p4
    OpenVMS usage:mask_longword
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Longword of shutdown flags to specify the disposition of pending
    I/O operations on the socket. The p4 argument is used only with
    the IO$M_SHUTDOWN function modifier. The following table lists
    available shutdown flags.
    Shutdown Flag   Description
    TCPIP$C_DSC_    Discards messages from the receive queue and
    RCV             disallows further receiving. Pending messages
                    in the receive queue for this connection are
                    discarded.
    TCPIP$C_DSC_    Discards messages from the send queue and
    SND             disallows sending new messages. Pending messages
                    in the transmit queue for this connection are
                    discarded.
    TCPIP$C_DSC_    Discards all messages and disallows both
    ALL             sending and receiving. All pending messages are
                    discarded.
                    Specifying this flag has the same effect as
                    issuing a $CANCEL QIO followed by an IO$_DEACCESS
                    QIO without specifying any flags.