SendMessage

받는 쪽

#define WM_MESSGAE WM_USER + 1

LRESULT OnRequestFromClient( WPARAM wParam, LPARAM lParam );

ON_MESSAGE( WM_MESSAGE, OnRequestFromClient)

보내는 쪽

HWND hWnd = ::FindWindow( NULL, "Process Name" );

::SendMessage( hWnd, WM_MESSAGE, 0, 0 );