C++ :: Void As Function Argument

I was reading about void as function argument, but I did not fully understand it's meaning in C.

In C++
void foo(void) {}
and
void foo() {}

are the same. It means no arguments for foo function. But in C it's different. First function means the same as in C++, but second means

In C, an empty parameter list means that the number and type of the function arguments are unknown. But if it is unknown you can't use this arguments if user specifies same. Because here are no variables to store them. So doesn't result are the some? You do not get any arguments. O do I can get this arguments from some hidden variable?

For example.

void foo() {
printf("%d", var);
}
foo(5);

It is very unclear for me. Do this apply to main function too?

int main(void)
int main()

or can I use arguments given to int main() like given to int main(int argc, char* argv[])