You assume that the compiler can determine what behavior is undefined. It can't. C compilers don't just look at some individual line of the program and say "oh, that's undefined, unleash the nasal demons". C compilers look at code, reason that if such-and-such variable has a certain value (say, a null or invalid pointer), then such-and-such operation is undefined (say, dereferencing that variable), and therefore on the next line that variable can be assumed not to have that bad value. Despite all the FUD, this is a very limited power. C compilers don't usually know the actual values in question, all they do is exclude some invalid ones.