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Groups > comp.lang.c > #110416
| From | Ben Bacarisse <ben.usenet@bsb.me.uk> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Help on this typedef |
| Date | 2017-05-19 21:35 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <8760gwio07.fsf@bsb.me.uk> (permalink) |
| References | (7 earlier) <kfninl2nqqy.fsf@x-alumni2.alumni.caltech.edu> <799899ab-ccf3-49a4-a928-d405dc142fff@googlegroups.com> <lnwp9e15td.fsf@kst-u.example.com> <1c41ec70-5868-4a28-a724-58f4bb70e593@googlegroups.com> <lno9uo24cj.fsf@kst-u.example.com> |
Keith Thompson <kst-u@mib.org> writes:
<snip>
> Here's a demo program, based on your declaration above with the syntax
> error corrected:
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> int main(void) {
> typedef struct string_header_s {
> short length; /* limited length */
> char text[ 1 ];
> } string_header_t;
>
> string_header_t *p = malloc(sizeof *p + 10); /* more than we need */
> if (p == NULL) exit(EXIT_FAILURE);
> p->length = 4;
> strcpy(p->text, "abcd");
> printf("p->text[3] = '%c'\n", p->text[3]);
> }
>
> When I compile and run this on my system, I get no compile-time
> diagnostics and the output, as expected, is:
>
> p->text[3] = 'd'
>
> But both the strcpy() call and the reference to p->text[3] have
> undefined behavior. Concentrating on the latter, the indexing
> operator [] is defined in terms of pointer addition, in this case
> (p->text + 3). The description of pointer addition, in N1570
> 6.5.6p8, says:
>
> If both the pointer operand and the result point to elements
> of the same array object, or one past the last element of the
> array object, the evaluation shall not produce an overflow;
> otherwise, the behavior is undefined.
>
> The array object in question is p->text, which is of type char[1].
> We've used malloc to allocate additional memory past the end of
> that array object, but it's not part of the array object.
I think something might be missing from the standard in the area. I
think no one disputes that this is defined behaviour:
int *ip = malloc(10);
if (ip) ip[3] = 42;
but what is the array in question here? The storage allocated by malloc
has no effective type (yet) so how can ip and ip+3 point to elements of
the same array? (I've used and int * because there are special
dispensations for accessing any objects byte as a sequence of chars.)
And does that mean we can make the struct hack valid like this:
p->text[3] = 'x'; // invalid
char *cp = (void *)p;
cp += offsetof(sting_header_t, text);
cp[3] = 'x'; // this one OK?
I'd say yes and I think my answer would stay the same if the 'text'
member and the pointer access where to int types rather than char.
(You'd need a char * to do the first offsetof addition, of course.)
<snip>
--
Ben.
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Help on this typedef fl <rxjwg98@gmail.com> - 2017-05-10 19:12 -0700
Re: Help on this typedef Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-05-10 21:06 -0600
Re: Help on this typedef Keith Thompson <kst-u@mib.org> - 2017-05-10 20:11 -0700
Re: Help on this typedef David Brown <david.brown@hesbynett.no> - 2017-05-11 10:03 +0200
Re: Help on this typedef supercat@casperkitty.com - 2017-05-11 08:35 -0700
Re: Help on this typedef David Brown <david.brown@hesbynett.no> - 2017-05-12 10:34 +0200
Re: Help on this typedef joel.rees@gmail.com - 2017-05-12 06:01 -0700
Re: Help on this typedef Keith Thompson <kst-u@mib.org> - 2017-05-12 09:38 -0700
Re: Help on this typedef joel.rees@gmail.com - 2017-05-12 05:08 -0700
Re: Help on this typedef Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-13 14:25 -0700
Re: Help on this typedef joel.rees@gmail.com - 2017-05-14 23:36 -0700
Re: Help on this typedef Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-15 07:23 -0700
Re: Help on this typedef joel.rees@gmail.com - 2017-05-18 07:40 -0700
Re: Help on this typedef Keith Thompson <kst-u@mib.org> - 2017-05-18 09:35 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-18 10:59 -0700
Re: Help on this typedef joel.rees@gmail.com - 2017-05-18 23:40 -0700
