void as function argument

[Shinigami]

Hi,
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.

P.S. 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[])

[Caligulaminus]

Shinigami wrote:

But if it is unknown you can't use this arguments if user specifies same. Because here are no variables to store them.

It's as simple as that.

[Shinigami]

So, this specifications in C makes bugs which can't be caught by compiler?

[Caligulaminus]

Bug? Feature? That might be open to dispute.

[MikeyBoy]

In C, it is legal for your function prototypes to not specify the arguments to the function. So it is legal to have in your header file:

/* My function prototypes */
void Function1();
int Function2();

And in the function definitions:

/* My function definitions */
void Function1(int a)
{
  printf("The value of a is %d", a);
}

int Function2(char *message)
{
  printf("%s", message);

  return 42;
}

Obviously, for the function definitions, you need to define what the arguments are. But you don't need them in the prototypes. Of course, if you do this, then the compiler can't check whether you're passing the correct arguments, which could lead to bugs. So it's a lazy and, frankly, a stupid thing to do.

(Note that I'm not knowledgeable about modern C standards, so this may have changed in modern standards.)

EDIT: There's more discussion under this StackOverflow topic, if you're interested:

http://stackoverflow.com/questions/13950642/why-does-a-function-with-no-parameters-compared-to-the-actual-function-definiti