Re: malloc attributes and realloc

[Jeff Sturm <jsturm at one-point dot com>]

On 1 Jan 2004, Gabriel Dos Reis wrote:
> | >> Can __attribute__((__malloc__)) safely be used on realloc-type
> | >> functions?
> | >
> | > My non-expert understanding is "no"...
> | >
> | >> /* Reallocate memory without fail.  This works like xmalloc.  Note,
> | >>    realloc type functions are not suitable for attribute malloc since
> | >>    they may return the same address across multiple calls. */
> | >
> | > This comment is misleading.  The pointer address returned by a
> | > malloc-type
> | > function isn't significant to alias analysis.
> |
> | This isn't true.
> | It's quite significant, because it means it can't be the same pointer
> | as before.
>
> Well, technically, it could be as hinted by the standard:
>
>        [#4] The realloc function  returns  a  pointer  to  the  new
>        object  (which  may  have the same value as a pointer to the
>        old object), or a null pointer if the new object  could  not
>        be allocated.

So, out of curiosity, what happens in an example like:

#include <stdlib.h>
int f(void) {
  int *p, *q;
  p = malloc(sizeof(int));
  *p = 0;
  q = realloc(p, sizeof(int));
  if (p == q) {
    *p = 1;
    return *q;
  }
  return 0;
}

Does the alias machinery determine that p,q alias after all?  Or is this
illegal for some reason?

Jeff