Well, it's the return value in realloc(ptr, 0) case that is implementation-specific, but its behavior _is_ well-defined - it is expected to "deallocate the old object".

My point was that in reality the behavior varies. "Undefined" in a common tongue sense, not in the terms of the C standard.

No... maybe? It's probably arguable, even if it's not an interesting argument.

Either way, realloc's behaviour is well-defined when the pointer is null.