int foo (int argc, …) vs int foo() vs int foo(void) in C

Question

So today I figured (for the first time admittedly) that int foo() is in fact different from int foo(void) in that the first one allows any

Answer 1

First, take int foo(). This form is a function declaration that does not provide a prototype - that is, it doesn't specify the number and types of its arguments. It is an archaic form of function declaration - it's how functions were originally declared in pre-ANSI C. Such a function however does have a fixed number and type of arguments - it's just that the declaration doesn't tell the compiler what it is, so you are responsible for getting it right where you call the function. You declare such a function with arguments like so:

int foo(a, b)
    int a;
    char *b;
{
    return a + strlen(b);
}

This form of function declaration is now obsolete.

int foo(void) is a modern declaration that provides a prototype; it declares a function that takes no arguments. int foo(int argc, ...) also provides a prototype; it declares a function that has one fixed integer argument and a variable argument list.