Function declared twice in C header file

Question

I'm aware of the declaration of C header files with #ifdef and the meaning of extern before variables and functions. But recently I've got a third party library for an embedded device with the following scheme:

/* "lib.h" */
#ifndef LIB_H_
#define LIB_H_

   #ifdef LIB_C

      void function1();
      /* ... */

   #else

      extern void function1();
      /* ... */

   #endif

#endif /* LIB_H_ */

And additionally I've got a corresponding C source file:

/* lib.c */
#define LIB_C
#include "lib.h"

void function1()
   {
   /* ... */
   }

/* ... */

So here I am and a bit confused. What is the reason to declare all functions twice in the header in this way?

Answer 1

It's either an affectation, or a compatibility hack for some non-conforming or ancient compiler. You don't need the extern version, but using it is also fine, because function declarations are extern by default.

In other words, it's cruft, but maybe someone needs that cruft. We can't know for sure.