
the type signed int and the type unsigned int.
LONG_MIN
This is the minimum value that can be represented by a signed
long int. On most machines that the GNU C system runs on, long
integers are 32-bit quantities, the same size as int.
LONG_MAX
ULONG_MAX
These are the maximum values that can be represented by a
signed long int and unsigned long int, respectively.
LONG_LONG_MIN
This is the minimum value that can be represented by a signed
long long int. On most machines that the GNU C system runs on,
long long integers are 64-bit quantities.
LONG_LONG_MAX
ULONG_LONG_MAX
These are the maximum values that can be represented by a signed
long long int and unsigned long long int, respectively.
WCHAR_MAX
This is the maximum value that can be represented by a wchar_t.
@xref{Wide Character Intro}.
The header file `limits.h' also defines some additional constants that parameterize various operating system and file system limits. These constants are described in section System Configuration Parameters.
The specific representation of floating point numbers varies from machine to machine. Because floating point numbers are represented internally as approximate quantities, algorithms for manipulating floating point data often need to take account of the precise details of the machine's floating point representation.
Some of the functions in the C library itself need this information; for example, the algorithms for printing and reading floating point numbers (see section Input/Output on Streams) and for calculating trigonometric and irrational functions (see section Mathematics) use it to avoid round-off error and loss of accuracy. User programs that implement numerical analysis techniques also often need this information in order to minimize or compute error bounds.
The header file `float.h' describes the format used by your machine.
This section introduces the terminology for describing floating point representations.
You are probably already familiar with most of these concepts in terms
of scientific or exponential notation for floating point numbers. For
example, the number 123456.0 could be expressed in exponential
notation as 1.23456e+05, a shorthand notation indicating that the
mantissa 1.23456 is multiplied by the base 10 raised to
power 5.
More formally, the internal representation of a floating point number can be characterized in terms of the following parameters:
-1 or 1.
1. This is a constant for a particular representation.
Sometimes, in the actual bits representing the floating point number, the exponent is biased by adding a constant to it, to make it always be represented as an unsigned quantity. This is only important if you have some reason to pick apart the bit fields making up the floating point number by hand, which is something for which the GNU library provides no support. So this is ignored in the discussion that follows.
Many floating point representations have an implicit hidden bit in the mantissa. This is a bit which is present virtually in the mantissa, but not stored in memory because its value is always 1 in a normalized number. The precision figure (see above) includes any hidden bits.
Again, the GNU library provides no facilities for dealing with such low-level aspects of the representation.
The mantissa of a floating point number actually represents an implicit
fraction whose denominator is the base raised to the power of the
precision. Since the largest representable mantissa is one less than
this denominator, the value of the fraction is always strictly less than
1. The mathematical value of a floating point number is then the
product of this fraction, the sign, and the base raised to the exponent.
We say that the floating point number is normalized if the
fraction is at least 1/b, where b is the base. In
other words, the mantissa would be too large to fit if it were
multiplied by the base. Non-normalized numbers are sometimes called
denormal; they contain less precision than the representation
normally can hold.
If the number is not normalized, then you can subtract 1 from the
exponent while multiplying the mantissa by the base, and get another
floating point number with the same value. Normalization consists
of doing this repeatedly until the number is normalized. Two distinct
normalized floating point numbers cannot be equal in value.
(There is an exception to this rule: if the mantissa is zero, it is
considered normalized. Another exception happens on certain machines
where the exponent is as small as the representation can hold. Then
it is impossible to subtract 1 from the exponent, so a number
may be normalized even if its fraction is less than 1/b.)
These macro definitions can be accessed by including the header file `float.h' in your program.
Macro names starting with `FLT_' refer to the float type,
while names beginning with `DBL_' refer to the double type
and names beginning with `LDBL_' refer to the long double
type. (Currently GCC does not support long double as a distinct
data type, so the values for the `LDBL_' constants are equal to the
corresponding constants for the double type.)
Of these macros, only FLT_RADIX is guaranteed to be a constant
expression. The other macros listed here cannot be reliably used in
places that require constant expressions, such as `#if'
preprocessing directives or in the dimensions of static arrays.
Although the ANSI C standard specifies minimum and maximum values for most of these parameters, the GNU C implementation uses whatever values describe the floating point representation of the target machine. So in principle GNU C actually satisfies the ANSI C requirements only if the target machine is suitable. In practice, all the machines currently supported are suitable.
FLT_ROUNDS
-1
0
1
2
3
Any other value represents a machine-dependent nonstandard rounding mode.
On most machines, the value is 1, in accordance with the IEEE
standard for floating point.
Here is a table showing how certain values round for each possible value
of FLT_ROUNDS, if the other aspects of the representation match
the IEEE single-precision standard.
0 1 2 3
1.00000003 1.0 1.0 1.00000012 1.0
1.00000007 1.0 1.00000012 1.00000012 1.0
-1.00000003 -1.0 -1.0 -1.0 -1.00000012
-1.00000007 -1.0 -1.00000012 -1.0 -1.00000012
FLT_RADIX digits in the floating point
mantissa for the float data type. The following expression
yields 1.0 (even though mathematically it should not) due to the
limited number of mantissa digits:
float radix = FLT_RADIX; 1.0f + 1.0f / radix / radix / ... / radix
where radix appears FLT_MANT_DIG times.
FLT_RADIX digits in the floating point
mantissa for the data types double and long double,
respectively.
This is the number of decimal digits of precision for the float
data type. Technically, if p and b are the precision and
base (respectively) for the representation, then the decimal precision
q is the maximum number of decimal digits such that any floating
point number with q base 10 digits can be rounded to a floating
point number with p base b digits and back again, without
change to the q decimal digits.
The value of this macro is supposed to be at least 6, to satisfy
ANSI C.
These are similar to FLT_DIG, but for the data types
double and long double, respectively. The values of these
macros are supposed to be at least 10.
float.
More precisely, is the minimum negative integer such that the value
FLT_RADIX raised to this power minus 1 can be represented as a
normalized floating point number of type float.
These are similar to FLT_MIN_EXP, but for the data types
double and long double, respectively.
10 raised to this
power minus 1 can be represented as a normalized floating point number
of type float. This is supposed to be -37 or even less.
FLT_MIN_10_EXP, but for the data types
double and long double, respectively.
float. More
precisely, this is the maximum positive integer such that value
FLT_RADIX raised to this power minus 1 can be represented as a
floating point number of type float.
FLT_MAX_EXP, but for the data types
double and long double, respectively.
10 raised to this
power minus 1 can be represented as a normalized floating point number
of type float. This is supposed to be at least 37.
FLT_MAX_10_EXP, but for the data types
double and long double, respectively.
The value of this macro is the maximum number representable in type
float. It is supposed to be at least 1E+37. The value
has type float.
The smallest representable number is - FLT_MAX.
These are similar to FLT_MAX, but for the data types
double and long double, respectively. The type of the
macro's value is the same as the type it describes.
The value of this macro is the minimum normalized positive floating
point number that is representable in type float. It is supposed
to be no more than 1E-37.
These are similar to FLT_MIN, but for the data types
double and long double, respectively. The type of the
macro's value is the same as the type it describes.
This is the minimum positive floating point number of type float
such that 1.0 + FLT_EPSILON != 1.0 is true. It's supposed to
be no greater than 1E-5.
These are similar to FLT_EPSILON, but for the data types
double and long double, respectively. The type of the
macro's value is the same as the type it describes. The values are not
supposed to be greater than 1E-9.
Here is an example showing how the floating type measurements come out for the most common floating point representation, specified by the IEEE Standard for Binary Floating Point Arithmetic (ANSI/IEEE Std 754-1985). Nearly all computers designed since the 1980s use this format.
The IEEE single-precision float representation uses a base of 2. There is a sign bit, a mantissa with 23 bits plus one hidden bit (so the total precision is 24 base-2 digits), and an 8-bit exponent that can represent values in the range -125 to 128, inclusive.
