== 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.

== __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);
}}}