Data alignment questin, structures

From	mathog <mathog@caltech.edu>
Newsgroups	comp.lang.c
Subject	Data alignment questin, structures
Date	2013-01-12 10:15 -0800
Organization	A noiseless patient Spider
Message-ID	<kcs982$r3p$1@dont-email.me> (permalink)

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(Apologies if this is a duplicate post, glitch on first attempt.)

Consider the following struct:

typedef struct {
   uint16_t one;
   uint16_t twothree[2];
   uint16_t four;
   uint16_t five;
} Mystruct;

It contains data storage which is actually:

   { uint16_t, uint32_t, uint16_t, uint16_t  }

Accessing 'twothree" as a uint32_t directly, for instance with:

   Mystruct instance;
   printf("twothree is:%d\n",*(uint32_t *)&(instance.twothree[0]);

will be a nonaligned memory access.  That will do nothing untoward on 
x86 CPUs but will blow up on many others.  OK so far?

Now, what happens when this struct is embedded in a binary file stored 
in a character array "buffer[]" such that its position "i" is not 
guaranteed to be aligned on the same boundary that

   Mystract *instance2 = malloc(sizeof(Mystruct));

would have used, but will always be on a multiple of 2.

1.  Will this always work?

   function1((Mystruct *) &(buffer[i]));

2.  How about this?

   function2(*(Mystruct *) &(buffer[i]));

Assume that within the function access to the data is by memcpy() to the 
appropriate offset for the uint32_t and by "=" for the uint16_t fields. 
  My guess is that (2) is likely to blow up more often than not, since 
it is trying to pass a Mystruct unaligned.

What does the language standard say should happen in these two cases?

Thanks,

David Mathog

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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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