And probably -Werror (all warnings become errors, breaking compilation).

Fun post. Now I know why everyone was raving about the clang error-messages:

    echo "void main() {}" > main.c


    gcc -Wall -Werror --std=c99 main.c 
    main.c:1:6: error: return type of ‘main’ is not ‘int’ [-Werror=main]
    cc1: all warnings being treated as errors


    clang -Wall -Werror --std=c99 main.c 
    main.c:1:1: error: 'main' must return 'int'
    void main() {}
    ^
    1 error generated.

    (not showing clangs pretty colours)

instead of -ansi, i'd argue that -std=c99 (or -std=c11) would be more appropriate. also note the -pedantic flag.

"-Weverything" if the code is compiling perfectly, but you're just bored. If you're not a complete masochist try turning off some of the sillier ones (-Wno-padded -Wno-unused-parameter -Wno-conversion)