Re: Microcontroller software stacks

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From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: Microcontroller software stacks
Date	Sun, 23 Aug 2026 10:54:11 -0700
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antispam@fricas.org (Waldek Hebisch) writes:

> Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:

>>> [on deciding which type to use for a bit-field member]

>> To me it seems more logical to use the smallest basic type that
>> suffices for the width of the associated bit-field, assuming
>> the compiler allows it, or 'unsigned' for compilers that don't
>> provide those extensions.
>
> Conider the following two structs:
>
> typedef struct {char a:5; char b:5; char c:5;} bf1;
>
> typedef struct {short a:5; short b:5; short c:5;} bf2;
>
> gcc allocates 3 bytes to first one and 2 bytes for the second one.

That's interesting;  thank you for the research.  I wouldn't
expect code to use char for a bit-field type but presumably
unsigned char or signed char would give the same result.

> Bitfields are frequently used to converve memory and the second
> one is better in this aspect.  So it is hard the avoid notion
> of storage unit and using type to control size of storage unit.

In the early days of C conserving space may have been a common
consideration.  These days I expect it hardly ever is, except
perhaps in the case of lots of _Bool bit-fields.

Furthermore there are advantages to using a type where the
members occupy separate bytes rather than being packed into a
single 16-bit object.  The generated code may be better.  Also
using a single byte for each member reduces the change of
undefined behavior when accessing these bit-fields.

> Of course, if you can not count on compiler behaving in specific
> way, abd you can not tolerate different behaviour, then it is better
> to use access if given needed size and masks and shifts to access
> the bits.

I hope it goes without saying that whenever it is necessary to
conform to an externally imposed layout, and bit-fields are used
to effect that, that the layout produced by the compiler(s)
should be checked against the external specification, and that
whatever type-selection heuristics are used should be adjusted to
ensure the needed conformance does in fact occur.  Using masks
and shifts -- and probably macros -- can provide a more reliable
alternative in some cases;  but that approach also carries its
own set of downsides, which need to be weighed against other
approaches.

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 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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