Hi,
First, I assume we are talking about C realloc here, not just a
"realloc-like" function which may have other semantics and for which
__attribute_malloc__ may not be appropriate.
> > It looks like gcc assumes a functon marked with DECL_IS_MALLOC won't
> > return an address which can alias something else. But it isn't true
> > for realloc. Now, the qestions are
> >
> > 1. Can gcc make such an assumption?
>
> No, it can't. The returned memory may alias the original memory.
After realloc(p, s) has returned non-NULL, the "original object"
doesn't exist any more, hence there can't be any aliases.
> > 2. Can realloc be marked as DECL_IS_MALLOC.
>
> ... with DECL_IS_MALLOC the following
>
> int *p;
> p = malloc (4);
> *p = 0;
> p = realloc (p, 4);
> *p = 1;
>
> will have VOPs that do not prevent re-ordering of the two stores.
By that reasoning, consider:
int *p;
p = malloc (4);
*p = 0;
free(p);
p = malloc (4); /* this is very likely to return the same address as before */
*p = 1;
What prevents the reordering of the stores in this case?
Should we also remove __attribute_malloc__ from malloc :-)?
IMHO this is an object lifetime issue not an aliasing issue.
> > BTW, glibc also marks realloc with __attribute_malloc__.
>
> Which is wrong as well.
I disagree.
Of course, the gcc developers get to define the semantics of
__attribute_malloc__, but according to the gcc manual, the attribute
only refers to the _result_ of the attributed function, hence I would
intuitively expect that I can safely mark:
int *destroy_something_and_allocate_anotherthing(int *p)
{
free(p);
return malloc(sizeof(int)); /* again very likely to return the
same as the previous p */
}
as __attribute_malloc__.
Regards,
Wolfram.
