INFO: User-Defined CRT Function Generates Warning or Errors

• The C Run-time (CRT) included with: - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

An attempt to replace a C run-time (CRT) function with a user-defined version may cause the linker to issue a warning or error message when linking with the CRT dynamic-link library (DLL) MSVCRTx0.DLL. With Visual C++ version 4.0 and MSVCRT40.DLL, the following errors are generated:

   MSVCRT.LIB(MSVCRT40.dll) : error LNK2005: <function_name> already
      defined in <file.obj>
   <target_name> : fatal error LNK1169: one or more multiply defined
      symbols found

With Visual C++ version 2.x and MSVCRT20.DLL, the following error is generated:

   MSVCRT.LIB(MSVCRT20.dll) : error LNK2001: unresolved external symbol
      "<symbol>"

With Visual C++ version 1.0 and MSVCRT10.DLL, the following warning is generated:

   msvcrt.lib(msvcrt10.def) : warning LNK4006: <function_name>
      already defined in <file.obj>; second definition ignored

MORE INFORMATION

The /MD compiler switch is used when creating applications that use the DLL version of the CRT. The /MD switch causes the compiler driver to define the _MT and _DLL symbols so that both the multithreaded and DLL versions of the run-time routines are selected from the standard header files. /MD also causes the compiler to place the library name MSVCRT.LIB in the object file. MSVCRT.LIB is the import library for the CRT DLL that is named MSVCRTx0.DLL. (In the name of the DLL, "x" is a digit referring to the DLL's version. This number is the same as the major version number of the corresponding Visual C++ with which it was distributed. For Visual C++ version 1.0, the name is MSVCRT10.DLL. For Visual C++ versions 2.x and 4.0, the names are MSVCRT20.DLL and MSVCRT40.DLL, respectively.)

Most function prototypes in the standard CRT header files are prefaced with the _CRTIMP symbol. When code is compiled without the /MD switch and the _DLL symbol is not defined, _CRTIMP is therefore defined to be nothing. However, when code is compiled with the /MD switch and _DLL is defined, _CRTIMP is defined to be __declspec(dllimport). The __declspec(dllimport) tells the compiler that this routine actually resides in a DLL. For example, the following is the prototype in IO.H for the _commit CRT function:

   _CRTIMP int __cdecl _commit(int);

If a user-defined version of a CRT function is compiled so that it can be statically linked into applications, the linker errors or warning listed above will be generated when that function is linked into an application that was compiled with /MD. The linker generates the error because it has encountered two definitions of the CRT function. With Visual C++ versions 1.0 and 2.x, the user-defined version of the function is not called because the linker preferentially looks for the version of the function that is decorated as an imported DLL entry point. With Visual C++ version 4.0, the linker terminates, forcing the user to implement one of the resolutions presented below or to specifiy /FORCE:MULTIPLE on the link command line. This option tells the linker to create a valid .EXE or .DLL file whether or not it finds more than one definition for a symbol. But, be warned, doing so will generate the following link warning:

   MSVCRT.LIB(MSVCRT40.dll) : warning LNK4006: <function_name> already
      defined in <file.obj>; second definition ignored
   <target_name> : warning LNK4088: image being generated due to /FORCE
      option; image may not run

   cl /c /MD MAIN.C
   cl /c COMMIT.C
   link MAIN.OBJ COMMIT.obj /NOD MSVCRT.LIB KERNEL32.LIB

The code shown in Sample Code 2 below illustrates this method. To build this sample, use the following compile and link commands:

   cl /c /MD MAIN.C
   cl /MD /LD COMMIT.C
   link MAIN.OBJ COMMIT.LIB

Sample Code 1

   //////////
   // MAIN.C
   //
   // Compile options needed:    /c /MD

   #include <stdio.h>
   #include <process.h>

   // Provide a non-__declspec(dllimport) prototype.
   int __cdecl _commit(int filedes );

   FILE *stream;

   void main( void )
   {
       // Open a stream for writing.
       if( (stream = fopen( "data.txt", "w+" )) == NULL )
       {
           printf( "The file 'data.txt' was not opened\n" );
           exit( 1 );
       }
       else
           printf( "The file 'data.txt' was opened\n" );

       // Write to the file stream.
       fprintf( stream, "This is some text" );

       // Flush the CRT buffer.
          fflush( stream );

       // Tell the operating system to commit the data to the file.
       if(_commit( _fileno( stream ) ))
           printf( "There was a problem committing to disk\n" );
       else
           printf( "The data was successfully committed to disk\n" );

       // Close the stream.
       if( fclose( stream ) )
           printf( "The file 'data.txt' was not closed\n" );
   }

   ////////////
   // COMMIT.C
   //
   // Compile options needed:    /c
   //
   // This code replaces the version of _commit() provided by the C
   // run-time library.

   #include <stdio.h>

   int __cdecl _commit(int filedes )
   {
       printf( "This is from the user-defined _commit()\n" );
       return 0;
   }

Sample Code 2

   //////////
   // MAIN.C
   //
   // Compile options needed:    /c /MD

   #include <stdio.h>
   #include <process.h>
   #include <io.h>

   FILE *stream;

   void main( void )
   {
       // Open a stream for writing.
       if( (stream = fopen( "data.txt", "w+" )) == NULL )
       {
           printf( "The file 'data.txt' was not opened\n" );
           exit( 1 );
       }
       else
           printf( "The file 'data.txt' was opened\n" );

       // Write to the file stream.
       fprintf( stream, "This is some text" );

       // Flush the CRT buffer.
          fflush( stream );

       // Tell the operating system to commit the data to the file.
       if(_commit( _fileno( stream ) ))
           printf( "There was a problem committing to disk\n" );
       else
           printf( "The data was successfully committed to disk\n" );

       // Close the stream.
       if( fclose( stream ) )
           printf( "The file 'data.txt' was not closed\n" );
   }

   ////////////
   // COMMIT.C
   //
   // Compile options needed:    /c /MD /LD
   //
   // This code replaces the version of _commit() provided by the C
   // run-time library.

   #include <stdio.h>

   __declspec(dllexport) int __cdecl _commit(int filedes );

   int __cdecl _commit(int filedes )
   {
       printf( "This is from the user-defined _commit()\n" );
       return 0;
   }

Keywords          : CRTIss
Version           : WINDOWS NT:1.0,2.0,2.1,4.0,5.0;
Platform          : NT WINDOWS
Issue type        : kbinfo