If you need to understand the entire project, the best place to start is the main() function. If the code is reasonably structured this might give you a good to

If you need to understand the entire project, the best place to start is the main() function. If the code is reasonably structured this might give you a good top-level view on what the main components are. And from there you can basically dive into any direction of calls that you like. I would really not bother with the headers. Headers are for the most part only there for type declarations, so that a compiler can compile an isolated code unit (.c file) with the information on types and function signatures in other code units. This type information is needed for the compiler because otherwise linking will fail after compilation.