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Groups > comp.lang.c > #24006

Re: casting unsigned int to void*

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Fred K <fred.l.kleinschmidt@gmail.com> writes:
> The Xpm pixmap library contains code that casts an unsigned int
> to a (void *), passing that as an argument to a function
> that expects a (void *):
>
> /* Function prototype: */
> xpmHashIntern (xpmHashTable *table, char *tag, void *data));
>
> /* Calling it, casting 3rd argument: */
> int a = <something>;
> xpmHashIntern(hashtable, color->string, (void *)a );
>
> On a 64-bit platform where pointers are 64 bits and ints
> are 32 bits, is this safe?

Others have addressed the safety (or lack thereof) of doing this.

The "void *data" parameter is intended for passing arbitrary information
into the function.  A safer way to pass an int value is to pass the
address of an int object:

    int a = <something>;
    int *p = malloc(sizeof *p);
    /* check for allocation error */
    xpmHashIntern(hashtable, color->string, p);

I used malloc to ensure that the object continues to exist as long as
it's needed; if it's a one-off object, declaring "a" as static would
also work.

This assumes, of course, that the code that retrieves the value from the
hash table knows to treat it as a pointer to an int object.

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

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Thread

casting unsigned int to void* Fred K <fred.l.kleinschmidt@gmail.com> - 2012-07-30 10:33 -0700
  Re: casting unsigned int to void* Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-07-30 13:59 -0400
    Re: casting unsigned int to void* falk@rahul.net (Edward A. Falk) - 2012-07-30 19:03 +0000
  Re: casting unsigned int to void* James Kuyper <jameskuyper@verizon.net> - 2012-07-30 14:14 -0400
    Re: casting unsigned int to void* Fred K <fred.l.kleinschmidt@gmail.com> - 2012-07-30 11:23 -0700
      Re: casting unsigned int to void* pacman@kosh.dhis.org (Alan Curry) - 2012-07-30 18:46 +0000
        Re: casting unsigned int to void* Fred K <fred.l.kleinschmidt@gmail.com> - 2012-07-30 12:09 -0700
          Re: casting unsigned int to void* Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-07-30 21:02 +0100
  Re: casting unsigned int to void* bartek szurgot <basz@no.spam> - 2012-07-30 20:47 +0200
  Re: casting unsigned int to void* "China Blue [Tor], Meersburg" <chine.bleu@yahoo.com> - 2012-07-30 13:04 -0700
    Re: casting unsigned int to void* James Kuyper <jameskuyper@verizon.net> - 2012-07-30 16:22 -0400
      Re: casting unsigned int to void* "China Blue [Tor], Meersburg" <chine.bleu@yahoo.com> - 2012-07-30 14:55 -0700
        Re: casting unsigned int to void* James Kuyper <jameskuyper@verizon.net> - 2012-07-30 18:15 -0400
          Re: casting unsigned int to void* "China Blue [Tor], Meersburg" <chine.bleu@yahoo.com> - 2012-07-30 15:18 -0700
            Re: casting unsigned int to void* Mark Bluemel <mark_bluemel@pobox.com> - 2012-07-31 08:30 +0100
              Re: casting unsigned int to void* "China Blue [Tor], Meersburg" <chine.bleu@yahoo.com> - 2012-07-31 01:17 -0700
    Re: casting unsigned int to void* Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-07-30 23:36 +0100
    Re: casting unsigned int to void* Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-07-30 20:45 -0400
      Re: casting unsigned int to void* James Kuyper <jameskuyper@verizon.net> - 2012-07-31 07:23 -0400
  Re: casting unsigned int to void* Keith Thompson <kst-u@mib.org> - 2012-07-30 16:28 -0700
  Re: casting unsigned int to void* James Kuyper <jameskuyper@verizon.net> - 2012-07-31 07:26 -0400
    Re: casting unsigned int to void* 88888 Dihedral <dihedral88888@googlemail.com> - 2012-08-05 09:16 -0700
      Re: casting unsigned int to void* Ike Naar <ike@sdf.lonestar.org> - 2012-08-05 20:50 +0000
        Re: casting unsigned int to void* Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-08-10 11:05 +0300

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