Re: Data alignment questin, structures

From	Bart van Ingen Schenau <bart@ingen.ddns.info.invalid>
Newsgroups	comp.lang.c
Subject	Re: Data alignment questin, structures
Date	2013-01-21 10:19 +0000
Organization	A noiseless patient Spider
Message-ID	<kdj4o7$fjk$2@dont-email.me> (permalink)
References	(3 earlier) <kdftp6$i66$1@dont-email.me> <kdh9it$kjm$1@dont-email.me> <kdhct5$bmb$1@dont-email.me> <kdhlfn$6fc$1@dont-email.me> <kdig35$1m1$1@dont-email.me>

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On Sun, 20 Jan 2013 20:27:11 -0800, mathog wrote:

> James Kuyper wrote:
>> On 01/20/2013 01:26 PM, mathog wrote: ...
>>> ...  As for
>>> bytes that are not 8 bits, does any modern hardware exist where that
>>> is not true?
>>
>> I gather that there is VERY modern hardware in the embedded world where
>> CHAR_BIT == 16; in fact, it's a fairly common feature in that world.

See this answer on stackoverflow for some examples: 
http://stackoverflow.com/a/6971919/430719

> 
> On this hardware is memory access also 16 bits at a time?

Yes, in units of 16-bits or larger.

> I wonder what
> it does if it reads a network packet with an odd number of 8 bit bytes,
> or worse yet, needs to send one?

Those devices typically don't have to deal with that situation.
Many processors that have `CHAR_BIT == 16` (or event `CHAR_BIT == 32`) 
are DSPs (Digital Signal Processors) that don't communicate with octet-
oriented peripherals (such as networks).
And when they do, it turns out that all the relevant protocols were 
designed to be friendly for such processors (all multi-octet fields 
aligned to a multiple of their size, protocol headers a multiple of 4 
octets, etc.)

What I have also seen with such a processor is that each octet received 
from outside is put into a separate 16-bits char. It was up to the 
programmer to combine the values from multiple chars into a single value 
if the protocol actually contained a multi-octet value at that point.
No amount of memory mapping technique is going to help you there.

> 
> Admittedly pragma pack gets most of what I need from a memstruct in a
> struct.  Has that pragma made it into some version of the C standard, or
> is it still a compiler specific extension (albeit a common one)?

The #pragma directive is specified in the C standard, but in the most 
part it is defined to have implementation-defined effects.

