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Groups > comp.lang.c > #138116
| From | Ben Bacarisse <ben.usenet@bsb.me.uk> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] |
| Date | 2018-11-19 15:06 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <87y39pgl8q.fsf@bsb.me.uk> (permalink) |
| References | <20181117143819.43938ee34006bbcc1826bf0c@gmail.com> <20181119022614.5cf93af72a29cb41cdc86910@gmail.com> <87va4tiat8.fsf@bsb.me.uk> <20181119153734.485dbb156e26d9843e8305f6@g{oogle}mail.com> |
Anton Shepelev <anton.txt@g{oogle}mail.com> writes:
> Ben Bacarisse to Anton Sheplev:
>
>>Only the size element might be a problem -- the others are
>>guaranteed to be aligned. And below, I argue that all
>>three should be of the same type anyway and hence they will
>>all be aligned.
>
> What problem can there be if I access all three through a
> char*? My only consideration about their alignment was that
> of performance. Is it the problem you refer to?
Yes. Only the size (in the header) might be misaligned, so only the
size needs special access.
The data are also possibly miss-aligned. If you want to use old C, you
will probably have to say what assumptions you are prepared to make.
>>Alignment might be an issue for the data, though. From the
>>point of view of portable access the optimal layout is
>>
>> +----------+----------+----------+---+--- ------ --+
>> | length | capacity | size |...| ... data ... |
>> +----------+----------+----------+---+--- ------ --+
>> ^
>> | padding
>>
>>where a possibly zero-sized gap is left after the three
>>equal-sized meta-data items so as to re-gain maximal
>>alignment.
>
> I hadn't thought of this. Is it right that a pointer to int
> cannot contain an arbitrary byte address and work correctly?
> In other words, can I access an allocated object throught a
> pointer to int from an arbitrary byte offset?
Not in 100% portable code.
>>The simplest way to achieve this in modern C define a
>>struct for the header:
>>
>> struct meta_data {
>> int length;
>> int capacity;
>> size_t size;
>> max_align_t data[];
>> };
>
> Do you propose to use this header for the calculation of
> offsets or for the actual access to the array's data and
> metadata?
For the access. Why would you not use the C features that are designed
for this purpose? Calculating offsets and converting pointers looks so
last century!
<snip>
>>>union intalias
>>>{ char cdata[sizeof(int)];
>>> int idata;
>>>};
>>
>>I don't see the point of this union. The effective type of
>>the allocated storage is determined by the first modifying
>>access, so if you store an int at start, that will be the
>>effective type of that part of the storage.
>
> As far I as have understood, the effective type is
> determined by the *last* non-character modyfying access,
> which is why I have gone to great length to access metadata
> exclusively through a char*. I have found no other way to
> access metadata without breaking the rules of aliasing.
But why not access it as what it is? It's int (or size_t) data so use
an lvalue expression of that type.
<snip>
>>>#define SHIFT(A,S) (((char*)A) - (S))
>>>#define OFFSET ( sizeof(size_t) + 2*sizeof(int))
>>>#define ORIGIN(A) SHIFT(A, OFFSET)
>>>#define GETLEN(A) getint (ORIGIN(A))
>>>#define GETSIZE(A) getsize(SHIFT((A),sizeof(size_t)))
>>>#define GETCAP(A) getint (SHIFT((A),sizeof(size_t)+sizeof(int)))
>>>#define SETLEN(A,L) setint (ORIGIN(A),(L))
>>>#define SETSIZE(A,S) setsize(SHIFT((A),sizeof(size_t)),S)
>>>#define SETCAP(A,C) setint (SHIFT((A),sizeof(size_t)+sizeof(int)),(C))
>>
>>I'd simplify all this with a struct for the header.
>
> I am sure the rules of aliasing allow it.
Yes, so am I. I may not follow every detail, but if you allocate a
struct (even with some extra data added on) you can certainly access the
members of such a struct at the start of the allocated storage.
--
Ben.
