My mistake. I could have sworn that only C++ defined 0 to be equivalent to a null pointer. I guess my confusion stemmed from the fact that the NULL preprocessor macro in C++ is actually defined to be just "0" (or a magical-seeming "__null" in recent versions of g++), whereas in any modern C implementation I've seen, it's "((void *)0)".

Though there are cases in which passing an unadorned "0" as a null pointer can be wrong; there's a null-terminated varargs function example here:

http://c-faq.com/null/null2.html