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Re: Storage needed when there are bit-field members

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From Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups comp.lang.c
Subject Re: Storage needed when there are bit-field members
Date Sun, 23 Aug 2026 07:53:23 -0700
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scott@slp53.sl.home (Scott Lurndal) writes:

> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>
>> 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:
>>>>>
>>>>> 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.
>
> I generally only use the packed attribute when creating
> a C struct to match a hardware register, data structure or
> data packet.

I would use the packed attribute if it were necessary to conform
to some externally imposed layout, such as a hardware register or
network packet.  I don't know of any cases were it is necessary.
Maybe the data formats you need to match are more exotic than
ones I am used to.  Can you give an example of a format where the
format cannot be matched using only language features available
in standard C?

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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> - 2026-08-23 07:53 -0700
                Re: Storage needed when there are bit-field members David Brown <david.brown@hesbynett.no> - 2026-08-23 17:33 +0200
            Re: Storage needed when there are bit-field members antispam@fricas.org (Waldek Hebisch) - 2026-08-19 19:34 +0000
              Re: Storage needed when there are bit-field members Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-08-19 15:26 -0700
                Re: Storage needed when there are bit-field members David Brown <david.brown@hesbynett.no> - 2026-08-20 11:00 +0200
                Re: Storage needed when there are bit-field members Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-08-20 03:30 -0700
                Re: Storage needed when there are bit-field members David Brown <david.brown@hesbynett.no> - 2026-08-20 13:58 +0200
                Re: Storage needed when there are bit-field members Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-08-20 13:37 -0700
                Re: Storage needed when there are bit-field members Michael S <already5chosen@yahoo.com> - 2026-08-21 14:31 +0300
                Re: Storage needed when there are bit-field members David Brown <david.brown@hesbynett.no> - 2026-08-21 13:41 +0200
                Re: Storage needed when there are bit-field members scott@slp53.sl.home (Scott Lurndal) - 2026-08-21 15:41 +0000
                Re: Storage needed when there are bit-field members Michael S <already5chosen@yahoo.com> - 2026-08-21 14:58 +0300
                Re: Storage needed when there are bit-field members scott@slp53.sl.home (Scott Lurndal) - 2026-08-20 14:50 +0000
                Re: Storage needed when there are bit-field members "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-08-20 12:31 -0700
                Re: Storage needed when there are bit-field members antispam@fricas.org (Waldek Hebisch) - 2026-08-20 19:55 +0000
                Re: Storage needed when there are bit-field members David Brown <david.brown@hesbynett.no> - 2026-08-21 08:57 +0200
                Re: Storage needed when there are bit-field members scott@slp53.sl.home (Scott Lurndal) - 2026-08-21 14:59 +0000
                Re: Storage needed when there are bit-field members Michael S <already5chosen@yahoo.com> - 2026-08-20 22:15 +0300
          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) cross@spitfire.i.gajendra.net (Dan Cross) - 2026-08-18 20:32 +0000
  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> - 2026-08-23 07:56 -0700
                Re: Microcontroller software stacks David Brown <david.brown@hesbynett.no> - 2026-08-23 17:37 +0200
                Re: Microcontroller software stacks scott@slp53.sl.home (Scott Lurndal) - 2026-08-23 16:09 +0000
                Re: Microcontroller software stacks "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-08-23 19:42 -0700
                Re: Microcontroller software stacks David Brown <david.brown@hesbynett.no> - 2026-08-24 10:27 +0200
                Re: Microcontroller software stacks scott@slp53.sl.home (Scott Lurndal) - 2026-08-23 16:08 +0000
        Re: Microcontroller software stacks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-15 16:16 -0700
          Re: Microcontroller software stacks cross@spitfire.i.gajendra.net (Dan Cross) - 2026-08-18 20:34 +0000
          Re: Microcontroller software stacks antispam@fricas.org (Waldek Hebisch) - 2026-08-19 19:57 +0000
            Re: Microcontroller software stacks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-23 10:54 -0700
              Re: Microcontroller software stacks antispam@fricas.org (Waldek Hebisch) - 2026-08-23 21:22 +0000
                Re: Microcontroller software stacks David Brown <david.brown@hesbynett.no> - 2026-08-24 10:37 +0200
        Re: Microcontroller software stacks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-15 17:31 -0700
  Re: Microcontroller software stacks (was Re: this girl calls c ugly) Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> - 2026-08-20 04:51 +0800

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