Commenting C code, header and source files

I suggest adopting the conventions imposed by a tool like Doxygen. Then instead of just code comments, you can also generate HTML/PDF/Latex etc documentation and its gives you good conventions.

Agree with Thomas about the cpp files

The header is meant for users of the code. So in there I document the interface: how to use it, preconditions and postconditions, etcetera.

The .c file is for maintainers. In there, I document the implementation: how things work internally, and why they work that way.