> However, this optimization is valid because writing to a null pointer is undefined... if writing to a null pointer trapped (SIGFAULT), which is what it actual

> However, this optimization is valid because writing to a null pointer is undefined... if writing to a null pointer trapped (SIGFAULT), which is what it actually does at runtime, then the optimization would be incorrect... because you could observe the intermediate value of x->a = 5. I'm not sure this is a good example, because there's no way for you to read them from the same thread (and reading from another thread would be a race). Reading them from a signal handler would yield unspecified values (unless you're using lock-free atomic types, in which case we're probably not worried about optimizing these assignments). > When the processing of the abstract machine is interrupted by receipt of a signal, the values of objects that are neither lock-free atomic objects nor of type volatile sig_atomic_t are unspecified, as is the state of the floating-point environment.