Description

   When your symbiont calls the SMB$INITIALIZE routine to initialize
   the interface between the symbiont and the job controller, you
   can choose to have requests from the job controller delivered
   by means of an AST. If you choose not to use ASTs, your symbiont
   must call SMB$CHECK_FOR_MESSAGE during the processing of tasks in
   order to see if a message from the job controller is waiting to
   be read. If a message is waiting, SMB$CHECK_FOR_MESSAGE returns a
   success code; if not, it returns a zero.

   If a message is waiting, the symbiont should call SMB$READ_
   MESSAGE to read it to determine if immediate action should be
   taken (as in the case of STOP_TASK, RESET_STREAM or PAUSE_TASK).

   If a message is not waiting, SMB$CHECK_MESSAGE returns a zero.
   If this condition is detected, the symbiont should continue
   processing the request at hand.