Re: Help on this typedef jadill33@gmail.com - 2017-05-19 08:12 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-19 08:41 -0700
Re: Help on this typedef jadill33@gmail.com - 2017-05-19 09:41 -0700
Re: Help on this typedef Keith Thompson <kst-u@mib.org> - 2017-05-19 09:34 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-19 10:18 -0700
Re: Help on this typedef Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-19 21:35 +0100
Re: Help on this typedef supercat@casperkitty.com - 2017-05-19 14:16 -0700
Re: Help on this typedef Ike Naar <ike@iceland.freeshell.org> - 2017-05-19 21:15 +0000
Re: Help on this typedef Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-19 23:54 +0100
Re: Help on this typedef Keith Thompson <kst-u@mib.org> - 2017-05-19 14:25 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-19 15:18 -0700
Re: Help on this typedef Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-19 23:52 +0100
Re: Help on this typedef Keith Thompson <kst-u@mib.org> - 2017-05-19 16:42 -0700
Re: Help on this typedef Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-20 01:16 +0100
Re: Help on this typedef Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-20 01:58 -0700
Re: Help on this typedef joel.rees@gmail.com - 2017-05-20 02:25 -0700
Re: Help on this typedef joel.rees@gmail.com - 2017-05-19 22:09 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-19 23:23 -0700
Re: Help on this typedef "James R. Kuyper" <jameskuyper@verizon.net> - 2017-05-19 13:33 -0400
Re: Help on this typedef supercat@casperkitty.com - 2017-05-19 13:44 -0700
Re: Help on this typedef jameskuyper@verizon.net - 2017-05-19 19:20 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-19 23:30 -0700
Re: Help on this typedef jameskuyper@verizon.net - 2017-05-20 07:20 -0700
Re: Help on this typedef jameskuyper@verizon.net - 2017-05-20 07:46 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-22 10:50 -0700
Re: Help on this typedef "James R. Kuyper" <jameskuyper@verizon.net> - 2017-05-18 12:37 -0400
Re: Help on this typedef Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-18 20:02 +0100
Re: Help on this typedef joel.rees@gmail.com - 2017-05-19 00:20 -0700
Re: Help on this typedef Keith Thompson <kst-u@mib.org> - 2017-05-19 09:51 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-19 10:31 -0700
Re: Help on this typedef Ike Naar <ike@iceland.freeshell.org> - 2017-05-19 19:48 +0000
Re: Help on this typedef Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-20 08:47 -0700
Re: Help on this typedef "Patrick.Schluter" <Patrick.Schluter@free.fr> - 2017-05-27 18:50 +0200
Re: Help on this typedef joel.rees@gmail.com - 2017-05-12 06:17 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-12 08:46 -0700
Re: Help on this typedef joel.rees@gmail.com - 2017-05-12 18:11 -0700
Re: Help on this typedef Keith Thompson <kst-u@mib.org> - 2017-05-12 18:37 -0700
Re: Help on this typedef Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-13 09:44 -0700
Re: Help on this typedef Keith Thompson <kst-u@mib.org> - 2017-05-13 13:47 -0700
Re: Help on this typedef Robert Wessel <robertwessel2@yahoo.com> - 2017-05-14 00:28 -0500
Re: Help on this typedef Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-14 13:39 -0700
Re: Help on this typedef raltbos@xs4all.nl (Richard Bos) - 2017-05-15 13:02 +0000
Re: Help on this typedef Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-15 07:30 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-12 20:45 -0700
Re: Help on this typedef Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-13 09:57 -0700
Re: Help on this typedef Philip Lantz <prl@canterey.us> - 2017-05-13 12:07 -0700
Re: Help on this typedef David Brown <david.brown@hesbynett.no> - 2017-05-14 15:34 +0200
Re: Help on this typedef Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-14 12:48 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-15 07:39 -0700
Re: Help on this typedef Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-24 08:23 -0700
Re: Help on this typedef supercat@casperkitty.com - 2017-05-24 09:17 -0700
Re: Help on this typedef Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-27 06:15 -0700
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