Anything can happen because the compiler will assume it doesn't happen. "Undefined behavior" essentially means, "If you try to do this, you have written a progr

Anything can happen because the compiler will assume it doesn't happen. "Undefined behavior" essentially means, "If you try to do this, you have written a program that is not C, and you should not be handing it to a C compiler expecting it to do anything correctly." If you were to try to write a program that accurately detected whether the given input is a meaningful C program, you'd have to solve the Halting Problem (and more, once you take IO into account.) Thus undefined behavior must exist in the places where your program is syntactically valid but has no definite semantics. (And you can't enumerate all of these places in a Turing complete language.) Not even a C compiler will be able to tell you if it actually has been given C code; all it can say is "I parsed it, and here're some semantics for it."