The source code contained in this book is covered by the GNU General Public License (GPL), which is listed in Appendix F, "GNU General Public License." A great deal of free software, especially GNU/Linux software, is licensed under it. For instance, the Linux kernel itself is licensed under the GPL, as are many other GNU programs and libraries you'll find in GNU/Linux distributions. If you use the source code in this book, be sure to read and understand the terms of the GPL.
The GNU Project Web site includes an extensive discussion of the GPL (http://www.gnu.org/copyleft/) and other free software licenses. You can find information about open source software licenses at http://www.opensource.org/licenses/index.html.