Re: Passing an integer to a function that accepts a void pointer

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Passing an integer to a function that accepts a void pointer
Date	2011-05-11 08:24 -0700
Organization	None to speak of
Message-ID	<lnhb91l0p9.fsf@nuthaus.mib.org> (permalink)
References	<80905e4d-6b11-4843-b4e7-96f0c7db8b23@l18g2000yql.googlegroups.com> <chine.bleu-0AF5C7.04030711052011@reserved-multicast-range-not-delegated.example.com> <slrnisl271.p2s.fis@iris.zem.fi> <chine.bleu-B2F785.06465311052011@reserved-multicast-range-not-delegated.example.com>

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Columbus sailed the ocean China Blue <chine.bleu@yahoo.com> writes:
> In article <slrnisl271.p2s.fis@iris.zem.fi>,
>  Heikki Kallasjoki <see@end.of.message.invalid> wrote:
[...]
>> The above is not entirely true.  The intptr_t and uintptr_t types, if
>> they exist (they are optional), are capable of holding any valid
>> pointer-to-void value, but not the other way around.  To quote the
>> C99 draft,
>> 
>> "The following type [intptr_t] designates a signed integer type with the
>> property that any valid pointer to void can be converted to this type,
>> then converted back to pointer to void, and the result will compare
>> equal to the original pointer"
>> 
>> In other words, the conversion to intptr_t and back is safe only for
>> valid pointers.  Arbitrary integer values, even if they are stored in an
>> intptr_t object, are not guaranteed to be safely convertible to
>> pointers.
>
> That's nice. Any compiler that screws this up will break so much
> software that it will be rejected.

I'm sure there's plenty of software that assumes it can safely
convert integers to pointers.  Much of it probably does so
unnecessarily, and much of it makes non-portable assumptions about
the relative sizes of integer and pointer types, and is likely to
break on 64-bit systems.  I suspect that most programmers who are
aware of the existence of intptr_t and uintptr_t are experienced
enough to know that they're rarely the right solution.

But how much software depends on being able to convert arbitrary
integer values to pointers?  Why would you even want to do that?

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Nokia
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

