[NumLock]

The BN_CLICKED notification message is sent when the user clicks a button. The parent window of the button receives this notification message through the WM_COMMAND message. Unlike the other button notification messages, this message is intended for applications written for any version of Windows.

BN_CLICKED
idButton = (int) LOWORD(wParam); // identifier of button
hwndButton = (HWND) lParam; // handle of button

http://msdn.microsoft.com/en-us/libr...(v=vs.85).aspx