Re: Help on this typedef

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From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Help on this typedef
Date	Fri, 19 May 2017 14:25:56 -0700
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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
[...]
> 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.)

N1570 7.22.3p1:

    The pointer returned if the allocation succeeds is suitably
    aligned so that it may be assigned to a pointer to any type of
    object with a fundamental alignment requirement and then used
    to access such an object or an array of such objects in the
    space allocated (until the space is explicitly deallocated).

The wording does seem to imply that the pointer may be used
to access "such an object or an array of such objects" merely
*because* it's suitably aligned, which I'm not comfortable with,
but the intent seems to be that the allocation functions can and
do create accessible objects.

So a pointer to malloc()ed memory can be treated as a pointer to an
object or array of arbitrary types, but I don't think that implies that
an array member within a structure can be treated as if it were a bigger
array.

> 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>

I'd say that doesn't take advantage of the explicit permission granted
for malloc() and friends, so the behavior is undefined.

-- 
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"

Thread

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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