So, for an implementation that uses this representation for the
float data type, appropriate values for the corresponding
parameters are:
FLT_RADIX 2 FLT_MANT_DIG 24 FLT_DIG 6 FLT_MIN_EXP -125 FLT_MIN_10_EXP -37 FLT_MAX_EXP 128 FLT_MAX_10_EXP +38 FLT_MIN 1.17549435E-38F FLT_MAX 3.40282347E+38F FLT_EPSILON 1.19209290E-07F
Here are the values for the double data type:
DBL_MANT_DIG 53 DBL_DIG 15 DBL_MIN_EXP -1021 DBL_MIN_10_EXP -307 DBL_MAX_EXP 1024 DBL_MAX_10_EXP 308 DBL_MAX 1.7976931348623157E+308 DBL_MIN 2.2250738585072014E-308 DBL_EPSILON 2.2204460492503131E-016
You can use offsetof to measure the location within a structure
type of a particular structure member.
Macro: size_t offsetof (type, member)
This expands to a integer constant expression that is the offset of the
structure member named member in a the structure type type.
For example, offsetof (struct s, elem) is the offset, in bytes,
of the member elem in a struct s.
This macro won't work if member is a bit field; you get an error from the C compiler in that case.
This appendix is a complete list of the facilities declared within the header files supplied with the GNU C library. Each entry also lists the standard or other source from which each facility is derived, and tells you where in the manual you can find more information about how to use it.
char *tzname[2]
AF_FILE
AF_INET
AF_UNIX
AF_UNSPEC
ALTWERASE
int ARG_MAX
B0
B110
B1200
B134
B150
B1800
B19200
B200
B2400
B300
B38400
B4800
B50
B600
B75
B9600
int BC_BASE_MAX
int BC_DIM_MAX
int BC_DIM_MAX
int BC_SCALE_MAX
int BC_STRING_MAX
BRKINT
int BUFSIZ
CCTS_OFLOW
CHAR_BIT
CHAR_MAX
CHAR_MIN
int CHILD_MAX
int CLK_TCK
CLOCAL
int CLOCKS_PER_SEC
int COLL_WEIGHTS_MAX
CREAD
CRTS_IFLOW
CS5
CS6
CS7
CS8
CSIZE
CSTOPB
DBL_DIG
DBL_EPSILON
DBL_MANT_DIG
DBL_MAX
DBL_MAX_10_EXP
DBL_MAX_EXP
DBL_MIN
DBL_MIN_10_EXP
DBL_MIN_EXP
DIR
int E2BIG
int EACCES
int EADDRINUSE
int EADDRNOTAVAIL
int EAFNOSUPPORT
int EAGAIN
int EALREADY
int EBADF
int EBUSY
int ECHILD
ECHO
ECHOCTL
ECHOE
ECHOK
ECHOKE
ECHONL
ECHOPRT
int ECONNABORTED
int ECONNREFUSED
int ECONNRESET
int ED
int EDEADLK
int EDESTADDRREQ
int EDOM
int EDQUOT
int EEXIST
int EFAULT
int EFBIG
int EGRATUITOUS
int EHOSTDOWN
int EHOSTUNREACH
int EINPROGRESS
int EINTR
int EINVAL
int EIO
int EISCONN
int EISDIR
int ELOOP
int EMFILE
int EMLINK
int EMSGSIZE
int ENAMETOOLONG
int ENETDOWN
int ENETRESET
int ENETUNREACH
int ENFILE
int ENOBUFS
int ENODEV
int ENOENT
int ENOEXEC
int ENOLCK
int ENOMEM
int ENOPROTOOPT
int ENOSPC
int ENOSYS
int ENOTBLK
int ENOTCONN
int ENOTDIR
int ENOTEMPTY
int ENOTSOCK
int ENOTTY
int ENXIO
int EOF
int EOPNOTSUPP
int EPERM
int EPFNOSUPPORT
int EPIPE
int EPROTONOSUPPORT
int EPROTOTYPE
int ERANGE
int EREMOTE
int EROFS
int ESHUTDOWN
int ESOCKTNOSUPPORT
int ESPIPE
int ESRCH
int ESTALE
int ETIMEDOUT
int ETXTBSY
int EUSERS
int EWOULDBLOCK
int EXDEV
int EXIT_FAILURE
int EXIT_SUCCESS
int EXPR_NEST_MAX
int FD_CLOEXEC
void FD_CLR (int filedes, fd_set *set)
int FD_ISSET (int filedes, fd_set *set)
void FD_SET (int filedes, fd_set *set)
int FD_SETSIZE
void FD_ZERO (fd_set *set)
FILE
int FILENAME_MAX
FLT_DIG
FLT_EPSILON
FLT_MANT_DIG
FLT_MAX
FLT_MAX_10_EXP
FLT_MAX_EXP
FLT_MIN
FLT_MIN_10_EXP
FLT_MIN_EXP
FLT_RADIX
FLT_ROUNDS
FLUSHO
FNM_CASEFOLD
FNM_FILE_NAME
FNM_LEADING_DIR
FNM_NOESCAPE
FNM_PATHNAME
FNM_PERIOD
int FOPEN_MAX
FPE_DECOVF_TRAP
FPE_FLTDIV_FAULT
FPE_FLTDIV_TRAP
FPE_FLTOVF_FAULT
FPE_FLTOVF_TRAP
FPE_FLTUND_FAULT
FPE_FLTUND_TRAP
FPE_INTDIV_TRAP
FPE_INTOVF_TRAP
FPE_SUBRNG_TRAP
int F_DUPFD
int F_GETFD
int F_GETFL
int F_GETLK
int F_GETOWN
int F_OK
F_RDLCK
int F_SETFD
int F_SETFL
int F_SETLK
int F_SETLKW
int F_SETOWN
F_UNLCK
F_WRLCK
GLOB_ABORTED
glob.
GLOB_APPEND
GLOB_DOOFFS
GLOB_ERR
GLOB_MARK
GLOB_NOCHECK
GLOB_NOESCAPE
GLOB_NOMATCH
glob.
GLOB_NOSORT
GLOB_NOSPACE
glob.
HOST_NOT_FOUND
double HUGE_VAL
HUPCL
ICANON
ICRNL
IEXTEN
IGNBRK
IGNCR
IGNPAR
IMAXBEL
unsigned long int INADDR_ANY
INLCR
INPCK
INT_MAX
INT_MIN
int IPPORT_RESERVED
int IPPORT_USERRESERVED
ISIG
ISTRIP
ITIMER_PROF
ITIMER_REAL
ITIMER_VIRTUAL
IXANY
IXOFF
IXON
LANG
LC_ALL
LC_COLLATE
LC_CTYPE
LC_MONETARY
LC_NUMERIC
LC_RESPONSE
LC_TIME
int LINE_MAX
int LINK_MAX
LONG_LONG_MAX
LONG_LONG_MIN
LONG_MAX
LONG_MIN
L_INCR
L_SET
L_XTND
int L_ctermid
int L_cuserid
int L_tmpnam
int MAXNAMLEN
int MAX_CANON
int MAX_INPUT
int MB_CUR_MAX
int MB_LEN_MAX
MDMBUF
int MSG_DONTROUTE
int MSG_OOB
int MSG_PEEK
int NAME_MAX
double NAN
int NCCS
int NGROUPS_MAX
NOFLSH
NOKERNINFO
NO_ADDRESS
NO_RECOVERY
int NSIG
void * NULL
int ONLCR
int ONOEOT
int OPEN_MAX
int OPOST
int OXTABS
int O_ACCMODE
O_APPEND
O_APPEND
O_CREAT
O_EXCL
O_NDELAY
O_NOCTTY
O_NONBLOCK
O_NONBLOCK
O_RDONLY
O_RDWR
O_TRUNC
O_WRONLY
PARENB
PARMRK
PARODD
int PATH_MAX
PA_CHAR
PA_DOUBLE
PA_FLAG_LONG
PA_FLAG_LONG_DOUBLE
PA_FLAG_LONG_LONG
int PA_FLAG_MASK
PA_FLAG_PTR
PA_FLAG_SHORT
PA_FLOAT
PA_INT
PA_LAST
PA_POINTER
PA_STRING
PENDIN
int PF_FILE
int PF_INET
int PF_UNIX
int PIPE_BUF
PRIO_MAX
PRIO_MIN
PRIO_PGRP
PRIO_PROCESS
PRIO_USER
char * P_tmpdir
int RAND_MAX
REG_BADBR
REG_BADPAT
REG_BADRPT
REG_EBRACE
REG_EBRACK
REG_ECOLLATE
REG_ECTYPE
REG_EESCAPE
REG_EPAREN
REG_ERANGE
REG_ESPACE
REG_ESPACE
REG_ESUBREG
REG_EXTENDED
REG_ICASE
REG_NEWLINE
REG_NOMATCH
REG_NOSUB
REG_NOTBOL
REG_NOTEOL
int RE_DUP_MAX
RLIMIT_CORE
RLIMIT_CPU
RLIMIT_DATA
RLIMIT_FSIZE
RLIMIT_OPEN_FILES
RLIMIT_RSS
RLIMIT_STACK
RLIM_NLIMITS
RUSAGE_CHILDREN
RUSAGE_SELF
int R_OK
int SA_NOCLDSTOP
sigaction.
