2.1. Setting Up Your Test System

Starting with this chapter, we present example modules to demonstrate programming concepts. (All of these examples are available on O'Reilly's FTP site, as explained in Chapter 1.) Building, loading, and modifying these examples are a good way to improve your understanding of how drivers work and interact with the kernel.
The example modules should work with almost any 2.6.x kernel, including those provided by distribution vendors. However, we recommend that you obtain a "mainline" kernel directly from the kernel.org mirror network, and install it on your system. Vendor kernels can be heavily patched and divergent from the mainline; at times, vendor patches can change the kernel API as seen by device drivers. If you are writing a driver that must work on a particular distribution, you will certainly want to build and test against the relevant kernels. But, for the purpose of learning about driver writing, a standard kernel is best.
Regardless of the origin of your kernel, building modules for 2.6.x requires that you have a configured and built kernel tree on your system. This requirement is a change from previous versions of the kernel, where a current set of header files was sufficient. 2.6 modules are linked against object files found in the kernel source tree; the result is a more robust module loader, but also the requirement that those object files be available. So your first order of business is to come up with a kernel source tree (either from the kernel.org network or your distributor's kernel source package), build a new kernel, and install it on your system. For reasons we'll see later, life is generally easiest if you are actually running the target kernel when you build your modules, though this is not required.
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You should also give some thought to where you do your module experimentation, development, and testing. We have done our best to make our example modules safe and correct, but the possibility of bugs is always present. Faults in kernel code can bring about the demise of a user process or, occasionally, the entire system. They do not normally create more serious problems, such as disk corruption. Nonetheless, it is advisable to do your kernel experimentation on a system that does not contain data that you cannot afford to lose, and that does not perform essential services. Kernel hackers typically keep a "sacrificial" system around for the purpose of testing new code.
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So, if you do not yet have a suitable system with a configured and built kernel source tree on disk, now would be a good time to set that up. We'll wait. Once that task is taken care of, you'll be ready to start playing with kernel modules.