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by 1718627440
2 days ago
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No, it's also removed if you dereference it later on, that's why it is said, that UB has time traveling behaviour. This means, that the compiler can emit a program, that let's you access data without a security check, while the crash only comes later. Also the crash can be removed, because dereferencing it is UB anyway, so the compiler does not need to emit it. |
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If there were UB, a compiler in C is not allowed to move (time-travel) UB before any observable behavior. This was never allowed by in C (int contrast to C++), but the wording was not clear, which we fixed in C23.
If there is no check, you are right that the access itself is UB and there is no requirement to trap in ISO C, but this is something compilers explicitly support. (and it might be required by POSIX, but I am not sure)