Just a quick hat tip to FCTX, a library I've found invaluable these past few months. FCTX provides header-only unit testing for C. Sure, if you're in C++ land there's a ton of xUnit-like frameworks available (with Boost.Test being my favorite), but for vanilla C projects FCTX wins hands down.
As an example, here's something I put together for a Stack Overflow response: The logic isn't rocket science, of course. But testing it in C without resorting to external libraries and complicated makefiles shouldn't be rocket science either. Provided that fct.h is in the same directory, this source will compile and run.