On Fri, Jul 05, 2024 at 06:32:26PM GMT, Alejandro Colomar wrote:
> Hi Xi,
>
> On Fri, Jul 05, 2024 at 11:55:05PM GMT, Xi Ruoyao wrote:
> > On Fri, 2024-07-05 at 17:23 +0200, Alejandro Colomar wrote:
> > > > strtol does have a "char * restrict * restrict" though, so the
> > > > situation is different. A "char **" and a "const char *"
> > > > shouldn't alias anyway.
> > >
> > > Pedantically, it is actually declared as 'char **restrict' (the inner
> > > one is not declared as restrict, even though it will be restricted,
> > > since there are no other unrestricted pointers).
> >
> > So how's the following implementation of strtol (10-based, no negative
> > number handling, no overflow handling, ASCII-only) wrong?
> >
> > long int my_strtol(const char *restrict nptr, char **restrict endptr)
> > {
> > long ret = 0;
> >
> > while (isdigit(*nptr))
> > ret = ret * 10 + (*nptr++ - '0');
> >
> > *endptr = (char *)nptr;
> > return ret;
> > }
> >
> > There's no dumb thing, there's no restrict violation (unless it's called
> > in a stupid way, see below), and there **shouldn't** be a -Wrestrict
> > warning.
> >
> > If you do
> >
> > char *x = NULL;
> > strtol((char *)&x, &x, 10);
>
> The restrict in `char **restrict endptr` already protects you from this.
> You don't need to make the first parameter also restricted. See:
>
> $ cat r.c
> long alx_strtol(const char *nptr, char **restrict endp);
>
> int main(void)
> {
> char x = 3;
> char *xp = &x;
>
> alx_strtol(xp, &xp); // Fine.
> alx_strtol(xp, (char **) xp); // Bug.
> alx_strtol((char *) &xp, &xp); // Bug.
> }
> $ cc -Wall -Wextra -S r.c
> r.c: In function ‘main’:
> r.c:9:24: warning: passing argument 2 to ‘restrict’-qualified parameter aliases with argument 1 [-Wrestrict]
> 9 | alx_strtol(xp, (char **) xp); // Bug.
> | ^~~~~~~~~~~~
> r.c:10:34: warning: passing argument 2 to ‘restrict’-qualified parameter aliases with argument 1 [-Wrestrict]
> 10 | alx_strtol((char *) &xp, &xp); // Bug.
> | ~~~~~~~~~~~~ ^~~
>
> Using my proposed prototype wouldn't case any warnings with a powerful
> -fanalyzer that would be able to emit diagnostics with the current
> prototype.
>
> In strtol(3), there are 3 pointers:
>
> - nptr
> - *endptr
> - endptr
>
> The first two should be allowed to alias each other (and at the end of
> the call, they definitely alias each other). It is only the third one
> which must not alias any of the other, which is why my patch (v2) keeps
(Whoops; forget about that v2; that was about a similar patch to
strsep(3). In this case we're in patch v1; which already had that into
consideration. Please read it as s/v2/v1/.)
> that restrict, but removes the other one.
>
> Does that make sense?
>
> Cheers,
> Alex
>
> >
> > it'll violate restrict. Nobody sane should write this, and it's warned
> > anyway:
> >
> > t.c: In function 'main':
> > t.c:6:28: warning: passing argument 2 to 'restrict'-qualified parameter
> > aliases with argument 1 [-Wrestrict]
> > 6 | strtol((char *)&x, &x, 10);
> > | ~~~~~~~~~~ ^~
> >
> > --
> > Xi Ruoyao <xry111@xxxxxxxxxxx>
> > School of Aerospace Science and Technology, Xidian University
>
> --
> <https://www.alejandro-colomar.es/>
--
<https://www.alejandro-colomar.es/>
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