IPC on Windows CE using SendMessage

How does one send data between applications on a windows CE system? Having searched for two days now I still can't send a single message... This is roughly how I try to send a simple message from app A(sender) to app B(receiver): app A: int MY_MSG = RegisterWindowMessage( "WM_MY_MSG" ); hWnd = FindWindow( null, "app A" ); SendMessage( hWnd, MY_MSG, 0 , 0 ); app B: override MessageWindow class to catch Windows messages with WndProc int MY_MSG = RegisterWindowMessage( "WM_MY_MSG" ); if( msg.Msg == MY_MSG ) then // take action Well, even this simple setup doesn't work at all! App B never receives my message, so obviously I'm wrong somewhere, but where I can't find out (likely my sending method but I can't see where I'm going wrong there). The next step would be to send actual data using WM_COPYDATA from and to (using structs with strings and integers), but this problem should be tackled first. Can someone tell me where I'm going wrong here? Thank you