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.)

Answer 2

OO techniques can be applied to C code, they just require more discipline.

• Use opaque handles to operate on objects. One good example of how this is done is the stdio library -- everything is organised around the opaque FILE* handle. Many successful libraries are organised around this principle (e.g. zlib, apr)
• Because all members of structs are implicitly public in C, you need a convention + programmer discipline to enforce the useful technique of information hiding. Pick a simple, automatically checkable convention such as "private members end with '_'".
• Interfaces can be implemented using arrays of pointers to functions. Certainly this requires more work than in languages like C++ that provide in-language support, but it can nevertheless be done in C.