int SA_ONSTACK
sigaction.
int SA_RESTART
sigaction.
SCHAR_MAX
SCHAR_MIN
int SEEK_CUR
int SEEK_END
int SEEK_SET
SHRT_MAX
SHRT_MIN
int SIGABRT
int SIGALRM
int SIGBUS
int SIGCHLD
int SIGCONT
int SIGFPE
int SIGHUP
int SIGILL
int SIGINT
int SIGIO
int SIGKILL
int SIGPIPE
int SIGPROF
int SIGQUIT
int SIGSEGV
int SIGSTOP
int SIGTERM
int SIGTSTP
int SIGTTIN
int SIGTTOU
int SIGURG
int SIGUSR1
int SIGUSR2
int SIGVTALRM
SIG_BLOCK
sighandler_t SIG_ERR
SIG_SETMASK
SIG_UNBLOCK
int SOCK_DGRAM
int SOCK_RAW
int SOCK_RDM
int SOCK_SEQPACKET
int SOCK_STREAM
int SOL_SOCKET
SO_BROADCAST
SO_DEBUG
SO_DONTROUTE
SO_ERROR
SO_KEEPALIVE
SO_LINGER
SO_OOBINLINE
SO_RCVBUF
SO_REUSEADDR
SO_SNDBUF
SO_STYLE
SO_TYPE
int SSIZE_MAX
STDERR_FILENO
STDIN_FILENO
STDOUT_FILENO
int STREAM_MAX
int SV_INTERRUPT
int SV_ONSTACK
int SV_RESETHAND
S_IEXEC
S_IFBLK
S_IFCHR
S_IFDIR
S_IFIFO
S_IFLNK
int S_IFMT
S_IFREG
S_IFSOCK
S_IREAD
S_IRGRP
S_IROTH
S_IRUSR
S_IRWXG
S_IRWXO
S_IRWXU
int S_ISBLK (mode_t m)
int S_ISCHR (mode_t m)
int S_ISDIR (mode_t m)
int S_ISFIFO (mode_t m)
S_ISGID
int S_ISLNK (mode_t m)
int S_ISREG (mode_t m)
int S_ISSOCK (mode_t m)
S_ISUID
S_ISVTX
S_IWGRP
S_IWOTH
S_IWRITE
S_IWUSR
S_IXGRP
S_IXOTH
S_IXUSR
TCSADRAIN
TCSAFLUSH
TCSANOW
TCSASOFT
TEMP_FAILURE_RETRY (expression)
int TMP_MAX
TOSTOP
TRY_AGAIN
int TZNAME_MAX
UCHAR_MAX
UINT_MAX
ULONG_LONG_MAX
ULONG_MAX
USHRT_MAX
int VDISCARD
int VDSUSP
int VEOF
int VEOL
int VEOL2
int VERASE
int VINTR
int VKILL
int VLNEXT
int VMIN
int VQUIT
int VREPRINT
int VSTART
int VSTATUS
int VSTOP
int VSUSP
int VTIME
int VWERASE
WCHAR_MAX
int WCOREDUMP (int status)
int WEXITSTATUS (int status)
int WIFEXITED (int status)
int WIFSIGNALED (int status)
int WIFSTOPPED (int status)
WRDE_APPEND
WRDE_BADCHAR
wordexp.
WRDE_BADVAL
wordexp.
WRDE_CMDSUB
wordexp.
WRDE_DOOFFS
WRDE_NOCMD
WRDE_NOSPACE
wordexp.
WRDE_REUSE
WRDE_SHOWERR
WRDE_SYNTAX
wordexp.
WRDE_UNDEF
int WSTOPSIG (int status)
int WTERMSIG (int status)
int W_OK
int X_OK
_CS_PATH
int _IOFBF
int _IOLBF
int _IONBF
_PC_CHOWN_RESTRICTED
pathconf.
_PC_LINK_MAX
pathconf.
_PC_MAX_CANON
pathconf.
_PC_MAX_INPUT
pathconf.
_PC_NAME_MAX
pathconf.
_PC_NO_TRUNC
pathconf.
_PC_PATH_MAX
pathconf.
_PC_PIPE_BUF
pathconf.
_PC_VDISABLE
pathconf.
_POSIX2_BC_BASE_MAX
_POSIX2_BC_DIM_MAX
_POSIX2_BC_SCALE_MAX
_POSIX2_BC_STRING_MAX
_POSIX2_COLL_WEIGHTS_MAX
int _POSIX2_C_DEV
long int _POSIX2_C_VERSION
_POSIX2_EXPR_NEST_MAX
int _POSIX2_FORT_DEV
int _POSIX2_FORT_RUN
_POSIX2_LINE_MAX
int _POSIX2_LOCALEDEF
_POSIX2_RE_DUP_MAX
int _POSIX2_SW_DEV
_POSIX_ARG_MAX
_POSIX_CHILD_MAX
int _POSIX_CHOWN_RESTRICTED
int _POSIX_JOB_CONTROL
_POSIX_LINK_MAX
_POSIX_MAX_CANON
_POSIX_MAX_INPUT
_POSIX_NAME_MAX
_POSIX_NGROUPS_MAX
int _POSIX_NO_TRUNC
_POSIX_OPEN_MAX
_POSIX_PATH_MAX
_POSIX_PIPE_BUF
int _POSIX_SAVED_IDS
_POSIX_SSIZE_MAX
_POSIX_STREAM_MAX
_POSIX_TZNAME_MAX
unsigned char _POSIX_VDISABLE
long int _POSIX_VERSION
_SC_2_C_DEV
sysconf Parameters.
_SC_2_FORT_DEV
sysconf Parameters.
_SC_2_FORT_RUN
sysconf Parameters.
_SC_2_LOCALEDEF
sysconf Parameters.
_SC_2_SW_DEV
sysconf Parameters.
_SC_2_VERSION
sysconf Parameters.
_SC_ARG_MAX
sysconf Parameters.
_SC_BC_BASE_MAX
sysconf Parameters.
_SC_BC_DIM_MAX
sysconf Parameters.
_SC_BC_SCALE_MAX
sysconf Parameters.
_SC_BC_STRING_MAX
sysconf Parameters.
_SC_CHILD_MAX
sysconf Parameters.
_SC_CLK_TCK
sysconf Parameters.
_SC_COLL_WEIGHTS_MAX
sysconf Parameters.
_SC_EXPR_NEST_MAX
sysconf Parameters.
_SC_JOB_CONTROL
sysconf Parameters.
_SC_LINE_MAX
sysconf Parameters.
_SC_NGROUPS_MAX
sysconf Parameters.
_SC_OPEN_MAX
sysconf Parameters.
_SC_SAVED_IDS
sysconf Parameters.
_SC_STREAM_MAX
sysconf Parameters.
_SC_TZNAME_MAX
sysconf Parameters.
_SC_VERSION
sysconf Parameters.
_SC_VERSION
sysconf Parameters.
