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Re: what is defined, was for or against equality

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On Sat, 8 Jan 2022 09:31:06 -0000 (UTC)
Thomas Koenig <tkoenig@netcologne.de> wrote:
> Spiros Bousbouras <spibou@gmail.com> schrieb:
> > On Thu, 6 Jan 2022 16:43:05 -0000 (UTC)
> > Thomas Koenig <tkoenig@netcologne.de> wrote:
> >> This is a rather C-centric view of things.  The Fortran standard
> >> uses a different model.
> >>
> >> There are constraints, which are numbered.  Any violation of such
> >> a constraint needs to be reported by the compiler ("processor",
> >> in Fortran parlance).  If it fails to do so, this is a bug in
> >> the compiler.
> >>
> >> There are also phrases which have "shall" or "shall not".  If this
> >> is violated, this is an error in the program.  Catching such a
> >> violation is a good thing from quality of implementation standpoint,
> >> but is not required.  Many run-time errors such as array overruns
> >> fall into this category.
> >
> > This seems to me exactly like the C model. What difference do you see ?
>
> First, I see a difference in result.  Highly intelligent and
> knowledgable people argue vehemently if a program should be able
> to use undefined behavior or not, and lot of vitriol is directed
> against compiler writers who use the assumption that undefined
> behavior cannot happen in their compilers for optimization,
> especially if it turns out that existing code was broken and no
> longer works after a compiler upgrade (Just read a few of Linus
> Torvald's comments on that matter).
>
> I see C conflating two separate concepts:  Programm errors and
> behavior that is outside the standard.  "Undefined behavior is
> always a programming error" does not work; that would make

The C standard is in no position to say that some programme is in
error. This would require near omniscience from the standard
writers.

> #include <unistd.h>
> #include <string.h>
>
> int main()
> {
>   char a[] = "Hello, world!\n";
>   write (1, a, strlen(a));
>   return 0;
> }
>
> not more and not less erroneous than
>
> int main()
> {
>   int *p = 0;
>   *p = 42;
> }
>
> whereas I would argue that there is an important difference between
> the two.

The only difference I see between the two is that the first is defined
by POSIX and the second is not. According to POSIX the first is required
to print something on stdout. I cannot imagine any extension which
would make the second programme do something useful and a conforming
implementation may well compile it as essentially a no-op.

But with something like

int main(voidd) {
    int *p = 0 ;
    *p = 42 ;
    .... do other stuff ...
    return 0 ;
}

the C standard allows for a conforming implementation to do something
useful like perhaps store 42 to address 0.

> If the C standard replaced "the behavior is undefined" with "the
> program is in error, and the subsequent behavior is undefined"
> or something along those lines, the discussion would be much
> muted.
>
> (Somebody may point out to me that this what the standard is
> actually saying.  If so, that would sort of reinforce my argument
> that it should be clearer :-)

No , it most definitely does not say that nor could it possibly say
that.

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Re: for or against equality, was Why are ambiguous grammars usually a bad idea? Martin Ward <martin@gkc.org.uk> - 2022-01-05 10:25 +0000
  Re: for or against equality, was Why are ambiguous grammars usually a bad idea? David Brown <david.brown@hesbynett.no> - 2022-01-06 09:11 +0100
    Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-06 16:43 +0000
      Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-07 12:06 +0100
      Re: what is defined, was for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-07 13:21 +0000
        Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-08 09:31 +0000
          Re: what is defined, was for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-08 22:28 +0000
            Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-09 00:09 +0000
              Re: what is defined, was for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-09 21:30 +0000
          Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-09 23:00 +0100
            Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-10 12:04 +0000
              Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-11 18:16 +0100
                Re: what is defined, was for or against equality Kaz Kylheku <480-992-1380@kylheku.com> - 2022-01-11 19:19 +0000
                Re: what is defined, was for or against equality gah4 <gah4@u.washington.edu> - 2022-01-11 14:18 -0800
                Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-12 19:02 +0000
                Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-13 08:24 +0100
                Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-13 11:17 +0000
          Re: what is defined, was for or against equality gah4 <gah4@u.washington.edu> - 2022-01-10 16:58 -0800
    Re: for or against equality, was Why are ambiguous grammars usually a bad idea? Robert Prins <robert@prino.org> - 2022-01-06 19:07 +0000
    Undefined behaviour, was: for or against equality Martin Ward <martin@gkc.org.uk> - 2022-01-07 14:02 +0000
      Re: Undefined behaviour, was: for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-08 03:41 +0000
    Re: Undefined behaviour, was: for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-07 15:56 +0100
      Re: Undefined behaviour, was: for or against equality anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2022-01-08 17:52 +0000
        Re: Undefined behaviour, was: for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-09 23:53 +0100
        Re: Undefined behaviour, was: for or against equality Kaz Kylheku <480-992-1380@kylheku.com> - 2022-01-11 16:55 +0000
          Re: Undefined behaviour, was: for or against equality George Neuner <gneuner2@comcast.net> - 2022-01-11 22:01 -0500

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