Comment by jolux

See "useful" — it may not be not quite as strong as you're thinking. It may be possible to write a minimal C program without UB, but I'm thinking of larger programs, more than a few hundred lines. Common UB includes: array access out of bounds, dereferencing a null pointer, use after free, use of uninitialized variables. -Wall -Werror can catch some instances of some types of UB, and runtime libraries like UBSan can catch more. But they're not exhaustive.