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Groups > comp.lang.c > #401223
| From | Tim Rentsch <tr.17687@z991.linuxsc.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Storage needed when there are bit-field members |
| Date | 2026-08-15 16:10 -0700 |
| Organization | A noiseless patient Spider |
| Message-ID | <86cxvj3rq1.fsf@linuxsc.com> (permalink) |
| References | (9 earlier) <86v7b27h20.fsf_-_@linuxsc.com> <111sgeq$3vq40$1@kst.eternal-september.org> <111soae$1eq8$1@kst.eternal-september.org> <86o6f45pu7.fsf@linuxsc.com> <5GMfS.13614$EDc8.5150@fx24.iad> |
scott@slp53.sl.home (Scott Lurndal) writes:
> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>
>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>>
>>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>>>
>>>> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>>>>
>>>>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>>>
>>> [...]
>>>
>>>>>> It would make a lot more sense *to me* if an N-bit bit field were
>>>>>> simply N bits.
>>>
>>> [...]
>
> <snip>
>
>>> I had gotten the impression that the behavior is imposed by ABIs,
>>> but my copy of the "System V Application Binary Interface AMD64
>>> Architecture Processor Supplement" just says:
>>>
>>> - bit-fields are allocated from right to left
>>
>> I think that means they are allocated in order of low-to-high,
>> because the AMD64 architecture is little-endian.
>>
>>> - bit-fields must be contained in a storage unit appropriate for
>>> its declared type
>>> - bit-fields may share a storage unit with other struct / union
>>> members
>>>
>>> which doesn't seem to be enough to specify the behavior I see
>>> (and I find it annoyingly vague).
>>>
>>> Is there a document (ABI, compiler document, whatever) that specifies
>>> the (odd, to me) behavior I'm seeing?
>>
>> As best I can tell the layout you are seeing is consistent with
>> the rules stated above. Perhaps the rules are deliberately meant
>> to be an under-specification (which IMO is not a bad thing).
>>
>>> Here's a test program:
>>>
>>> #include <stdio.h>
>>> #include <stddef.h>
>>> int main(void) {
>>> struct s1 { unsigned char bf:1; unsigned char c; };
>>> struct s2 { unsigned short bf:1; unsigned char c; };
>>> struct s3 { unsigned int bf:1; unsigned char c; };
>>> struct s4 { unsigned long bf:1; unsigned char c; };
>>> struct s5 { unsigned long long bf:1; unsigned char c; };
>
> FWIW, adding GCC's __attribute__((packed)) to each of those, we see:
>
>> type size offset struct-size
>> unsigned char 1 1 2
>> unsigned short 2 1 2
>> unsigned int 4 1 4
>> unsigned long 8 1 8
>> unsigned long long 8 1 8
>
> type size offset struct-size
> unsigned char 1 1 2
> unsigned short 2 1 2
> unsigned int 4 1 2
> unsigned long 8 1 2
> unsigned long long 8 1 2
>
> Which doesn't seem to violate the ABI rules you quoted.
That's good to know I guess, although I'm not sure what it
tells me. My impression is that using attribute__((packed))
produces code that may be less portable than not using it.
Generally I try to write code that avoids compiler-specific
constructs whenever feasible.
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Re: Microcontroller software stacks (was Re: this girl calls c ugly) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-06-21 14:13 -0700
Re: Microcontroller software stacks (was Re: this girl calls c ugly) David Brown <david.brown@hesbynett.no> - 2026-06-22 08:58 +0200
Re: Microcontroller software stacks (was Re: this girl calls c ugly) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-22 03:35 -0700
Re: Microcontroller software stacks (was Re: this girl calls c ugly) cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-22 10:50 +0000
Re: Microcontroller software stacks (was Re: this girl calls c ugly) David Brown <david.brown@hesbynett.no> - 2026-06-22 12:59 +0200
Storage needed when there are bit-field members Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-06-28 09:42 -0700
Re: Storage needed when there are bit-field members Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-28 18:06 -0700
Re: Storage needed when there are bit-field members Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-28 20:20 -0700
Re: Storage needed when there are bit-field members Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-14 14:56 -0700
Re: Storage needed when there are bit-field members scott@slp53.sl.home (Scott Lurndal) - 2026-08-14 22:30 +0000
Re: Storage needed when there are bit-field members Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-15 16:10 -0700
Re: Storage needed when there are bit-field members scott@slp53.sl.home (Scott Lurndal) - 2026-08-16 15:20 +0000
Re: Storage needed when there are bit-field members Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-14 14:19 -0700
Re: Microcontroller software stacks (was Re: this girl calls c ugly) cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-22 10:45 +0000
Re: Microcontroller software stacks (was Re: this girl calls c ugly) scott@slp53.sl.home (Scott Lurndal) - 2026-06-22 15:23 +0000
Re: Microcontroller software stacks (was Re: this girl calls c ugly) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-14 13:23 -0700
Re: Microcontroller software stacks (was Re: this girl calls c ugly) scott@slp53.sl.home (Scott Lurndal) - 2026-06-22 15:04 +0000
Re: Microcontroller software stacks (was Re: this girl calls c ugly) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-22 13:02 -0700
Re: Microcontroller software stacks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-14 12:51 -0700
Re: Microcontroller software stacks Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> - 2026-08-15 05:02 +0800
Re: Microcontroller software stacks bart <bc@freeuk.com> - 2026-08-15 01:18 +0100
Re: Microcontroller software stacks Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> - 2026-08-16 13:52 +0800
Re: Microcontroller software stacks Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-08-14 14:06 -0700
Re: Microcontroller software stacks scott@slp53.sl.home (Scott Lurndal) - 2026-08-14 21:42 +0000
Re: Microcontroller software stacks Michael S <already5chosen@yahoo.com> - 2026-08-15 22:13 +0300
Re: Microcontroller software stacks scott@slp53.sl.home (Scott Lurndal) - 2026-08-15 19:39 +0000
Re: Microcontroller software stacks Michael S <already5chosen@yahoo.com> - 2026-08-15 23:22 +0300
Re: Microcontroller software stacks scott@slp53.sl.home (Scott Lurndal) - 2026-08-16 14:38 +0000
Re: Microcontroller software stacks Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-08-16 16:11 -0700
Re: Microcontroller software stacks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-15 16:16 -0700
Re: Microcontroller software stacks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-15 17:31 -0700
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