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

From	Tim Rentsch <txr@alumni.caltech.edu>
Newsgroups	comp.lang.c
Subject	Re: [union] Pointers to inherited structs are valid ?
Date	2013-01-03 15:31 -0800
Organization	A noiseless patient Spider
Message-ID	<kfnd2xldetc.fsf@x-alumni2.alumni.caltech.edu> (permalink)
References	<afef3988-7b31-4d55-bf9a-29ab7a835403@x20g2000vbf.googlegroups.com> <kc04nr$38p$1@dont-email.me> <kfnpq1nctkv.fsf@x-alumni2.alumni.caltech.edu> <kc2kvg$in7$1@dont-email.me>

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Shao Miller <sha0.miller@gmail.com> writes:

> 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.

These ideas aren't consistent with how the Standard uses the
notion of having the same representation in other instances.  For
example, an object of type (int) has the same representation and
alignment requirements as an object of type (const int).  Yet it's
ridiculous to think that loading an (int) object through a pointer
of type (const int *) might cause a trap when accessing the object
just as a plain int wouldn't, despite the two types being distinct
and not compatible.

> 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?

No.  I expect you're thinking of "representation" as more or less
synonymous with "format", but representation means more than that.
The representation of a type is the mapping from the bits (ie, the
byte values of the object representation) to values in the type's
abstract value space, including trap values.  If two types have
the same representation, that means the two mappings produce
corresponding values (ie, for each object representatioon) in the
two abstract value spaces.  For C, corresponding values are what
would be produced by conversion between the two types in question.
In other words, if types A and B have the same representation,
then copying the bytes (eg, with memcpy()) from an 'A a;' into a
'B b;' must give the same results as 'b = (B) a;'.  Any change in
behavior between the two cases means the two representations are
not the same.  Accessing via type B using a union member access
works the same way that the memcpy() would.

For pointers, there is the additional concern that the converted
or corresponding value be a non-trap value in the abstract value
space of the new pointer type.  However, in the particular example
here (ie, in the original posting, even though since disappeared
in the subthread), we know the pointer conversions have to work
because of the way the particular structs being pointed to are
nested.

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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