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Re: Typecasting Pointers on a 64 bit System

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On 2011-11-10, christian.bau <christian.bau@cbau.wanadoo.co.uk> wrote:
> On Nov 10, 8:21 pm, Quentin Pope <qp19...@hotmail.NOSPAM.com> wrote:
>> What is the best way to handle this warning:
>>
>> warning: cast from pointer to integer of different size
>
> On MacOS X 64 bit, an int or unsigned int cast to a pointer will
> _never_ produce a valid pointer. So the warning is very much
> justified. 

The warning is about a different size, not about never producing
a valid pointer.

> Now imagine you wanted to pass a long double. 

Then you'd be writing a different program, requiring a different
approach. 

Conversions between pointer and long double are not even a standard feature of
the C language; a diagnostic is required.

> Or a long
> long on a system with 32 bit pointers.

In what situation would you need a POSIX file descriptor to be held in a long
long?  Descriptor values are allocated into the smallest available position, so
if your program is ever to achieve a file descriptor value of N, it must at
least momentarily create a situation where there N+1 descriptors simultaneously
open.

Single Unix Spec Issue 6, on the subject of the open function:

``Upon successful completion, the function shall open the file and return a
non-negative integer representing the lowest numbered unused file descriptor.
Otherwise, -1 shall be returned and errno set to indicate the error. No files
shall be created or modified if the function returns -1.''

> Your approach just cannot work
> in these cases.

Casting a small long long value to a 32 bit pointer and back to recover
that value is something that is /widely/ portable.

> The safe thing to do is to allocate some memory with malloc, store
> whatever you want to pass in the allocated memory, and pass the
> pointer, so someone else can then safely pick up the data.

There is nothing unsafe about the conversions when they work.

They're only unsafe on some fictitious machine where you get a trap
representation, etc.

"Less than maximally portable" and "unsafe" are totally different concepts.

Getting it running on the weird machine is the porter's responsibility. He
hopes to make X dollars by porting the program to a weird platform Y, guessing
that he can do so with an investment of Z dollars in an amount of time T and
takes his chances accordingly.

