Re: Nice way of allocating flexible struct.

From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: Nice way of allocating flexible struct.
Date	2025-12-15 11:24 -0800
Organization	A noiseless patient Spider
Message-ID	<86cy4ftuoi.fsf@linuxsc.com> (permalink)
References	<20251007233002.852@kylheku.com> <10c63rl$1n5qf$1@dont-email.me> <87347thx9h.fsf@example.invalid>

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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> BGB <cr88192@gmail.com> writes:
>
>> On 10/8/2025 1:35 AM, Kaz Kylheku wrote:
>>
>>> Jonas Lund of https://whizzter.woorlic.org/ mentioned this
>>> trick in a HackerNews comment:
>>> Given:
>>>    struct S {
>>>      // ...
>>>      T A[];
>>>    };
>>> Don't do this:
>>>    malloc(offsetof(S, A) + n * sizeof (T));
>>> But rather this:
>>>    malloc(offsetof(S, A[n]));
>>> It's easy to forget that the second argument of offsetof is a
>>> designator, not simply a member name.
>>
>> This is assuming offsetof and can deal with general expressions (vs
>> just field names).  IIRC, it is only required to work with field names
>> (and with plain structs).
>
> I just read that part of the standard, and it's not clear whether
> the second argument to offsetof() has to be a member name or whether
> it can be something more elaborate.
>
> Quoting the N3096 draft of C23, 7.21:
>
>     offsetof(type, member-designator)
>
>     which expands to an integer constant expression that has type
>     `size_t`, the value of which is the offset in bytes, to the
>     subobject (designated by *member-designator*), from the beginning
>     of any object of type *type*. The type and member designator
>     shall be such that given
>
>         static type t;
>
>     then the expression &(t.  *member-designator*) evaluates to
>     an address constant.  If the specified *type* defines a new
>     type or if the specified member is a bit-field, the behavior
>     is undefined.
>
> The requirements imply that the type can be a struct or a union.
>
> The term "member designator" is not used elsewhere in the standard.
> If the term to be taken literally, then it has to designate a
> *member*, not an element of a member.  But the term "subobject",
> along with the address constant requirement, could imply that it
> could be an arbitrary sequence of members and array elements.

Clearly the italicized token member-designator is just a placeholder
with no semantics implied, and the controlling text here is the word
"subobject", which applies recursively.

Note also that the phrase "from the beginning of any object of type
*type*" precludes the use of 'offsetof(S, A[n])', if n is too large,
since A[n] will not be a subobject of an arbitrary object of type
*type*.

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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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