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Groups > comp.lang.c > #163555
| From | "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: on why declare a struct with a single array in it |
| Date | 2021-11-21 12:59 -0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <snebus$mv8$1@dont-email.me> (permalink) |
| References | (11 earlier) <Y2UlJ.79170$ya3.75666@fx38.iad> <874k87q35r.fsf@nosuchdomain.example.com> <snaq53$voh$1@dont-email.me> <875ysmjsfz.fsf@nosuchdomain.example.com> <sndi8b$3ht$1@dont-email.me> |
On 11/21/2021 5:40 AM, David Brown wrote: > On 21/11/2021 00:52, Keith Thompson wrote: >> David Brown <david.brown@hesbynett.no> writes: >>> On 19/11/2021 21:52, Keith Thompson wrote: >> [...] >>>> Perhaps -- but typically that information is not stored in the pointer >>>> itself. If it were, all void* values would have to store that extra >>>> information. >>>> >>>> (I think it's easier to store bookkeeping data before the allocated >>>> space. If you store it after the space, it's hard to know where it is.) >>> >>> That's the most common way for a simple malloc/free system. >>> >>> It has always struck me as a mistake that C did things that way - in my >>> opinion, "free" should have taken an additional parameter of the size >>> for the deallocation. >> >> Meh. That would have introduced another rich source of programming >> errors (passing the wrong size). Probably most implementations would >> have kept track of the size anyway, so they could detect errors. As >> long as they're keeping track, why make the user do so as well? > > It would have lead to some different programming errors, perhaps, but I > don't think it would have lead to more. If you are at the point in your > code where you can correctly call "free(p);", it seems highly unlikely > that you don't also have the size of the allocation there too. (To be > clear, I expect you to have the size used when calling malloc - if the > heap implementation rounds that up to a block size in some way, then it > would do the same for sized free.) > > You know the size used in your malloc() call. Either you are getting > space for a single object (such as a large struct), or an array - and > you know the size when you are using the object or array later. [...] Fwiw, I am quite fond of creating a big chunk of memory, and aligning it on a "large" boundary. Say 4096*4 bytes... Now, every address in the memory can be rounded down to get at the boundary. There is a structure sitting right before this boundary, a header: [header][aligned_memory] Every memory allocation less than the size of a pointer, is rounded up to the size of a pointer. So, on a 32-bit system, 0...3 are rounded up to 4. That's it. Now, wrt freeing something, we take the address, round down to boundary, subtract the header, then we are at the header that can contain a free list to link the pointer in. It's quite nice, and can be on a per-thread basis. headers per-thread... ;^)
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on why declare a struct with a single array in it Meredith Montgomery <mmontgomery@levado.to> - 2021-11-12 15:48 -0300
Re: on why declare a struct with a single array in it Guillaume <message@bottle.org> - 2021-11-12 19:56 +0100
Re: on why declare a struct with a single array in it Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-11-12 19:31 +0000
Re: on why declare a struct with a single array in it David Brown <david.brown@hesbynett.no> - 2021-11-12 21:07 +0100
Re: on why declare a struct with a single array in it pozz <pozzugno@gmail.com> - 2021-11-18 15:53 +0100
Re: on why declare a struct with a single array in it David Brown <david.brown@hesbynett.no> - 2021-11-18 16:13 +0100
Re: on why declare a struct with a single array in it Manfred <noname@add.invalid> - 2021-11-18 16:26 +0100
Re: on why declare a struct with a single array in it pozz <pozzugno@gmail.com> - 2021-11-18 17:22 +0100
Re: on why declare a struct with a single array in it Manfred <noname@add.invalid> - 2021-11-18 18:04 +0100
Re: on why declare a struct with a single array in it Meredith Montgomery <mmontgomery@levado.to> - 2021-11-19 16:00 -0300
Re: on why declare a struct with a single array in it David Brown <david.brown@hesbynett.no> - 2021-11-18 22:58 +0100
Re: on why declare a struct with a single array in it Guillaume <message@bottle.org> - 2021-11-19 17:53 +0100
Re: on why declare a struct with a single array in it Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-19 11:27 -0800
Re: on why declare a struct with a single array in it scott@slp53.sl.home (Scott Lurndal) - 2021-11-19 20:46 +0000
Re: on why declare a struct with a single array in it Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-19 12:52 -0800
Re: on why declare a struct with a single array in it David Brown <david.brown@hesbynett.no> - 2021-11-20 13:36 +0100
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-11-20 14:49 +0000
Re: on why declare a struct with a single array in it Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-20 15:52 -0800
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-11-21 00:19 +0000
Re: on why declare a struct with a single array in it Richard Damon <Richard@Damon-Family.org> - 2021-11-20 20:44 -0500
Re: on why declare a struct with a single array in it David Brown <david.brown@hesbynett.no> - 2021-11-21 14:51 +0100
Re: on why declare a struct with a single array in it Manfred <noname@add.invalid> - 2021-11-21 19:32 +0100
Re: on why declare a struct with a single array in it David Brown <david.brown@hesbynett.no> - 2021-11-21 21:51 +0100
Re: on why declare a struct with a single array in it Manfred <noname@add.invalid> - 2021-11-22 03:17 +0100