__free_hook
__malloc_hook
__realloc_hook
void _exit (int status)
int _tolower (int c)
int _toupper (int c)
void abort (void)
int abs (int number)
int accept (int socket, struct sockaddr *addr, size_t *length_ptr)
int access (const char *filename, int how)
double acos (double x)
double acosh (double x)
int adjtime (const struct timeval *delta, struct timeval *olddelta)
unsigned int alarm (unsigned int seconds)
void * alloca (size_t size);
char * asctime (const struct tm *brokentime)
double asin (double x)
double asinh (double x)
int asprintf (char **ptr, const char *template, ...)
void assert (int expression)
double atan (double x)
double atan2 (double y, double x)
double atanh (double x)
int atexit (void (*function) (void))
double atof (const char *string)
int atoi (const char *string)
long int atol (const char *string)
int bcmp (const void *a1, const void *a2, size_t size)
void * bcopy (void *from, const void *to, size_t size)
int bind (int socket, struct sockaddr *addr, size_t length)
void * bsearch (const void *key, const void *array, size_t count, size_t size, comparison_fn_t compare)
void * bzero (void *block, size_t size)
double cabs (struct { double real, imag; } z)
void * calloc (size_t count, size_t eltsize)
double cbrt (double x)
cc_t
double ceil (double x)
speed_t cfgetispeed (const struct termios *termios_p)
speed_t cfgetospeed (const struct termios *termios_p)
int cfmakeraw (struct termios *termios_p)
void cfree (void *ptr)
malloc.
int cfsetispeed (struct termios *termios_p, speed_t speed)
int cfsetospeed (struct termios *termios_p, speed_t speed)
int cfsetspeed (struct termios *termios_p, speed_t speed)
int chdir (const char *filename)
int chmod (const char *filename, mode_t mode)
int chown (const char *filename, uid_t owner, gid_t group)
void clearerr (FILE *stream)
clock_t clock (void)
clock_t
int close (int filedes)
int closedir (DIR *dirstream)
size_t confstr (int parameter, char *buf, size_t len)
int connect (int socket, struct sockaddr *addr, size_t length)
cookie_close_function
cookie_read_function
cookie_seek_function
cookie_write_function
double copysign (double value, double sign)
double cos (double x)
double cosh (double x)
int creat (const char *filename, mode_t mode)
char * ctermid (char *string)
char * ctime (const time_t *time)
char * cuserid (char *string)
int daylight
dev_t
double difftime (time_t time1, time_t time0)
div_t div (int numerator, int denominator)
div_t
double drem (double numerator, double denominator)
int dup (int old)
int dup2 (int old, int new)
void endgrent (void)
void endhostent ()
void endnetent (void)
void endprotoent (void)
void endpwent (void)
void endservent (void)
char ** environ
volatile int errno
int execl (const char *filename, const char *arg0, ...)
int execle (const char *filename, const char *arg0, char *const env[], ...)
int execlp (const char *filename, const char *arg0, ...)
int execv (const char *filename, char *const argv[])
int execve (const char *filename, char *const argv[], char *const env[])
int execvp (const char *filename, char *const argv[])
void exit (int status)
double exp (double x)
double expm1 (double x)
double fabs (double number)
int fchmod (int filedes, int mode)
int fchown (int filedes, int owner, int group)
int fclean (stream)
int fclose (FILE *stream)
int fcntl (int filedes, int command, ...)
fd_set
FILE * fdopen (int filedes, const char *opentype)
int feof (FILE *stream)
int ferror (FILE *stream)
int fflush (FILE *stream)
int fgetc (FILE *stream)
struct group * fgetgrent (FILE *stream)
int fgetpos (FILE *stream, fpos_t *position)
struct passwd * fgetpwent (FILE *stream)
char * fgets (char *s, int count, FILE *stream)
int fileno (FILE *stream)
int finite (double x)
flock
double floor (double x)
FILE * fmemopen (void *buf, size_t size, const char *opentype)
double fmod (double numerator, double denominator)
int fnmatch (const char *pattern, const char *string, int flags)
FILE * fopen (const char *filename, const char *opentype)
FILE * fopencookie (void *cookie, const char *opentype, struct cookie_functions io_functions)
pid_t fork (void)
long int fpathconf (int filedes, int parameter)
pathconf.
fpos_t
int fprintf (FILE *stream, const char *template, ...)
int fputc (int c, FILE *stream)
int fputs (const char *s, FILE *stream)
size_t fread (void *data, size_t size, size_t count, FILE *stream)
void free (void *ptr)
malloc.
FILE * freopen (const char *filename, const char *opentype, FILE *stream)
double frexp (double value, int *exponent)
int fscanf (FILE *stream, const char *template, ...)
int fseek (FILE *stream, long int offset, int whence)
int fsetpos (FILE *stream, const fpos_t position)
int fstat (int filedes, struct stat *buf)
long int ftell (FILE *stream)
size_t fwrite (const void *data, size_t size, size_t count, FILE *stream)
int getc (FILE *stream)
int getchar (void)
char * getcwd (char *buffer, size_t size)
ssize_t getdelim (char **lineptr, size_t *n, int delimiter, FILE *stream)
gid_t getegid (void)
char * getenv (const char *name)
uid_t geteuid (void)
gid_t getgid (void)
struct group * getgrent (void)
struct group * getgrgid (gid_t gid)
struct group * getgrnam (const char *name)
int getgroups (int count, gid_t *groups)
struct hostent * gethostbyaddr (const char *addr, int length, int format)
struct hostent * gethostbyname (const char *name)
struct hostent * gethostent ()
long int gethostid (void)
int gethostname (char *name, size_t size)
int getitimer (int which, struct itimerval *old)
ssize_t getline (char **lineptr, size_t *n, FILE *stream)
char * getlogin (void)
struct netent * getnetbyaddr (long net, int type)
struct netent * getnetbyname (const char *name)
struct netent * getnetent (void)
int getopt (int argc, char **argv, const char *options)
int getopt_long (int argc, char **argv, const char *shortopts, struct option *longopts, int *indexptr)
int getpeername (int socket, struct sockaddr *addr, size_t *length_ptr)
pid_t getpgrp (pid_t pid)
pid_t getpgrp (void)
pid_t getpid (void)
pid_t getppid (void)
int getpriority (int class, int id)
struct protoent * getprotobyname (const char *name)
struct protoent * getprotobynumber (int protocol)
struct protoent * getprotoent (void)
struct passwd * getpwent (void)
struct passwd * getpwnam (const char *name)
struct passwd * getpwuid (uid_t uid)
int getrlimit (int resource, struct rlimit *rlp)
int getrusage (int processes, struct rusage *rusage)
char * gets (char *s)
struct servent * getservbyname (const char *name, const char *proto)
struct servent * getservbyport (int port, const char *proto)
struct servent * getservent (void)
int getsockname (int socket, struct sockaddr *addr, size_t *length_ptr)
int getsockopt (int socket, int level, int optname, void *optval, size_t *optlen_ptr)
int gettimeofday (struct timeval *tp, struct timezone *tzp)
uid_t getuid (void)
mode_t getumask (void)
int getw (FILE *stream)
char * getwd (char *buffer)
gid_t
int glob (const char *pattern, int flags, int (*errfunc) (const char *filename, int error-code), glob_t *vector_ptr)
glob.
glob_t
glob.
