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| From | Ken Brody <kenbrody@spamcop.net> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: I want to ask a question... |
| Date | 2015-08-10 12:46 -0400 |
| Organization | A noiseless patient Spider |
| Message-ID | <mqakda$srg$1@dont-email.me> (permalink) |
| References | <mq2uol$dpj$3@dont-email.me> <87pp2y7wl7.fsf@gmail.com> <lnmvy2g291.fsf@kst-u.example.com> |
On 8/7/2015 6:30 PM, Keith Thompson wrote: > Gareth Owen <gwowen@gmail.com> writes: >> Ramine <ramine@1.1> writes: >>> Suppose that you are using a pointer that points to an allocated chunk >>> of memory, and suppose that you deallocated this chunk of memory and >>> you have keeped the old address of this pointer inside another pointer >>> , is there a problem to access the old address of the pointer and >>> content of the memory even if it was deallocated ? >> >> You can access the pointer itself, but you can't access the memory it >> points to. Sometimes it might work - and it simple programs it probably >> will work more ofhen than not - but you can't rely on it. >> >> Don't do that. > > That's not quite correct. > > Accessing a pointer value after the object it points to no longer exists > has undefined behavior, even if you don't dereference it. It's not > likely to do anything nasty on most systems, but in principle an > implementation could mark the pointer value as invalid and trap on any > reference to it. [...] In old Intel "386 protected mode", where memory is accessed via a selector+offset, merely loading a selector register with an invalid value would trigger a fault. In an implementation where "far" allocated memory would be given its own selector, free()ing that memory would make the selector invalid. Unless a subsequent allocation gave the same selector, simply accessing the pointer, even without dereferencing it, could trigger that fault. (Actually, I think that may still be true, but given today's 32- and 64-bit offsets, the notion of "far pointers" is usually irrelevant.) -- Kenneth Brody
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Re: I want to ask a question... Gareth Owen <gwowen@gmail.com> - 2015-08-07 20:00 +0100
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-07 19:04 +0000
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-07 15:30 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-07 22:52 +0000
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-07 16:46 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-07 23:29 +0000
Re: I want to ask a question... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-08-08 01:37 +0100
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-07 18:55 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-08 05:51 +0000
Re: I want to ask a question... Robert Wessel <robertwessel2@yahoo.com> - 2015-08-08 01:44 -0500
Re: I want to ask a question... Ken Brody <kenbrody@spamcop.net> - 2015-08-10 12:46 -0400
Re: I want to ask a question... supercat@casperkitty.com - 2015-08-19 16:43 -0700
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-19 18:09 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-20 01:00 +0000
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-19 20:25 -0700
Re: I want to ask a question... supercat@casperkitty.com - 2015-08-20 08:15 -0700
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-20 11:38 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-20 18:47 +0000
Re: I want to ask a question... supercat@casperkitty.com - 2015-08-20 14:01 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-20 16:43 +0000
Re: I want to ask a question... James Kuyper <jameskuyper@verizon.net> - 2015-08-20 13:02 -0400
Re: I want to ask a question... Tonton Th <tTh@nowhere.invalid> - 2015-08-20 17:40 +0000
Re: I want to ask a question... James Kuyper <jameskuyper@verizon.net> - 2015-08-20 15:27 -0400
Re: I want to ask a question... David Brown <david.brown@hesbynett.no> - 2015-08-20 22:58 +0200
Re: I want to ask a question... James Kuyper <jameskuyper@verizon.net> - 2015-08-20 23:24 -0400
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-21 06:19 +0000
Re: I want to ask a question... Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-08-21 09:20 -0600
Re: I want to ask a question... supercat@casperkitty.com - 2015-08-22 12:25 -0700
Re: I want to ask a question... Robert Wessel <robertwessel2@yahoo.com> - 2015-08-23 01:33 -0500
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-20 19:28 +0000
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-20 12:11 -0700
Re: I want to ask a question... David Brown <david.brown@hesbynett.no> - 2015-08-20 10:02 +0200
Re: I want to ask a question... raltbos@xs4all.nl (Richard Bos) - 2015-08-22 13:42 +0000
Re: I want to ask a question... supercat@casperkitty.com - 2015-08-22 12:23 -0700
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