As far as standard ISO C goes, nothing. That's just how UB can work in theory, though in practice compiler (ab)use of UB is somewhat more indirect (optimizing u

As far as standard ISO C goes, nothing. That's just how UB can work in theory, though in practice compiler (ab)use of UB is somewhat more indirect (optimizing under the assumption that UB is not present in the program, rather than observing UB and making decisions based on that). Beyond that, I believe Linux and other kernels technically use a slight variant of C by taking advantage of compiler extensions/flags to better fit their use cases. For example, Linux compiles (compiled?) with -fno-strict-aliasing and -fwrapv and uses a GCC extensions that allows type punning via unions [0], so that they can compile what the standard calls "incorrect" C code without worry. I'm not sure whether recent versions of C have changed their stance on this particular UB, but since it'll probably be a while until they're adopted (if ever) kernels will be making do with their workarounds for a while longer. [0]: https://lkml.org/lkml/2018/6/5/769 https://lkml.org/lkml/2018/6/5/769