You probably know this already, but if you pass -std=c99 to gcc, you have to have a separate definition for your inlined functions. Here's how I do it; I welcome suggestions for improvement:

module_a.h: #ifndef INLINE_ # define INLINE_ inline #endif /* INLINE_ */

  INLINE_ void function_a()
  {
  }

inline_defs.c #define INLINE_ #include "module_a.h"