Is the behavior of subtracting two NULL pointers defined?

Question

Is the difference of two non-void pointer variables defined (per C99 and/or C++98) if they are both NULL valued?

For instance, say I have a buffer struc

Answer 1

Edit: This answer is only valid for C, I didn't see the C++ tag when I answered.

No, pointer arithmetic is only allowed for pointers that point within the same object. Since by definition of the C standard null pointers don't point to any object, this is undefined behavior.

(Although, I'd guess that any reasonable compiler will return just 0 on it, but who knows.)