Passing an integer to a function that accepts a void pointer pozz <pozzugno@gmail.com> - 2011-05-11 03:01 -0700
  Re: Passing an integer to a function that accepts a void pointer Tom St Denis <tom@iahu.ca> - 2011-05-11 03:52 -0700
    Re: Passing an integer to a function that accepts a void pointer pete <pfiland@mindspring.com> - 2011-05-11 08:10 -0400
    Re: Passing an integer to a function that accepts a void pointer pozz <pozzugno@gmail.com> - 2011-05-11 18:28 +0200
      Re: Passing an integer to a function that accepts a void pointer Shao Miller <sha0.miller@gmail.com> - 2011-05-11 22:10 -0500
        Re: Passing an integer to a function that accepts a void pointer pozz <pozzugno@gmail.com> - 2011-05-12 19:44 +0200
          Re: Passing an integer to a function that accepts a void pointer Shao Miller <sha0.miller@gmail.com> - 2011-05-12 14:16 -0400
            Re: Passing an integer to a function that accepts a void pointer pozz <pozzugno@gmail.com> - 2011-05-12 21:21 +0200
              Re: Passing an integer to a function that accepts a void pointer Shao Miller <sha0.miller@gmail.com> - 2011-05-12 16:05 -0400
                Re: Passing an integer to a function that accepts a void pointer pozz <pozzugno@gmail.com> - 2011-05-13 23:39 +0200
                Re: Passing an integer to a function that accepts a void pointer Dr Nick <3-nospam@temporary-address.org.uk> - 2011-05-14 09:41 +0100
          Re: Passing an integer to a function that accepts a void pointer Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-12 22:07 -0400
            Re: Passing an integer to a function that accepts a void pointer pozz <pozzugno@gmail.com> - 2011-05-13 23:12 +0200
              Re: Passing an integer to a function that accepts a void pointer Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-13 23:30 -0400
  Re: Passing an integer to a function that accepts a void pointer Columbus sailed the ocean China Blue <chine.bleu@yahoo.com> - 2011-05-11 04:03 -0700
    Re: Passing an integer to a function that accepts a void pointer Heikki Kallasjoki <see@end.of.message.invalid> - 2011-05-11 13:03 +0000
      Re: Passing an integer to a function that accepts a void pointer Columbus sailed the ocean China Blue <chine.bleu@yahoo.com> - 2011-05-11 06:46 -0700
        Re: Passing an integer to a function that accepts a void pointer Keith Thompson <kst-u@mib.org> - 2011-05-11 08:24 -0700
          Re: Passing an integer to a function that accepts a void pointer Columbus sailed the ocean China Blue <chine.bleu@yahoo.com> - 2011-05-11 08:44 -0700
            Re: Passing an integer to a function that accepts a void pointer Keith Thompson <kst-u@mib.org> - 2011-05-11 08:58 -0700
              Re: Passing an integer to a function that accepts a void pointer gazelle@shell.xmission.com (Kenny McCormack) - 2011-05-11 17:38 +0000
                Re: Passing an integer to a function that accepts a void pointer Kenneth Brody <kenbrody@spamcop.net> - 2011-05-11 14:57 -0400
                Re: Passing an integer to a function that accepts a void pointer Paul N <gw7rib@aol.com> - 2011-05-11 14:06 -0700
                Re: Passing an integer to a function that accepts a void pointer Michael Press <rubrum@pacbell.net> - 2011-05-12 13:59 -0700
                Re: Passing an integer to a function that accepts a void pointer Keith Thompson <kst-u@mib.org> - 2011-05-12 14:12 -0700
                Re: Passing an integer to a function that accepts a void pointer Columbus sailed the ocean China Blue <chine.bleu@yahoo.com> - 2011-05-12 15:27 -0700
                Re: Passing an integer to a function that accepts a void pointer Seebs <usenet-nospam@seebs.net> - 2011-05-12 22:37 +0000
                Re: Passing an integer to a function that accepts a void pointer Ian Collins <ian-news@hotmail.com> - 2011-05-13 10:41 +1200
                Re: Passing an integer to a function that accepts a void pointer Shao Miller <sha0.miller@gmail.com> - 2011-05-12 17:57 -0500
                Re: Passing an integer to a function that accepts a void pointer Ian Collins <ian-news@hotmail.com> - 2011-05-13 11:05 +1200
                Re: Passing an integer to a function that accepts a void pointer Seebs <usenet-nospam@seebs.net> - 2011-05-12 23:19 +0000
                Re: Passing an integer to a function that accepts a void pointer Keith Thompson <kst-u@mib.org> - 2011-05-12 16:32 -0700
                Re: Passing an integer to a function that accepts a void pointer Shao Miller <sha0.miller@gmail.com> - 2011-05-12 19:13 -0500
                Re: Passing an integer to a function that accepts a void pointer Keith Thompson <kst-u@mib.org> - 2011-05-12 18:18 -0700
                Re: Passing an integer to a function that accepts a void pointer Keith Thompson <kst-u@mib.org> - 2011-05-12 16:18 -0700
                Re: Passing an integer to a function that accepts a void pointer Tobias Blass <tobiasblass@gmx.net> - 2011-05-13 08:24 +0000
                Re: Passing an integer to a function that accepts a void pointer Columbus sailed the ocean China Blue <chine.bleu@yahoo.com> - 2011-05-13 01:33 -0700
                Re: Passing an integer to a function that accepts a void pointer Seebs <usenet-nospam@seebs.net> - 2011-05-13 08:36 +0000
                Re: Passing an integer to a function that accepts a void pointer "madamemandm@yahoo.com" <madamemandm@yahoo.com> - 2011-05-13 07:08 -0700
                Re: Passing an integer to a function that accepts a void pointer Keith Thompson <kst-u@mib.org> - 2011-05-12 15:59 -0700
            Re: Passing an integer to a function that accepts a void pointer Seebs <usenet-nospam@seebs.net> - 2011-05-11 21:46 +0000
              Re: Passing an integer to a function that accepts a void pointer Columbus sailed the ocean China Blue <chine.bleu@yahoo.com> - 2011-05-12 00:56 -0700
                Re: Passing an integer to a function that accepts a void pointer Ian Collins <ian-news@hotmail.com> - 2011-05-12 20:06 +1200
                Re: Passing an integer to a function that accepts a void pointer Heikki Kallasjoki <see@end.of.message.invalid> - 2011-05-12 08:59 +0000
                Re: Passing an integer to a function that accepts a void pointer Ian Collins <ian-news@hotmail.com> - 2011-05-13 10:39 +1200
        Re: Passing an integer to a function that accepts a void pointer Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-12 06:51 -0400
          Re: Passing an integer to a function that accepts a void pointer Columbus sailed the ocean China Blue <chine.bleu@yahoo.com> - 2011-05-12 04:08 -0700
            Re: Passing an integer to a function that accepts a void pointer Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-12 07:33 -0400
            Re: Passing an integer to a function that accepts a void pointer Keith Thompson <kst-u@mib.org> - 2011-05-12 07:58 -0700
              Re: Passing an integer to a function that accepts a void pointer gazelle@shell.xmission.com (Kenny McCormack) - 2011-05-12 15:23 +0000
              Re: Passing an integer to a function that accepts a void pointer Shao Miller <sha0.miller@gmail.com> - 2011-05-12 13:36 -0400
                Re: Passing an integer to a function that accepts a void pointer Keith Thompson <kst-u@mib.org> - 2011-05-12 12:03 -0700
            Re: Passing an integer to a function that accepts a void pointer Seebs <usenet-nospam@seebs.net> - 2011-05-12 15:57 +0000
          Re: Passing an integer to a function that accepts a void pointer Michael Press <rubrum@pacbell.net> - 2011-05-12 14:14 -0700
            Re: Passing an integer to a function that accepts a void pointer Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-12 22:20 -0400
              Re: Passing an integer to a function that accepts a void pointer Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-14 02:39 +0100
  Re: Passing an integer to a function that accepts a void pointer Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-11 07:37 -0400
  Re: Passing an integer to a function that accepts a void pointer James Kuyper <jameskuyper@verizon.net> - 2011-05-11 09:42 -0400
  Re: Passing an integer to a function that accepts a void pointer Datesfat Chicks <datesfat.chicks@gmail.com> - 2011-05-11 11:53 -0400
    Re: Passing an integer to a function that accepts a void pointer Fred <fred.l.kleinschmidt@boeing.com> - 2011-05-11 10:44 -0700
      Re: Passing an integer to a function that accepts a void pointer Datesfat Chicks <datesfat.chicks@gmail.com> - 2011-05-12 07:58 -0400
    Re: Passing an integer to a function that accepts a void pointer Dr Nick <3-nospam@temporary-address.org.uk> - 2011-05-11 21:27 +0100

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