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| From | Shao Miller <sha0.miller@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Data alignment questin, structures |
| Date | 2013-01-12 17:45 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <kcsp35$2tq$1@dont-email.me> (permalink) |
| References | <kcs982$r3p$1@dont-email.me> |
On 1/12/2013 13:15, mathog wrote:
> (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?
If you have implementation-specific knowledge of what will happen, then
I'd suggest that you do whatever you want to accomplish your goal.
If you actually care about portability (you seem to), I'd suggest
formally serializing and deserializing data to and from the file.
You might be interested in modern C, which is C11. It includes the
'_Alignof' and '_Alignas' keywords, which allow you to consider and
choose alignments, respectively.
Prior to C11, you could try to use something like:
#include <stddef.h> /* For 'offsetof' */
#define Alignof(type) (offsetof(struct { char c; type t; }, t))
to consider alignment, and could use unions to choose alignment, if you
know the complete object type whose alignment needs to be satsified:
union {
char buffer[512];
MyStruct as_mystruct;
} buffer_and_mystruct;
Here, you know that 'buffer_and_mystruct.buffer' will be properly
aligned for an instance of your structure type.
You can always use 'memcpy' to copy some bytes into a member of an
instance of your structure type; then it'll be safe to examine that
member, as long as it hasn't been populated with a trap representation.
--
- Shao Miller
--
"Thank you for the kind words; those are the kind of words I like to hear.
Cheerily," -- Richard Harter
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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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