struct tm * gmtime (const time_t *time)
int gsignal (int signum)
unsigned long int htonl (unsigned long int hostlong)
unsigned short int htons (unsigned short int hostshort)
double hypot (double x, double y)
unsigned long int inet_addr (const char *name)
int inet_lnaof (struct in_addr addr)
struct in_addr inet_makeaddr (int net, int local)
int inet_netof (struct in_addr addr)
unsigned long int inet_network (const char *name)
char * inet_ntoa (struct in_addr addr)
double infnan (int error)
int initgroups (const char *user, gid_t gid)
void * initstate (unsigned int seed, void *state, size_t size)
ino_t
int RLIM_INFINITY
int isalnum (int c)
int isalpha (int c)
int isascii (int c)
int isatty (int filedes)
int isblank (int c)
int iscntrl (int c)
int isdigit (int c)
int isgraph (int c)
int isinf (double x)
int islower (int c)
int isnan (double x)
int isprint (int c)
int ispunct (int c)
int isspace (int c)
int isupper (int c)
int isxdigit (int c)
jmp_buf
int kill (pid_t pid, int signum)
int killpg (int pgid, int signum)
long int labs (long int number)
double ldexp (double value, int exponent)
ldiv_t ldiv (long int numerator, long int denominator)
ldiv_t
int link (const char *oldname, const char *newname)
int listen (int socket, unsigned int n)
struct lconv * localeconv (void)
struct tm * localtime (const time_t *time)
double log (double x)
double log10 (double x)
double log1p (double x)
double logb (double x)
void longjmp (jmp_buf state, int value)
off_t lseek (int filedes, off_t offset, int whence)
int lstat (const char *filename, struct stat *buf)
void * malloc (size_t size)
int mblen (const char *string, size_t size)
size_t mbstowcs (wchar_t *wstring, const char *string, size_t size)
int mbtowc (wchar_t *result, const char *string, size_t size)
void mcheck (void (*abortfn) (void))
void * memalign (size_t size, int boundary)
void * memccpy (void *to, const void *from, int c, size_t size)
void * memchr (const void *block, int c, size_t size)
int memcmp (const void *a1, const void *a2, size_t size)
void * memcpy (void *to, const void *from, size_t size)
void * memmem (const void *needle, size_t needle_len,
const void *haystack, size_t haystack_len)
void * memmove (void *to, const void *from, size_t size)
void memory_warnings (void *start, void (*warn_func) (char *))
void * memset (void *block, int c, size_t size)
int mkdir (const char *filename, mode_t mode)
int mkfifo (const char *filename, mode_t mode)
int mknod (const char *filename, int mode, int dev)
time_t mktime (struct tm *brokentime)
mode_t
double modf (double value, double *integer_part)
struct mstats mstats (void)
malloc.
int nice (int increment)
nlink_t
unsigned long int ntohl (unsigned long int netlong)
unsigned short int ntohs (unsigned short int netshort)
void obstack_1grow (struct obstack *obstack_ptr, char c)
void obstack_1grow_fast (struct obstack *obstack_ptr, char c)
int obstack_alignment_mask (struct obstack *obstack_ptr)
void * obstack_alloc (struct obstack *obstack_ptr, size_t size)
void * obstack_base (struct obstack *obstack_ptr)
void obstack_blank (struct obstack *obstack_ptr, size_t size)
void obstack_blank_fast (struct obstack *obstack_ptr, size_t size)
size_t obstack_chunk_size (struct obstack *obstack_ptr)
void * obstack_copy (struct obstack *obstack_ptr, void *address, size_t size)
void * obstack_copy0 (struct obstack *obstack_ptr, void *address, size_t size)
void * obstack_finish (struct obstack *obstack_ptr)
void obstack_free (struct obstack *obstack_ptr, void *object)
void obstack_grow (struct obstack *obstack_ptr, void *data, size_t size)
void obstack_grow0 (struct obstack *obstack_ptr, void *data, size_t size)
void obstack_init (struct obstack *obstack_ptr)
void * obstack_next_free (struct obstack *obstack_ptr)
size_t obstack_object_size (struct obstack *obstack_ptr)
size_t obstack_object_size (struct obstack *obstack_ptr)
int obstack_printf (struct obstack *obstack, const char *template, ...)
size_t obstack_room (struct obstack *obstack_ptr)
int obstack_vprintf (struct obstack *obstack, const char *template, va_list ap)
off_t
size_t offsetof (type, member)
int on_exit (void (*function)(int status, void *arg), void *arg)
int open (const char *filename, int flags[, mode_t mode])
FILE * open_memstream (char **ptr, size_t *sizeloc)
FILE * open_obstack_stream (struct obstack *obstack)
DIR * opendir (const char *dirname)
char * optarg
int opterr
int optind
int optopt
size_t parse_printf_format (const char *template, size_t n, int *argtypes)
long int pathconf (const char *filename, int parameter)
pathconf.
int pause ()
pause.
int pclose (FILE *stream)
void perror (const char *message)
pid_t
int pipe (int filedes[2])
FILE * popen (const char *command, const char *mode)
double pow (double base, double power)
int printf (const char *template, ...)
printf_arginfo_function
printf_function
char * program_invocation_name
char * program_invocation_short_name
void psignal (int signum, const char *message)
ptrdiff_t
int putc (int c, FILE *stream)
int putchar (int c)
int putenv (const char *string)
int putpwent (const struct passwd *p, FILE *stream)
int puts (const char *s)
int putw (int w, FILE *stream)
void qsort (void *array, size_t count, size_t size, comparison_fn_t compare)
void * r_alloc (void **handleptr, size_t size)
void r_alloc_free (void **handleptr)
void * r_re_alloc (void **handleptr, size_t size)
int raise (int signum)
int rand ()
long int random ()
ssize_t read (int filedes, void *buffer, size_t size)
struct dirent * readdir (DIR *dirstream)
int readlink (const char *filename, char *buffer, size_t size)
void * realloc (void *ptr, size_t newsize)
int recv (int socket, void *buffer, size_t size, int flags)
int recvfrom (int socket, void *buffer, size_t size, int flags, struct sockaddr *addr, size_t *length_ptr)
int recvmsg (int socket, struct msghdr *message, int flags)
int regcomp (regex_t *compiled, const char *pattern, int cflags)
size_t regerror (int errcode, regex_t *compiled, char *buffer, size_t length)
regex_t
int regexec (regex_t *compiled, char *string, size_t nmatch, regmatch_t matchptr [], int eflags)
void regfree (regex_t *compiled)
int register_printf_function (int spec, printf_function handler_function, printf_arginfo_function arginfo_function)
regmatch_t
regoff_t
int remove (const char *filename)
int rename (const char *oldname, const char *newname)
void rewind (FILE *stream)
void rewinddir (DIR *dirstream)
double rint (double x)
int rmdir (const char *filename)
double scalb (double value, int exponent)
int scanf (const char *template, ...)
void seekdir (DIR *dirstream, off_t pos)
int select (int nfds, fd_set *read_fds, fd_set *write_fds, fd_set *except_fds, struct timeval *timeout)
int send (int socket, void *buffer, size_t size, int flags)
int sendmsg (int socket, const struct msghdr *message, int flags)
int sendto (int socket, void *buffer. size_t size, int flags, struct sockaddr *addr, size_t length)
void setbuf (FILE *stream, char *buf)
void setbuffer (FILE *stream, char *buf, size_t size)
int setgid (gid_t newgid)
void setgrent (void)
int setgroups (size_t count, gid_t *groups)
void sethostent (int stayopen)
int sethostid (long int id)
int sethostname (const char *name, size_t length)
int setitimer (int which, struct itimerval *old, struct itimerval *new)
int setjmp (jmp_buf state)
void setlinebuf (FILE *stream)
char * setlocale (int category, const char *locale)
void setnetent (int stayopen)
int setpgid (pid_t pid, pid_t pgid)
int setpgrp (pid_t pid, pid_t pgid)
int setpriority (int class, int id, int priority)
void setprotoent (int stayopen)
void setpwent (void)
int setregid (gid_t rgid, fid_t egid)
int setreuid (uid_t ruid, uid_t euid)
int setrlimit (int resource, struct rlimit *rlp)
void setservent (int stayopen)
pid_t setsid (void)
int setsockopt (int socket, int level, int optname, void *optval, size_t optlen)
void * setstate (void *state)
int settimeofday (const struct timeval *tp, const struct timezone *tzp)
int setuid (uid_t newuid)
int setvbuf (FILE *stream, char *buf, int mode, size_t size)
int shutdown (int socket, int how)
sig_atomic_t
int sigaction (int signum, const struct sigaction *action, struct sigaction *old_action)
int sigaddset (sigset_t *set, int signum)
int sigblock (int mask)
int sigdelset (sigset_t *set, int signum)
int sigemptyset (sigset_t *set)
int sigfillset (sigset_t *set)
sighandler_t
int siginterrupt (int signum, int failflag)
int sigismember (const sigset_t *set, int signum)
sigjmp_buf
void siglongjmp (sigjmp_buf state, int value)
int sigmask (int signum)
sighandler_t signal (int signum, sighandler_t action)
int sigpause (int mask)
int sigpending (sigset_t *set)
int sigprocmask (int how, const sigset_t *set, sigset_t *oldset)
sigset_t
int sigsetjmp (sigjmp_buf state, int savesigs)
int sigsetmask (int mask)
int sigstack (const struct sigstack *stack, struct sigstack *oldstack)
int sigsuspend (const sigset_t *set)
sigsuspend.
