PRB: /MD Causes C4273 Warning When Replacing

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:

112297 INFO: User Defined CRT Function Causes Unresolved External