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| From | glen herrmannsfeldt <gah@ugcs.caltech.edu> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Data alignment questin, structures |
| Date | 2013-01-18 21:11 +0000 |
| Organization | Aioe.org NNTP Server |
| Message-ID | <kdcdqm$301$1@speranza.aioe.org> (permalink) |
| References | <kcs982$r3p$1@dont-email.me> <kcshq5$7e8$1@dont-email.me> <kdc426$f0i$1@dont-email.me> |
mathog <dmathog@gmail.com> wrote:
> Eric Sosman wrote:
>>> 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.
(snip)
> 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;
> typedef struct {
> uint16_t one;
> uint16_t two;
> uint16_t three;
> uint16_t four;
> uint16_t five;
> } Mystruct5;
> The first would presumably be 8 bytes and the second 10. Is it safe to
> access these in a memory buffer via a pointer so long as that pointer
> is aligned on a 2 byte boundary?
Consider Alpha, where all memory accesss are on four byte boundaries.
Now, there are special instructions to access 16 byte quantities on
two byte, or even (I believe) one byte boundaries, but it seems to me
that the compiler could optimize this case if it knew the structs
was allocated on a four byte boundary, and padded the end, such that
arrays of the struct aligned them on four byte boundaries. (Or,
similarly, for other boundaries.)
> I would assume yes because all of the
> data within each struct has that alignment, and I am also assuming that
> this code should always work:
> Mystruct5 array[10]; /* valid */
> Mystruct5 *aptr;
> uint16_t x;
> array[1].four=20;
> aptr = &array[1];
> /* the next three lines should all print: 1,4 is:20 */
> printf("1,4 is:%u\n",aptr->five);
> somefunction1(aptr);
> somefunction2(*aptr);
> ...
> void somefunction1(Mystruct5 *aptr){
> printf("1,4 is:%u\n",aptr->four);
> }
> void somefunction2(Mystruct5 aptr){
> printf("1,4 is:%u\n",aptr.four);
> }
> Or may a compiler make some assumption for Mystruct4 that requires it to
> be aligned on a 4 byte boundary too? That would not prevent it from
> being used in an array, and would not break the above example (after
> changing Mystruct5 -> Mystruct4). But it would break both of the two
> function calls if aptr was pointing to a buffer in memory where the data
> was only 2 byte aligned for the structure. Conversely, adding a 4 byte
> boundary alignment requirement on Mystruct5 would add two pad bytes, for
> no obvious reason, but it would not break the above code example. I see
> no reason for the compiler to add the 4 byte alignment
> requirement for these structures, but is it nevertheless free to do so?
Well, on 32 bit word addressed machines it is somewhat obvious what
might happen. Alpha is byte addressed but, at least until MS got into
it, required access addresses to be multiples of four.A
Given a pointer to an arbitrary (16 bit) short, the compiler has to use
the general case, but it seems to me that it could optimize the struct
access case.
-- glen
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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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