Ref:

• It’s a pure rip-off from https://goyalankit.com/blog/linux-device-drivers kept here for my convinence:

Environment setup

• You need linux headers to be able to compile new module.

• To make sure you have linux headers, see if the following path exists:

    $ ls /usr/src/linux-headers-`uname -r`
    arch   crypto         drivers   fs       init  Kbuild   kernel
    ...
  

• If the above directory is not present, you can install the headers using:

  • sudo apt-get update && sudo apt-get install linux-headers-$(uname -r)

Once the headers are installed you are good to go.

Writing hello module

• To start off, create the following file in any directory.

    // hello.c
    #include <linux/module.h>
    MODULE_LICENSE("Dual BSD/GPL");
  

• Create the following Makefile to compile your module.

    obj-m := hello.o
    KDIR := /usr/src/linux-headers-3.2.0-23-generic
  
    all:
        make -C $(KDIR) M=$(PWD) modules
    clean:
        make -C $(KDIR) M=$(PWD) clean
  

• After running make, you should see following:

• Check that .ko is created.

    vagrant@precise64:~$ ls
    hello.c  hello.ko  hello.mod.c  hello.mod.o  hello.o  Makefile
  

Install the module.

• vagrant@precise64:~$ sudo insmod hello.ko

• Confirm the it’s installed.

    vagrant@precise64:~$ lsmod | grep hello
    hello                   8217  0
  

Remove the module.

• vagrant@precise64:~$ sudo rmmod hello

Confirm that it’s removed.

• vagrant@precise64:~$ lsmod | grep hello

Expanding further, each module needs to have an init and exit method.

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

MODULE_LICENSE("Dual BSD/GPL");

static int hello_init(void) {
  printk("<1> Hola Mundo!\n");
  return 0;
}

static void hello_exit(void) {
  printk("<1>Hasta Luego\n");
}

module_init(hello_init);
module_exit(hello_exit);

Again, use inmod to install the command and you should see Hola Mundo! in $ dmesg. printk is important here; as kernel will print it to the syslog. Pretty useful in understanding other drivers.