Re: Packed structs (was Re: Some kind of contextual modulo operator?)

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From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Packed structs (was Re: Some kind of contextual modulo operator?)
Date	Mon, 04 Dec 2017 09:23:29 -0800
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scott@slp53.sl.home (Scott Lurndal) writes:
> David Brown <david.brown@hesbynett.no> writes:
>>In my experience, "#pragma pack" and similar directives are almost 
>>always used incorrectly - or at least, they are an inefficient solution.
>
> In my experience, packed structures are used for compatability with
> externally defined data structures (e.g. foreign filesystems, network
> data, etc)

Plausible

>            and are only useful on systems that don't trap misaligned
> accesses.

Not necessarily.  gcc and other compilers support #pragma pack.  On
target systems that trap on misaligned accesses, the compiler generates
whatever code is needed to access misaligned members correctly.

There is a problem, though.  For example:

int main(void) {
    struct foo {
        char c;
        int i;
    };
    struct foo obj;
    obj.i = 10; // ok
    int *p = &obj.i;
    *p = 20;    // kaboom
}

On the first assignment, the compiler knows that i is misaligned and
generates whatever code is needed to access it.

On the second, that information has been lost, and the compiler assumes
*p is correctly aligned.  On x86, that's not a problem.  On other
systems, it could be.

https://stackoverflow.com/q/8568432/827263
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51628
https://bugs.llvm.org/show_bug.cgi?id=22821

