Hello World Kernel Module

Hello World Kernel

file hello.c

#include <linux/module.h>
#include <linux/init.h>


#define HELLO_AUTHOR "Rajesh6115 <sahoorajesh.d@gmail.com>"
#define HELLO_DESC "A Simple Hello World Module"

static int __init hello_init(void);
static void __exit hello_exit(void);

MODULE_LICENSE("GPL");
MODULE_AUTHOR(HELLO_AUTHOR); /* Who wrote this module? */
MODULE_DESCRIPTION(HELLO_DESC); /* What does this module do */

static int __init hello_init(void){
    printk(KERN_INFO "Hello World Linux Kernel --------INIT Message");
return 0;
}

static void __exit hello_exit(void){
    printk(KERN_INFO "Good Bye Linux Kernel --------EXIT Message");
}

module_init(hello_init);
module_exit(hello_exit);

BUILD AND RUNNIG A Hello MODULE

For Building a Module We Normally Take the help of Kernel Build Script.
which present in '/lib/modules/<kernel version>/build' path.

Make File for Hello Module

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

Compile by make command

Out put in my system

rajesh@ubuntu:~/rajesh/kernelmod/hello$ make
make -C /lib/modules/3.12.6-rajesh-first/build M=/home/rajesh/rajesh/kernelmod/hello modules
make[1]: Entering directory `/host/linux-3.12.6'
  CC [M]  /home/rajesh/rajesh/kernelmod/hello/hello.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /home/rajesh/rajesh/kernelmod/hello/hello.mod.o
  LD [M]  /home/rajesh/rajesh/kernelmod/hello/hello.ko
make[1]: Leaving directory `/host/linux-3.12.6'
rajesh@ubuntu:~/rajesh/kernelmod/hello$

Some New files will generate in same directory
And our driver file is 'hello.ko'

Run/Load the module by using 'insmod' Command
To execute that Command you should be root user or use sudo to execute

rajesh@ubuntu:~/rajesh/kernelmod/hello$ sudo insmod ./hello.ko
[sudo] password for rajesh:
rajesh@ubuntu:~/rajesh/kernelmod/hello$

'printk' function log the message to kernel log file.And we can see the same by 'dmesg' command

rajesh@ubuntu:~/rajesh/kernelmod/hello$ dmesg | tail -5
[ 1094.480372] audit_printk_skb: 177 callbacks suppressed
[ 1094.480375] type=1400 audit(1392792445.131:71): apparmor="DENIED" operation="capable" parent=1 profile="/usr/sbin/cupsd" pid=797 comm="cupsd" pid=797 comm="cupsd" capability=36  capname="block_suspend"
[ 1599.091190] perf samples too long (2510 > 2500), lowering kernel.perf_event_max_sample_rate to 50000
[ 6380.140339] Hello World Linux Kernel --------INIT Message

Unload/Remove the module by using 'rmmod' Command
To execute that Command you should be root user or use sudo to execute

rajesh@ubuntu:~/rajesh/kernelmod/hello$ sudo rmmod hello

Again we will see exit message using 'dmesg'

rajesh@ubuntu:~/rajesh/kernelmod/hello$ dmesg | tail -5
[ 1094.480375] type=1400 audit(1392792445.131:71): apparmor="DENIED" operation="capable" parent=1 profile="/usr/sbin/cupsd" pid=797 comm="cupsd" pid=797 comm="cupsd" capability=36  capname="block_suspend"
[ 1599.091190] perf samples too long (2510 > 2500), lowering kernel.perf_event_max_sample_rate to 50000
[ 6380.140339] Hello World Linux Kernel
[ 6423.920972] Good Bye Linux Kernel --------EXIT Message

Find Same Code in github at 

https://github.com/rajesh6115/kernelStudy/tree/master/hello