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From: Michael S <already5chosen@yahoo.com>
Newsgroups: comp.lang.c
Subject: Re: question about linker
Date: Wed, 4 Dec 2024 15:36:38 +0200
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On Wed, 4 Dec 2024 13:15:53 +0100
David Brown <david.brown@hesbynett.no> wrote:

> On 04/12/2024 12:56, Michael S wrote:
> > On Tue, 3 Dec 2024 17:42:20 -0000 (UTC)
> > antispam@fricas.org (Waldek Hebisch) wrote:
> >   
> >> David Brown <david.brown@hesbynett.no> wrote:  
> >>> On 01/12/2024 17:57, Michael S wrote:  
> >>>> On Sun, 1 Dec 2024 15:34:04 +0100
> >>>> David Brown <david.brown@hesbynett.no> wrote:  
> >>>>>
> >>>>> I can see some advantages in a language being happy with any
> >>>>> order of function definition, without requiring forward
> >>>>> declarations to use a function before it is defined.  But C is
> >>>>> not like that, and I cannot honestly say it bothers me one way
> >>>>> or the other.  And apparently, it does not particularly bother
> >>>>> many people - there is, I think, no serious impediment or
> >>>>> backwards compatibility issue that would prevent C being
> >>>>> changed in this way.  Yet no one has felt the need for it - at
> >>>>> least not strongly enough to fight for it going in the standard
> >>>>> or being a common compiler extension.
> >>>>>
> >>>>>
> >>>>>     
> >>>>
> >>>> I think, arguing in favor of such change would be easier on top
> >>>> of the changes made in C23.
> >>>> Before C23 there were, as you put it "no serious impediment or
> >>>> backwards compatibility issue". After C23 we could more
> >>>> categorical claim that there are no new issues.
> >>>>      
> >>>
> >>> Does that mean there was something that you think was allowed in C
> >>> before C23, but not after C23, that would potentially be a problem
> >>> here?
> >>>
> >>> What, specifically, are you thinking of?  
> >>
> >> Michael probably meant 'constexpr'.  
> > 
> > No, I am afraid of cases where function is used without prototype
> > and then there is conflicting definition later in the module.
> > Of course, it's UB, but in practice it could often work fine.
> > Something like that:
> > 
> > static int bar();
> > int foo(void)
> > {
> >    return bar(42);
> > }
> > 
> > static int bar(int a, int b)
> > {
> >    if (a == 42)
> >      return -1;
> >    return a - b;
> > }
> > 
> > 
> > Under c23 rules the code above is illegal, but before c23 it's
> > merely a UB.
> >   
> 
> I think it is always better to have a hard error than to allow UB!
> 
> But this is not actually anything to do with ordering of functions or 
> declarations.  You could omit "foo" entirely and the code is still an 
> error in C23, because "static int bar();" /is/ a prototype
> declaration in C23 - it means the same as "static int bar(void);".
> 

In case you lost the context, in the post above I am explaining why I
think that the suggested change that turns any static function
definition into "retroactive" module-scale prototype declaration can
potentially break working pre-C23 code. On the other hand, so far I was
not able to imagine a case in which such change would break C23 code.
And that is the reason why I think that the change is easier to
introduce on top of C23.