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Thread

Typecasting Pointers on a 64 bit System Quentin Pope <qp19433@hotmail.NOSPAM.com> - 2011-11-10 20:21 +0000
  Re: Typecasting Pointers on a 64 bit System Lauri Alanko <la@iki.fi> - 2011-11-10 20:29 +0000
    Re: Typecasting Pointers on a 64 bit System Kaz Kylheku <kaz@kylheku.com> - 2011-11-10 20:51 +0000
      Re: Typecasting Pointers on a 64 bit System Kaz Kylheku <kaz@kylheku.com> - 2011-11-10 20:58 +0000
      Re: Typecasting Pointers on a 64 bit System James Kuyper <jameskuyper@verizon.net> - 2011-11-10 16:08 -0500
        Re: Typecasting Pointers on a 64 bit System Kaz Kylheku <kaz@kylheku.com> - 2011-11-10 21:35 +0000
          Re: Typecasting Pointers on a 64 bit System James Kuyper <jameskuyper@verizon.net> - 2011-11-10 17:18 -0500
            Re: Typecasting Pointers on a 64 bit System Kaz Kylheku <kaz@kylheku.com> - 2011-11-10 23:14 +0000
              Re: Typecasting Pointers on a 64 bit System Richard Damon <news.x.richarddamon@xoxy.net> - 2011-11-10 19:36 -0500
  Re: Typecasting Pointers on a 64 bit System Ben Pfaff <blp@cs.stanford.edu> - 2011-11-10 12:32 -0800
    Re: Typecasting Pointers on a 64 bit System James Kuyper <jameskuyper@verizon.net> - 2011-11-10 15:44 -0500
    Re: Typecasting Pointers on a 64 bit System Keith Thompson <kst-u@mib.org> - 2011-11-10 12:58 -0800
      Re: Typecasting Pointers on a 64 bit System blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-11-17 15:53 +0000
  Re: Typecasting Pointers on a 64 bit System Kaz Kylheku <kaz@kylheku.com> - 2011-11-10 20:45 +0000
    Re: Typecasting Pointers on a 64 bit System Kaz Kylheku <kaz@kylheku.com> - 2011-11-10 21:00 +0000
  Re: Typecasting Pointers on a 64 bit System Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-11-10 15:14 -0700
  Re: Typecasting Pointers on a 64 bit System Ian Collins <ian-news@hotmail.com> - 2011-11-11 11:22 +1300
    Re: Typecasting Pointers on a 64 bit System William Ahern <william@wilbur.25thandClement.com> - 2011-11-14 11:10 -0800
      Re: Typecasting Pointers on a 64 bit System Ian Collins <ian-news@hotmail.com> - 2011-11-15 08:28 +1300
        Re: Typecasting Pointers on a 64 bit System James Kuyper <jameskuyper@verizon.net> - 2011-11-14 14:48 -0500
          Re: Typecasting Pointers on a 64 bit System Ian Collins <ian-news@hotmail.com> - 2011-11-15 10:15 +1300
            Re: Typecasting Pointers on a 64 bit System jacob navia <jacob@spamsink.net> - 2011-11-14 22:18 +0100
              Re: Typecasting Pointers on a 64 bit System Ian Collins <ian-news@hotmail.com> - 2011-11-15 10:26 +1300
            Re: Typecasting Pointers on a 64 bit System James Kuyper <jameskuyper@verizon.net> - 2011-11-14 16:46 -0500
              Re: Typecasting Pointers on a 64 bit System Ian Collins <ian-news@hotmail.com> - 2011-11-15 10:58 +1300
                Re: Typecasting Pointers on a 64 bit System James Kuyper <jameskuyper@verizon.net> - 2011-11-14 17:15 -0500
                Re: Typecasting Pointers on a 64 bit System blp@cs.stanford.edu (Ben Pfaff) - 2011-11-14 14:18 -0800
                Re: Typecasting Pointers on a 64 bit System Ian Collins <ian-news@hotmail.com> - 2011-11-15 11:29 +1300
                Re: Typecasting Pointers on a 64 bit System James Kuyper <jameskuyper@verizon.net> - 2011-11-14 18:14 -0500
                Re: Typecasting Pointers on a 64 bit System Seebs <usenet-nospam@seebs.net> - 2011-11-14 23:01 +0000
                Re: Typecasting Pointers on a 64 bit System Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-11-16 10:39 +0200
                Re: Typecasting Pointers on a 64 bit System Seebs <usenet-nospam@seebs.net> - 2011-11-16 20:18 +0000
                Re: Typecasting Pointers on a 64 bit System Fritz Wuehler <fritz@spamexpire-201111.rodent.frell.theremailer.net> - 2011-11-17 02:54 +0100
                Re: Typecasting Pointers on a 64 bit System pete <pfiland@mindspring.com> - 2011-11-16 21:29 -0500
                Re: Typecasting Pointers on a 64 bit System Seebs <usenet-nospam@seebs.net> - 2011-11-17 02:31 +0000
                Re: Typecasting Pointers on a 64 bit System Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-11-18 14:31 +0200
                Re: Typecasting Pointers on a 64 bit System Kaz Kylheku <kaz@kylheku.com> - 2011-11-17 02:44 +0000
                Re: Typecasting Pointers on a 64 bit System Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-11-16 21:33 -0700
                Re: Typecasting Pointers on a 64 bit System "io_x" <a@b.c.invalid> - 2011-11-17 04:01 +0100
                Re: Typecasting Pointers on a 64 bit System blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-11-17 15:46 +0000
                Re: Typecasting Pointers on a 64 bit System Seebs <usenet-nospam@seebs.net> - 2011-11-17 18:48 +0000
                Re: Typecasting Pointers on a 64 bit System blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-11-18 14:48 +0000
                Re: Typecasting Pointers on a 64 bit System Ian Collins <ian-news@hotmail.com> - 2011-11-15 14:37 +1300
                Re: Typecasting Pointers on a 64 bit System Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-11-16 10:23 +0200
                Re: Typecasting Pointers on a 64 bit System James Kuyper <jameskuyper@verizon.net> - 2011-11-16 06:53 -0500
                Re: Typecasting Pointers on a 64 bit System James Kuyper <jameskuyper@verizon.net> - 2011-11-16 11:15 -0500
                Re: Typecasting Pointers on a 64 bit System Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-11-16 17:08 +0000
                Re: Typecasting Pointers on a 64 bit System Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-11-18 15:09 +0200
                Re: Typecasting Pointers on a 64 bit System Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-11-18 14:24 +0200
                Re: Typecasting Pointers on a 64 bit System James Kuyper <jameskuyper@verizon.net> - 2011-11-18 10:08 -0500
                Re: Typecasting Pointers on a 64 bit System Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-11-18 17:52 +0200
            Re: Typecasting Pointers on a 64 bit System Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-11-14 15:45 -0700
  Re: Typecasting Pointers on a 64 bit System "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-11-10 15:19 -0800
    Re: Typecasting Pointers on a 64 bit System Kaz Kylheku <kaz@kylheku.com> - 2011-11-10 23:41 +0000
      Re: Typecasting Pointers on a 64 bit System Keith Thompson <kst-u@mib.org> - 2011-11-10 20:48 -0800
      Re: Typecasting Pointers on a 64 bit System "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-11-14 03:39 -0800
        Re: Typecasting Pointers on a 64 bit System Lauri Alanko <la@iki.fi> - 2011-11-14 13:08 +0000
          Re: Typecasting Pointers on a 64 bit System James Kuyper <jameskuyper@verizon.net> - 2011-11-14 09:21 -0500
        Re: Typecasting Pointers on a 64 bit System Keith Thompson <kst-u@mib.org> - 2011-11-14 12:54 -0800

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