Hello World (part 3): The __init and __exit Macros

This demonstrates a feature of kernel 2.2 and later. Notice the change in the definitions of the init and cleanup functions. The __init macro causes the init function to be discarded and its memory freed once the init function finishes for built-in drivers, but not loadable modules. If you think about when the init function is invoked, this makes perfect sense.

There is also an __initdata which works similarly to __init but for init variables rather than functions.

The __exit macro causes the omission of the function when the module is built into the kernel, and like __exit, has no effect for loadable modules. Again, if you consider when the cleanup function runs, this makes complete sense; built-in drivers don't need a cleanup function, while loadable modules do.

These macros are defined in linux/init.h and serve to free up kernel memory. When you boot your kernel and see something like Freeing unused kernel memory: 236k freed, this is precisely what the kernel is freeing.

/*  hello-3.c - Illustrating the __init, __initdata and __exit macros.
 *
 *  Copyright (C) 2001 by Peter Jay Salzman
 *
 *  08/02/2006 - Updated by Rodrigo Rubira Branco <rodrigo@kernelhacking.com>
 */

/* Kernel Programming */
#define MODULE
#define LINUX
#define __KERNEL__

#include <linux/module.h>      /* Needed by all modules */
#include <linux/kernel.h>      /* Needed for KERN_ALERT */
#include <linux/init.h>        /* Needed for the macros */

static int hello3_data __initdata = 3;


static int __init hello_3_init(void)
{
   printk(KERN_ALERT "Hello, world %d\n", hello3_data);
   return 0;
}


static void __exit hello_3_exit(void)
{
   printk(KERN_ALERT "Goodbye, world 3\n");
}


module_init(hello_3_init);
module_exit(hello_3_exit);

MODULE_LICENSE("GPL");

By the way, you may see the directive "__initfunction()" in drivers written for Linux 2.2 kernels:

 __initfunction(int init_module(void))
{
   printk(KERN_ALERT "Hi there.\n");
   return 0;
}

This macro served the same purpose as __init, but is now very deprecated in favor of __init. I only mention it because you might see it modern kernels. As of 2.4.18, there are 38 references to __initfunction(), and of 2.4.20, there are 37 references. However, don't use it in your own code.