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From: "bruno at clisp dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/109990] [12/13/14 Regression] Bogus -Wuse-after-free
 warning after realloc
Date: Fri, 26 May 2023 16:27:12 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109990

--- Comment #4 from Bruno Haible <bruno at clisp dot org> ---
> >=20
> >                   char *new_pool =3D (char *) realloc (string_space, ne=
w_size);
> >                   if (new_pool =3D=3D ((void *)0))
> >                     goto out;
> >                   if (__builtin_expect (string_space !=3D new_pool, 0))
> >                     {
> >                       size_t i;
> >                       for (i =3D 0; i < nmap; i++)
> >                         {
> >                           map[i].alias +=3D new_pool - string_space;
> >                           map[i].value +=3D new_pool - string_space;
> >                         }
> >                     }
> >                   string_space =3D new_pool;

> Also I think `new_pool - string_space` is undefined really.  That is
> subtracting two unrelated arrays is undefined. You can only compare equal=
ity
> on them.

That is the only way of keeping track of pointers _into_ the string_space a=
rea,
when it is reallocated. How else would you want to do it?=