> 
> Thanks,
> 
> David Mathog

Bart v Ingen Schenau

Thread

Data alignment questin, structures mathog <mathog@caltech.edu> - 2013-01-12 10:15 -0800
  Re: Data alignment questin, structures Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-12 18:54 +0000
    Re: Data alignment questin, structures Jorgen Grahn <grahn+nntp@snipabacken.se> - 2013-01-15 20:14 +0000
      Re: Data alignment questin, structures Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-15 16:29 -0500
  Re: Data alignment questin, structures Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-12 15:41 -0500
    Re: Data alignment questin, structures mathog <dmathog@gmail.com> - 2013-01-18 10:24 -0800
      Re: Data alignment questin, structures James Kuyper <jameskuyper@verizon.net> - 2013-01-18 14:05 -0500
      Re: Data alignment questin, structures Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-18 14:46 -0500
      Re: Data alignment questin, structures glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-18 21:11 +0000
      Re: Data alignment questin, structures Shao Miller <sha0.miller@gmail.com> - 2013-01-18 17:14 -0500
        Re: Data alignment questin, structures glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-18 23:09 +0000
      Re: Data alignment questin, structures Keith Thompson <kst-u@mib.org> - 2013-01-18 16:23 -0800
      Re: Data alignment questin, structures mathog <mathog@caltech.edu> - 2013-01-19 21:02 -0800
        Re: Data alignment questin, structures Shao Miller <sha0.miller@gmail.com> - 2013-01-20 01:25 -0500
        Re: Data alignment questin, structures glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-20 07:53 +0000
        Re: Data alignment questin, structures Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-20 14:02 +0000
        Re: Data alignment questin, structures Keith Thompson <kst-u@mib.org> - 2013-01-20 07:35 -0800
          Re: Data alignment questin, structures 88888 Dihedral <dihedral88888@googlemail.com> - 2013-01-20 11:49 -0800
        Re: Data alignment questin, structures James Kuyper <jameskuyper@verizon.net> - 2013-01-20 12:30 -0500
          Re: Data alignment questin, structures mathog <mathog@caltech.edu> - 2013-01-20 10:26 -0800
            Re: Data alignment questin, structures Shao Miller <sha0.miller@gmail.com> - 2013-01-20 15:11 -0500
            Re: Data alignment questin, structures "BartC" <bc@freeuk.com> - 2013-01-20 20:34 +0000
            Re: Data alignment questin, structures James Kuyper <jameskuyper@verizon.net> - 2013-01-20 15:53 -0500
              Re: Data alignment questin, structures 88888 Dihedral <dihedral88888@googlemail.com> - 2013-01-20 13:16 -0800
              Re: Data alignment questin, structures mathog <mathog@caltech.edu> - 2013-01-20 20:27 -0800
                Re: Data alignment questin, structures Shao Miller <sha0.miller@gmail.com> - 2013-01-21 00:37 -0500
                Re: Data alignment questin, structures Dr Nick <nospam-4@temporary-address.org.uk> - 2013-01-21 10:07 +0000
                Re: Data alignment questin, structures mathog <mathog@caltech.edu> - 2013-01-21 10:21 -0800
                Re: Data alignment questin, structures Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-21 10:34 -0800
                Re: Data alignment questin, structures Keith Thompson <kst-u@mib.org> - 2013-01-21 13:07 -0800
                Re: Data alignment questin, structures "BartC" <bc@freeuk.com> - 2013-01-21 22:24 +0000
                Re: Data alignment questin, structures mathog <dmathog@gmail.com> - 2013-01-22 10:18 -0800
                Re: Data alignment questin, structures glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-22 20:57 +0000
                Re: Data alignment questin, structures Keith Thompson <kst-u@mib.org> - 2013-01-22 14:51 -0800
                Re: Data alignment questin, structures Ian Collins <ian-news@hotmail.com> - 2013-01-22 11:12 +1300
                Re: Data alignment questin, structures glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-21 20:53 +0000
                Re: Data alignment questin, structures Keith Thompson <kst-u@mib.org> - 2013-01-20 22:06 -0800
                Re: Data alignment questin, structures Philip Lantz <prl@canterey.us> - 2013-01-22 00:27 -0800
                Re: Data alignment questin, structures James Kuyper <jameskuyper@verizon.net> - 2013-01-22 06:44 -0500
                Re: Data alignment questin, structures Bart van Ingen Schenau <bart@ingen.ddns.info.invalid> - 2013-01-21 10:19 +0000
            Re: Data alignment questin, structures glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-20 22:19 +0000
          Re: Data alignment questin, structures glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-20 22:14 +0000
          Re: Data alignment questin, structures "BartC" <bc@freeuk.com> - 2013-01-20 22:48 +0000
            Re: Data alignment questin, structures Les Cargill <lcargill99@comcast.com> - 2013-01-20 18:11 -0600
            Re: Data alignment questin, structures Ian Collins <ian-news@hotmail.com> - 2013-01-21 14:07 +1300
  Re: Data alignment questin, structures Shao Miller <sha0.miller@gmail.com> - 2013-01-12 17:45 -0500
    Re: Data alignment questin, structures mathog <mathog@caltech.edu> - 2013-01-13 10:17 -0800
      Re: Data alignment questin, structures Shao Miller <sha0.miller@gmail.com> - 2013-01-13 17:16 -0500

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