Re: RFC: -Wall by default

[Vincent Lefevre <vincent+gcc at vinc17 dot org>]

On 2012-04-10 14:48:05 +0100, Andrew Haley wrote:
> On 04/05/2012 12:30 PM, Vincent Lefevre wrote:
> > On 2012-04-05 11:55:45 +0100, Andrew Haley wrote:
> >> On 04/05/2012 11:50 AM, Vincent Lefevre wrote:
> >>> On 2012-04-04 20:01:27 +0100, Andrew Haley wrote:
> >>>> On 04/04/2012 07:11 PM, Gabriel Dos Reis wrote:
> >>>>> Really?  Such as what?
> >>>>
> >>>> Such as "I wrote a perfectly legal C program, and gcc spewed out
> >>>> a ton of messages."
> >>>
> >>> What's a "legal C program"?
> >>
> >> It's generally used to mean one that is fully defined by the
> >> specifications in effect, often some combination of POSIX and ISO C,
> >> with perhaps some vendor extensions.  Why do you ask?
> > 
> > Because: What if the specifications in effect say (as some vendor
> > extension) that some construct will generate some warning?
> 
> Interesting.  I had not considered that, but I have never seen such a
> specification.  Usually warnings are for things that are permitted by
> a specification, but not "good" in some way.  That's why warnings are
> usually optional, AFAIK; the program is well-defined, but has some
> properties that are, in the opinion of the compiler writer,
> undesirable.

There's also the case where the program is well-defined for some
specifications, but will have a different behavior with other
specifications, e.g. a newer standard (like C99 vs C89) or a
different one (like C++ vs C -- e.g. with MPFR, one allows it
to be built with a C++ compiler). In particular in the former
case, it's just C in both standards, but the concept of a
"legal C program" will vary (note that the broken[*] -std cannot
be used to tell GCC that the user wants to support both C89 and
C99).

[*] because it is not possible to distinguish between what GCC
supports and what GCC allows (there have already been discussions
about this).

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <http://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)