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Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-17 14:38 +0300
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-17 17:37 +0300
Re: Pointer conversion and alignment Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-17 21:01 +0000
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-18 00:43 +0300
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-18 01:20 +0300
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-11-17 18:12 -0500
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-18 02:32 +0300
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-11-17 18:59 -0500
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-18 12:40 +0300
Re: Pointer conversion and alignment Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-18 02:14 +0000
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-18 13:23 +0300
Re: Pointer conversion and alignment Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-18 11:28 +0000
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-18 15:31 +0300
Re: Pointer conversion and alignment Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-18 15:03 +0000
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-18 23:00 +0300
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-18 00:49 +0300
Re: Pointer conversion and alignment Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2018-11-17 06:52 -0800
Re: Pointer conversion and alignment Ian Collins <ian-news@hotmail.com> - 2018-11-18 09:18 +1300
Re: Pointer conversion and alignment Keith Thompson <kst-u@mib.org> - 2018-11-17 13:49 -0800
Re: Pointer conversion and alignment Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2018-11-17 13:57 -0800
Re: Pointer conversion and alignment Ian Collins <ian-news@hotmail.com> - 2018-11-18 11:08 +1300
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-18 01:13 +0300
Re: Pointer conversion and alignment Keith Thompson <kst-u@mib.org> - 2018-11-17 18:37 -0800
Re: Pointer conversion and alignment Philipp Klaus Krause <pkk@spth.de> - 2018-11-18 14:14 +0100
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-11-17 10:05 -0500
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-18 01:08 +0300
Re: Pointer conversion and alignment "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-11-17 14:31 -0800
Re: Pointer conversion and alignment "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-11-17 16:10 -0800
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-18 15:01 +0300
Re: Pointer conversion and alignment James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-11-18 07:25 -0500
Re: Pointer conversion and alignment "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-11-19 21:30 -0800
Re: Pointer conversion and alignment "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-11-20 14:20 -0800
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-11-17 18:04 -0500
Re: Pointer conversion and alignment Philipp Klaus Krause <pkk@spth.de> - 2018-11-18 14:18 +0100
Re: Pointer conversion and alignment scott@slp53.sl.home (Scott Lurndal) - 2018-11-18 19:49 +0000
Re: Pointer conversion and alignment Tim Rentsch <txr@alumni.caltech.edu> - 2018-11-23 18:27 -0800
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-11-23 21:59 -0500
Re: Pointer conversion and alignment Tim Rentsch <txr@alumni.caltech.edu> - 2018-11-24 00:32 -0800
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-11-24 08:17 -0500
Re: Pointer conversion and alignment Tim Rentsch <txr@alumni.caltech.edu> - 2018-11-29 09:55 -0800
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-11-29 21:48 -0500
Re: Pointer conversion and alignment Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-05 04:14 -0800
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-12-05 08:14 -0500
Re: Pointer conversion and alignment Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2018-12-05 05:29 -0800
Re: Pointer conversion and alignment David Brown <david.brown@hesbynett.no> - 2018-12-05 14:44 +0100
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-12-05 21:58 -0500
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-12-05 16:38 +0300
Re: Pointer conversion and alignment scott@slp53.sl.home (Scott Lurndal) - 2018-12-05 14:23 +0000
Re: Pointer conversion and alignment Keith Thompson <kst-u@mib.org> - 2018-12-05 14:35 -0800
Re: Pointer conversion and alignment Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-08 19:28 -0800
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-12-09 08:18 -0500
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-12-09 14:35 -0500
Re: Pointer conversion and alignment Keith Thompson <kst-u@mib.org> - 2018-12-09 14:04 -0800
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-12-09 20:40 -0500
Re: Pointer conversion and alignment Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-17 07:18 -0800
Re: Pointer conversion and alignment Keith Thompson <kst-u@mib.org> - 2018-12-17 12:35 -0800
Re: Pointer conversion and alignment Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-17 12:48 -0800
Re: Pointer conversion and alignment "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-11-24 18:33 -0800
My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-19 02:26 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-19 11:08 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-11-19 15:37 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-11-19 15:39 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-19 15:07 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-19 22:24 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] jameskuyper@alumni.caltech.edu - 2018-11-19 12:33 -0800
