#Compiling a Linux Device driver

<div class="text-success lead">
This article illustrates how to compile the "Hello World" example on page 16 chapter II on the <b>Linux Device Driver 3rd Edition</b> book written by Jonathan Corbet, Alessandro Rubini and Greg Kroah-Hartman for O'Reilly 
</div>

<img src="ldd3.jpg"/>

This book is the bible for who wants to write Linux Device Drivers. You can read it for free from here:

*  [PDF version](http://lwn.net/Kernel/LDD3/)

Before proceede you have to properly configure and built the Linux Kernel for your Acme board as explained on these articles:

* @article='compile_linux_3_16'

##Hello World example

This is the original source code of the basic Hello World example

<pre class="prettyprint" markdown="1">
	#include <linux/init.h>
	#include <linux/module.h>
	 
	MODULE_LICENSE("Dual BSD/GPL");
	 
	static int hello_init(void)
	{
		printk(KERN_ALERT "Hello, world\n");
		return 0;
	}
	 
	static void hello_exit(void)
	{
		printk(KERN_ALERT "Goodbye, cruel world\n");
	} 
	module_init(hello_init);
	module_exit(hello_exit);
</pre>

Create a directory to store your module in your home page on your Linux Ubuntu PC.

<pre class="minicom">
~$ <b>mkdir ldd3</b>
~$ <b>cd ldd3</b>
~/ldd3$
</pre> 

and save here the example code in __hello.c__.

##Create the Makefile

Create a file called **Makefile** in the same source directory with a single line:

<pre class="prettyprint" markdown="1">
	obj-m:= hello.o
</pre>

The fully explanation on how it works is illustrated on 
__page 23 chapter II__ or in __/Documentation/kbuild__ in the Linux Kernel sources.

Now issue the kernel module compilation by typing: 

<pre class="minicom">
~/ldd3$ <b>make -C ~/linux-3.16.1 ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- M=`pwd` modules</b>
make: Entering directory `/home/tanzilli/linux-3.13'
  CC [M]  /home/tanzilli/ldd3/hello.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /home/tanzilli/ldd3/hello.mod.o
  LD [M]  /home/tanzilli/ldd3/hello.ko
make: Leaving directory `/home/tanzilli/linux-3.13'
</pre>

Change <b>~/linux-3.16.1</b> with the Linux source directory on your PC.

The **hello.ko** file you will obtain is the kernel module ready to be 
launched on your board. Copy this file into the board file system and 
launch it by typing:

<pre class="minicom">
~# <b>insmod hello.ko</b>
Hello, world
</pre>

Get the modules list by typing:

<pre class="minicom">
~# <b>lsmod</b>
Module                  Size  Used by                                           
hello                    820  0         
...
</pre>

Remove the module by typing:

<pre class="minicom">
~# <b>rmmod hello</b>
Goodbye, cruel world  
</pre>

#Related links

* [Anatomy of Linux loadable kernel modules](http://www.ibm.com/developerworks/linux/library/l-lkm/index.html)
* [The Linux Kernel Module Programming Guide](http://tldp.org/LDP/lkmpg/2.6/html)
* [Linux Device Drivers, Third Edition](http://lwn.net/Kernel/LDD3/)
* [Writing device drivers in Linux](http://www.freesoftwaremagazine.com/articles/drivers_linux)

@include='bio_tanzilli'