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| From | David Brown <david.brown@hesbynett.no> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: local arrays and stack |
| Date | 2015-03-24 09:55 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <mer8p1$7tk$1@dont-email.me> (permalink) |
| References | (6 earlier) <mepdmh$qso$1@dont-email.me> <87fv8vu1sx.fsf@kst-u.example.com> <mepfa1$26r$1@dont-email.me> <1f3963cb-74d2-4b38-9241-fd6ce6c5179c@googlegroups.com> <mepjd8$kb1$1@dont-email.me> |
On 23/03/15 18:44, Bartc wrote: > On 23/03/2015 17:12, Xcott Craver wrote: >> On Monday, March 23, 2015 at 12:35:07 PM UTC-4, Bart wrote: >>> >>> I don't know what gcc in particular does. But if allocating a VLA, >>> perhaps inside a block which is the body of a loop, and the size is >>> different each time, then wouldn't it have to take of such details >>> anyway? Including dealing with goto, break, longjmp and return. >> >> If doing this on the stack, allocating a VLA means decrementing the >> stack pointer, and deallocating it means resetting the stack pointer >> to a previous value. > > How? The VLA size is a runtime expression. If it's a complicated one, > then it would need to be stored somewhere (presumably also on the > stack), as it would be awkward to push the stack pointer itself. Just assume that the VLA is allocated with alloca() rather than malloc() - with all the advantages and disadvantages that implies. Ideally, this means copying the SP to the BP register (so that the compiler can access data already on the stack with BP+offset after SP changes), then change SP to make enough space on the stack for the VLA. Deallocation is done by copying the BP back into the SP. This is exactly the same procedure as used whenever a function needs a full stack frame (rather than the simplified versions typically used these days). Jumping around inside the function (goto, break, etc.) or leaving the function is handled just like normal. Longjmp and setjmp are messy as always, but not worse than usual. > > See, already it's a bit more elaborate than it looks at first ... > >> Even if you are declaring an array in a loop with a different length >> each time, the allocation code should not require anything more >> complicated than altering a register at the beginning of the loop, >> and altering it again at the end. > > ... and it can get worse: > > With gcc, local arrays with a fixed size over about 4KB involve a > function call to ensure the remaining stack is big enough (so that there > is no page fault). That will depend on the target and the OS - maybe Windows requires some sort of check like that. In general, however, a compiler will simply pretend that the stack is as big as needed and rely on the OS and MMU to sort out the details (such as killing the program if it breaks the rules of the OS). > > What about VLAs over 4KB? The trouble is it won't know whether a VLA is > bigger than 4KB, and whether to call that __chkstk_ms() function, until > runtime. Now it gets complicated enough that using malloc()/free() might > be a simpler solution. >
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local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-22 01:51 -0700
local arrays and stack asetofsymbols@gmail.com - 2015-03-22 02:23 -0700
Re: local arrays and stack Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-03-22 13:22 -0700
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-23 01:55 -0700
Re: local arrays and stack Xcott Craver <xcottcraver@gmail.com> - 2015-03-22 15:07 -0700
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-23 01:50 -0700
Re: local arrays and stack Robert Wessel <robertwessel2@yahoo.com> - 2015-03-23 04:26 -0500
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-23 06:53 -0700
Re: local arrays and stack Xcott Craver <xcottcraver@gmail.com> - 2015-03-23 07:29 -0700
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-23 08:34 -0700
Re: local arrays and stack Robert Wessel <robertwessel2@yahoo.com> - 2015-03-23 19:57 -0500
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-24 04:01 -0700
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-24 04:37 -0700
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-24 13:04 +0100
Re: local arrays and stack glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-03-24 17:29 +0000
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-24 20:01 +0100
Re: local arrays and stack Keith Thompson <kst-u@mib.org> - 2015-03-23 08:24 -0700
