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);