DOCUMENT:Q119458  29-JUL-2001  [visualc]
TITLE   :Result of _fpreset() Is Not _CW_DEFAULT
PRODUCT :Microsoft C Compiler
PROD/VER:winnt:
OPER/SYS:
KEYWORDS:kbCRT kbVC

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - The C Run-Time (CRT), included with:
    - Microsoft Visual C++, versions 1.0, 1.50 
    - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, versions 1.0, 2.0, 2.1 
-------------------------------------------------------------------------------

SUMMARY
=======

Calling _fpreset() reinitializes the floating-point math package. The header
file FLOAT.H contains the following definition of _CW_DEFAULT:

       /* initial Control Word value */ 

      #ifdef  _M_IX86

      #define _CW_DEFAULT ( _RC_NEAR + _PC_64 + _EM_INVALID + _EM_ZERODIVIDE +
         _EM_OVERFLOW + _EM_UNDERFLOW + _EM_INEXACT )

      #endif

You might expect that if you call _fpreset() and then call _controlfp(0,0) to get
the control word, that the control word will be _CW_DEFAULT; however, that is
not the case. _CW_DEFAULT does not actually correspond to the default value of
the x87 control word. The difference is that _CW_DEFAULT does not mask the
denormal exception. The _fpreset() routine masks all exceptions, including
denormal.

The denormal exception does not belong to the set of floating-point exceptions
specified by the IEEE Standard. You cannot mask or unmask the exception using
_controlfp(). The abstract control word used by the C run- time reserves the
upper bits for machine-specific features. Therefore, you should not compare full
control-word encodings; instead, you should compare the bits that you are
interested in.

Additional query words: kbinf 1.00 1.50 2.00 2.10

======================================================================
Keywords          : kbCRT kbVC 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : winnt:

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND.  MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN NO
EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR
ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.  SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES
SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 2001.