Including C-DLL from C++ - c++

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

I know this is a very old question, but I am having exactly the same issues but in relation to writing a generic wrapper template for wrapping calls to LoadLibrary() and GetProcAddress()
Taking https://blog.benoitblanchon.fr/getprocaddress-like-a-boss/ as inspiration, it looks like he is taking FARPROC as a kind of "void* for Windows functions" and then casting it to the correct type subsequently.
I needed to tweak that code a little to work for me, and reproduce it here:
class ProcPtr
{
public:
explicit ProcPtr(FARPROC ptr) : m_ptr(ptr) {}
template <typename T>
operator T* () const { return reinterpret_cast<T*>(m_ptr); }
private:
FARPROC m_ptr;
};
class DllHelper
{
public:
explicit DllHelper(LPCTSTR filename) : m_module(LoadLibrary(filename)) {}
~DllHelper() { FreeLibrary(m_module); }
ProcPtr operator[](LPCSTR proc_name) const
{
return ProcPtr(::GetProcAddress(m_module, proc_name));
}
private:
HMODULE m_module;
};
So, with that helper code now available we can use it to write a wrapper class that encapsulates several functions in the Advapi32 library:
class Advapi32
{
public:
Advapi32() : m_dll(TEXT("Advapi32"))
{
getUserName = m_dll["GetUserNameA"];
openSCManager = m_dll["OpenSCManagerA"];
bogusFunction = m_dll["BogusFunctionThatDoesNotExist"];
}
decltype(GetUserNameA)* getUserName;
decltype(OpenSCManagerA)* openSCManager;
decltype(GetWindowsDirectoryA)* bogusFunction;
private:
DllHelper m_dll;
};
bogusFunction is a function with the same signature as GetWindowsDirectoryA but which doesn't exist in Advapi32. This is what I was trying to achieve - graceful fallback on an older OS which might not have a certain function.
So, finally a test app...
int main()
{
Advapi32 advapi32;
auto func1 = advapi32.getUserName;
if (func1)
{
TCHAR infoBuf[256];
DWORD bufCharCount = sizeof(infoBuf);
if (func1(infoBuf, &bufCharCount))
{
std::cout << "Username: " << infoBuf << std::endl;
}
}
auto func2 = advapi32.openSCManager;
if (func2)
{
SC_HANDLE handle = func2(NULL, NULL, SC_MANAGER_CONNECT);
if (handle)
{
std::cout << "opened SC Manager" << std::endl;
}
}
auto func3 = advapi32.bogusFunction;
if (func3)
{
std::cerr << "This should not happen!" << std::endl;
}
else
{
std::cout << "Function not supported" << std::endl;
}
}
Output:
Username: TestAccount
opened SC Manager
Function not supported
Note: This was compiled as a Windows 32-bit console application with MBCS rather than Unicode, under VS2019 with the VS2015_XP toolset, since that is what I am needing to target (don't ask).

Answer 2

The FARPROC type is a function pointer for a function that takes no parameters. You should declare EXPORTED_functionNameP like so (replacing void with whatever the function really returns):
extern void (*EXPORTED_functionNameP)(int);
And initialize it like so (the returned value from GetProcAddress() pretty much always needs to be cast to the correct type):
EXPORTED_functionNameP = (void (*)(int)) GetProcAddress(drvsHANDLE, "originalFunctionName");
A typedef for the funciton type might make things a bit more readable.

Answer 3

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.

Answer 4

After a quick Google search, it seems that FARPROC is defined as this:
typedef int (FAR WINAPI *FARPROC)();
That is, FARPROC is a function that returns an int and takes no arguments. So you can't use it for any other case.
Instead declare EXPORTED_functionNameP like this:
extern void (*EXPORTED_functionNameP)(int);
Now EXPORTED_functionNameP is a pointer to a function that takes an int argument and returns no value.

Answer 5

It is because of FARPROC is defined as:
int (FAR WINAPI * FARPROC) ()
So you can not pass any parameters to such function in C++. For fix it you should define EXPORTED_functionNameP as pointer to function with equal semantics as defined in DLL-library. For example:
typedef (void* EXPORTED_functionNameP)(int value);
EXPORTED_functionNameP ExportedFns;
...
ExportedFns = GetProcAddress(drvsHANDLE, "originalFunctionName");

Answer 6

FARPROC is defined as
typedef int (FAR WINAPI *FARPROC)();
When you pass an additional argument although the argument list of the prototype is empty you get the error.
You need a proper prototype definition for PORTED_functionNameP and cas the result from GetProcAddress to that type in your GetDLLPopinters functions.