According to the fine standard, an implementation may #define NULL as either

  #define NULL ((void *) 0)

or just plain

  #define NULL 0

The latter works correctly because a literal 0 in pointer context is interpreted by the compiler as a null pointer.

If it is 0, the printf() provides no pointer context, so the 0 is interpreted as an int. That works... poorly if sizeof (int) < sizeof (void *)