int sigvec (int signum, const struct sigvec *action,struct sigvec *old_action)
double sin (double x)
double sinh (double x)
size_t
unsigned int sleep (unsigned int seconds)
int snprintf (char *s, size_t size, const char *template, ...)
int socket (int namespace, int style, int protocol)
int socketpair (int namespace, int style, int protocol, int filedes[2])
speed_t
int sprintf (char *s, const char *template, ...)
double sqrt (double x)
void srand (unsigned int seed)
void srandom (unsigned int seed)
int sscanf (const char *s, const char *template, ...)
sighandler_t ssignal (int signum, sighandler_t action)
ssize_t
int stat (const char *filename, struct stat *buf)
FILE * stderr
FILE * stdin
FILE * stdout
char * stpcpy (char *to, const char *from)
int strcasecmp (const char *s1, const char *s2)
char * strcat (char *to, const char *from)
char * strchr (const char *string, int c)
int strcmp (const char *s1, const char *s2)
int strcoll (const char *s1, const char *s2)
char * strcpy (char *to, const char *from)
size_t strcspn (const char *string, const char *stopset)
char * strdup (const char *s)
char * strerror (int errnum)
size_t strftime (char *s, size_t size, const char *template, const struct tm *brokentime)
size_t strlen (const char *s)
int strncasecmp (const char *s1, const char *s2, size_t n)
char * strncat (char *to, const char *from, size_t size)
int strncmp (const char *s1, const char *s2, size_t size)
char * strncpy (char *to, const char *from, size_t size)
char * strpbrk (const char *string, const char *stopset)
char * strrchr (const char *string, int c)
char * strsignal (int signum)
size_t strspn (const char *string, const char *skipset)
char * strstr (const char *haystack, const char *needle)
double strtod (const char *string, char **tailptr)
char * strtok (char *newstring, const char *delimiters)
long int strtol (const char *string, char **tailptr, int base)
unsigned long int strtoul (const char *string, char **tailptr, int base)
struct cookie_io_functions
struct dirent
struct group
struct hostent
struct in_addr
struct itimerval
struct lconv
struct linger
struct msghdr
struct mstats
malloc.
struct netent
struct obstack
struct option
struct passwd
struct printf_info
struct protoent
struct rlimit
struct rusage
struct servent
struct sigaction
struct sigstack
struct sigvec
struct sockaddr
struct sockaddr_in
struct sockaddr_un
struct stat
struct termios
struct timeval
struct timezone
struct tm
struct tms
struct utimbuf
struct utsname
size_t strxfrm (char *to, const char *from, size_t size)
int symlink (const char *oldname, const char *newname)
long int sysconf (int parameter)
sysconf.
int system (const char *command)
double tan (double x)
double tanh (double x)
int tcdrain (int filedes)
tcflag_t
int tcflow (int filedes, int action)
int tcflush (int filedes, int queue)
int tcgetattr (int filedes, struct termios *termios_p)
pid_t tcgetpgrp (int filedes)
int tcsendbreak (int filedes, int duration)
int tcsetattr (int filedes, int when, const struct termios *termios_p)
int tcsetpgrp (int filedes, pid_t pgid)
off_t telldir (DIR *dirstream)
char * tempnam (const char *dir, const char *prefix)
time_t time (time_t *result)
time_t
clock_t times (struct tms *buffer)
long int timezone
FILE * tmpfile (void)
char * tmpnam (char *result)
int toascii (int c)
int tolower (int c)
int toupper (int c)
char * ttyname (int filedes)
void tzset (void)
uid_t
mode_t umask (mode_t mask)
int uname (struct utsname *info)
int ungetc (int c, FILE *stream)
ungetc To Do Unreading.
union wait
int unlink (const char *filename)
int utime (const char *filename, const struct utimbuf *times)
int utimes (const char *filename, struct timeval tvp[2])
va_alist
type va_arg (va_list ap, type)
va_dcl
void va_end (va_list ap)
va_list
void va_start (va_list ap)
void va_start (va_list ap, last_required)
void * valloc (size_t size)
int vasprintf (char **ptr, const char *template, va_list ap)
pid_t vfork (void)
int vfprintf (FILE *stream, const char *template, va_list ap)
int vfscanf (FILE *stream, const char *template, va_list ap)
int vprintf (const char *template, va_list ap)
int vscanf (const char *template, va_list ap)
int vsnprintf (char *s, size_t size, const char *template, va_list ap)
int vsprintf (char *s, const char *template, va_list ap)
int vsscanf (const char *s, const char *template, va_list ap)
pid_t wait (int *status_ptr)
pid_t wait3 (union wait *status_ptr, int options, struct rusage *usage)
pid_t wait4 (pid_t pid, union wait *status_ptr, int options, struct rusage *usage)
pid_t waitpid (pid_t pid, int *status_ptr, int options)
wchar_t
size_t wcstombs (char *string, const wchar_t wstring, size_t size)
int wctomb (char *string, wchar_t wchar)
int wordexp (const char *words, wordexp_t *word-vector-ptr, int flags)
wordexp.
wordexp_t
wordexp.
void wordfree (wordexp_t *word-vector-ptr)
wordexp.
ssize_t write (int filedes, const void *buffer, size_t size)
Installation of the GNU C library is relatively simple.
You need the latest version of GNU make. Modifying the GNU C
Library to work with other make programs would be so hard that we
recommend you port GNU make instead. Really.
To configure the GNU C library for your system, run the shell script
`configure' with sh. Use an argument which is the
conventional GNU name for your system configuration--for example,
`sparc-sun-sunos4.1', for a Sun 4 running Sunos 4.1.
See section 'Installing GNU CC' in Using and Porting GNU CC, for a full description of standard GNU configuration
names.
The GNU C Library currently supports configurations that match the following patterns:
sparc-sun-sunos4.n m68k-hp-bsd4.3 m68k-sun-sunos4.n m68k-sony-bsd4.3 mips-dec-ultrix4.n i386-bsd4.3 i386-sysv i386-sysv4
While no other configurations are supported, there are handy aliases for these few. (These aliases work in other GNU software as well.)
sun4-sunos4.n hp320-bsd4.3 sun3-sunos4.n news decstation-ultrix i386-svr4
Here are some options that you should specify (if appropriate) when
you run configure:
ld to link programs with
the GNU C Library. (We strongly recommend that you do.)
gas, when
building the GNU C Library. On some systems, the library may not build
properly if you do not use gas.
Use this option if your computer lacks hardware floating point support.
The simplest way to run configure is to do it in the directory
that contains the library sources. This prepares to build the library
in that very directory.
You can prepare to build the library in some other directory by going
to that other directory to run configure. In order to run
configure, you will have to specify a directory for it, like this:
mkdir ../hp320 cd ../hp320 ../src/configure hp320-bsd4.3
configure looks for the sources in whatever directory you
specified for finding configure itself. It does not matter where
in the file system the source and build directories are--as long as you
specify the source directory when you run configure, you will get
the proper results.
This feature lets you keep sources and binaries in different
directories, and that makes it easy to build the library for several
different machines from the same set of sources. Simply create a
build directory for each target machine, and run configure in
that directory specifying the target machine's configuration name.
The library has a number of special-purpose configuration parameters. These are defined in the file `Makeconfig'; see the comments in that file for the details.
