No, implementation defined means the standard said, This is allowed, but we do not specify what will happen. That's up to the compiler to decide. Undefined mean

No, implementation defined means the standard said, This is allowed, but we do not specify what will happen. That's up to the compiler to decide. Undefined means This is not allowed. All bets are off. The problem is that programmers probably assume that many things which are undefined are implementation defined. An example of something that is implementation defined are struct layouts: the standard (I think) does say that the order must be the same as defined in the struct definition, but it allows the implementation to put as much space as it wants inbetween those values (for optimal architecture alignment.) Things that are implementation defined are typically going to be things which must happen, but if the standard defined exactly how, it would unnecessarily constrain the implementation (such as being able to perform optimizations).