Foreword

Here there be dragons. Medieval mapmakers wrote that about unknown or dangerous places, and that is likely the feeling you get the first time you type:

 cd /usr/src/linux ; ls 

"Where do I start?" you wonder. "What exactly am I looking at? How does it all hang together and actually work?"

Modern, full-featured operating systems are big and complex. The number of subsystems is large, and their interactions are many and often subtle. And while it's great that you have the Linux kernel source code (more about that in a moment), knowing where to start, what to look at, and in what order, is far from self-evident.

That is the purpose of this book. Step by step, you will learn about the different kernel components, how they work, and how they relate to each other. The authors are intimately familiar with the kernel, and this knowledge shows through; by the end of the book, you and the kernel will at least be good friends, with the prospect of a deeper relationship ahead of you.

The Linux kernel is "Free" (as in freedom) Software. In The Free Software Definition,^[1] Richard Stallman defines the freedoms that make software Free (with a capital F). Freedom 0 is the freedom to run the software. This is the most fundamental freedom. But immediately after that is Freedom 1, the freedom to study how a program works. This freedom is often overlooked. However, it is very important, because one of the best ways to learn how to do something is by watching other people do it. In the software world, that means reading other peoples' programs and seeing what they did well as well as what they did poorly. The freedoms of the GPL are, at least in my opinion, one of the most fundamental reasons that GNU/Linux systems have become such an important force in modern computing. Those freedoms benefit you every moment you use your GNU/Linux system, and it's a good idea to stop and think about that every once in awhile.

^[1] http://www.gnu.org/philosophy/free-sw.html

With this book, we take advantage of Freedom 1 to give you the opportunity to study the Linux kernel source code in depth. You will see things that are done well, and other things that are done, shall we say, less well. But because of Freedom 1, you will see it all, and you will be able to learn from it.

And that brings me to the Prentice Hall Open Source Software Development Series, of which this book is one of the first members. The idea for the series developed from the principle that reading programs is one of the best ways to learn. Today, the world is blessed with an abundance of Free and Open Source softwarewhose source code is just waiting (maybe even eager!) to be read, understood, and appreciated. The aim of the series is to be your guide up the software development learning curve, so to speak, and to help you learn by showing you as much real code as possible.

I sincerely hope that you will enjoy this book and learn a lot. I also hope that you will be inspired to carve out your own niche in the Free Software and Open Source worlds, which is definitely the most enjoyable way to participate in them.

Have fun!

Arnold Robbins
Series Editor