Re: Is this a bad pointer bug?

[Daniel Jacobowitz <drow at mvista dot com>]

On Fri, Jul 18, 2003 at 07:12:13AM -0700, H. J. Lu wrote:
> On Fri, Jul 18, 2003 at 09:43:43AM +0200, Marcel Cox wrote:
> > "H. J. Lu" <hjl@lucon.org> wrote in message
> > 20030718041719.GA21789@lucon.org">news:20030718041719.GA21789@lucon.org...
> > > # cat x.c
> > > #include <stdlib.h>
> > >
> > > struct a
> > > {
> > >   unsigned int x[11];
> > > };
> > >
> > > unsigned long i;
> > > struct a *c=( struct a*)0x3f736080UL, *b= (struct a*)0x1000048UL;
> > >
> > > int
> > > main()
> > > {
> > >   i = (0x3f736080UL-0x1000048UL)/sizeof (struct a);
> > >   if (i != (c - b))
> > >     abort ();
> > >   return 0;
> > > }
> > > # gcc x.c
> > > # ./a.out
> > > zsh: 21264 abort      ./a.out
> > >
> > > Intel compiler works fine. Is this a really bad pointer bug?
> > >
> > 
> > Your pointers c and b are not even a multiple of 44 apart. So their
> > difference is absolutely meaningless.
> > 
> 
> There is no guarantee that pointers c and b will be a multiple of 44
> apart. I got
> 
> [hjl@gnu pointer]$ cat y.c
> #include <stdlib.h>
> #include <stdio.h>
>  
> struct a
> {
>   unsigned int x[11];
> };
>  
> unsigned long i;
> struct a *c, *b;
> char *p;
>  
> int
> main()
> {
>   b = (struct a *) malloc (sizeof (struct a) * 2);
>   p = (char *) malloc (sizeof (struct a) / 2 + sizeof (struct a));
>   c = (struct a *) malloc (sizeof (struct a) * 3);
>   i = ((char *) c - (char *) b)/sizeof (struct a);
>   printf ("%lx, %lx, %lx\n", c, p, b);
>   printf ("%lx, %lx\n", i, c - b);
>   if (i != (c - b))
>     abort ();
>   return 0;
> }
> [hjl@gnu pointer]$ ./a.out
> 8049768, 8049720, 80496c0
> 3, 8ba2e8be
> Aborted
> [hjl@gnu pointer]$ icc y.c
> [hjl@gnu pointer]$ ./a.out
> 804b900, 804b8b8, 804b858
> 3, 3
> 
> What does C standard say on this?

That there's no bug.

       [#9]  When  two pointers are subtracted, both shall point to
       elements of the same array object,  or  one  past  the  last
       element of the array object; the result is the difference of
       the subscripts of the two array elements.


-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer