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Re: casting constant value from float to unsigned short - compiler bugs?

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On 12/02/2011 04:06 AM, news.aon.at wrote:
> Hello,
> 
> I try to cast a constant value (-32766.5) to float and then to an unsigned 
> short.
> Once directly and once with an intermediate variable.
> 
>  unsigned short s=(unsigned short)((float)(-32766.5));
> 
> vs.
> 
>  float f=((float)(-32766.5));
>  unsigned short s=(unsigned short)f;
> 
> I use different compilers for this:
> 
> a) IBM Visual Age 3.5 (very old, 1995)
> b) Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 12.00.8804 for 
> 80x86  (MS-C6)
> c) Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 
> 80x86 (VS2010)
> d) gcc version 2.95.2-6 19991024 (cygwin experimental)
> e) mingw32-gcc (GCC) 3.4.5 (mingw special) [Copyright (C) 2004 Free Software 
> Foundation, Inc.]
> f) WindRiver Vxw6.7 CrossCompiler: ccpentium (Wind River VxWorks G++ 
> DWARF-EH 4.1-131) 4.1.2 [Copyright (C) 2006 Free Software Foundation, Inc.]
> 
> a,b,c,d : give 0x8002 in both cases.
> 
> e,f give 0 (!) in the first case and 0x8002 in the 2nd cases
> 
> ??

"When a finite value of real floating type is converted to an integer
type other than _Bool, the fractional part is discarded (i.e., the value
is truncated toward zero)" (6.3.1.4p1), so -32766.5 is converted to
-32766. That section goes on to say that "If the value of
the integral part cannot be represented by the integer type, the
behavior is undefined." Since -32766 cannot be represented in any
unsigned type, all of the results you've gotten are equally conforming.
A conforming implementation could also have formatted your hard disk or
sent pornographic e-mail to your boss, though those are far less likely
to occur.

> Note : when i cast to a SHORT instead of an unsigned short in the first 
> case, ALL compilers give 0x8002

That's because short can can represent -32766. There is still a
conversion to unsigned short, which can't represent that value; but for
conversions from signed integer types to unsigned integer types, the
rules are different than for floating point types. If a value cannot be
represented in the new type, "the value is converted by repeatedly
adding or subtracting one more than the maximum value that can be
represented in the new type until the value is in the range of the new
type." (6.3.1.3p2). In this case, that means adding or subtracting one
more than USHORT_MAX, which is commonly 65536 (though it could be
larger). 65536-32766 == 0x8002.
-- 
James Kuyper

