Re: what is defined, was for or against equality

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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