Hi Martin,
On Thu, Aug 14, 2025 at 01:37:55PM +0200, Martin Uecker wrote:
> Am Donnerstag, dem 14.08.2025 um 12:19 +0200 schrieb Alejandro Colomar:
> > Hi Martin,
> >
> > On Thu, Aug 14, 2025 at 11:01:05AM +0200, Alejandro Colomar wrote:
> > > +void
> > > +decay_obvious (int a[2])
> > > +{
> > > + _Countof (typeof (a)); /* { dg-error "invalid" } */
> > > + _Countof (a + 1); /* { dg-error "invalid" } */
> >
> > It didn't occur to me the following test:
> >
> > alx@devuan:~/tmp$ cat c.c | nl -ba
> > 1 int
> > 2 main(int argc, char *argv[argc + 1])
> > 3 {
> > 4 argv++;
> > 5
> > 6 return _Countof(argv);
> > 7 }
> > alx@devuan:~/tmp$ /opt/local/gnu/gcc/countof_ap/bin/gcc -Wall -Wextra
> > c.c
> > alx@devuan:~/tmp$ ./a.out
> > alx@devuan:~/tmp$ echo $?
> > 2
> >
> > This is a weak point, which requires -farray-parameters-are-const as
> > proposed in:
> > <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121271>
> >
> > I'm willing to accept it as a known limitation of _Countof(), and
> > tell that programmers to use const (or use -farray-parameters-are-const
> > once we provide it) to ensure safety.
>
> You could also add a warning when _Countof is applied to a
> non-constant pointer.Yup, I should add that. I wouldn't enable it in any groups flags, though. I don't want people to start writing [const n]. Instead, I expect them to use -farray-parameters-are-const in the future. > > So, I stand by this patch even with this known limitation. > > > > On the other, hand, it may be a GCC existing bug? I see that > > _Countof(a+0) remembers the array number of elements but _Countof(a+1) > > doesn't. Maybe once you do a++ you should also forget the number of > > elements (or update it)? > > Difficult, as this is then flow-sensitive and not simply a > property of 'a'. That a + 0 works because the expression > is folded before. Maybe you could change it to take the expression > before folding? Hmmm, yeah, that would be good. It would fix the _surprise() test cases. Do you expect any issues by removing the call to c_fully_fold()? Thanks! Have a lovely day! Alex -- <https://www.alejandro-colomar.es/>
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