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casting constant value from float to unsigned short - compiler bugs? "news.aon.at" <mario_semo@hotmail.com> - 2011-12-02 10:06 +0100
  Re: casting constant value from float to unsigned short - compiler bugs? jacob navia <jacob@spamsink.net> - 2011-12-02 10:34 +0100
    Re: casting constant value from float to unsigned short - compiler bugs? Vincenzo Mercuri <vincenzo.mercuri@hotmail.it> - 2011-12-02 18:10 +0100
    Re: casting constant value from float to unsigned short - compiler bugs? Quentin Pope <qp19433@hotmail.NOSPAM.com> - 2011-12-02 22:36 +0000
      Re: casting constant value from float to unsigned short - compiler bugs? jacob navia <jacob@spamsink.net> - 2011-12-03 00:45 +0100
        Re: casting constant value from float to unsigned short - compiler  bugs? pete <pfiland@mindspring.com> - 2011-12-02 19:43 -0500
          Re: casting constant value from float to unsigned short - compiler   bugs? pete <pfiland@mindspring.com> - 2011-12-02 19:49 -0500
        Re: casting constant value from float to unsigned short - compiler bugs? Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-12-03 01:51 +0100
          Re: casting constant value from float to unsigned short - compiler bugs? James Kuyper <jameskuyper@verizon.net> - 2011-12-03 11:51 -0500
            Re: casting constant value from float to unsigned short - compiler bugs? Vincenzo Mercuri <vincenzo.mercuri@hotmail.it> - 2011-12-03 19:08 +0100
              Re: casting constant value from float to unsigned short - compiler bugs? Quentin Pope <qp19433@hotmail.NOSPAM.com> - 2011-12-03 19:06 +0000
                Re: casting constant value from float to unsigned short - compiler bugs? Kaz Kylheku <kaz@kylheku.com> - 2011-12-03 19:14 +0000
                Re: casting constant value from float to unsigned short - compiler bugs? Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-12-04 17:19 +0200
                Re: casting constant value from float to unsigned short - compiler bugs? James Kuyper <jameskuyper@verizon.net> - 2011-12-04 10:39 -0500
      Re: casting constant value from float to unsigned short - compiler bugs? Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-12-03 01:41 +0100
        Re: casting constant value from float to unsigned short - compiler bugs? Michael Press <rubrum@pacbell.net> - 2012-01-03 12:31 -0800
          Re: casting constant value from float to unsigned short - compiler bugs? blp@cs.stanford.edu (Ben Pfaff) - 2012-01-03 12:38 -0800
            Re: casting constant value from float to unsigned short - compiler bugs? Michael Press <rubrum@pacbell.net> - 2012-01-06 11:38 -0800
              Re: casting constant value from float to unsigned short - compiler bugs? James Kuyper <jameskuyper@verizon.net> - 2012-01-06 15:10 -0500
          Re: casting constant value from float to unsigned short - compiler bugs? Keith Thompson <kst-u@mib.org> - 2012-01-03 12:49 -0800
            Re: casting constant value from float to unsigned short - compiler bugs? Michael Press <rubrum@pacbell.net> - 2012-01-04 15:14 -0800
              Re: casting constant value from float to unsigned short - compiler bugs? James Kuyper <jameskuyper@verizon.net> - 2012-01-04 19:12 -0500
                Re: casting constant value from float to unsigned short - compiler bugs? Michael Press <rubrum@pacbell.net> - 2012-01-04 23:04 -0800
                Re: casting constant value from float to unsigned short - compiler bugs? Keith Thompson <kst-u@mib.org> - 2012-01-05 02:43 -0800
                Re: casting constant value from float to unsigned short - compiler bugs? James Kuyper <jameskuyper@verizon.net> - 2012-01-05 07:14 -0500
                Re: casting constant value from float to unsigned short - compiler bugs? Michael Press <rubrum@pacbell.net> - 2012-01-06 11:39 -0800
                Re: casting constant value from float to unsigned short - compiler bugs? Michael Press <rubrum@pacbell.net> - 2012-01-05 16:14 -0800
                Re: casting constant value from float to unsigned short - compiler bugs? Keith Thompson <kst-u@mib.org> - 2012-01-05 16:44 -0800
                Re: casting constant value from float to unsigned short - compiler bugs? Michael Press <rubrum@pacbell.net> - 2012-01-06 11:30 -0800
                Re: casting constant value from float to unsigned short - compiler bugs? James Kuyper <jameskuyper@verizon.net> - 2012-01-06 15:24 -0500
                Re: casting constant value from float to unsigned short - compiler bugs? Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-26 09:14 -0800
              Re: casting constant value from float to unsigned short - compiler bugs? gwowen <gwowen@gmail.com> - 2012-01-05 04:09 -0800
              Re: casting constant value from float to unsigned short - compiler bugs? "io_x" <a@b.c.invalid> - 2012-01-05 16:32 +0100
          Re: casting constant value from float to unsigned short - compiler bugs? Nobody <nobody@nowhere.com> - 2012-01-04 09:11 +0000
            Re: casting constant value from float to unsigned short - compiler bugs? Michael Press <rubrum@pacbell.net> - 2012-01-04 23:08 -0800
              Re: casting constant value from float to unsigned short - compiler bugs? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-05 11:15 +0000
                Re: casting constant value from float to unsigned short - compiler bugs? James Kuyper <jameskuyper@verizon.net> - 2012-01-05 07:27 -0500
                Re: casting constant value from float to unsigned short - compiler bugs? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-05 13:03 +0000
                Re: casting constant value from float to unsigned short - compiler bugs? Michael Press <rubrum@pacbell.net> - 2012-01-05 16:28 -0800
                Re: casting constant value from float to unsigned short - compiler bugs? James Kuyper <jameskuyper@verizon.net> - 2012-01-06 07:17 -0500
                Re: casting constant value from float to unsigned short - compiler bugs? lawrence.jones@siemens.com - 2012-01-06 12:04 -0500
                Re: casting constant value from float to unsigned short - compiler bugs? Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-26 09:08 -0800
                Re: casting constant value from float to unsigned short - compiler bugs? Michael Press <rubrum@pacbell.net> - 2012-01-05 16:17 -0800
      Re: casting constant value from float to unsigned short - compiler bugs? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-03 01:50 +0000
  Re: casting constant value from float to unsigned short - compiler bugs? James Kuyper <jameskuyper@verizon.net> - 2011-12-02 06:41 -0500
    Re: casting constant value from float to unsigned short - compiler bugs? Vincenzo Mercuri <vincenzo.mercuri@hotmail.it> - 2011-12-02 16:26 +0100

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