> Saying that it has "undefined behavior" means that the C language standard says nothing about how it behaves. If its behavior for the current compiler happens

> Saying that it has "undefined behavior" means that the C language standard says nothing about how it behaves. If its behavior for the current compiler happens to satisfy the language requirements for offsetof, then it's a legitimate implementation. Code that implements the standard library is free to do whatever it likes, as long as the resulting behavior is correct. Undefined is not the same as implementation-defined. Yes, a given implementation is free to do whatever it wants, but code that includes a null pointer dereference is not C code.