This section briefly outlines the procedure for generating a bootable kernel image from source code.

Note that you can set the default kernel for the next reboot only by re-running lilo with the -R switch followed by the appropriate image label.

The printk() function can be used to output tracing/debugging info to the kernel logs.

See forthcoming revised Linux Module Programming Guide.

The easiest way to add boot time options is to find the existing kernel option that matches your requirements most closely and copy that. Take a look at and edit the Config.in file in the associated directory. Then surround your option specific code with inclusion guards.

It is possible to pass information to the kernel at boot time in the form of command line options. For information about the current list of options, see ~/Documentation/kernel-parameters.txt and the Linux BootPrompt-HOWTO.

Some drivers make use of this facility to set base addresses where hardware auto-detection is not possible or not implemented. Boot time parameters are especially useful when drivers or features are compiled into the kernel and not as a module, where options can be given when the module is loaded.

At version 2.3.13 the kernel initialisation code changed significantly for the better. Previously, the initialisation code was covered in #ifdef SOME_CONFIG_OPTION statements, to ensure only the required initialisation code is compiled into the final image. Now a different approach is taken, known as "init calls", where the initialisation functions are registered and stored in a special location in the ELF executable format. At system start up, each of the registered functions is called in turn (although it is not possible to specify the order of execution, so some #ifdef statements remain).

A similar approach is used to parse boot time options. To associate a particular boot time string with its own handling function, a call is made to __setup(). This could be done from ~/init/main.c.

For example, to add support for a command line option uptime, which sets initial uptime to a specified number of jiffies:

static int __init riguptime(char *line){
	jiffies = simple_strtol(line, (char **)(line + sizeof(line)), (unsigned int)10);
	return 0;
}

__setup("uptime=", riguptime);
			

So if the kernel was booted with uptime=12345 as a command line option, *line (passed to riguptime()) would point to a null-terminated string containing "12345". This string is converted to a value of type long by the simple_strtol() macro.

It should be possible to do just about everything you could need to without doing this. Really.

OK so you still want to? ("Don't try this at home kids!")

See this tutorial (but needs adapting for 2.4.x).

One of the ways to avoid adding another system call is to use the /proc interface. This section will describe a simple example.