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| From | mathog <mathog@caltech.edu> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Data alignment questin, structures |
| Date | 2013-01-19 21:02 -0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <kdftp6$i66$1@dont-email.me> (permalink) |
| References | <kcs982$r3p$1@dont-email.me> <kcshq5$7e8$1@dont-email.me> <kdc426$f0i$1@dont-email.me> |
>>> 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]));
>>
(Eric Sosman wrote:)
>> Not "always," no. 6.3.2.3p7: "A pointer to an object type
>> may be converted to a pointer to a different object type. If the
>> resulting pointer is not correctly aligned for the referenced
>> type, the behavior is undefined. [...]" There's no guarantee
>> that the alignment of `buffer' suffices for `Mystruct'.
>
> On further consideration, how does one know what "correctly aligned"
> means for a given struct? Consider these two examples:
>
> typedef struct {
> uint16_t one;
> uint16_t two;
> uint16_t three;
> uint16_t four;
> } Mystruct4;
>
Taking into account the various answers in this thread to the preceding
question it would seem that there is still no safe (safe meaning here
"code that will work on any platform") method to use a C struct
to extract binary data directly from memory, since the compiler can
throw in padding and alignment requirements whenever it feels like it.
Even if the data is reduced to byte representation, something like this
(I know it isn't the same as Mystruct4 above):
typedef struct {
uint8_t one[2]; /* actually uint16_t */
uint8_t two[4]; /* actually uint32_t */
int8_t three[2]; /* etc. */
int8_t four;
} Mystruct4b;
it seems not to be safe to pass a "Mystruct4b *" pointer to a function
which references this data at an arbitrary location in memory. Instead
the only safe method is to pass "char *" pointers and take the data
apart with memcpy() at a very low level, moving it from memory to the
structure, or vice versa.
What an odd situation. I would have thought with all of the data that
does show up in C programming as "a series of bytes in a buffer arranged
in some particular manner", that is, pretty much any data which is
passed from machine to machine, C would have developed a method for
simplifying this sort of code. Perhaps something along these lines
/* declare memory organization, no padding, no alignment requirement */
typedef memstruct {
uint16_t one;
uint32_t two;
int16_t three;
uint8_t four;
} Mystruct4c;
Specifically so that one could pass a "Mystruct4c *" pointer to a
function, like so:
myfunction3a(Mystruct4c *ptr){
ptr->two = 5;
printf("value of three:%d\n",ptr->three);
}
and the compiler would do the "right thing", using memcpy or whatever,
to hide all of the cruft that is platform specific, up to and including
loading the magic N bytes of memory (as in the Alpha issue Glen
mentioned), shifting, and masking and so forth, to handle data types
smaller than is native for the CPU, without the programmer having to
ever care about the details. One might even dream that the compiler
could be induced to do:
Mystruct4c native,ncopy;
Mystruct4b *foreign
Mystruct4b acopy;
/* field names/sizes must match for the following statement */
alternate_representations {Mystruct4b, Mystruct4c}
char *buffer;
/* fill buffer from a file or network */
foreign = (Mystruct4b *)buffer[123];
native = *foreign; /* field to field copy, NOT a memcpy*/
/* change some data in native*/
*foreign = native; /* field to field copy */
ncopy = native; /* this is a memcpy */
acopy = *foreign; /* as is this */
*foreign = acopy; /* as is this */
ncopy = acopy; /* field to field copy */
That is, when a memstruct pointer is dereferenced it does not mean quite
the same thing as when a struct pointer is dereferenced. A few more
rules for the compiler, a lot less work for the programmer.
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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