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-20 00:34 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] jameskuyper@alumni.caltech.edu - 2018-11-19 14:38 -0800
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-20 00:43 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-11-19 21:44 -0500
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] jadill33@gmail.com - 2018-11-20 07:11 -0800
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] bart4858@gmail.com - 2018-11-20 10:02 -0800
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-19 22:20 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-11-20 14:01 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Melzzzzz <Melzzzzz@zzzzz.com> - 2018-11-20 11:07 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-11-20 15:12 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-11-20 08:15 -0500
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-20 23:08 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] jameskuyper@alumni.caltech.edu - 2018-11-20 14:13 -0800
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-11-21 16:38 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-11-21 09:05 -0500
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-11-21 20:28 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] jameskuyper@alumni.caltech.edu - 2018-11-21 12:58 -0800
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-22 01:47 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] jameskuyper@alumni.caltech.edu - 2018-11-21 15:50 -0800
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-24 19:37 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-21 21:22 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-21 01:15 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-19 15:06 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] mark.bluemel@gmail.com - 2018-11-19 08:05 -0800
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] scott@slp53.sl.home (Scott Lurndal) - 2018-11-19 16:49 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] jameskuyper@alumni.caltech.edu - 2018-11-19 10:35 -0800
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-20 00:45 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] jameskuyper@alumni.caltech.edu - 2018-11-19 14:24 -0800
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-11-20 15:05 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-11-20 08:31 -0500
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Richard Damon <Richard@Damon-Family.org> - 2018-11-20 20:02 -0500
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-21 22:47 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] jameskuyper@alumni.caltech.edu - 2018-11-21 13:08 -0800
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-22 00:31 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-21 21:36 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-11-19 16:04 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-19 14:56 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-24 19:24 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-24 21:31 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-24 21:46 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-25 01:12 +0300
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-11-25 01:39 +0000
Re: My dynamic arrays module [was:Re: Pointer conversion and alignment] Anton Shepelev <anton.txt@gmail.com> - 2018-11-25 14:19 +0300
Re: Pointer conversion and alignment jadill33@gmail.com - 2018-11-19 08:44 -0800
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-20 00:02 +0300
Re: Pointer conversion and alignment Keith Thompson <kst-u@mib.org> - 2018-11-19 13:51 -0800
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-20 01:08 +0300
Re: Pointer conversion and alignment jameskuyper@alumni.caltech.edu - 2018-11-19 14:31 -0800
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-11-20 15:21 +0300
Re: Pointer conversion and alignment James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-11-20 08:59 -0500
Re: Pointer conversion and alignment Keith Thompson <kst-u@mib.org> - 2018-11-20 11:06 -0800
Re: Pointer conversion and alignment jameskuyper@alumni.caltech.edu - 2018-11-20 12:43 -0800
Re: Pointer conversion and alignment Keith Thompson <kst-u@mib.org> - 2018-11-20 14:33 -0800
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-11-21 14:33 +0300
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-11-21 07:49 -0500
Re: Pointer conversion and alignment Keith Thompson <kst-u@mib.org> - 2018-11-21 12:42 -0800
Re: Pointer conversion and alignment James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-11-21 08:41 -0500
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-25 15:06 +0300
Re: Pointer conversion and alignment James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-11-25 10:47 -0500
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-11-26 12:42 +0300
Re: Pointer conversion and alignment Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2018-11-26 03:07 -0800
Re: Pointer conversion and alignment Keith Thompson <kst-u@mib.org> - 2018-11-26 12:56 -0800
Re: Pointer conversion and alignment Richard Damon <Richard@Damon-Family.org> - 2018-11-26 18:17 -0500
Re: Pointer conversion and alignment jameskuyper@alumni.caltech.edu - 2018-11-26 09:06 -0800
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-26 23:52 +0300
Re: Pointer conversion and alignment jameskuyper@alumni.caltech.edu - 2018-11-26 13:13 -0800
Re: Pointer conversion and alignment Anton Shepelev <anton.txt@gmail.com> - 2018-11-27 00:31 +0300
Re: Pointer conversion and alignment jameskuyper@alumni.caltech.edu - 2018-11-26 14:15 -0800
Re: Pointer conversion and alignment jadill33@gmail.com - 2018-11-19 14:33 -0800
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