Re: [Diagnostic Patch] don't print column zero

[Martin Sebor <msebor at gmail dot com>]

On 10/26/2017 12:23 PM, Nathan Sidwell wrote:
> On 10/26/2017 02:12 PM, Eric Gallager wrote:
> > On 10/26/17, Nathan Sidwell <nathan@acm.org> wrote:
> > > On 10/26/2017 10:34 AM, David Malcolm wrote:
>
> > > > Possibly a silly question, but is it OK to have a formatted string
> > > > call in which some of the arguments aren't consumed? (here "col" is 
> > > > only
> > > > consumed for the true case, which consumes 2 arguments; it's not 
> > > > consumed
> > > > for the false case).
> > >
> > > Yes.
> >
> > I think I remember clang disagreeing; I remember it printing warnings
> > from -Wformat-extra-args in a similar situation in gnulib's
> > error_at_line module
>
> C++ 21.10.1 defers to C.  C-99 7.15.1 has no words saying va_arg must be 
> applied to exactly all arguments captured by va_list object. (and I'm 
> pretty sure scanf can bail early)
>
> Now, it might be sensible to warn about:
>    printf ("", 5);
> because printf's semantics are known.  But that's not ill-formed, just 
> inefficient.  And in this case we're doing the equivalent of:
>    printf (not-compile-time-constant, 5);

C says excess arguments are ignored:

  If the format is exhausted while arguments remain, the excess
  arguments are evaluated (as always) but are otherwise ignored.

-Wformat normally warns on this case when the format string is
constant.  It doesn't when the string is non-constant but that's
likely just a consequence of the warning running very early.  If
it ran later on it would warn.  It would be quite useful to have
-Wformat run later to catch mistakes in conditional format strings
(like the growing number of GCC's own uses of conditionals in
warning_at type of calls).  If/when -Wformat is ever moved to run
later, these cases will either have to be fixed or the warning
relaxed to allow them.  There have been requests to add
a portability level to -Wformat so this case could be moved into
some pedantic level if one were ever added.

Martin