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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Issue on assigning address to pointers |
| Date | 2015-11-17 09:41 -0800 |
| Organization | None to speak of |
| Message-ID | <lnegfojzfu.fsf@kst-u.example.com> (permalink) |
| References | <dc62f43a-1725-496b-b36a-c3e196015b7d@googlegroups.com> |
Alla _ <modelling.data@gmail.com> writes:
> I have seen many times the following declaration of pointers to arrays,
> advised without any caveats:
>
> int numbers[8];
> int *ptr;
>
> ptr = numbers + 8; // to point at a value stored at numbers[8]
>
> Please, take a look at a paragraph from K&R, stating that this method
> doesn't work on "multi-tasking machines". Given K&R was published in
> 1999, I wonder if that statement still holds.
>
> QUOTE:
> char arrayname[5];
> char *ptr;
> for (ptr = arrayname; ptr <= arrayname+4; ptr++)
> {
> *ptr = 0;
> }
> The code above sets the array arrayname to zero. This method of getting
> at array data is not recommended by this author except in very simple
> computer environments. If a program is running on a normal microcomputer,
> then there should be few problems with this alternative method of handling
> arrays. On the hand, if the microcomputer is multi-tasking, or the program
> is running on a larger system which has a limited manager, then memory
> ceases to be something which can be thought of as a sequence of boxes
> standing next to one another. A multi-tasking system shares memory with
> Questions 175
> other programs and it takes what it can find, where it can find it. The upshot
> of this is that it is not possible to guarantee that arrays will be stored in one
> simple string of memory locations: it might be scattered around in different
> places. So
> ptr = arrayname + 5;
> might not be a pointer to the fifth character in a character array. This could
> be found instead using the '&' operator. A pointer to the fifth element can
> be reliably found with:
> ptr = &(arrayname[5]);
> Be warned!
> END OF QUOTE
That code is perfectly safe as far as I can tell.
I cannot find that passage in K&R2 (I have the Kindle edition and I
searched for several words in the quoted section), nor do I see anything
relevant in the official errata. Are you *sure* that's from (a
legitimate copy of) K&R2? Can you cite the section number?
BTW, in your example, ptr points past the end of the array. That's
legal, but the pointer may not be dereferenced.
--
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"
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Issue on assigning address to pointers Alla _ <modelling.data@gmail.com> - 2015-11-17 08:49 -0800
Re: Issue on assigning address to pointers James Kuyper <jameskuyper@verizon.net> - 2015-11-17 12:36 -0500
Re: Issue on assigning address to pointers Richard Heathfield <rjh@cpax.org.uk> - 2015-11-17 17:47 +0000
Re: Issue on assigning address to pointers Alla _ <modelling.data@gmail.com> - 2015-11-17 10:34 -0800
Re: Issue on assigning address to pointers Keith Thompson <kst-u@mib.org> - 2015-11-17 12:16 -0800
Re: Issue on assigning address to pointers James Kuyper <jameskuyper@verizon.net> - 2015-11-17 15:23 -0500
Re: Issue on assigning address to pointers Alla _ <modelling.data@gmail.com> - 2015-11-17 10:33 -0800
Re: Issue on assigning address to pointers Keith Thompson <kst-u@mib.org> - 2015-11-17 09:41 -0800
Re: Issue on assigning address to pointers Geoff <geoff@invalid.invalid> - 2015-11-17 10:03 -0800
Re: Issue on assigning address to pointers glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-17 20:31 +0000
Re: Issue on assigning address to pointers Geoff <geoff@invalid.invalid> - 2015-11-17 12:53 -0800
Re: Issue on assigning address to pointers Alla _ <modelling.data@gmail.com> - 2015-11-17 10:32 -0800
Re: Issue on assigning address to pointers Richard Heathfield <rjh@cpax.org.uk> - 2015-11-17 18:45 +0000
Re: Issue on assigning address to pointers Alla _ <modelling.data@gmail.com> - 2015-11-17 10:53 -0800
