Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]
| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: I want to ask a question... |
| Date | 2015-08-07 15:30 -0700 |
| Organization | None to speak of |
| Message-ID | <lnmvy2g291.fsf@kst-u.example.com> (permalink) |
| References | <mq2uol$dpj$3@dont-email.me> <87pp2y7wl7.fsf@gmail.com> |
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.
An example:
#include <stdlib.h>
#include <stdio.h>
int main(void) {
int *p = malloc(sizeof *p);
if (p == NULL) exit(EXIT_FAILURE);
*p = 42;
printf("p = %p\n", (void*)p); // OK
printf("*p = %d\n", *p); // OK, prints 42
free(p);
printf("p = %p\n", (void*)p);
// Undefined behavior, *probably* won't crash
printf("*p = %d\n", *p);
// Undefined behavior, *probably* won't crash
// The memory location may or may not have been clobbered.
return 0;
}
Don't access either the pointer itself *or* the memory it points to.
Of course a write access to the pointer object (such as assigning a new
value to it) is fine, since it doesn't access the stored value.
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
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
csiph-web