Thread: Access violation when loading function form dll

Hello everyone,
I am attempting to make a function call from a dll file, the function in question is suppose to load a separate window. However when I make the call I the windows is loaded(I can see the GUI), but crashes straight away.
I therefore attempted to write a small sample app with a function of same return type and number of parameters as the one I am suppose to be calling, just for testing purposes, however I am running into a memory access violation error while trying to implement the sample app to execute a function from a dll; leading me to conclude that there is a fundamental mistake in my code.

I have to source files,
File 1: LibTest.cpp which is as follow

Code:

// LibTest.cpp : Defines the exported functions for the DLL application.
//

#include "stdafx.h"
#include <Windows.h>

void DisplayMsg(){
	MessageBox(0, L"Hello from test lib", L"Test Lib", MB_OK);
}

With file one I was able to create a dll, please note that I must use dynamic loading since the ".lib" file is not available for my final task. Once the dll created, I created a second sample app to attempt to call the function above, here is the code of the procedure making the call:

Code:

#include <windows.h>
void LoadMfKbdDll() {
	//Loading the library
	HINSTANCE hDLL = LoadLibrary(TEXT("LibTest.dll"));

	if (hDLL != NULL){
		MessageBox(0, L"Test Lib Loaded", L"Test Lib", MB_OK);
		//Get The point to the Display Message function in the dll
		typedef void (__stdcall* pICFUNC)(); //building a template for the DisplayMsg function
		FARPROC lpfnGetProcessID = GetProcAddress(HMODULE(hDLL),"DisplayMsg");
		
		pICFUNC DisplayMsg;
		DisplayMsg =  pICFUNC(lpfnGetProcessID);
		DisplayMsg(); //calling the function
		FreeLibrary(hDLL); //releasing the dll
	}
	else {
		MessageBox(0, L"Error Loading libTest.dll", L"Login UI", MB_OK);
	}
}

Now my problem is every time I attempt to run the file, I run into a memory access violation, I tried to debug as much as I could but I have now hit a solid wall, any help or suggestions would be greatly appreciated. Thanks

Yan
Note:
1) I am using VS 2010, SP1 on a Win7 x64 SP1 machine.
2) I have posted this question in another forum also, I hope it is not against regulations.

Check the return value from GetProcAddress(). It returns a NULL value if it fails - dereferencing that value (i.e. calling the function) will generally yield a crash.

The first argument to GetProcAddress() would normally, in your code, be hDLL, not HMODULE(hMfdll) - whatever that is.

Since your DLL source file is named with a .cpp extension, it will be compiled as C++ by default (depending on compiler settings though). If it is compiled as C++, the name will be mangled - which means its name as exported from the DLL is not as simple as "DisplayMsg". To prevent the mangling, declare it as extern "C".

You also need to check the calling conventions match (although __stdcall is the default for VS).

There are multiple problems.

1. What is hMfDll? You should be using hDLL.

2. DLL exported functions must be declared extern "C", otherwise name mangling will bite you.

3. It's good practice for DLL exports to be declared with __stdcall convention (actually, WINAPI, but it's the same thing).

The default calling convention in Visual Studio is NOT __stdcall, it is __cdecl.