Re: Storage needed when there are bit-field members

From	"Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: Storage needed when there are bit-field members
Date	2026-08-20 12:31 -0700
Organization	A noiseless patient Spider
Message-ID	<1167km4$3ordo$1@dont-email.me> (permalink)
References	(9 earlier) <111sgeq$3vq40$1@kst.eternal-september.org> <111soae$1eq8$1@kst.eternal-september.org> <11650g9$19gjl$1@paganini.bofh.team> <1165ai6$314h2$2@kst.eternal-september.org> <RuEhS.43025$KXn4.4683@fx08.iad>

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On 8/20/2026 7:50 AM, Scott Lurndal wrote:
> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>> antispam@fricas.org (Waldek Hebisch) writes:
>   <snip>
>>> Other post mentioned gcc "packed".  gcc "packed" does not
>>> respect alignment rules, so in such cases 2 byte are enough
>>> regardless of type.
>>
>> gcc "packed" can also cause quiet crashes on some systems.  It can
>> result in, for example, an int member being allocated at an odd address.
>> Passing the address of such a member to a function that assumes the
>> pointed-to object is correctly aligned can result in Bad Things
>> Happening.  (On x86, as I understand it, misaligned accesses
>> are merely somewhat slower than aligned accesses.)
> 
> In modern systems, so long as the misaligned access is
> fully contained within a single cache line, there is
> zero additional latency for the misaligned  access.
> 

fully contained within a single cache line, that's step 1. We also need 
to make sure the cache line is aligned on a cache line boundary.

Thread

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 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 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 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> - 2026-08-15 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> - 2026-08-15 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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