Before digging into programming, we’d like to comment on the version numbering scheme used in Linux and which versions are covered.
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. Fortunately, almost all modern distributions support the upgrade of single packages by checking interpackage dependencies; the distribution’s package manager generally will not allow an upgrade until the dependencies are satisfied.
To run the examples we introduce during the discussion, you won’t need particular versions of any tool but the kernel; any recent Linux distribution can be used to run our examples. We won’t detail specific requirements, because the file Documentation/Changes in your kernel sources is the best source of such information if you experience any problem.
As far as the kernel is concerned, the even-numbered kernel versions (i.e., 2.2.x and 2.4.x) are the stable ones that are intended for general distribution. The odd versions (such as 2.3.x), on the contrary, are development snapshots and are quite ephemeral; the latest of them repr esents the current status of development, but becomes obsolete in a few days or so.
This artical covers versions 2.0 through 2.4 of the kernel. Our focus has been to show all the features available to device driver writers in 2.4, the current version at the time we are writing. We also try to cover 2.2 thoroughly, in those areas where the features differ between 2.2 and 2.4. We also note features that are not available in 2.0, and offer workarounds where space permits. In general, the code we show is designed to compile and run on a wide range of kernel versions; in particular, it has all been tested with version 2.4.4, and, where applicable, with 2.2.18 and 2.0.38 as well.
This text doesn’t talk specifically about odd-numbered kernel versions. General users will never have a reason to run development kernels. Developers experimenting with new features, however, will want to be running the latest development release. They will usually keep upgrading to the most recent version to pick up bug fixes and new implementations of features. Note, however, that there’s no guarantee on experimental kernels, * and nobody will help 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.
Linux is licensed with the GNU General Public License (GPL), a document devised for the GNU project by the Free Software Foundation. The GPL allows anybody to redistribute, and even sell, a product covered by the GPL, as long as the recipient is allowed to rebuild an exact copy of the binary files from source. Additionally, any software product derived from a product covered by the GPL must, if it is redistributed at all, be released under the GPL.
The main goal of such a license is to allow the growth of knowledge by permitting everybody to modify programs at will; at the same time, people selling software to the public can still do their job. Despite this simple objective, there’s a never-ending discussion about the GPL and its use. If you want to read the license, you can find it in several places in your system, including the directory /usr/sr c/linux, as a file called COPYING.
Third-party and custom modules are not part of the Linux kernel, and thus you’re not forced to license them under the GPL. A module uses the kernel through a well-defined interface, but is not part of it, similar to the way user programs use the kernel through system calls. Note that the exemption to GPL licensing applies only to modules that use only the published module interface. Modules that dig deeper into the kernel must adhere to the “derived work” terms of the GPL.
In brief, if your code goes in the kernel, you must use the GPL as soon as you release the code. Although personal use of your changes doesn’t force the GPL on you, if you distribute your code you must include the source code in the distribution — people acquiring your package must be allowed to rebuild the binary at will. If you write a module, on the other hand, you are allowed to distribute it in binary form. However, this is not always practical, as modules should in general be recompiled for each kernel version that they will be linked with . New kernel releases — even minor stable releases — often break compiled modules, requiring a recompile. Linus Torvalds has stated publicly that he has no problem with this behavior, and that binary modules should be expected to work only with the kernel under which they were compiled. As a module writer, you will generally serve your users better by making source available.