int main() is not a prototype. int main(void) is a prototype. int main() declares main with an unknown (at this point) but fixed number of arguments (i.e., non-variadic). Callers must guess the correct number and types of arguments, the compiler does not enforce anything. In contrast, int main(void) declares main with exactly 0 arguments, to be enforced by the compiler.

For main it doesn't matter much since usually you don't call it yourself, but consider:

    int f();    // no arguments, apparently

    int g(int a, int b, int c) {
        return f(a) + f(b, c);  // at least one of these is fishy...
    }

    int f(double d) {   // oh, the caller guessed wrong. twice.
        return (d != 0.0 ? 0 : 1);
    }

GCC and Clang do not complain about this program even with -Wall (they do with -Wstrict-prototypes).

Apparently void foo() will not error when called with arbitrary values - it's implicitly variadic. This surprised me too, but I confirmed it with gcc, even on -std=c1x and with -Wall -Wextra -pedantic.

Learnt something!