VMS Help TCPIP Services, Programming Interfaces, Sockets API, $CANCEL, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    To cancel I/O on a channel, the access mode of the calling
    process must be equal to or more privileged than the access mode
    of the process that made the original channel assignment.
    The $CANCEL service requires system dynamic memory and uses the
    process's buffered I/O limit (BIOLM) quota.
    When a request currently in progress is canceled, the driver is
    notified immediately. Actual cancellation may or may not occur
    immediately, depending on the logical state of the driver. When
    cancellation does occur, the action taken for I/O in progress is
    similar to that taken for queued requests. For example:
    o  The specified event flag is set.
    o  The first word of the I/O status block, if specified, is set
       to SS$_CANCEL if the I/O request is queued, or to SS$_ABORT if
       the I/O operation is in progress.
    o  If the asynchronous system trap (AST) is specified, it is
       queued.
    For proper synchronization between this service and the actual
    canceling of I/O requests to take place, the issuing process
    must wait for the I/O process to complete normally. Note that
    the I/O has been canceled. Outstanding I/O requests are canceled
    automatically at image exit.