If OpenBSD has taught us anything, it's that when you need to start hardening at that level, C stops becoming weakest link and actually the design of the broader UNIX ABIs are the bigger problem. This is why things like selinux and cgroups exist in Linux -- POSIX ABIs are about as secure as Win32 APIs in Windows and thus you need to take additional steps to isolate your running processes if you really care about them behaving.