Re: C: Add new warning -Wunprototyped-calls

[Richard Biener <richard dot guenther at gmail dot com>]

On Sat, Apr 6, 2013 at 11:50 PM, Andreas Schwab <schwab@linux-m68k.org> wrote:
> Tobias Burnus <burnus@net-b.de> writes:
>
>> gcc.dg/Wunprototyped-calls.c:13:3: warning: call to function ‘g’ without a
>> real prototype [-Wunprototyped-calls]
>
> What is a real prototype?

One reason I didn't bother to upstream that patch is language lawyer
legalise ...

We want to catch

int foo ();

int bar (T x)
{
   return foo (x);
}

int foo (U)
{
...
}

that is, calling foo () from a context where the definition or declaration with
argument specification is not visible.  This causes the C frontend to apply
var-args promotion rules to all arguments which may differ from promotion
rules that would be applied when a "real prototype" was visible at the point
of the function call.

I'd just say "without a prototype".  int foo();  or just foo(); is specified as
part of 6.7.5.3/14 as "The empty list in a function declarator that is not
part of a definition of that function specifies that no information about the
number or types of the parameters is supplied." (this appears mostly
in K&R style programs where the T D ( identifier-list(opt) ) form is valid).
I am not sure that GCC doing varargs style promotions to calls with
only this kind of declarator is valid or if the program would be rejected
by K&R (and only the GCC extension of varargs functions without
a first named arguments makes us do what we do ...).

The patch was implemented while hunting down "miscompiles" in
either X or ghostscript (I don't remember...).

Richard.

> Andreas.
>
> --
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
> "And now for something completely different."