Re: [union] Pointers to inherited structs are valid ?

From	Shao Miller <sha0.miller@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: [union] Pointers to inherited structs are valid ?
Date	2013-01-02 19:56 -0500
Organization	A noiseless patient Spider
Message-ID	<kc2kvg$in7$1@dont-email.me> (permalink)
References	<afef3988-7b31-4d55-bf9a-29ab7a835403@x20g2000vbf.googlegroups.com> <kc04nr$38p$1@dont-email.me> <kfnpq1nctkv.fsf@x-alumni2.alumni.caltech.edu>

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On 1/2/2013 13:45, Tim Rentsch wrote:
> Shao Miller <sha0.miller@gmail.com> writes:
>> But the implementation's actual pointer representation could be
>> complicated and so there's still no guarantee.  If you can dream up a
>> pointer representation, then you can dream up a counter-example to
>> your code's portability.
>
> The stipulation that all pointers to structs have the same
> representation and alignment requirements means that the
> type-punning union member access has to work.  That's what
> having the same represention means -- that the same object
> representation (ie, the same bytes) will have the same value.
> Any choice of representations for the two cases that doesn't
> produce identical results here means the two representations
> are not the same, ie, the implementation is not conforming
> (under C99/C11 rules).
>

Here are three examples that I would consider to be counter-examples:

1. A 'struct any *' pointer representation that is a simple index.

This could provide a level of indirection into a table.  The table 
element could have type and bounds information, along with some other 
form of address for the pointee.  When the representation (a simple 
index) is loaded into a 'struct bar *' instead of into a 'struct foo *', 
a trap could be generated.

2. A 'struct any *' pointer representation that encodes bounds 
information.  While the original post "has this covered" because the 
bounds of the the original pointee encompass the bounds of the members 
and sub-members, it's not safe in the general case.  When the 
representation is loaded into a 'struct bigger *' instead of a 'struct 
smaller *', the bounds mismatch could generate a trap.

3. A 'struct any *' pointer representation that encodes type 
information.  Maybe for the sole reason of generating a trap when the 
representation is loaded into an incompatible pointer type of object.

It seems clear to me that size, alignment, argument promotion (none) and 
format of 'struct foo *' and 'struct bar *' must be the same, but I 
don't yet understand how that ties into compatible types nor into 
defined behaviour, since

   "Certain object representations need not represent a value of the 
object type. If the stored value of an object has such a representation 
and is read by an lvalue expression that does not have character type, 
the behavior is undefined. If such a representation is produced by a 
side effect that modifies all or any part of the object by an lvalue 
expression that does not have character type, the behavior is 
undefined.41) Such a representation is called a trap representation."

Why can a valid 'struct foo *' value's representation represent a valid 
'struct foo *' value but not a trap for a 'struct bar *'?  For example, 
it might be useful to trap a 'const struct baz *' representation read 
into a 'struct baz *' object.  A single bit in the representation would 
be sufficient for that.  The representation would be the same, wouldn't it?

- Shao Miller

Thread

[union] Pointers to inherited structs are valid ? Maciej Labanowicz <m.labanowicz@gmail.com> - 2013-01-01 03:45 -0800
  Re: [union] Pointers to inherited structs are valid ? Barry Schwarz <schwarzb@dqel.com> - 2013-01-01 17:59 -0800
    Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-02 10:18 -0800
  Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-01 21:06 -0500
    Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-02 10:45 -0800
      Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-02 19:56 -0500
        Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-03 08:28 -0500
        Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-03 15:31 -0800
          Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-03 20:55 -0500
            Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-03 21:00 -0500
              Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-04 12:56 -0500
            Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-04 12:59 -0500
            Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 20:00 -0800
              Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 00:47 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 23:06 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 04:03 -0500
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 08:25 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-13 10:34 -0800
                Re: Off-Topic: Irritating Posting Patterns Shao Miller <sha0.miller@gmail.com> - 2013-01-13 16:00 -0500
                Re: Off-Topic: Irritating Posting Patterns Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-14 12:27 -0800
              Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 07:41 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-12 14:52 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-13 15:19 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-14 12:04 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-14 18:52 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-16 00:01 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-16 05:43 -0500
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-16 09:01 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-16 18:32 -0500
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-16 17:32 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-16 20:57 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-17 07:55 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-17 16:47 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-18 08:41 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-18 15:53 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-18 17:38 -0800
                Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-19 02:31 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-18 23:15 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-19 20:03 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 02:19 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-20 07:57 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 15:25 -0500
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-20 23:58 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 21:30 -0500
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 02:53 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 00:05 -0500
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 16:31 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 14:07 -0500
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 21:05 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 17:13 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 16:11 -0800
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 04:59 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 01:05 -0500
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 14:53 +0000
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:38 -0800
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-21 20:53 -0800
                Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-22 00:58 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:40 -0800
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 23:32 -0800
                Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-23 23:21 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-24 04:53 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-25 10:49 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:25 -0800
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 23:41 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 13:32 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 17:46 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 07:43 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:07 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 14:15 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 14:31 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 21:32 -0500
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-23 17:17 -0500
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-23 19:49 -0500
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-23 18:55 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-24 11:01 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-24 14:11 -0500
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-22 17:51 -0800
                Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-20 16:48 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 21:33 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-21 09:23 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-20 17:07 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 22:20 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-20 22:02 -0800
                Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-21 07:00 +0000
                Re: [union] Pointers to inherited structs are valid ? James Kuyper <jameskuyper@verizon.net> - 2013-01-21 08:17 -0500
                Re: [union] Pointers to inherited structs are valid ? Dr Nick <nospam-4@temporary-address.org.uk> - 2013-01-21 14:13 +0000
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 14:12 +0000
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 08:20 -0800
                Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-21 20:41 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 12:37 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 12:17 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 16:33 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 15:50 -0800
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-21 19:45 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 00:18 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:01 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-18 23:02 -0500
                Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-19 01:28 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-19 12:08 -0500
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-21 09:27 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 12:46 -0500
                Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-21 20:48 +0000
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-21 18:52 -0800
                Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-22 04:07 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 23:15 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-16 10:59 -0800
  Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-02 10:34 -0800

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