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Re: Address of array && address of pointer to array

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On Thu, 15 Sep 2011 05:36:48 -0700, Stanley Rice wrote:

> 1. when we use %p to print the pointer, but the argument is not
> explicitly converted, will it implicitly be converted to (void *)
> type?

No. The compiler doesn't know the types of printf's arguments (other than
the first one), so it can't know that it should be a void*.

> 2. Why the equal pointers will print differently? It's machine
> dependent or anything else?

The format used by "%p" is implementation-dependent.

One reason why equal pointers might print differently is if there are
multiple representations which are considered equal, and the output for
"%p" is based upon the representation without any form of normalisation.

E.g. on 8086, a (far) pointer contains a 16-bit segment and a 16-bit
offset. The actual memory address is "segment * 16 + offset", so e.g.
0100:2300 and 0200:1300 both refer to the address 0x03300. If
comparison normalises the pointers, they will compare equal. If printing
uses the raw representation, they will print differently.

> Can we say that, two pointers must be equal if they are printed the same.

No. The format used by "%p" is implementation-dependent. An
implementation is free to print e.g. "?" for every pointer.

> 3. In my previous post. If I change my code:
>     printf("the address of A+1: %p\n", (void *)A+1);     // 3'
>     printf("the address of &A+1: %p\n", (void *)&A+1);   // 4'
> 
> statement 3' and 4' will print the same thing. Why it that compared to
> my original code, without explicitly cast.

You're relying upon a implementation-specific extension. Type casts bind
more tightly than addition, so "(void *)A+1" is parsed as "((void *)A)+1".
This is invalid according to the standard ("void" doesn't have a size, so
you can't perform arithmetic on a "void*").

Implementations which allow arithmetic on void* (e.g. gcc) act as if
sizeof(void)==1 in such cases. Because you convert before adding, both
expressions have the same value.

The reason why the original expressions had different values is that, in
pointer arithmetic, the offset is the number of items rather than the
number of bytes, so the calculation scales the offset by the size of the
type being pointed to.

So "A+1" is one int past A ("A" is a pointer to int) while "&A+1" is one
array of 3 ints past A ("&A" is a pointer to an array of 3 ints).

To correctly apply an explicit cast to your original code, you should have
written:

     printf("the address of A+1: %p\n", (void *)(A+1));     // 3'
     printf("the address of &A+1: %p\n", (void *)(&A+1));   // 4'

This will (typically) produce the same result as before, except that this
version doesn't rely upon implementation-specific behavior (i.e. all
pointers using the same representation).

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Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-14 06:12 -0700
  Re: Address of array && address of pointer to array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-14 14:41 +0100
    Re: Address of array && address of pointer to array Paul N <gw7rib@aol.com> - 2011-09-14 14:15 -0700
    Re: Address of array && address of pointer to array Peter Nilsson <airia@acay.com.au> - 2011-09-14 19:25 -0700
      Re: Address of array && address of pointer to array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-15 10:54 +0100
        Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 05:36 -0700
          Re: Address of array && address of pointer to array Ike Naar <ike@sverige.freeshell.org> - 2011-09-15 13:46 +0000
          Re: Address of array && address of pointer to array Nobody <nobody@nowhere.com> - 2011-09-15 14:48 +0100
            Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-15 10:28 -0700
              Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 18:23 -0700
                Re: Address of array && address of pointer to array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-16 12:58 +0100
                Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-16 08:03 -0400
              Re: Address of array && address of pointer to array Nobody <nobody@nowhere.com> - 2011-09-16 13:09 +0100
                Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-16 12:05 -0700
                Re: Address of array && address of pointer to array gazelle@shell.xmission.com (Kenny McCormack) - 2011-09-16 19:35 +0000
                Re: Address of array && address of pointer to array Peter Nilsson <airia@acay.com.au> - 2011-09-19 15:45 -0700
                Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-19 17:47 -0700
          Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-15 10:21 -0400
    Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 05:22 -0700
      Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-15 10:23 -0400
        Re: Address of array && address of pointer to array pete <pfiland@mindspring.com> - 2011-09-15 15:26 -0400
          Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 18:14 -0700
            Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-16 02:09 -0700
              Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-16 08:39 -0400
              Re: Address of array && address of pointer to array pete <pfiland@mindspring.com> - 2011-09-16 18:08 -0400
            Re: Address of array && address of pointer to array Bart van Ingen Schenau <bart@ingen.ddns.info> - 2011-09-16 16:38 +0200
              Re: Address of array && address of pointer to array cri@tiac.net (Richard Harter) - 2011-09-16 22:04 +0000
          Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 18:08 -0700
            Re: Address of array && address of pointer to array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-16 13:12 +0100
            Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-16 08:30 -0400
      Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-15 11:51 -0700
        Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 18:13 -0700
          Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-15 21:18 -0400
            Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-16 05:37 -0700
              Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-16 09:46 -0400
              Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-16 12:08 -0700
          Re: Address of array && address of pointer to array Nobody <nobody@nowhere.com> - 2011-09-16 13:20 +0100

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