DOCUMENT:Q112337  27-FEB-2002  [visualc]
TITLE   :PRB: /MD Causes C4273 Warning When Replacing _matherr
PRODUCT :Microsoft C Compiler
PROD/VER::1.0,2.0
OPER/SYS:
KEYWORDS:kbCRT kbVC

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

 - The C Run-Time (CRT), included with:
    - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

Attempting to replace the default _matherr function with a user-defined version
and then linking with the C run-time dynamic-linked library (DLL) causes the
compiler to issue the following warning:

   warning C4273: '_matherr' : inconsistent dll linkage. dllexport assumed.

Running the resultant executable causes the default _matherr to be used rather
than the user-defined version.

CAUSE
=====

The compiler issues the C4273 warning because of the way the _matherr function
is prototyped in the MATH.H header file. _matherr has the following prototype:

      _CRTIMP int __cdecl _matherr(struct _exception *);

When code is being compiled to be used with the C run-time DLL, _CRTIMP is
defined to be the following:

      __declspec(dllimport)

When code is being compiled to be used with one of the statically linked C
run-time libraries, _CRTIMP is defined to be nothing.

The C4273 warning is generated because the compiler has detected a redefinition
of _matherr after already seeing a prototype that specifies that _matherr is
being imported.

RESOLUTION
==========

It is not possible to replace the default version of _matherr when using the
dynamically linked version of the C run-time library.

The README.WRI file supplied with Visual C++, 32-bit Edition, version 1.0, gives
the following information:

   Using _matherr with CRTDLL.DLL

   If you use the dynamically linked version of the C run-time
   library (CRTDLL.DLL), you cannot replace the default _matherr
   routine with a user-defined version. You can only install a
   custom _matherr routine if you use one of the statically linked C
   run-time libraries.

CRTDLL.DLL is the name of the C Run-time DLL that is supplied with Windows NT.
The C Run-time DLL supplied with version 1.0 of Visual C++, 32-bit Edition, is
called MSVCRT10.DLL and the name of the C Run-time DLL supplied with version 2.0
of Visual C++, 32-bit Edition is MSVCRT20.DLL. The limitation of not being able
to replace _matherr pertains only to using MSVCRT*.DLL.

MORE INFORMATION
================

The following sample code can be used to demonstrate the problem.

Sample Code
-----------

      /* Compile options needed: /MD
      */ 
      #include <math.h>
      #include <string.h>
      #include <stdio.h>
      void main()
      {
          // Do a math operation that causes an error
          printf( "log( -2.0 ) = %e\n", log( -2.0 ) );
      }
      int _matherr( struct _exception *except )
      {
          /* Handle _DOMAIN errors for log */ 
          if( except->type == _DOMAIN )
          {
              if( strcmp( except->name, "log" ) == 0 )
              {
                  except->retval = log( -(except->arg1) );
                  printf( "Special: using absolute value: %s: _DOMAIN "
                          "error\n", except->name );
                  return 1;
              }
          }
          else
          {
              printf( "Normal: " );
              return 0;    /* Else use the default actions */ 
          }
          return 0;
      }

REFERENCE
---------

For additional information, please see the following article in the Microsoft
Knowledge Base:

   Q112297 INFO: User Defined CRT Function Causes Unresolved External

Additional query words: _matherrl

======================================================================
Keywords          : kbCRT kbVC 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : :1.0,2.0
Issue type        : kbprb

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

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