It is something experts must fail to do. Documented extensions fall under "undefined behavior", and a lot of real-world coding requires them. Oh, and use of header files and functions not in ISO C is undefined behavior. On a POSIX system #include <fcntl.h> provides definitions of things like F_DUPFD. But there is no reason why on some non-POSIX system, #include <fcntl.h> might not cause the rest of the translation unit to be compiled as Fortran 77. The include mechanism per se has the well-defined behavior that, if the header is found, it replaces itself by the content, which is thereby incorporated into the translation unit. But if that header isn't coming from the program, or from ISO C, then the content is not defined by ISO C.