checking for NULL before calling free

Question

Many C code freeing pointers calls:

if (p)
  free(p);

But why? I thought C standard say the free function doesn\'t do anything

Answer 1

If you rely on that free(0) is OKAY, and it's normal for your pointer to be null at this point, please say so in comment // may be NULL

This may be merely self-explanatory code, saying yes I know, I also use p as a flag.