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Re: Alignment problems

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"BartC" <bc@freeuk.com> writes:
[...]
> Alternatively, don't compilers have a 'pack' directive for exactly this 
> purpose? Again this would be conditional. Ideally a way of controlling 
> alignment on a field-by-field basis is needed.

Yes, some compilers have a pack directive, but it doesn't always work.

For example, on SPARC, attempting to access a misaligned object causes
the program to crash.  Here's a program that uses gcc's
__attribute__((packed)) extension:

#include <stdio.h>
int main(void) {
    struct unpacked {
        char c;
        double d;
    };

    struct packed {
        char c;
        double d;
    } __attribute__((packed));

    struct unpacked u[2] = { { 'u', 1.2 }, { 'U', 3.4 } };
    struct packed   p[2] = { { 'p', 5.6 }, { 'P', 7.8 } };
    double *up0 = &u[0].d;
    double *up1 = &u[1].d;
    double *pp0 = &p[0].d;
    double *pp1 = &p[1].d;

    printf("*up0 = %g\n", *up0);
    printf("*up1 = %g\n", *up1);
    printf("*pp0 = %g\n", *pp0);
    printf("*pp1 = %g\n", *pp1);

    return 0;
}

And here's the output I got:

*up0 = 1.2
*up1 = 3.4
Bus error

If you refer to the misaligned member directly, so the compiler
knows that it's misaligned, it can access it correctly.  If you've
saved a pointer to the misaligned member, there's no way for the
compiler to know that it's misaligned.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
    Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

Alignment problems jacob navia <jacob@spamsink.net> - 2011-12-01 01:05 +0100
  Re: Alignment problems Ian Collins <ian-news@hotmail.com> - 2011-12-01 13:15 +1300
  Re: Alignment problems pacman@kosh.dhis.org (Alan Curry) - 2011-12-01 02:57 +0000
  Re: Alignment problems gordonb.hpln0@burditt.org (Gordon Burditt) - 2011-11-30 21:27 -0600
  Re: Alignment problems jacob navia <jacob@spamsink.net> - 2011-12-01 13:38 +0100
    Re: Alignment problems pacman@kosh.dhis.org (Alan Curry) - 2011-12-01 20:38 +0000
      Re: Alignment problems jacob navia <jacob@spamsink.net> - 2011-12-01 21:50 +0100
        Re: Alignment problems Ian Collins <ian-news@hotmail.com> - 2011-12-02 10:27 +1300
          Re: Alignment problems Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-12-04 00:23 +0000
            Re: Alignment problems Ian Collins <ian-news@hotmail.com> - 2011-12-04 13:31 +1300
        Re: Alignment problems ImpalerCore <jadill33@gmail.com> - 2011-12-01 14:34 -0800
  Re: Alignment problems "BartC" <bc@freeuk.com> - 2011-12-01 13:46 +0000
    Re: Alignment problems Keith Thompson <kst-u@mib.org> - 2011-12-01 14:15 -0800
      Re: Alignment problems David Thompson <dave.thompson2@verizon.net> - 2012-01-19 02:44 -0500
  Re: Alignment problems "James" <no@spam.invalid> - 2011-12-01 19:43 -0800
  Re: Alignment problems "Brain" <Brian@myhome.com> - 2011-12-04 02:47 -0600
    Re: Alignment problems Keith Thompson <kst-u@mib.org> - 2011-12-04 08:26 -0800
      Re: Alignment problems Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-12-08 00:12 +0200
        Re: Alignment problems Keith Thompson <kst-u@mib.org> - 2011-12-07 22:31 -0800
          Re: Alignment problems lovecreatesbeauty <lovecreatesbeauty@gmail.com> - 2011-12-08 21:43 -0800
            Re: Alignment problems Keith Thompson <kst-u@mib.org> - 2011-12-09 01:31 -0800
              Re: Alignment problems Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-12-09 14:15 +0200

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