What methods are there to modularize C code?

Question

What methods, practices and conventions do you know of to modularize C code as a project grows in size?

Answer 1

Create header files which contain ONLY what is necessary to use a module. In the corresponding .c file(s), make anything not meant to be visible outside (e.g. helper functions) static. Use prefixes on the names of everything externally visible to help avoid namespace collisions. (If a module spans multiple files, things become harder., as you may need to expose internal things and not be able hide them with "static")

(If I were to try to improve C, one thing I would do is make "static" the default scoping of functions. If you wanted something visible outside, you'd have to mark it with "export" or "global" or something similar.)