Making a device driver in linux system

There is a sample code to explain the process of making a device driver in linux system.

hello_printk.c

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

int init_module(void)
{
     printk(KERN_INFO "Hello world 1.\n");
 
     /*
     * A non 0 return means init_module failed; module can't be loaded.
     */
    
    /*
    *  return 0 means successfully loaded  
    */
 
return 0;
}

void cleanup_module(void)
{
    printk(KERN_INFO "Goodbye world 1.\n");
}

Makefile

obj-m += hello_printk.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




To run the files, you need to have a complete linux kernel installed in your system. You can find a list of kernel here: https://www.kernel.org/

For example, I installed linux 4.1.45

command:
$ wget https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.1.46.tar.xz

Now, you can go to the file folder and run make file.
$ make

You will find there is one .ko file. It is a kernel object file. Now, to add this module in your system, you need to attach it as a root user.

$ sudo insmod ./hello_printk.ko

To see your module in system,
$ cat /proc/modules

Another command to see or display your module with size and uses information:
$ lsmod

To check specific module information:
$ modinfo hello_printk.ko

To remove your module:
$ sudo rmmod hello_printk.ko

By the way, you can't see any text /output in your console. Because, the printk keeps messages in kernel. Hence, we can't see anything in user interface. To get the "Hello world" message, you need to look at kernel messages using 'dmesg' command or cat /var/log/syslog

$ dmesg

$ cat /var/log/syslog

Reference:

[1] http://www.tldp.org/LDP/lkmpg/2.6/lkmpg.pdf
[2] http://www.linuxdevcenter.com/pub/a/linux/2007/07/05/devhelloworld-a-simple-introduction-to-device-drivers-under-linux.html
[3] http://freesoftwaremagazine.com/articles/drivers_linux/