Here are a first few lines from an article on how to develop Kernel modules for FreeBSD.

FreeBSD 7.0 has already been released. If you are a real hacker, the best way to jump in and learn it is hacking together an introductory kernel module. In this article I’ll implement a very basic module that prints a message when it is loaded, and another when it is unloaded. I’ll also cover the mechanics of compiling our module using standard tools and rebuilding the stock FreeBSD kernel. Let’s do it!

There are some prerequisites for this article. I assume you have a little bit of C programming knowledge, though nothing too fancy. If I reference a pointer or a structure, I expect you to understand those concepts without much explanation. I also expect you to be familiar with UNIX-like operating systems and know your way around basic shell usage.

The article is at http://www.freesoftwaremagazine.com/articles/writing_a_kernel_module_for_freebsd