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-23 08:42 -0700
Re: local arrays and stack Bartc <bc@freeuk.com> - 2015-03-23 16:07 +0000
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-23 09:15 -0700
Re: local arrays and stack Keith Thompson <kst-u@mib.org> - 2015-03-23 09:16 -0700
Re: local arrays and stack Bartc <bc@freeuk.com> - 2015-03-23 16:34 +0000
Re: local arrays and stack Xcott Craver <xcottcraver@gmail.com> - 2015-03-23 10:12 -0700
Re: local arrays and stack Bartc <bc@freeuk.com> - 2015-03-23 17:44 +0000
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-23 11:47 -0700
Re: local arrays and stack Xcott Craver <xcottcraver@gmail.com> - 2015-03-23 12:06 -0700
Re: local arrays and stack Bartc <bc@freeuk.com> - 2015-03-23 20:25 +0000
Re: local arrays and stack Xcott Craver <xcottcraver@gmail.com> - 2015-03-23 14:10 -0700
Re: local arrays and stack Bartc <bc@freeuk.com> - 2015-03-23 22:22 +0000
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-23 23:57 -0700
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-24 10:28 +0100
Re: local arrays and stack Bartc <bc@freeuk.com> - 2015-03-24 10:44 +0000
Re: local arrays and stack Martin Shobe <martin.shobe@yahoo.com> - 2015-03-24 06:38 -0500
Re: local arrays and stack Martin Shobe <martin.shobe@yahoo.com> - 2015-03-24 09:57 -0500
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-24 13:22 +0100
Re: local arrays and stack Bartc <bc@freeuk.com> - 2015-03-25 16:07 +0000
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-25 17:42 +0100
Re: local arrays and stack Bartc <bc@freeuk.com> - 2015-03-25 19:06 +0000
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-25 12:26 -0700
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-25 13:20 -0700
Re: local arrays and stack Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-03-25 12:28 -0700
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-25 21:45 +0100
Re: local arrays and stack Keith Thompson <kst-u@mib.org> - 2015-03-25 12:50 -0700
Re: local arrays and stack Bartc <bc@freeuk.com> - 2015-03-25 22:33 +0000
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-25 23:57 +0100
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-25 21:42 +0100
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-25 09:55 -0700
Re: local arrays and stack Keith Thompson <kst-u@mib.org> - 2015-03-25 12:36 -0700
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-25 13:04 -0700
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-25 21:52 +0100
Re: local arrays and stack Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-03-25 15:18 -0700
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-26 00:02 +0100
Re: local arrays and stack Melzzzzz <mel@zzzzz.com> - 2015-03-26 00:47 +0100
Re: local arrays and stack Mel <mel@zzzzz.com> - 2015-03-24 14:58 +0100
Re: local arrays and stack Xcott Craver <xcottcraver@gmail.com> - 2015-03-24 06:58 -0700
Re: local arrays and stack Keith Thompson <kst-u@mib.org> - 2015-03-23 14:34 -0700
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-24 10:16 +0100
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-24 09:55 +0100
Re: local arrays and stack Noob <root@127.0.0.1> - 2015-03-24 10:23 +0100
Re: local arrays and stack Tim Rentsch <txr@alumni.caltech.edu> - 2015-03-24 12:18 -0700
Re: local arrays and stack Keith Thompson <kst-u@mib.org> - 2015-03-24 13:42 -0700
Re: local arrays and stack Tim Rentsch <txr@alumni.caltech.edu> - 2015-03-25 08:47 -0700
Re: local arrays and stack Xcott Craver <xcottcraver@gmail.com> - 2015-03-23 09:32 -0700
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-23 09:57 -0700
Re: local arrays and stack Keith Thompson <kst-u@mib.org> - 2015-03-23 10:37 -0700
Re: local arrays and stack Xcott Craver <xcottcraver@gmail.com> - 2015-03-23 10:49 -0700
Re: local arrays and stack Keith Thompson <kst-u@mib.org> - 2015-03-23 12:48 -0700
Re: local arrays and stack jt@toerring.de (Jens Thoms Toerring) - 2015-03-23 22:29 +0000
Re: local arrays and stack James Kuyper <jameskuyper@verizon.net> - 2015-03-23 19:03 -0400
Re: local arrays and stack jt@toerring.de (Jens Thoms Toerring) - 2015-03-23 23:36 +0000
Re: local arrays and stack fir <profesor.fir@gmail.com> - 2015-03-24 02:17 -0700
Re: local arrays and stack Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-03-23 11:56 -0700
Re: local arrays and stack Robert Wessel <robertwessel2@yahoo.com> - 2015-03-23 20:00 -0500
Re: local arrays and stack David Brown <david.brown@hesbynett.no> - 2015-03-24 10:39 +0100
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