You forgot "-pedantic" and "-Wextra". =P (And possibly a few more one-offs that aren't contained in the groups.) Though the key flag there to make it warn is -W

You forgot "-pedantic" and "-Wextra". =P (And possibly a few more one-offs that aren't contained in the groups.) Though the key flag there to make it warn is -Wall, so you can still compile with c99 and avoid the -ansi flag. Pretty much any compiler or language worth its salt that allows assignment in conditionals with the equals sign (so Python, Lisp are out) will warn you about it.