You are correct that such things are outside of the C abstract machine, but GCC provides a lot of extensions that are outside of the C abstract machine. For exa

You are correct that such things are outside of the C abstract machine, but GCC provides a lot of extensions that are outside of the C abstract machine. For example, __sync_synchronize and other atomic intrinsics (https://gcc.gnu.org/onlinedocs/gcc-4.6.2/gcc/Atomic-Builtins.html https://gcc.gnu.org/onlinedocs/gcc-4.6.2/gcc/Atomic-Builtins...). C had no memory model when these were introduced (which, of course, is why they were introduced), so they were outside of the C abstract machine.