Beware of bugs in the above code; I have only proved it correct, not tried it. - Donald Knuth Formal correctness and implementation correctness are still

Beware of bugs in the above code; I have only proved it correct, not tried it. - Donald Knuth Formal correctness and implementation correctness are still separate issues in compilers like gcc but that doesn't change the fact that there are correctness specifications for gcc. There are projects that try to bridge the gap: https://en.wikipedia.org/wiki/CompCert https://en.wikipedia.org/wiki/CompCert.