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Groups > comp.lang.c > #395811
| From | Tim Rentsch <tr.17687@z991.linuxsc.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Nice way of allocating flexible struct. |
| Date | 2025-12-14 23:55 -0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <86fr9cuqkz.fsf@linuxsc.com> (permalink) |
| References | <20251007233002.852@kylheku.com> <10c52nj$pnn0$1@dont-email.me> |
pozz <pozzugno@gmail.com> writes:
> Il 08/10/2025 08:35, Kaz Kylheku ha scritto:
>
>> 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.
>
> struct S {
> unsigned int size;
> unsigned char mode;
> unsigned char array[];
> }
>
> In a 32-bits integer machine, sizeof(struct S) is 8, because there are
> 3 bytes of padding after mode and array is considered empty.
>
> Now I want to store 9 bytes in array[]. I could use:
>
> malloc(sizeof(struct S) + 9 * sizeof(unsigned char))=malloc(17)
>
> or I could use:
>
> malloc(offsetof(struct S, array[9]))=malloc(14)
>
> Is the second better (and correct) than the first?
>
> And another question. Suppose I need an array of struct S. All
> elements have 7-bytes array[] member. How to allocate this array and
> access each element?
>
> I think I can't use the first malloc (17), neither the second
> (14). Both aren't a multiple of alignment length.
Right. Getting the (minimum) size exactly right is tricky enough
to merit writing a macro to compute the desired value.
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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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