VMS Help SMB Routines, SMB$CHECK_FOR_MESSAGE, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    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.