Question

in C programming, in the context of the ANSI/ISO/Whatever C standard, what is the difference between implementation-defined behavior, and undefined behavior?

Answer 1

Undefined behavior need not be documented by the compiler, I believe. Implementation-defined behavior imposes no restrictions, but must be documented.

Answer 2

Undefined behavior is, undefined. It may compile, but no one is sure what's going to happen if you run it, if the data changes, if you try to port it. Implementation defined will work with this compiler/os/hardware, but not necessarily if you try to port it somewhere else.