Re: Issue on assigning address to pointers Keith Thompson <kst-u@mib.org> - 2015-11-17 15:05 -0800
Re: Issue on assigning address to pointers Philip Lantz <prl@canterey.us> - 2015-11-19 21:16 -0800
Re: Issue on assigning address to pointers Anand Hariharan <mailto.anand.hariharan@gmail.com> - 2015-11-17 15:47 -0800
Re: Issue on assigning address to pointers Barry Schwarz <schwarzb@dqel.com> - 2015-11-17 15:53 -0800
Re: Issue on assigning address to pointers Richard Heathfield <rjh@cpax.org.uk> - 2015-11-18 04:05 +0000
Re: Issue on assigning address to pointers Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-17 19:41 +0000
Re: Issue on assigning address to pointers Stephen Sprunk <stephen@sprunk.org> - 2015-11-17 12:17 -0600
Re: Issue on assigning address to pointers Richard Heathfield <rjh@cpax.org.uk> - 2015-11-17 18:21 +0000
Re: Issue on assigning address to pointers Geoff <geoff@invalid.invalid> - 2015-11-17 10:36 -0800
Re: Issue on assigning address to pointers Richard Heathfield <rjh@cpax.org.uk> - 2015-11-17 18:46 +0000
Re: Issue on assigning address to pointers Alla _ <modelling.data@gmail.com> - 2015-11-17 10:49 -0800
Re: Issue on assigning address to pointers Alla _ <modelling.data@gmail.com> - 2015-11-17 10:47 -0800
Re: Issue on assigning address to pointers Richard Heathfield <rjh@cpax.org.uk> - 2015-11-17 18:51 +0000
Re: Issue on assigning address to pointers Alla _ <modelling.data@gmail.com> - 2015-11-17 10:55 -0800
Re: Issue on assigning address to pointers Geoff <geoff@invalid.invalid> - 2015-11-17 11:59 -0800
Re: Issue on assigning address to pointers Stephen Sprunk <stephen@sprunk.org> - 2015-11-17 12:58 -0600
Re: Issue on assigning address to pointers Richard Heathfield <rjh@cpax.org.uk> - 2015-11-17 19:12 +0000
Re: Issue on assigning address to pointers Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-17 19:33 +0000
Re: Issue on assigning address to pointers Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-11-17 13:25 -0500
Re: Issue on assigning address to pointers Richard Heathfield <rjh@cpax.org.uk> - 2015-11-17 18:31 +0000
Re: Issue on assigning address to pointers Alla _ <modelling.data@gmail.com> - 2015-11-17 10:39 -0800
Re: Issue on assigning address to pointers Alla _ <modelling.data@gmail.com> - 2015-11-17 10:40 -0800
Re: Issue on assigning address to pointers Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-17 19:40 +0000
Re: Issue on assigning address to pointers Geoff <geoff@invalid.invalid> - 2015-11-17 11:15 -0800
Re: Issue on assigning address to pointers James Kuyper <jameskuyper@verizon.net> - 2015-11-17 15:32 -0500
Re: Issue on assigning address to pointers Keith Thompson <kst-u@mib.org> - 2015-11-17 15:09 -0800
Re: Issue on assigning address to pointers supercat@casperkitty.com - 2015-11-17 15:41 -0800
Re: Issue on assigning address to pointers Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-17 21:28 -0800
Re: Issue on assigning address to pointers supercat@casperkitty.com - 2015-11-17 23:18 -0800
Re: Issue on assigning address to pointers Geoff <geoff@invalid.invalid> - 2015-11-17 17:18 -0800
Re: Issue on assigning address to pointers James Kuyper <jameskuyper@verizon.net> - 2015-11-17 22:37 -0500
Re: Issue on assigning address to pointers Lőrinczy Zsigmond <zsiga@nospam.for.me> - 2015-11-18 11:27 +0100
Re: Issue on assigning address to pointers Keith Thompson <kst-u@mib.org> - 2015-11-18 06:13 -0800
Re: Issue on assigning address to pointers guinness.tony@gmail.com - 2015-11-24 02:46 -0800
Re: Issue on assigning address to pointers Keith Thompson <kst-u@mib.org> - 2015-11-24 09:21 -0800
Re: Issue on assigning address to pointers John Bode <jfbode1029@gmail.com> - 2015-11-24 05:01 -0800
Re: Issue on assigning address to pointers Keith Thompson <kst-u@mib.org> - 2015-11-24 09:22 -0800
Re: Issue on assigning address to pointers John Bode <jfbode1029@gmail.com> - 2015-11-24 06:47 -0800
Re: Issue on assigning address to pointers Alla _ <modelling.data@gmail.com> - 2015-11-25 06:44 -0800
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