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WindowProc: The WindowProc function is an application-defined function that processes messages sent to a window. WPARAM and LPARAM are third and forth parameter in WindowProc() callback. Both WPARAM and LPARAM are dependent on message identifier. For some message identifiers these has values and for some others these are either NULL or 0.

 
LRESULT CALLBACK WindowProc(
  HWND hwnd,      // handle to window
  UINT uMsg,      // message identifier
  WPARAM wParam,  // first message parameter
  LPARAM lParam   // second message parameter
);
Parameters
  • hwnd - Handle to the window.
  • uMsg - Specifies the message.
  • wParam - Specifies additional message information. The contents of this parameter depend on the value of the uMsg parameter.
  • lParam - Specifies additional message information. The contents of this parameter depend on the value of the uMsg parameter.

Example: Mouse move or WM_MOUSEMOVE message has the following meaning for WPARAM and LPARAM

Message: WM_MOUSEMOVE 
Mouse Kyes: fwKeys = wParam;
Mouse horizontal position: xPos = LOWORD(lParam);
Mouse vertical position: yPos = HIWORD(lParam); 

Minimal Window Callback Function:

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
  switch (message) 
  {
    case WM_DESTROY:
      PostQuitMessage(0);
      break;
    default:
      return DefWindowProc(hWnd, message, wParam, lParam);
   }
   return 0;
}

int APIENTRY WinMain(HINSTANCE hInstance,
                     HINSTANCE hPrevInstance,
                     LPSTR     lpCmdLine,
                     int       nCmdShow)
{
  /*Define WNDCLASS*/

  /*RegisterClass*/

  /*CreateWindow*/

  /*ShowWindow*/
  
  /*UpdateWindow*/
  
  while (GetMessage(&msg, NULL, 0, 0)) 
  {
    DispatchMessage(&msg);
  }
  return 0;
}

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