Implementation-defined means that an implementation must choose a behavior, document that behavior, and then consistently apply that behavior. Undefined behavi

Implementation-defined means that an implementation must choose a behavior, document that behavior, and then consistently apply that behavior. Undefined behavior means that an implementation need not have consistent behavior, need not issue any warnings, need not be predictable, need not be repeatable. The classic example is that if you have signed integer overflow, the compiler is allowed to make demons fly out of your nose, and the C Standard would see nothing incorrect about that behavior. It doesn't matter that the compiler's output is x86. You are entirely correct that x86 has signed integer overflow, but you aren't writing x86 assembly that runs on an x86 processor. You're writing C code, and that runs on the C Abstract Machine. The compiler's job is to output x86 code that will produce the same output as the C Abstract Machine outputs. If the C Abstract Machine doesn't define a behavior, then that means any x86 is acceptable by the Standard.