Re: Nice way of allocating flexible struct.

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From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: Nice way of allocating flexible struct.
Date	Thu, 09 Oct 2025 19:50:43 -0700
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BGB <cr88192@gmail.com> writes:
> On 10/8/2025 8:39 PM, Kaz Kylheku wrote:
>> On 2025-10-08, Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:
>>> But in addition to that, in Kaz's example, n is not a constant
>>> expression, so `&(t.member-designator)` is not an address constant
>>> and therefore `offsetof(S, A[n])` has undefined behavior.
>> Great; I'd like to hear reasons to avoid it so I don't look foolish
>> for having overlooked it for manytyears. :)
>> 
>>> Every compiler I've tried handles this "correctly", and I tend to
>> I'm sure I've seen foo.bar expressions on the right of an offsetof,
>> but those still yield constants.
>> 
>
> I think it is a case of, it is not required to work...
>
> But, if the typical implementation is something like, say:
> #define offsetof(T, M)  ((long)(&(((T *)0)->M)))
>
> It is probably going to work without issue.

The cast needs to be (size_t), not (long).  With that change,
the behavior is still undefined, but it's likely to work in most
implementations, which is all that's required for code that's part
of the implementation.

Several implementations I've tried (gcc, clang, tcc) implement the
offsetof macro via "__builtin_offsetof".  Whatever compiler magic
is used to implement "__builtin_offsetof" typically works correctly
for Kaz's example (which is of course one of the possible results of
undefined behavior).  One other compiler I've tried has a #define
similar to yours (and also gets the type wrong, but the author of
that implementation is not interested in bug reports from me).

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */

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Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-08 06:35 +0000
  Re: Nice way of allocating flexible struct. pozz <pozzugno@gmail.com> - 2025-10-08 09:09 +0200
    Re: Nice way of allocating flexible struct. richard@cogsci.ed.ac.uk (Richard Tobin) - 2025-10-08 12:01 +0000
    Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-08 15:23 +0000
      Re: Nice way of allocating flexible struct. Michael S <already5chosen@yahoo.com> - 2025-10-08 19:04 +0300
        Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-08 20:05 +0000
    Re: Nice way of allocating flexible struct. Michael S <already5chosen@yahoo.com> - 2025-10-08 18:52 +0300
    Re: Nice way of allocating flexible struct. Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-12-14 23:55 -0800
  Re: Nice way of allocating flexible struct. Bonita Montero <Bonita.Montero@gmail.com> - 2025-10-08 11:09 +0200
    Re: Nice way of allocating flexible struct. Bonita Montero <Bonita.Montero@gmail.com> - 2025-10-08 11:23 +0200
      Re: Nice way of allocating flexible struct. Michael S <already5chosen@yahoo.com> - 2025-10-08 12:53 +0300
        Re: Nice way of allocating flexible struct. Bonita Montero <Bonita.Montero@gmail.com> - 2025-10-08 12:09 +0200
          Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-08 15:59 +0200
            Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-08 12:29 -0500
              Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-08 21:04 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-08 22:49 -0500
                Re: Nice way of allocating flexible struct. bart <bc@freeuk.com> - 2025-10-10 01:13 +0100
                Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-10 01:54 +0000
                Re: Nice way of allocating flexible struct. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-10-09 19:43 -0700
                Re: Nice way of allocating flexible struct. bart <bc@freeuk.com> - 2025-10-10 11:25 +0100
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-09 22:50 -0500
                Re: Nice way of allocating flexible struct. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-09 20:59 -0700
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-10 01:27 -0500
                Re: Nice way of allocating flexible struct. David Brown <david.brown@hesbynett.no> - 2025-10-10 12:06 +0200
                Re: Nice way of allocating flexible struct. Michael S <already5chosen@yahoo.com> - 2025-10-10 17:28 +0300
                Re: Nice way of allocating flexible struct. David Brown <david.brown@hesbynett.no> - 2025-10-10 17:47 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-10 16:32 -0500
                Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-11 00:02 +0000
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-13 06:20 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-10 15:01 -0500
                Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-10 23:45 +0000
                Re: Nice way of allocating flexible struct. Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-06 18:24 -0800
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-14 06:29 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-14 20:13 -0500
                Re: Nice way of allocating flexible struct. bart <bc@freeuk.com> - 2025-10-15 11:26 +0100
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-15 13:00 -0500
                Re: Nice way of allocating flexible struct. bart <bc@freeuk.com> - 2025-10-17 22:07 +0100
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-17 17:44 -0500
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-20 10:02 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-20 04:42 -0500
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-21 03:40 +0200
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-16 06:45 +0200
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-16 06:37 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-16 04:43 -0500
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-20 09:58 +0200
                Re: Nice way of allocating flexible struct. rbowman <bowman@montana.com> - 2025-10-20 18:36 +0000
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-20 16:44 -0500
                Re: Nice way of allocating flexible struct. rbowman <bowman@montana.com> - 2025-10-21 01:33 +0000
                Language-design, tradeoffs (was Re: Nice way of allocating flexible struct.) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-21 04:19 +0200
                Re: Language-design, tradeoffs (was Re: Nice way of allocating flexible struct.) BGB <cr88192@gmail.com> - 2025-10-21 04:27 -0500
                Re: Language-design, tradeoffs (was Re: Nice way of allocating flexible struct.) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-22 02:30 +0200
                Re: Language-design, tradeoffs (was Re: Nice way of allocating flexible struct.) BGB <cr88192@gmail.com> - 2025-10-22 02:10 -0500
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-21 03:51 +0200
                Re: Nice way of allocating flexible struct. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-20 19:21 -0700
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-21 04:53 +0200
                Re: Nice way of allocating flexible struct. rbowman <bowman@montana.com> - 2025-10-21 18:21 +0000
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-21 13:42 -0500
                Re: Nice way of allocating flexible struct. James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-10-21 06:17 -0400
                Re: Nice way of allocating flexible struct. rbowman <bowman@montana.com> - 2025-10-21 18:41 +0000
                Re: Nice way of allocating flexible struct. rbowman <bowman@montana.com> - 2025-10-21 18:12 +0000
            Re: Nice way of allocating flexible struct. Bonita Montero <Bonita.Montero@gmail.com> - 2025-10-08 19:36 +0200
              Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-08 19:51 +0200
    Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-08 15:29 +0000
  Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-08 11:33 -0500
    Re: Nice way of allocating flexible struct. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-08 14:57 -0700
      Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-09 01:39 +0000
        Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-08 22:25 -0500
          Re: Nice way of allocating flexible struct. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-09 19:50 -0700
            Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-10 04:20 +0000
      Re: Nice way of allocating flexible struct. Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-12-15 11:24 -0800
  Re: Nice way of allocating flexible struct. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-10-08 13:35 -0700
    Re: Nice way of allocating flexible struct. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-10-08 13:36 -0700
  Re: Nice way of allocating flexible struct. Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-12-14 22:48 -0800

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