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

ID: Q117428

The information in this article applies to:

The Microsoft C/C++ Compiler (CL.EXE)
- Microsoft C/C++ for MS-DOS, version 7.0
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52
- 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
Version : MS- DOS:7.0;WIN3X:1.0,1.5,1.51,1.52;WINNT:1.0,2.0,2.1,4.0,5.0;
Platform : MS-DOS NT WINDOWS
Issue type : kbinfo