pointers - Is NULL always zero in C?

Question

I was interviewing a guy for a mid-level software engineering position yesterday, and he mentioned that in C, NULL is not always zero and that he had seen implementations of C where NULL is not zero. I find this highly suspect, but I want to be sure. Anyone know if he is right?

(Responses will not affect my judgement on this candidate, I've already submitted my decision to my manager.)

Answer 1

I'm assuming you mean the null pointer. It is guaranteed to compare equal to 0.¹ But it doesn't have to be represented with all-zero bits.²

See also the comp.lang.c FAQ on null pointers.

---

_{See C99, 6.3.2.3. There's no explicit claim; but see the footnote for C99, 7.20.3 (thanks to @birryree in the comments).}