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/**
 * @file    hello.c
 * @author  Akshat Sinha
 * @date    10 Sept 2016
 * @version 0.1
 * @brief  An introductory "Hello World!" loadable kernel
 *  module (LKM) that can display a message in the /var/log/kern.log
 *  file when the module is loaded and removed. The module can accept
 *  an argument when it is loaded -- the name, which appears in the
 *  kernel log files.
*/
#include <linux/module.h>     /* Needed by all modules */
#include <linux/kernel.h>     /* Needed for KERN_INFO */
#include <linux/init.h>       /* Needed for the macros */

///< The license type -- this affects runtime behavior
MODULE_LICENSE("GPL");

///< The author -- visible when you use modinfo
MODULE_AUTHOR("Akshat Sinha");

///< The description -- see modinfo
MODULE_DESCRIPTION("A simple Hello world LKM!");

///< The version of the module
MODULE_VERSION("0.1");

static int __init hello_start(void)
{
    printk(KERN_INFO "Loading hello module...\n");
    printk(KERN_INFO "Hello world\n");
    return 0;
}

static void __exit hello_end(void)
{
    printk(KERN_INFO "Goodbye Mr.\n");
}

obj-m = hello.o
all:
        make -C /lib/modules/$(shell uname -r)/build/ M=$(PWD) modules
clean:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

ubuntu@ubuntu:/opt/test3$ make
make -C /lib/modules/5.4.0-42-generic/build/ M=/opt/test3 modules
make[1]: Entering directory '/usr/src/linux-headers-5.4.0-42-generic'
  CC [M]  /opt/test3/hello.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC [M]  /opt/test3/hello.mod.o
  LD [M]  /opt/test3/hello.ko
make[1]: Leaving directory '/usr/src/linux-headers-5.4.0-42-generic'
ubuntu@ubuntu:/opt/test3$
ubuntu@ubuntu:/opt/test3$ sudo insmod hello.ko
ubuntu@ubuntu:/opt/test3$
ubuntu@ubuntu:/opt/test3$ dmesg |tail -n 1
[256200.240481] Hello world
ubuntu@ubuntu:/opt/test3$