Question

This feels like a noob question, so if it’s a dupe, please point me to the right location 🙂

I tried including a DLL written in C into a C++ program. It didn’t work; gcc said

test.cpp: xxx: error: too many arguments to function.

Here’s a minimal working example:

Wrapper for DLL functions:

/* myWrapper.h */

#ifndef _MYWRAPPER_H
#define _MYWRAPPER_H
#include <windows.h>

#ifdef __cplusplus
extern "C" {
#endif

extern  FARPROC   EXPORTED_functionNameP;

int GetDLLpointers();

#ifdef __cplusplus
}
#endif

#endif

Implementation thereof:

/* myWrapper.c */
#include <windows.h>
#include "myHeader.h"

#ifdef __cplusplus
extern "C" {
#endif

HINSTANCE drvsHANDLE;

extern  FARPROC   EXPORTED_functionNameP;

int GetDLLpointers()
{
    static int result;

    drvsHANDLE = LoadLibrary("myLibrary.dll");
    if (drvsHANDLE == NULL) return (result=0);

    EXPORTED_functionNameP = GetProcAddress(
        drvsHANDLE, "originalFunctionName");    
    if (EXPORTED_functionNameP == NULL) return (result = 0);

    return (result = 1);
}

#ifdef __cplusplus
}
#endif

Naturally, I haven’t written these nor the library myself, and preferably, they should all stay untouched. I did however add the extern "C" lines.

Then, my main file:

// my Main
#include <windows.h>
#include "myHeader.h"

int main(int argc, char **argv)
{
    int arg = 1;
    EXPORTED_functionNameP(arg);

    return 0;
}

Build commands:

gcc -I. -c -o myHeader.o myHeader.c -L. -lmyLibrary
g++ -I. -o main.exe myMain.cpp myHeader.o -L. -lmyLibrary

It works fine if I rewrite my main.cpp into valid C and compile with gcc instead of g++.

I tried changing extern "C" into extern "C++" to no avail, I tried all permutations or gcc and g++ for the two build commands, nothing.

I know it’s something to do with name mangling, but I thought gcc would take care of that when you include the extern "C" lines…Can someone please explain what I’m missing here?

In case it matters —

Windows XP Pro (will be Win7 later on)

(MinGW) gcc 4.6.2

Answer 1

There is a difference between C and C++.

int (FAR WINAPI * FARPROC) () 

In C, the FARPROC declaration indicates a callback function that has an unspecified parameter list. In C++, however, the empty parameter list in the declaration indicates that a function has no parameters.

The MSDN page on CallWindowProc explains a bit more.