There is only one low level callback function which get called whenever LIBLINCKS have received a out-band-data from the database server. This callback function is call from within a signal handler and thus should be careful not to ``block'' and to do as little as possible. Use RCB_REGISTERCALLBACK() with RCB_PEN to register your own functions and notice this low level callback overrides any high level callback functions.
The low level callback function could a set flag which would be checked from the normal code. Using PEN_RETRIEVE_MESSAGE() (see section 11.7.4) from the non signal code to retrieve the queued PENs. Then retrieval function then has to act upon PEN information (PEN message type, node index and possible a list of users for CHECK and POLL notifications).