Comment by saagarjha
3 years ago
The program can logically undo the call to fflush, too. Mainly by not dispatching it at all–UB is a global program attribute, at least currently. (People have made proposals to change this, but I don't think they have gone anywhere.)
No, it cannot, and UB is not a global program property. The C standard defines valid program executions according to the behaviors of the abstract machine. UB is a property of an execution of the program given some inputs.
Yes, sorry for not being precise: UB applies to executions. When I said "global" I meant global over that entire execution, so if your path ends up hitting undefined behavior it can go back and logically undo its entire execution, including parts which it shared with a well-defined execution or where you'd generally expect side effects to be placed.
No, that logically doesn't make sense. The program cannot know whether it is going through a particular execution ahead of time without actually executing all the side effects along that path first (which in this case would include the fflush()). The very difference between a "program" and a "program execution" is the fact that an execution includes the interactions of the program with the external world (as defined by the standard, all of which I loosely called "inputs" in my previous comment). The interactions basically extend prefixes of the execution through performing the semantics of the program according to the abstract machine and observing the responses from the external world. You don't have an "execution" of the program until the point of UB, until the interactions (aka side effects) up to that point have first occurred (and the responses of the system observed for continuing the execution).
P.S. Have you ever seen a single example of a compiler time-traveling UB through observable behavior like this? I sure haven't. If you have, I'd love to see it, because despite all the crazy ways compilers take advantage of UB, I've never seen C/C++ compilers actually agree with the stance that this way would be somehow legal (if it's even logically possible).
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