Win32 API calls throwing access violation - c++

Question

I've got a simple piece of C++ code I'm exporting from a DLL.
DWORD WINAPI MessageBoxThread(LPVOID lpParam)
{
MessageBox(0, L"Test", L"Test", 0);
return 0;
}
Here's how I'm calling it
typedef DWORD(*MessageBoxThread)(LPVOID);
int StartMessageBoxThread() {
MessageBoxThread ShowMessageBox;
HMODULE testModule = LoadLibrary(L"C:\\Users\\david\\COMServer.dll");
ShowMessageBox = (MessageBoxThread)GetProcAddress(testModule, "MessageBoxThread");
ShowMessageBox(NULL);
FreeLibrary(testModule);
return 0;
}
I get an exception thrown in KernelBase.dll on the ShowMessageBox() line, involved an access violation when writing to a memory location.
I can't understand what I'm doing wrong. Both Visual Studio projects are set to Unicode, and I know using the L prefix denotes wide strings.
I can debug and step through into my DLL, I see the address of my function, so I can't see anything wrong with the code calling the function.

Answer 1

typedef DWORD(*MessageBoxThread)(LPVOID);
The prototype does not match the definition in dll. By default calling convention here is __cdecl whereas WINAPI is __stdcall
typedef DWORD(WINAPI *MessageBoxThread)(LPVOID);
Specifically, at the called end, since the convention is __stdcall(callee clears the stack), the function pops argument off the stack. At the caller end, it sees that the convention is __cdecl(caller clears the stack) and it also pops the argument from the stack, eventually corrupting the stack.

Answer 2

In your StartMessageBoxThread() code, MessageBoxThread is declared incorrectly. Specifically, it is missing a calling convention, so it uses the compiler's default convention, which is typically __cdecl rather than __stdcall (what WINAPI maps to). Calling convention mismatches are a common cause of crashes, call stack corruption, etc.
Also, the code has no error checking at all.
Try this instead:
typedef DWORD (WINAPI *MessageBoxThread)(LPVOID);
int StartMessageBoxThread()
{
HMODULE testModule = LoadLibrary(L"C:\\Users\\david\\COMServer.dll");
if (testModule)
{
MessageBoxThread ShowMessageBox = (MessageBoxThread) GetProcAddress(testModule, "MessageBoxThread");
if (ShowMessageBox)
ShowMessageBox(NULL);
FreeLibrary(testModule);
}
return 0;
}