IPC with WM_COPYDATA

Inter-process communications (IPC) in Windows can be accomplished in a variety of ways. Essentially these different implementations boil down to just 2 types: local and network based IPC. Local implementations like COM and WM_COPYDATA are all just forms of passing data with memory mapped files. Setting up a custom memory mapped file mechanism can be rather involved, and creating COM applications for a simple IPC operation between two applications can result in a bit of code and registration hassles.

This is where WM_COPYDATA can provide a quick IPC link between two applications. It really makes sense when the messages occur between the application windows themselves, but even that isn't a requirement. Here is an example where a request is sent to a server application by first finding the window handle of the server process and sending the data via a WM_COPYDATA message along with a handle to a window that will process any response, which occurs before the original SendMessage returns in this case. The code below would even work from multiple threads.


	char result[7];

	HWND hwnd = FindWindow(0, _T("MyServerWin"));

	if (hwnd)

	{

	   char data[] = "REQUEST:ACK";

	   TCHAR className[] = _T("MyMessageClass");

	   static bool registered = false;

	

	   if (!registered)

	   {

	      WNDCLASSEX wndClass = {0};

	      wndClass.lpfnWndProc = (WNDPROC)RespWindowProc;

	      wndClass.hInstance = g_hInstance;

	      wndClass.lpszClassName = className;

	      wndClass.cbSize = sizeof(wndClass);

	

	      RegisterClassEx(&wndClass);

	      registered = true;

	   }

	

	   HWND hwndResp = CreateWindow(className, _T(""), 0, 0, 0, 0, 0, 0, 0, 0, 0);

	   SetWindowLong(hwndResp, GWL_USERDATA, (LONG)result);

	

	   COPYDATASTRUCT request;

	   request.dwData = 0;

	   request.cbData = strlen(data) + 1;

	   request.lpData = data;

	   retval = SendMessage(hwnd, WM_COPYDATA, (WPARAM)hwndResp, (LPARAM)&request);

	

	   DestroyWindow(hwndResp);

	}

The window procedure processing the response would look something like this:


	LRESULT CALLBACK RespWindowProc(HWND HWnd, UINT nMsg, WPARAM wParam, LPARAM lParam)

	{

	   if (nMsg == WM_COPYDATA)

	   {

	      COPYDATASTRUCT *data = (COPYDATASTRUCT *)lParam;

	      char *result = (char *)GetWindowLong(HWnd, GWL_USERDATA);

	      strcpy_s(result, 7, (char *)data->lpData);

	      return 0;

	   }

	

	   return DefWindowProc(HWnd, nMsg, wParam, lParam);

	}

(TCHAR and _tcs functions are macros for MBCS/Unicode compatibility)

IPC with WM_COPYDATA

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