I was trying to call getsockopt(fd, SOL_LOCAL, LOCAL_PEERCRED, ...), and by mistake passed a wrong value for the second parameter where it should be SOL_LOCAL. But the call still succeeded. Then I did more experiments and passed more random values for the second parameter, all succeeded. It seems there is a lack of parameter check in the implementation of getsockopt() , where it should return errors if people pass invalid parameters instead of succeeding silently. Hope the Apple engineers can help to validate and fix it.