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Re: Pointer Equality for Different Array Objects

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On 2/2/2012 19:34, Shao Miller wrote:
> (More bounds-checking.)
>
> Wait a minute... N1256 6.5.9p6:
>
> "Two pointers compare equal if and only if both are null pointers, both
> are pointers to the same object (including a pointer to an object and a
> subobject at its beginning) or function, both are pointers to one past
> the last element of the same array object, or one is a pointer to one
> past the end of one array object and the other is a pointer to the start
> of a different array object that happens to immediately follow the first
> array object in the address space.94)"
>
> So if we have:
>
> union u_test {
>     int a[2][2];
>     int b[4];
>   };
> union u_test test = { { { 0 } } };
> int x = &test.a[1][1] == &test.b[3];
>
> Is 'x' zero or one?
>
> If we can claim, for the purposes of supporting bounds-checking, that
> '&test.a[1][1]' points into an 'int[2]' and nothing more, and that
> '&test.b[3]' points into an 'int[4]' and nothing more, then aren't these
> pointers pointing to distinct objects?
>
> If they're pointing to the same object, why might they not be for the
> purposes of 6.5.6p8?
>
> Just to illustrate an identical [declaration]:
>
> int x = (*(test.a + 1) + 1) == (test.b + 3);
>
>

Maybe using a 'typedef' makes it look different?

   typedef int array_of_two_ints[2];
   union u_test {
       array_of_two_ints a[2];
       int b[4];
     };
   union u_test test = { { { 0 } } };
   int x;
   array_of_two_ints * ptr_into_array_of_two_ints;
   int * ptr_into_int;

   /*
    * Point into the second element of 'a', so the
    * element type is 'array_of_two_ints'
    */
   ptr_into_array_of_two_ints = &test.a[1];


   /*
    * Point into the second element of the array object
    * with type 'array_of_two_ints', so the element
    * type is 'int'
    */
   ptr_into_int = &ptr_into_array_of_two_ints[1];

   /*
    * 'ptr_into_int' does _not_ point to:
    * - A subobject at the beginning of the 'test.b'
    *   array object
    * - The array object 'test.b'
    * - A function
    * - No object (it's non-null)
    * - One past the array object 'test.b'
    * - An array object that happens to immediately
    *   follow 'test.b' in the address space
    * - An object that is not an element of an array
    *   object
    * '&test.b[3] points to:
    * - The fourth element of the 'test.b' array
    *   object
    */
   x = ptr_into_int == &test.b[3];
   x = ptr_into_int - &test.b[3];

Perhaps similarly:

   struct s_common {
       double d;
       int i[2];
     };
   struct s_foo {
       struct s_common common;
       short s;
     };
   struct s_bar {
       struct s_common common;
       long l;
     };
   union u_foobar {
       struct s_foo foo;
       struct s_bar bar;
     };
   union u_foobar test;
   int * p;
   int * q;
   int x;

   p = &test.foo.common.i[1];
   q = &test.bar.common.i[1];

   /*
    * 'p' does _not_ point to:
    * - A subobject at the beginning of the
    *   'test.bar.common.i' array object
    * - The array object 'test.bar.common.i'
    * - A function
    * - No object (it's non-null)
    * - One past the array object 'test.bar.common.i'
    * - An array object that happens to immediately
    *   follow 'test.bar.common.i' in the
    *   address space
    * - An object that is not an element of an array
    *   object
    * 'q' points to:
    * - The second element of the 'test.bar.common.i'
    *   array object
    */
   x = p == q;
   x = p - q;

Fortunately, of course, they point to the same byte in the same 
contiguous region of data storage.

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Thread

Pointer Equality for Different Array Objects Shao Miller <sha0.miller@gmail.com> - 2012-02-02 19:34 -0500
  Re: Pointer Equality for Different Array Objects Shao Miller <sha0.miller@gmail.com> - 2012-02-02 19:40 -0500
  Re: Pointer Equality for Different Array Objects Kaz Kylheku <kaz@kylheku.com> - 2012-02-03 00:47 +0000
    Re: Pointer Equality for Different Array Objects Shao Miller <sha0.miller@gmail.com> - 2012-02-02 20:19 -0500
      Re: Pointer Equality for Different Array Objects pete <pfiland@mindspring.com> - 2012-02-02 20:45 -0500
        Re: Pointer Equality for Different Array Objects Shao Miller <sha0.miller@gmail.com> - 2012-02-02 21:15 -0500
        Re: Pointer Equality for Different Array Objects Kaz Kylheku <kaz@kylheku.com> - 2012-02-03 02:36 +0000
      Re: Pointer Equality for Different Array Objects Kaz Kylheku <kaz@kylheku.com> - 2012-02-03 02:35 +0000
        Re: Pointer Equality for Different Array Objects Shao Miller <sha0.miller@gmail.com> - 2012-02-03 01:27 -0500
          Re: Pointer Equality for Different Array Objects Shao Miller <sha0.miller@gmail.com> - 2012-02-03 01:29 -0500
          Re: Pointer Equality for Different Array Objects Kaz Kylheku <kaz@kylheku.com> - 2012-02-03 07:11 +0000
            Re: Pointer Equality for Different Array Objects Shao Miller <sha0.miller@gmail.com> - 2012-02-03 19:31 -0500
  Re: Pointer Equality for Different Array Objects Shao Miller <sha0.miller@gmail.com> - 2012-02-06 02:25 -0500

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