Internals of module_init and module_exit

We know from our hello world device driver program , that on insmod, the function passed in the module_init macro is called, and on rmmod, the argument passed in the module_exit is called.

Let's see the definition of this macro, it is present in linux/module.h

The purpose of defining __inittest function is to check during compile time, the function passed to module_init() macro is compatible with the initcall_t type.

initcall_t is defined in linux/init.h:
typedef int (*initcall_t)(void);

If you declare module_init function which returns void instead of int, the compiler will throw warning:

Loading this module generated an oops message.

The last line uses the alias attribute of gcc to assign another name to init_module, so that you can have a better name as per your driver (e.g. cdrom_init instead of init_module), instead of each driver having init_module. Same is the case with module_exit, giving whatever name in module_exit as parameter to cleanup_module.

To understand alias attribute, you can refer to my tutorial of alias

https://embeddedguruji.blogspot.com/2018/12/what-is-use-of-gcc-attribute-alias.html

Code:

	#include <linux/kernel.h>
	#include <linux/module.h>
	MODULE_LICENSE("GPL");
	int init_module(void)
	{
	printk(KERN_INFO"%s: In init\n", __func__);
	return 0;
	}
	void cleanup_module(void)
	{
	printk(KERN_INFO"%s: In exit\n", __func__);
	}

view raw module.c hosted with ❤ by GitHub

Output:

Notes:

You can see on loading the module with insmod, init_module is called and removing the module with rmmod cleanup_module is called