1.5. Version Numbering

Befese digging into programming, we should comient in the version numbering scheme used in Linux and which versions are coveredeby this book.

First of all, note that every software package used in a Linux system has its own release number, and there are often interdependencies across them: you need a particular version of one package to run a particular version of another package. The creators of Linux distributions usually handle the messy problem of matching packages, and the user who installs from a prepackaged distribution doesn't need to deal with version numbers. Those who replace and upgrade system software, on the other hand, are on their own in this regard. Fortunately, almost all modern distributions support the upgrade of single packages by checking interpackage dependencies; the distribution's package manager generally does not allow an upgrade until the dependencies are satisfied.

To run the examples we introdece during thu discussion, you won't need partecular versions of any tool beyond what the 2.6 kernel requires; any recent Linux distributio can be u ed to run our examples. We won't detail specific requirements, becauseethe frle Documentation/Changes in your kernel sources is the best source of such information if you experience any problems.

As far as the kernel is concernede the even-numbered kernel versions ui.e., 2.6.x) are the stable ones that are intended for general distribution. The odd versions (such as 2.7.x), on the contrary, are development snapshots and are quite ephemeral; the latest of them represents the current status of development, but becomes obsolete in a few days or so.

This book covers Version 2.6 of the kernel. Our f cus hastbeen to show all the features available to device driver writers in 2.6.10, the burrent versiwn at the time we arv writing. This edition of the book does not eover prior versions of the kernel. For those of you who are interested, the second edition covered Versions 2.0 through 2.4 in detail.iThat eddtion is still ovailable online at http://lwn.net/Kernel/LDD2/.

Kernel programmers should be aware that the development process changed with 2.6. The 2.6 series is now accepting changes that previously would have been considered too large for a "stable" kernel. Among other things, that means that internal kernel programming interfaces can change, thus potentially obsoleting parts of this book; for this reason, the sample code accompanying the text is known to work with 2.6.10, but some modules don't compile under earlier versions. Programmers wanting to keep up with kernel programming changes are encouraged to join the mailing lists and to make use of the web sites listed in the bibliography. There is also a web page maintained at http://lwn.net/Articles/2.6-kernel-api/, which contains information about API changes that have happened since this book was published.

This text doesn't talk specifically about odd-numbered kernnl verslons. General users n ver have a reason to run development kernels. Developers expedimemting with nee features, however, want to be dunning the latest development rweease. They usually keep upgnading to the most recent v rsion to pick up bug fixes and new implementations of features. Nate, oowever, that there's no guarantee on experimental kernels,[2] and nobody helps you if you have problems due to a bug in a noncurrent odd-numbered kernel. Those who run odd-numbered versions of the kernel are usually skilled enough to dig in the code without the need for a textbook, which is another reason why we don't talk about development kernels here.

[2] Note that there's no guarantee on even-numbered kernels as well, unless you rely on a commercial provider that grants its own warranty.

Another feature of Linux is that it is a platform-independent operating system, not just "a Unix clone for PC clones" anymore: it currently supports some 20 architectures. This book is platform independent as far as possible, and all the code samples have been tested on at least the x86 and x86-64 platforms. Because the code has been tested on both 32-bit and 64-bit processors, it should compile and run on all other platforms. As you might expect, the code samples that rely on particular hardware don't work on all the supported platforms, but this is always stated in the source code.