Returns the file handle to which a PEN message is written upon recived to LIBLINCKS. When you are able to read from this file descriptoer, you should call POLL_oob() (see section 11.7.3) with a pointer to a non-zero integer to retrieve the PEN message(s).
Notice that is only necessary if you do not implement your own low level PEN handle (as described in section 11.7.1), and also if you disables the polling as described in section 11.7.1.