Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #401188

Re: Storage needed when there are bit-field members

Show all headers | View raw

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:
>
> [...]
>
>>> But
>>>     uint32_t bf : 1;
>>> is meaningfully different from
>>>     unsigned bf : 1;
>>>
>>> only because in most implementations (and ABIs), the underlying type
>>> of a bit field affects the layout of the entire structure.
>
> [...]
>
>>> I accept that this is the case, but it's never made any sense to me,
>>> and there's no hint of it in the C standard.
>>
>> I think saying there is not even a hint is an overstatement.  The C
>> standard says that an implementation "may allocate any addressable
>> storage unit large enough to hold a bit-field."  It shouldn't be a
>> surprise that how much storage is allocated depends on the type of
>> the bit-field member.  For example, a bit-field of type 'unsigned'
>> might very well choose a larger storage unit than what is chosen
>> for a bit-field of type '_Bool'.  It seems obvious that the type of
>> a bit-field might affect what size and layout is chosen.
>
> I'm sure it seems obvious to you.  As I said, it's not at all
> obvious to me.
>
> Prior to C99, C didn't even require compilers to support bit-field
> types other than int, unsigned int, and signed int.

True, but allowing other types was listed as a common extension.

> The declared
> type might typically be used only to determine the signedness of the
> bit-field (though I *think* most compilers permitted other types).
>
> Implementations are certainly not *required* to use the declared
> type of a bit-field as a factor in deciding how to allocate it,
> or how to allocate the rest of the structure.  Allocating just one
> byte for an isolated 1-bit bit-field of any declared type would
> be conforming.  A conforming compiler could use the declared type
> only to determine the signedness and the maximum allowed width of
> a bit-field (and its conversion behavior in the case of bool)

Yes, it could.

>>> For example, if I write:
>>>     uint64_t bf : 1;
>>>
>>> then the containing struct is typically at least 64 bits, even
>>> though those other 63 bits aren't part of the bit field and other
>>> members can be allocated within them.
>>>
>>> It would make a lot more sense *to me* if an N-bit bit field were
>>> simply N bits.
>>
>> Two problems with that.  One, it seems to be in conflict with what
>> the C standard says about 0-width bit-fields.
>
> 0-width bit-fields are obviously a special case.

Sorry for not making my point more clear.  My comment is meant to
to raise the question of whether

    struct x {
        _Bool foo:1;
        _Bool :0;
        char  c;
    };

and

    struct y {
        unsigned foo:1;
        _Bool       :0;
        char  c;
    };

should be different.  I'm inclined to think they should be, by
which I mean my preference is for compilers where they would be.

>>                                                Two, the C standard
>> explicitly allows allocating bit-fields using a high-to-low order
>> or a low-to-high order (implementation-defined choice).  Presumably
>> this freedom is given to accommodate both big- and little-endian
>> platforms.  The idea that an N-bit bit-field should simply be N
>> bits doesn't work in big-endian environments.  It seems better to
>> allow little-endian implementations to choose a size that matches
>> what a big-endian implementation would use, rather than insisting
>> that they be different.
>
> I honestly don't understand your point here.  How does making
> N-bit bit-fields N bits not work in a big-endian environment?
> Can you elaborate?  Of course endianness can affect how bit-fields
> are allocated within a "storage unit".

Suppose we have a little endian machine where bit-fields are
allocated in a high-to-low order.  Further suppose that unsigned
ints are 32 bits.  In such an environment, I would expect (or
prefer) a definition like this

    struct foo {
        unsigned x:15;
    };

to be represented like so

    --------  --------  -XXXXXXX  XXXXXXXX

where the X's indicate where the bit-field goes, and the -'s
indicate where there are padding bits.  In such an environment,
I would find it counterintuitive if this type were represented
thus

    -XXXXXXX  XXXXXXXX

rather than as shown in the previous layout.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

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> - 2026-08-14 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

csiph-web