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Some kind of contextual modulo operator? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-11-29 10:41 -0800
  Re: Some kind of contextual modulo operator? jameskuyper@verizon.net - 2017-11-29 11:12 -0800
    Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-11-29 19:22 +0000
      Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-11-29 19:43 +0000
        Re: Some kind of contextual modulo operator? jameskuyper@verizon.net - 2017-11-29 12:33 -0800
          Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-11-29 23:22 +0000
            Re: Some kind of contextual modulo operator? jameskuyper@verizon.net - 2017-11-29 20:53 -0800
              Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-11-30 12:17 +0000
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-01 15:08 +0000
                Re: Some kind of contextual modulo operator? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-12-01 07:31 -0800
                Re: Some kind of contextual modulo operator? scott@slp53.sl.home (Scott Lurndal) - 2017-12-01 16:32 +0000
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-01 17:07 +0000
                Re: Some kind of contextual modulo operator? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-12-01 09:15 -0800
                Re: Some kind of contextual modulo operator? scott@slp53.sl.home (Scott Lurndal) - 2017-12-01 18:14 +0000
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-01 19:03 +0000
                Re: Some kind of contextual modulo operator? scott@slp53.sl.home (Scott Lurndal) - 2017-12-01 20:54 +0000
                Re: Some kind of contextual modulo operator? Keith Thompson <kst-u@mib.org> - 2017-12-01 12:09 -0800
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-01 21:56 +0000
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-01 23:11 +0000
                Re: Some kind of contextual modulo operator? supercat@casperkitty.com - 2017-12-01 15:16 -0800
                Re: Some kind of contextual modulo operator? David Brown <david.brown@hesbynett.no> - 2017-12-02 17:09 +0100
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-02 19:42 +0000
                Re: Some kind of contextual modulo operator? Ian Collins <ian-news@hotmail.com> - 2017-12-03 09:16 +1300
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-02 22:06 +0000
                Re: Some kind of contextual modulo operator? Ian Collins <ian-news@hotmail.com> - 2017-12-03 11:20 +1300
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-03 00:01 +0000
                Re: Some kind of contextual modulo operator? Ian Collins <ian-news@hotmail.com> - 2017-12-03 13:55 +1300
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-03 02:14 +0000
                Re: Some kind of contextual modulo operator? Ian Collins <ian-news@hotmail.com> - 2017-12-03 16:13 +1300
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-03 12:28 +0000
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-03 14:26 +0000
                Re: Some kind of contextual modulo operator? Ian Collins <ian-news@hotmail.com> - 2017-12-04 10:49 +1300
                Re: Some kind of contextual modulo operator? Ian Collins <ian-news@hotmail.com> - 2017-12-04 10:38 +1300
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-03 23:00 +0000
                Re: Some kind of contextual modulo operator? Ian Collins <ian-news@hotmail.com> - 2017-12-04 12:10 +1300
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-03 23:33 +0000
                Re: Some kind of contextual modulo operator? Ian Collins <ian-news@hotmail.com> - 2017-12-04 13:18 +1300
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-04 12:19 +0000
                Re: Some kind of contextual modulo operator? Ian Collins <ian-news@hotmail.com> - 2017-12-05 07:58 +1300
                Re: Some kind of contextual modulo operator? David Brown <david.brown@hesbynett.no> - 2017-12-04 00:30 +0100
                Packed structs (was Re: Some kind of contextual modulo operator?) scott@slp53.sl.home (Scott Lurndal) - 2017-12-04 15:02 +0000
                Re: Packed structs (was Re: Some kind of contextual modulo operator?) David Brown <david.brown@hesbynett.no> - 2017-12-04 16:34 +0100
                Re: Packed structs (was Re: Some kind of contextual modulo operator?) Keith Thompson <kst-u@mib.org> - 2017-12-04 09:23 -0800
                Re: Packed structs (was Re: Some kind of contextual modulo operator?) scott@slp53.sl.home (Scott Lurndal) - 2017-12-04 18:31 +0000
                Re: Packed structs (was Re: Some kind of contextual modulo operator?) supercat@casperkitty.com - 2017-12-04 15:37 -0800
                Re: Some kind of contextual modulo operator? supercat@casperkitty.com - 2017-12-04 08:39 -0800
                Re: Some kind of contextual modulo operator? "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2017-12-03 18:13 -0800
                Re: Some kind of contextual modulo operator? Ian Collins <ian-news@hotmail.com> - 2017-12-03 14:50 +1300
                Re: Some kind of contextual modulo operator? scott@slp53.sl.home (Scott Lurndal) - 2017-12-04 15:00 +0000
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-04 15:53 +0000
                Re: Some kind of contextual modulo operator? scott@slp53.sl.home (Scott Lurndal) - 2017-12-04 18:26 +0000
                Re: Some kind of contextual modulo operator? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-12-04 10:34 -0800
                Re: Some kind of contextual modulo operator? David Brown <david.brown@hesbynett.no> - 2017-12-03 19:40 +0100
                Re: Some kind of contextual modulo operator? David Kleinecke <dkleinecke@gmail.com> - 2017-12-03 11:35 -0800
                Re: Some kind of contextual modulo operator? scott@slp53.sl.home (Scott Lurndal) - 2017-12-04 14:59 +0000
                Re: Some kind of contextual modulo operator? Keith Thompson <kst-u@mib.org> - 2017-12-01 08:56 -0800
                Re: Some kind of contextual modulo operator? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-12-01 08:58 -0800
                Re: Some kind of contextual modulo operator? scott@slp53.sl.home (Scott Lurndal) - 2017-12-01 18:16 +0000
                Re: Some kind of contextual modulo operator? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-12-01 10:28 -0800
                Re: Some kind of contextual modulo operator? David Brown <david.brown@hesbynett.no> - 2017-12-01 20:19 +0100
                Re: Some kind of contextual modulo operator? Gareth Owen <gwowen@gmail.com> - 2017-12-01 22:22 +0000
                Re: Some kind of contextual modulo operator? jameskuyper@verizon.net - 2017-12-01 07:49 -0800
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-01 16:54 +0000
                Re: Some kind of contextual modulo operator? jameskuyper@verizon.net - 2017-12-01 09:34 -0800
                Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-12-01 18:48 +0000
                Re: Some kind of contextual modulo operator? jameskuyper@verizon.net - 2017-12-01 11:27 -0800
      Re: Some kind of contextual modulo operator? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-11-29 12:17 -0800
        Re: Some kind of contextual modulo operator? bartc <bc@freeuk.com> - 2017-11-30 12:27 +0000
          Re: Some kind of contextual modulo operator? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-11-30 06:11 -0800
    Re: Some kind of contextual modulo operator? "James R. Kuyper" <jameskuyper@verizon.net> - 2017-11-29 15:02 -0500
  Some kind of contextual modulo operator? mcheung63@gmail.com - 2017-12-08 23:20 -0800

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