By Peter Jay Salzman and Ori Pomerantz
Ori's original document was good about supporting earlier versions of Linux, going all the way back to the 2.0 days. I had originally intended to keep with the program, but after thinking about it, opted out. My main reason to keep with the compatibility was for Linux distributions like LEAF, which tended to use older kernels. However, even LEAF uses 2.2 and 2.4 kernels these days.
Both Ori and I use the x86 platform. For the most part, the source code and discussions should apply to other architectures, but I can't promise anything. One exception is Chapter 12, Interrupt Handlers, which should not work on any architecture except for x86.
What Is A Kernel Module?
So, you want to write a kernel module. You know C, you've written a few normal programs to run as processes, and now you want to get to where the real action is, to where a single wild pointer can wipe out your file system and a core dump means a reboot.
What exactly is a kernel module? Modules are pieces of code that can be loaded and unloaded into the kernel upon demand. They extend the functionality of the kernel without the need to reboot the system. For example, one type of module is the device driver, which allows the kernel to access hardware connected to the system. Without modules, we would have to build monolithic kernels and add new functionality directly into the kernel image. Besides having larger kernels, this has the disadvantage of requiring us to rebuild and reboot the kernel every time we want new functionality.