Driver-tutorial/LKM/hello.c

#include <linux/module.h>   // included for all kernel module
#include <linux/kernel.h>   // included for KERN_INFO
#include <linux/init.h>     // included for __init and __exit macros

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Junet Hossain");
MODULE_DESCRIPTION("A Simple Hello World module");

static int __init hello_init(void){
    printk(KERN_INFO "Hello world!\n");
    return 0; // Non-zero return means that the module couldn't be loaded.
}

static void  __exit hello_cleanup(void){
    printk(KERN_INFO "Cleaning up hello module.\n");
}

module_init(hello_init);
module_exit(hello_cleanup);

/**
 *  Use these macros for efficient and optimized kernel modules.
 *  
 *  __init: It's a macro which is used for marking
 *          the function which is executed when the
 *          kernel is loaded. After execution, its memory
 *          is freed (for built-in modules). For module
 *          compiled as a loadable kernel module (LKM),
 *          __init has no effect (because the memory
 *          is allocated dynamically). If not used for built
 *          -in kernel, the memory for the init function
 *          remains allocated even after module initializaiton
 *          (wasted memory)
 * 
 *  __exit: It's a macro which is used to mark the function
 *          which is executed when the module is unloaded
 *          from the kernel. It is ignored for built-in kernel modules
 *          (since they can't be removed at runtime). For
 *          dynamically loaded modules, it ensures proper
 *          cleanup. If __exit is not used, the function will
 *          still run when removing the module, but marking
 *          it properly makes the code cleaner and optimized.
 */