But don't edit the file `Makeconfig' yourself--instead, create a file `configparms' in the directory where you are building the library, and define in that file the parameters you want to specify. `configparms' should not be an edited copy of `Makeconfig'; specify only the parameters that you want to override.
Some of the machine-dependent code for some machines uses extensions in the GNU C compiler, so you may need to compile the library with GCC. (In fact, all of the existing complete ports require GCC.)
The current release of the C library contains some header files that the compiler normally provides: `stddef.h', `stdarg.h', and several files with names of the form `va-machine.h'. The versions of these files that came with older releases of GCC do not work properly with the GNU C library. The `stddef.h' file in release 2.2 and later of GCC is correct. If you have release 2.2 or later of GCC, use its version of `stddef.h' instead of the C library's. To do this, put the line `override stddef.h =' in `configparms'. The other files are corrected in release 2.3 and later of GCC. `configure' will automatically detect whether the installed `stdarg.h' and `va-machine.h' files are compatible with the C library, and use its own if not.
There is a potential problem with the size_t type and versions of
GCC prior to release 2.4. ANSI C requires that size_t always be
an unsigned type. For compatibility with existing systems' header
files, GCC defines size_t in `stddef.h' to be whatever type
the system's `sys/types.h' defines it to be. Most Unix systems
that define size_t in `sys/types.h', define it to be a
signed type. Some code in the library depends on size_t being an
unsigned type, and will not work correctly if it is signed.
The GNU C library code which expects size_t to be unsigned is
correct. The definition of size_t as a signed type is incorrect.
We plan that in version 2.4, GCC will always define size_t as an
unsigned type, and the `fixincludes' script will massage the
system's `sys/types.h' so as not to conflict with this.
In the meantime, we work around this problem by telling GCC explicitly
to use an unsigned type for size_t when compiling the GNU C
library. `configure' will automatically detect what type GCC uses
for size_t arrange to override it if necessary.
To build the library, type make lib. This will produce a lot of
output, some of which looks like errors from make (but isn't).
Look for error messages from make containing `***'. Those
indicate that something is really wrong. Using the `-w' option to
make may make the output easier to understand (this option tells
make to print messages telling you what subdirectories it is
working on).
To install the library and header files, type make install, after
setting the installation directories in `configparms'. This will
build things if necessary, before installing them.
There are probably bugs in the GNU C library. If you report them, they will get fixed. If you don't, no one will ever know about them and they will remain unfixed for all eternity, if not longer.
To report a bug, first you must find it. Hopefully, this will be the hard part. Once you've found a bug, make sure it's really a bug. A good way to do this is to see if the GNU C library behaves the same way some other C library does. If so, probably you are wrong and the libraries are right (but not necessarily). If not, one of the libraries is probably wrong.
Once you're sure you've found a bug, try to narrow it down to the smallest test case that reproduces the problem. In the case of a C library, you really only need to narrow it down to one library function call, if possible. This should not be too difficult.
The final step when you have a simple test case is to report the bug. When reporting a bug, send your test case, the results you got, the results you expected, what you think the problem might be (if you've thought of anything), your system type, and the version of the GNU C library which you are using.
If you think you have found some way in which the GNU C library does not conform to the ANSI and POSIX standards (see section Standards and Portability), that is definitely a bug. Report it!
Send bug reports to the Internet address `[email protected]' or the UUCP path `mit-eddie!prep.ai.mit.edu!bug-glibc'. If you have other problems with installation, use, or the documentation, please report those as well.
The process of building the library is driven by the makefiles, which
make heavy use of special features of GNU make. The makefiles
are very complex, and you probably don't want to try to understand them.
But what they do is fairly straightforward, and only requires that you
define a few variables in the right places.
The library sources are divided into subdirectories, grouped by topic. The `string' subdirectory has all the string-manipulation functions, `stdio' has all the standard I/O functions, etc.
Each subdirectory contains a simple makefile, called `Makefile',
which defines a few make variables and then includes the global
makefile `Rules' with a line like:
include ../Rules
The basic variables that a subdirectory makefile defines are:
subdir
headers
routines
aux
routines for
modules that define functions in the library, and aux for
auxiliary modules containing things like data definitions. But the
values of routines and aux are just concatenated, so there
really is no practical difference.
tests
others
install-lib
install-data
install
install-data are installed in the directory specified by
`datadir' in `configparms' or `Makeconfig'. Files listed
in install are installed in the directory specified by
`bindir' in `Makeconfig'.
distribute
distribute if there are files used in an unusual way
that should go into the distribution.
generated
extra-objs
others or tests.
The GNU C library is written to be easily portable to a variety of machines and operating systems. Machine- and operating system-dependent functions are well separated to make it easy to add implementations for new machines or operating systems. This section describes the layout of the library source tree and explains the mechanisms used to select machine-dependent code to use.
All the machine-dependent and operating system-dependent files in the library are in the subdirectory `sysdeps' under the top-level library source directory. This directory contains a hierarchy of subdirectories (see section The Layout of the `sysdeps' Directory Hierarchy).
Each subdirectory of `sysdeps' contains source files for a particular machine or operating system, or for a class of machine or operating system (for example, systems by a particular vendor, or all machines that use IEEE 754 floating-point format). A configuration specifies an ordered list of these subdirectories. Each subdirectory implicitly appends its parent directory to the list. For example, specifying the list `unix/bsd/vax' is equivalent to specifying the list `unix/bsd/vax unix/bsd unix'. A subdirectory can also specify that it implies other subdirectories which are not directly above it in the directory hierarchy. If the file `Implies' exists in a subdirectory, it lists other subdirectories of `sysdeps' which are appended to the list, appearing after the subdirectory containing the `Implies' file. Lines in an `Implies' file that begin with a `#' character are ignored as comments. For example, `unix/bsd/Implies' contains:
# BSD has Internet-related things. unix/inetand `unix/Implies' contains:
posix
So the final list is `unix/bsd/vax unix/bsd vax unix/inet unix posix'.
`sysdeps' has two "special" subdirectories, called `generic'
and `stub'. These two are always implicitly appended to the list
of subdirectories (in that order), so you needn't put them in an
`Implies' file, and you should not create any subdirectories under
them. `generic' is for things that can be implemented in
machine-independent C, using only other machine-independent functions in
the C library. `stub' is for stub versions of functions
which cannot be implemented on a particular machine or operating system.
The stub functions always return an error, and set errno to
ENOSYS (Function not implemented). See section Error Reporting.
A source file is known to be system-dependent by its having a version in `generic' or `stub'; every system-dependent function should have either a generic or stub implementation (there is no point in having both).
If you come across a file that is in one of the main source directories (`string', `stdio', etc.), and you want to write a machine- or operating system-dependent version of it, move the file into `sysdeps/generic' and write your new implementation in the appropriate system-specific subdirectory. Note that if a file is to be system-dependent, it must not appear in one of the main source directories.
There are a few special files that may exist in each subdirectory of `sysdeps':
make
conditional directives based on the variable `subdir' (see above) to
select different sets of variables and rules for different sections of
the library. It can also set the make variable
`sysdep-routines', to specify extra modules to be included in the
library. You should use `sysdep-routines' rather than adding
modules to `routines' because the latter is used in determining
what to distribute for each subdirectory of the main source tree.Each makefile in a subdirectory in the ordered list of subdirectories to be searched is included in order. Since several system-dependent makefiles may be included, each should append to `sysdep-routines' rather than simply setting it:
sysdep-routines := $(sysdep-routines) foo bar
Use this when there are whole new sets of routines and header files that should go into the library for the system this subdirectory of `sysdeps' implements. For example, `sysdeps/unix/inet/Subdirs' contains `inet'; the `inet' directory contains various network-oriented operations which only make sense to put in the library on systems that support the Internet.
That is the general system for how system-dependencies are isolated. The next section explains how to decide what directories in `sysdeps' to use. section Porting the GNU C Library to Unix Systems, has some tips on porting the library to Unix variants.
