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Groups > comp.lang.c > #35713

Re: Struct with unaligned fields

From Ian Collins <ian-news@hotmail.com>
Newsgroups comp.lang.c
Subject Re: Struct with unaligned fields
Date 2013-08-25 09:51 +1200
Message-ID <b7so6eF4n0U2@mid.individual.net> (permalink)
References (5 earlier) <kv989m$ub6$1@dont-email.me> <lnioyvec66.fsf@nuthaus.mib.org> <kvagqf$e88$1@dont-email.me> <lnbo4merqn.fsf@nuthaus.mib.org> <kvb8r8$4q1$1@dont-email.me>

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Les Cargill wrote:
> Keith Thompson wrote:
>> Les Cargill <lcargill99@comcast.com> writes:
>>> Keith Thompson wrote:
>>>> Les Cargill <lcargill99@comcast.com> writes:
>>>>> Stephen Sprunk wrote:
>> [...]
>>>>>> Some CPUs (e.g. most RISCs) don't allow unaligned accesses at all; if
>>>>>> you try it, the program crashes with a bus error.
>>>>>
>>>>> Perhaps I have lived a charmed life, then.
>>>>> I hate to be that guy, but can I have a concrete example?
>>>> [...]
>>>>
>>>> On a SPARC, this program:
>>>>
>> [snip]
>>>>
>>>> produces this output:
>>>>
>>>> ptr  = ffbff351
>>>> obj.f.x = 10
>>>> Bus error
>>>>
>>>> (I needed the union to force the `struct foo` object to be word-aligned;
>>>> without it, the compiler places it at an odd address, causing the `int x`
>>>> member to be word-aligned.)
>>>
>>> Thanks, Keith. Such sadistic hardware!
>>
>> Sadistic?  I don't think so.
>>
>
> I forgot to smiley. Here: :)
>
>> My understanding is that supporting misaligned access requires
>> extra work in designing the CPU hardware.
>
> So they push this off to the toolchain. But the toolchain
> doesn't support it...

In the case of SPARC, yes it does, but at a significant performance 
cost.  Even with the appropriate hardware support, misaligned access is 
still a performance killer.

-- 
Ian Collins

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Thread

Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-22 19:06 +0100
  Re: Struct with unaligned fields glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-08-22 18:18 +0000
  Re: Struct with unaligned fields James Kuyper <jameskuyper@verizon.net> - 2013-08-22 14:57 -0400
    Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-22 21:05 +0100
      Re: Struct with unaligned fields James Kuyper <jameskuyper@verizon.net> - 2013-08-22 16:45 -0400
      Re: Struct with unaligned fields Keith Thompson <kst-u@mib.org> - 2013-08-22 15:12 -0700
    Re: Struct with unaligned fields glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-08-22 22:22 +0000
    Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-22 22:07 -0500
  Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-22 14:13 -0500
    Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-22 22:08 -0500
      Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 09:36 +0100
        Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-23 12:48 -0500
    Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 09:34 +0100
      Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-23 04:47 -0500
        Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 15:31 +0100
          Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-23 13:31 -0500
            Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 20:46 +0100
            Re: Struct with unaligned fields glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-08-23 20:48 +0000
  Re: Struct with unaligned fields Jorgen Grahn <grahn+nntp@snipabacken.se> - 2013-08-22 20:01 +0000
    Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 12:54 +0100
      Re: Struct with unaligned fields Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-08-23 08:17 -0400
      Re: Struct with unaligned fields Jorgen Grahn <grahn+nntp@snipabacken.se> - 2013-08-23 20:00 +0000
      Re: Struct with unaligned fields Tim Rentsch <txr@alumni.caltech.edu> - 2013-08-24 16:50 -0700
  Re: Struct with unaligned fields "BartC" <bc@freeuk.com> - 2013-08-22 21:59 +0100
    Re: Struct with unaligned fields Keith Thompson <kst-u@mib.org> - 2013-08-22 15:02 -0700
    Re: Struct with unaligned fields Tim Rentsch <txr@alumni.caltech.edu> - 2013-08-24 16:42 -0700
  Re: Struct with unaligned fields Siri Cruise <chine.bleu@yahoo.com> - 2013-08-22 14:21 -0700
  Re: Struct with unaligned fields Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-08-22 17:23 -0400
    Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 13:03 +0100
      Re: Struct with unaligned fields Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-08-23 08:43 -0400
        Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 20:20 +0100
          Re: Struct with unaligned fields James Kuyper <jameskuyper@verizon.net> - 2013-08-23 16:15 -0400
          Re: Struct with unaligned fields Ian Collins <ian-news@hotmail.com> - 2013-08-24 11:47 +1200
            Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-24 14:01 +0100
              Re: Struct with unaligned fields Robert Wessel <robertwessel2@yahoo.com> - 2013-08-24 14:26 -0500
                Re: Struct with unaligned fields Ian Collins <ian-news@hotmail.com> - 2013-08-25 08:55 +1200
                Re: Struct with unaligned fields Robert Wessel <robertwessel2@yahoo.com> - 2013-08-25 04:17 -0500
                Re: Struct with unaligned fields Ian Collins <ian-news@hotmail.com> - 2013-08-25 21:22 +1200
                Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-24 16:16 -0500
                Re: Struct with unaligned fields Robert Wessel <robertwessel2@yahoo.com> - 2013-08-25 04:43 -0500
                Re: Struct with unaligned fields David Thompson <dave.thompson2@verizon.net> - 2013-08-28 22:27 -0400
                Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-29 14:30 -0500
                Re: Struct with unaligned fields Robert Wessel <robertwessel2@yahoo.com> - 2013-08-29 16:41 -0500
                Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-09-01 23:13 -0500
                Re: Struct with unaligned fields Richard Damon <Richard@Damon-Family.org> - 2013-09-02 09:48 -0400
                Re: Struct with unaligned fields David Thompson <dave.thompson2@verizon.net> - 2013-09-04 00:01 -0400
      Re: Struct with unaligned fields "BartC" <bc@freeuk.com> - 2013-08-23 23:55 +0100
  Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-22 22:05 -0500
    Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-23 00:29 -0500
      Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-23 12:53 -0500
        Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-23 15:15 -0500
          Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-23 22:14 -0500
            Re: Struct with unaligned fields Ian Collins <ian-news@hotmail.com> - 2013-08-24 16:24 +1200
              Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-23 23:47 -0500
            Re: Struct with unaligned fields Keith Thompson <kst-u@mib.org> - 2013-08-24 01:16 -0700
              Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-24 09:45 -0500
                Re: Struct with unaligned fields Keith Thompson <kst-u@mib.org> - 2013-08-24 13:52 -0700
                Re: Struct with unaligned fields Tim Prince <tprince@computer.org> - 2013-08-24 17:07 -0400
                Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-24 16:35 -0500
                Re: Struct with unaligned fields Ian Collins <ian-news@hotmail.com> - 2013-08-25 09:51 +1200
                Re: Struct with unaligned fields Keith Thompson <kst-u@mib.org> - 2013-08-24 16:04 -0700
                Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-24 21:03 -0500
          Re: Struct with unaligned fields Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-08-24 08:22 -0400
  Re: Struct with unaligned fields Nobody <nobody@nowhere.com> - 2013-08-23 17:51 +0100
  Re: Struct with unaligned fields falk@rahul.net (Edward A. Falk) - 2013-08-24 01:43 +0000

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