= What are the __init* and __exit* macros ? =

The '_​_​init*' and '_​_​exit*' macros are widely used in the kernel. These macros are defined in include/linux/init.h :
{{{
#define __init          __attribute__ ((__section__ (".init.text")))
#define __initdata      __attribute__ ((__section__ (".init.data")))
#define __exitdata      __attribute__ ((__section__(".exit.data")))
#define __exit_call     __attribute_used__ __attribute__ ((__section__ (".exitcall.exit")))
 
#ifdef MODULE
#define __exit          __attribute__ ((__section__(".exit.text")))
#else
#define __exit          __attribute_used__ __attribute__ ((__section__(".exit.text")))
#endif
}}}

== __init* macros ==


It tells the compiler to put the variable or the function in a special section, which is declared in vmlinux.lds.
_​_​init puts the function in the ".init.text" section and _​_​initdata puts the data in the ".init.data" section.

For example, the following declaration means that the variable md_setup_ents will be put in the the init data section.
{{{
static int md_setup_ents _​_​initdata;
}}}
But why must you use these macros ?

Let's take an example, with the following function, defined in mm/slab.c :
{{{
void _​_​init kmem_cache_init(void)
}}}		
This function initializes the slab system : it's only used once, at the boot of the kernel. So the code of this function should be freed from the memory after the first call. It's the goal of free_initmem().
	The function free_initmem() will free the entire text and data init sections and so the code of your function, if it has been declared as '_​_​init'. All kernel text (code) and data that are marked with '_​_​init' or '_​_​initdata' are freed and cannot be used later on.

== __exit* macro ==

The _​_​exit macro tells the compiler to put the function in the ".exit.text" section.
The _​_​exit_data macro tells the compiler to put the function in the ".exit.data" section.

exit.* sections make sense only for the modules : exit functions will never be called if compiled statically.
That's why there is a ifdef : exit.* sections will be discarded only if modules support is disabled.

== Prototype of a module ==

A module must use the _​_​init and _​_​exit macros. Here is a prototype of a module :
{{{
#include <linux/module.h>
#include <linux/kernel.h>

#define MODULE_AUTHOR "tyler@agat.net"
#define MODULE_DESC "Description of the module"

int _​_​init init_function(void)
{
	/* Do something */
	if (err)
		return -ERR;
	return 0;
}

void _​_​exit exit_function()
{
	/* Do something */
}

module_init(init_function);
module_exit(exit_function);

MODULE_LICENSE("GPL");
MODULE_AUTHOR(MODULE_AUTHOR);
MODULE_DESCRIPTION(MODULE_DESC);
}}}