If you’re designing a stable API for external users, you might want to lock down your API even more by forward declaring the struct types as opaque in the public header file and only defining the struct members in a private library header file. This prevents users from messing with your library’s private state and allows you to change implementation details later without breaking binary compatibility. The disadvantage is that users can’t control how the structs are allocated or embed them in their own structs.

  /* foo.h */
  struct foo_object;

  struct foo_object* foo_create(int flags, …);

  /* foo.c */
  #include “foo.h”

  struct foo_object {
    int flags,
    …
  };