Driver-tutorial/LKM/hello_func.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


void func(void);
EXPORT_SYMBOL_GPL(func);

/**
 *  EXPORT_SYMBOL_GPL:
 *  Since, in user space to use variable/function of one
 *  file to another file we use extern or header file. But
 *  for kernel space such thing is not allowed. But to use
 *  variable or function in another module/file we use
 *  EXPORT_SYMBOL_GPL(<fun or var name>) to export the 
 *  function or variable to global scope for all the modules.
 */

int val = 300;

void func(){

    printk(KERN_INFO"func invoked\n ");
    printk(KERN_INFO" val = %d \n", val);
}

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

static int __init hello_init(void){
    printk(KERN_INFO "Init Hello func module\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 func module.\n");
}

module_init(hello_init);
module_exit(hello_cleanup);