Comment by throw-qqqqq

> Undefined behavior only means that ISO C doesn't give requirements, not that nobody gives requirements. Many useful extensions are instances where undefined behavior is documented by an implementation.

True, most compilers have sane defaults in many cases for things that are technically undefined (like take sizeof(void) or do pointer arithmetic on something other than a char). But not all of these cases can be saved by sane defaults.

Undefined behavior means the compiler can replace the code with whatever. So if you e.g. compile optimizing for size, the compiler will rip out the offending code, as replacing it with nothing yields the greatest size optimization.

See also John Regehr's collection of UB-Canaries: https://github.com/regehr/ub-canaries

Snippets of software exhibiting undefined behavior, executing e.g. both the true and the false branch of an if-statement or none etc. UB should not be taken lightly IMO...