A GNU configuration name has three parts: the CPU type, the manufacturer's name, and the operating system. `configure' uses these to pick the list of system-dependent directories to look for. If the `--nfp' option is not passed to `configure', the directory `machine/fpu' is also used. The operating system often has a base operating system; for example, if the operating system is `sunos4.1', the base operating system is `unix/bsd'. The algorithm used to pick the list of directories is simple: `configure' makes a list of the base operating system, manufacturer, CPU type, and operating system, in that order. It then concatenates all these together with slashes in between, to produce a directory name; for example, the configuration `sparc-sun-sunos4.1' results in `unix/bsd/sun/sparc/sunos4.1'. `configure' then tries removing each element of the list in turn, so `unix/bsd/sparc' and `sun/sparc' are also tried, among others. Since the precise version number of the operating system is often not important, and it would be very inconvenient, for example, to have identical `sunos4.1.1' and `sunos4.1.2' directories, `configure' tries successively less specific operating system names by removing trailing suffixes starting with a period.
Here is the complete list of directories that would be tried for the configuration `sparc-sun-sunos4.1':
sparc/fpu unix/bsd/sun/sunos4.1/sparc unix/bsd/sun/sunos4.1 unix/bsd/sun/sunos4/sparc unix/bsd/sun/sunos4 unix/bsd/sun/sparc unix/bsd/sun unix/bsd/sunos4.1/sparc unix/bsd/sunos4.1 unix/bsd/sunos4/sparc unix/bsd/sunos4 unix/bsd/sparc unix/bsd sun/sunos4.1/sparc sun/sunos4.1 sun/sunos4/sparc sun/sunos4 sun/sparc sun sunos4.1/sparc sunos4.1 sunos4/sparc sunos4 sparc
Different machine architectures are generally at the top level of the `sysdeps' directory tree. For example, `sysdeps/sparc' and `sysdeps/m68k'. These contain files specific to those machine architectures, but not specific to any particular operating system. There might be subdirectories for specializations of those architectures, such as `sysdeps/m68k/68020'. Code which is specific to the floating-point coprocessor used with a particular machine should go in `sysdeps/machine/fpu'.
There are a few directories at the top level of the `sysdeps' hierarchy that are not for particular machine architectures.
float is IEEE 754 single-precision format, and
double is IEEE 754 double-precision format. Usually this
directory is referred to in the `Implies' file in a machine
architecture-specific directory, such as `m68k/Implies'.
socket and related functions on Unix systems.
The `inet' top-level subdirectory is enabled by `unix/inet/Subdirs'.
`unix/common' implies `unix/inet'.
Most Unix systems are fundamentally very similar. There are variations between different machines, and variations in what facilities are provided by the kernel. But the interface to the operating system facilities is, for the most part, pretty uniform and simple.
The code for Unix systems is in the directory `unix', at the top level of the `sysdeps' hierarchy. This directory contains subdirectories (and subdirectory trees) for various Unix variants.
The functions which are system calls in most Unix systems are implemented in assembly code in files in `sysdeps/unix'. These files are named with a suffix of `.S'; for example, `__open.S'. Files ending in `.S' are run through the C preprocessor before being fed to the assembler.
These files all use a set of macros that should be defined in `sysdep.h'. The `sysdep.h' file in `sysdeps/unix' partially defines them; a `sysdep.h' file in another directory must finish defining them for the particular machine and operating system variant. See `sysdeps/unix/sysdep.h' and the machine-specific `sysdep.h' implementations to see what these macros are and what they should do.
The system-specific makefile for the `unix' directory, `sysdeps/unix/Makefile', gives rules to generate several files from the Unix system you are building the library on (which is assumed to be the target system you are building the library for). All the generated files are put in the directory where the object files are kept; they should not affect the source tree itself. The files generated are `ioctls.h', `errnos.h', `sys/param.h', and `errlist.c' (for the `stdio' section of the library).
The GNU C library was written almost entirely by Roland McGrath. Some parts of the library were contributed by other people.
getopt function and related code were written by
Richard Stallman, David J. MacKenzie, and Roland McGrath.
All code incorporated from 4.4 BSD is under the following copyright:
Copyright (C) 1991 Regents of the University of California. All rights reserved.Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
- Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
- All advertising materials mentioning features or use of this software must display the following acknowledgement:
This product includes software developed by the University of California, Berkeley and its contributors.- Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
random, srandom,
setstate and initstate, which are also the basis for the
rand and srand functions, were written by Earl T. Cohen
for the University of California at Berkeley and are copyrighted by the
Regents of the University of California. They have undergone minor
changes to fit into the GNU C library and to fit the ANSI C standard,
but the functional code is Berkeley's.
qsort was written by Michael J. Haertel.
qsort was written
by Douglas C. Schmidt.
malloc, realloc and
free and related code were written by Michael J. Haertel.
memcpy,
strlen, etc.) were written by
Granlund.
Mach Operating System Copyright (C) 1991,1990,1989 Carnegie Mellon University All Rights Reserved.Permission to use, copy, modify and distribute this software and its documentation is hereby granted, provided that both the copyright notice and this permission notice appear in all copies of the software, derivative works or modified versions, and any portions thereof, and that both notices appear in supporting documentation.
CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS" CONDITION. CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
Carnegie Mellon requests users of this software to return to
Software Distribution Coordinator School of Computer Science Carnegie Mellon University Pittsburgh PA 15213-3890or `[email protected]' any improvements or extensions that they make and grant Carnegie Mellon the rights to redistribute these changes.
mips-dec-ultrix4)
was contributed by Brendan Kehoe and Ian Lance Taylor.
crypt and related functions were
contributed by Michael Glad.
ftw function was contributed by Ian Lance Taylor.
mktime function was contributed by Noel Cragg.
i386-sequent-bsd) was contributed by Jason Merrill.
Copyright (C) 1991 Free Software Foundation, Inc. 675 Mass Ave, Cambridge, MA 02139, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. [This is the first released version of the library GPL. It is numbered 2 because it goes with version 2 of the ordinary GPL.]
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public Licenses are intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users.
This license, the Library General Public License, applies to some specially designated Free Software Foundation software, and to any other libraries whose authors decide to use it. You can use it for your libraries, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the library, or if you modify it.
For example, if you distribute copies of the library, whether gratis or for a fee, you must give the recipients all the rights that we gave you. You must make sure that they, too, receive or can get the source code. If you link a program with the library, you must provide complete object files to the recipients so that they can relink them with the library, after making changes to the library and recompiling it. And you must show them these terms so they know their rights.
Our method of protecting your rights has two steps: (1) copyright the library, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the library.
Also, for each distributor's protection, we want to make certain that everyone understands that there is no warranty for this free library. If the library is modified by someone else and passed on, we want its recipients to know that what they have is not the original version, so that any problems introduced by others will not reflect on the original authors' reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that companies distributing free software will individually obtain patent licenses, thus in effect transforming the program into proprietary software. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.
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It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice.
This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.
Each version is given a distinguishing version number. If the Library specifies a version number of this License which applies to it and "any later version", you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Library does not specify a license version number, you may choose any version ever published by the Free Software Foundation.
If you develop a new library, and you want it to be of the greatest possible use to the public, we recommend making it free software that everyone can redistribute and change. You can do so by permitting redistribution under these terms (or, alternatively, under the terms of the ordinary General Public License).
To apply these terms, attach the following notices to the library. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.
one line to give the library's name and an idea of what it does. Copyright (C) year name of author This library is free software; you can redistribute it and/or modify it under the terms of the GNU Library General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public License for more details. You should have received a copy of the GNU Library General Public License along with this library; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
Also add information on how to contact you by electronic and paper mail.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the library, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the library `Frob' (a library for tweaking knobs) written by James Random Hacker. signature of Ty Coon, 1 April 1990 Ty Coon, President of Vice
That's all there is to it!
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alloca disadvantages
alloca function
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printf
printf conversions
alloca
malloc
exec functions
printf
fcntl function
select
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sigaction
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scanf
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malloc function
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printf
pause function
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setuid programs
sigaction flags
sigaction function
SIGCHLD, handling of
signal function
SIGTTIN, from background job
SIGTTOU, from background job
printf
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scanf)
volatile declarations