Question

Maybe I’m missing something, but could some please explain the “logic” behind the following code?

#ifndef _PTRDIFF_T
#ifndef _T_PTRDIFF_
#ifndef _T_PTRDIFF
#ifndef __PTRDIFF_T
#ifndef _PTRDIFF_T_
#ifndef _BSD_PTRDIFF_T_
#ifndef ___int_ptrdiff_t_h
#ifndef _GCC_PTRDIFF_T
#define _PTRDIFF_T
#define _T_PTRDIFF_
#define _T_PTRDIFF
#define __PTRDIFF_T
#define _PTRDIFF_T_
#define _BSD_PTRDIFF_T_
#define ___int_ptrdiff_t_h
#define _GCC_PTRDIFF_T
#ifndef __PTRDIFF_TYPE__
#define __PTRDIFF_TYPE__ long int
#endif
typedef __PTRDIFF_TYPE__ ptrdiff_t;
#endif /* _GCC_PTRDIFF_T */
#endif /* ___int_ptrdiff_t_h */
#endif /* _BSD_PTRDIFF_T_ */
#endif /* _PTRDIFF_T_ */
#endif /* __PTRDIFF_T */
#endif /* _T_PTRDIFF */
#endif /* _T_PTRDIFF_ */
#endif /* _PTRDIFF_T */

Why is this preferred over a simple:

#ifndef xyz
#define xyz
#endif

???

I can see they are nested, but it is very confusing. …and btw, just what are they defining, as there are no values after the identifiers???

Answer 1

The logic is to only define the macros if none of them is defiend. However, I would certainly refactor it into:

#if !defined(_PTRDIFF_T) && !defined(_T_PTRDIFF_) && //... well, you get the idea
  #define _PTRDIFF_T
  #define _T_PTRDIFF_
  //...
  #ifndef __PTRDIFF_TYPE__
    #define __PTRDIFF_TYPE__ long int
  #endif
  typedef __PTRDIFF_TYPE__ ptrdiff_t;
#endif

As to the second question: you can define a macro without a replacement text (it would just expand to nothing if used). Such macros are often used for conditional inclusion of code based on #ifdef or #ifndef.