Re: on why declare a struct with a single array in it David Brown <david.brown@hesbynett.no> - 2021-11-22 11:31 +0100
Re: on why declare a struct with a single array in it Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-12-10 04:21 -0800
Re: on why declare a struct with a single array in it David Brown <david.brown@hesbynett.no> - 2021-12-10 14:43 +0100
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-11-22 11:56 +0000
Re: on why declare a struct with a single array in it Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-10 01:47 -0800
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-12-10 10:55 +0000
Re: on why declare a struct with a single array in it David Brown <david.brown@hesbynett.no> - 2021-12-10 14:51 +0100
Re: on why declare a struct with a single array in it Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-11 06:56 -0800
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-12-11 16:04 +0000
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-12-11 17:23 +0000
Re: on why declare a struct with a single array in it Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-21 05:30 -0800
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2022-01-21 13:49 +0000
Re: on why declare a struct with a single array in it "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-12-11 12:17 -0800
Re: on why declare a struct with a single array in it gazelle@shell.xmission.com (Kenny McCormack) - 2021-12-12 13:00 +0000
Re: on why declare a struct with a single array in it Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-12 08:25 -0800
Re: on why declare a struct with a single array in it gazelle@shell.xmission.com (Kenny McCormack) - 2021-12-12 17:38 +0000
Re: on why declare a struct with a single array in it Dick <thiebauddick2@aol.com> - 2021-12-12 16:19 -0500
Re: on why declare a struct with a single array in it gazelle@shell.xmission.com (Kenny McCormack) - 2021-12-13 08:06 +0000
Re: on why declare a struct with a single array in it Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-12-13 01:30 -0800
Re: on why declare a struct with a single array in it David Brown <david.brown@hesbynett.no> - 2021-12-13 09:34 +0100
Re: on why declare a struct with a single array in it Robert Latest <boblatest@yahoo.com> - 2021-12-13 18:53 +0000
Re: on why declare a struct with a single array in it gazelle@shell.xmission.com (Kenny McCormack) - 2021-12-13 20:38 +0000
Re: on why declare a struct with a single array in it Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-15 00:38 -0800
Re: on why declare a struct with a single array in it gazelle@shell.xmission.com (Kenny McCormack) - 2021-12-15 11:21 +0000
Re: on why declare a struct with a single array in it Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-15 10:26 -0800
Re: on why declare a struct with a single array in it Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-15 10:45 -0800
Re: on why declare a struct with a single array in it Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-10 10:52 -0800
Re: on why declare a struct with a single array in it Guillaume <message@bottle.org> - 2021-11-22 04:06 +0100
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-11-22 11:32 +0000
Re: on why declare a struct with a single array in it Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-22 13:00 -0800
Re: on why declare a struct with a single array in it "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-11-21 13:08 -0800
Re: on why declare a struct with a single array in it Richard Damon <Richard@Damon-Family.org> - 2021-11-20 20:44 -0500
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-11-21 12:22 +0000
Re: on why declare a struct with a single array in it Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2021-11-20 20:13 -0700
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-11-21 12:35 +0000
Re: on why declare a struct with a single array in it Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2021-11-21 10:53 -0700
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-11-21 18:49 +0000
Re: on why declare a struct with a single array in it "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-11-21 13:04 -0800
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-11-21 14:42 +0000
Re: on why declare a struct with a single array in it Guillaume <message@bottle.org> - 2021-11-21 01:34 +0100
Re: on why declare a struct with a single array in it Bart <bc@freeuk.com> - 2021-11-21 00:48 +0000
Re: on why declare a struct with a single array in it "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-11-20 16:58 -0800
Re: on why declare a struct with a single array in it David Brown <david.brown@hesbynett.no> - 2021-11-21 14:40 +0100
Re: on why declare a struct with a single array in it "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-11-21 12:59 -0800
Re: on why declare a struct with a single array in it scott@slp53.sl.home (Scott Lurndal) - 2021-11-21 15:58 +0000
Re: on why declare a struct with a single array in it gazelle@shell.xmission.com (Kenny McCormack) - 2021-11-20 10:55 +0000
Re: on why declare a struct with a single array in it Jorgen Grahn <grahn+nntp@snipabacken.se> - 2021-11-13 08:47 +0000
Re: on why declare a struct with a single array in it Guillaume <message@bottle.org> - 2021-11-13 18:23 +0100
Re: on why declare a struct with a single array in it Kaz Kylheku <480-992-1380@kylheku.com> - 2021-11-13 17:55 +0000
Re: on why declare a struct with a single array in it Anton Shepelev <anton.txt@g{oogle}mail.com> - 2021-11-19 13:14 +0300
Re: on why declare a struct with a single array in it Meredith Montgomery <mmontgomery@levado.to> - 2021-11-19 16:07 -0300
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