C / C++ / MFC

// Handle to the mutex object
HANDLE MutexHandle;


// Check whether an instance of the app is already running
BOOL IsAlreadyRunning()
{
	// Create a mutex object
	MutexHandle = CreateMutex(NULL, TRUE, "YOURAPPNAME");
	// Does the mutex exist?
	if(GetLastError() == ERROR_ALREADY_EXISTS)
	{	// The mutex exists, the app is running
		return(true);
	}
	else
	{	// The mutex does not exist, go ahead with the app
		return(false);
	}
}



void CloseThisInstance()
{
	ReleaseMutex(MutexHandle);
	CloseHandle(MutexHandle);
}



...


// In your InitInstance or somewhere
	// Is the app already running?
	if(IsAlreadyRunning()) 
	{	// Yes, stop right now
		MessageBox(NULL, _T("The app is already running!"), _T("Your app name"), MB_OK);
	}
	else	// App not running, go ahead
	{
		// Your code
	}



...


// In ExitInstance or somewhere
	// We are done, so perform all the cleanup now
	CloseThisInstance();

static CMutex mutex(FALSE, "YOURAPPNAME");
if (::GetLastError() == ERROR_ALREADY_EXISTS)    	
   return FALSE;  // already running 

if(IsWindow(hwnd))
{
  CThing *pThing = new CThing;
  PostMessage(hwnd, WM_NEW_THING, 0, (LPARAM)pThing);
}

long CMyWnd::OnNewThing(WPARAM wParam, LPARAM lParam)
{
  ASSERT(lParam);
  CThing *pThing = (CThing *)lParam;
  ...
  delete pThing;
}

MSG msg;
while(::PeekMessage(&msg, (HWND)NULL, WM_NEW_THING, WM_NEW_THING, PM_REMOVE))
{
  CThing *pThing = (CThing *)msg.lParam;
  delete pThing;
}