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

The approach that Pidgin (formerly Gaim) uses is they created a Plugin struct. Each plugin populates a struct with callbacks for initialization and teardown, along with a bunch of other descriptive information. Pretty much everything except the struct is declared as static, so only the Plugin struct is exposed for linking.

Then, to handle loose coupling of the plugin communicating with the rest of the app (since it'd be nice if it did something between setup and teardown), they have a signaling system. Plugins can register callbacks to be called when specific signals (not standard C signals, but a custom extensible kind [identified by string, rather than set codes]) are issued by any part of the app (including another plugin). They can also issue signals themselves.

This seems to work well in practice - different plugins can build upon each other, but the coupling is fairly loose - no direct invocation of functions, everything's through the signaling stystem.