(j3.2006) Interesting F2003-ism

[Andy Vaught]

On Fri, 23 Feb 2007, Malcolm Cohen wrote:

> Andy Vaught said of Michael's though experiment:
> >   But you can't implement a C ABI that way.
> 
> Possibly not, but there is actually quite a lot of leeway.
> 
> >  Variable argument lists have
> > to be passed in the same manner as fixed argument lists.
> 
> You say that, but I am less than unconvinced.
> 
> >  Otherwise, a
> > variable argument list function won't work when invoked through a function
> > pointer that has been cast around a bit.
> 
> And many of those cases are illegal according to the C standard.
> 
> How far the few legal cases constrain how the ABI is forced to be
> implemented is not entirely clear.  I've certainly heard of implementations
> that work similarly (not quite the same) to what Mike was suggesting.
> Of course there need not even be an ABI...
 
> What *is* entirely clear is that you have to call a function that is
> defined with an ellipsis with a prototype, and that prototype must
> use ellipsis the same as the function definition.
> 
> This follows reasonably straightforwardly from p120 of c99.
> 
> So your printf examples are just wrong.
> 
> Whether gcc "lets them work" is a rather uninteresting question.
> We didn't add BIND(GCC), we added BIND(C).
>
> In fact my copy of gcc 3.4.4 doesn't allow it anyway:
> % cat junkz.c
> s1() { extern printf(char *,int); printf("%d",3); }
> s2() { extern printf(char *,float); printf("%e",3.5f); }
> % gcc junkz.c
> junkz.c: In function `s1':
> junkz.c:1: warning: conflicting types for built-in function 'printf'
> junkz.c: In function `s2':
> junkz.c:2: error: conflicting types for 'printf'
> junkz.c:1: error: previous declaration of 'printf' was here

And yet, after all this, if a C compiler sees something like:

    void sub(void) {
       foo(1, 2, 3);  /* No prototype of foo.  Variable or fixed? */
    }

Somehow, it has to call foo().  It sounds like someone could make a
conforming C compiler that would compile this program into something that
would crash but no one will.

And footnote 125 on p99 of C99 is: 

   125) If both function types are "old style", parameter types are not
        compared.

Probably not enough for you, but I'm going to allow people to do clever
things.

        Andy