Q112337: PRB: /MD Causes C4273 Warning When Replacing _matherr

Article: Q112337
Product(s): Microsoft C Compiler
Version(s): 1.0,2.0
Operating System(s): 
Keyword(s): kbCRT kbVC
Last Modified: 27-FEB-2002

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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 
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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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