Q117428: INFO: GetProcAddress(), Function Pointers, and C++

Article: Q117428
Product(s): Microsoft C Compiler
Version(s): 1.0,2.0,2.1,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbLangCPP kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 11-FEB-2002

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

- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0 
-------------------------------------------------------------------------------

SUMMARY
=======

When porting code that uses GetProcAddress() from C to C++, the C++ compiler for
MS-DOS can return the following error message:

  error C2564: formal/actual parameters mismatch in call through pointer to
  function

The error message that is returned with the 32-bit compiler is:

  error C2197: 'int (__stdcall *)(void )' : too many actual parameters

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

In a traditional C application, use GetProcAddress() to obtain the address of a
function to be called. Declare a variable of type FARPROC, initialize the
pointer with the value returned from GetProcAddress(), and then call the
function through a pointer as shown:

  void func1(void)
  {
     HINSTANCE       hLib;
     FARPROC         lpfnDLLProc;
     UINT            param1 = 1;
     int             param2;

     hLib = LoadLibrary ("dll1.dll");
     if (hLib)
     {
        lpfnDLLProc = GetProcAddress (hLib, "DLLProc");
        (*lpfnDLLProc) (param1, (LPINT)&param2);
        FreeLibrary (hLib);
     }
  }

When not compiling with STRICT, FARPROC is defined in the WINDOWS.H file as
follows:

  typedef int (CALLBACK* FARPROC)();

When the sample code above is converted to C++, a type-mismatch error occurs
because C and C++ have a fundamental difference in the way they interpret empty
parentheses in function declarations. In C, a function declared as follows:

  int (*funcptr)();

declares a function that accepts an unknown number of arguments. In C++, the same
declaration represents a function that accepts no arguments. In other words, in
C++, the statement is equivalent to:

  int (*funcptr)(void);

Because of this difference, when a pointer of type FARPROC is used to call a
function with parameters in C, no error occurs. In C++, when the function being
passed to GetProcAddress() has parameters, the formal/actual-parameter-mismatch
error occurs because the function of type FARPROC is defined as a function that
has void parameters rather than as a function that accepts parameters.

To eliminate the error, define the function pointer as a pointer to a function
with the correct number of parameters and then typecast the return value from
GetProcAddress() to the appropriate type:

  typedef void (CALLBACK *ULPRET)(UINT,LPINT);

  void func1(void)
  {
     HINSTANCE       hLib;
     ULPRET          lpfnDLLProc;
     UINT            param1 = 1;
     int             param2;

     hLib = LoadLibrary ("dll1.dll");
     if (hLib)
     {
        lpfnDLLProc = (ULPRET) GetProcAddress (hLib,"DLLProc");
        (*lpfnDLLProc) (param1, (LPINT)&param2);
        FreeLibrary (hLib);
     }
  }

Additional query words: 8.00 8.00c 9.00 9.10

======================================================================
Keywords          : kbLangCPP kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbvc100 kbVC500 kbVC600 kbVC200 kbVC210 kbVC32bitSearch kbVC500Search
Version           : :1.0,2.0,2.1,4.0,5.0,6.0
Issue type        : kbinfo

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

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 1986-2002.