Except that the C standard explicitly makes no distinction between explicit and implicit undefined behavior. If a "shall" or "shall not" requirement that a

Except that the C standard explicitly makes no distinction between explicit and implicit undefined behavior. If a "shall" or "shall not" requirement that appears outside of a constraint or runtime constraint is violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International Standard by the words "undefined behavior" or by the omission of any explicit definition of behavior. There is no difference in emphasis among these three; they all describe "behavior that is undefined". N1570 section 4 paragraph 2.