A smart way to prototype functions

Question

I am looking for a (clean) way of writing a function definition and a function prototype without duplicating code. Since DRY has established itself as a good idea and manual prototyping in header files is a clear violation, this seems like a reasonable requirement.

The sample code below shows a (rough) way to solve a preprocessor problem. It seems unlikely to be optimal, but it seems to work correctly.

Using separate files and duplication:

foo.h:
#ifndef FOO_H
  #define FOO_H
  // Normal header file stuff
  int dofoo(int a);
#endif /* FOO_H */

foo.c:
#include "foo.h"
int dofoo(int a) {
  return a * 2;
}

Using C preprocessor:

foo.h:
#ifndef FOO_H
  #define FOO_H

  // Normal header file stuff

  #ifdef PROTOTYPE // if incorrect:
  // No consequences for this test case, but we lose a sanity check
    #error "PROTOTYPE set elsewhere, include mechanism will fall over"
  #endif

  #define PROTOTYPE // if incorrect:
  // "error: redefinition of 'dofoo'" in clang & gcc, 
  // referring to int dofoo() line in foo.c
    #include "foo.c"
  #undef PROTOTYPE //if incorrect:
  // No warnings, but should trigger the earlier #error statement if
  // this method is used in more than one file

#endif /* FOO_H */

foo.c:
#include "foo.h"

int dofoo (int a)
#ifdef PROTOTYPE // if incorrect:
// "error: redefinition of 'dofoo'" in clang & gcc, 
// referring to int dofoo() line in foo.c
  ;
#else
  {
    return a * 2;
  }
#endif

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Answer 1

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Answer 2

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