efn

    OpenVMS usage:ef_number
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Number of the event flag specified in the call to the system
    service whose completion status is to be checked by $SYNCH.
    The efn argument is a longword containing this number; however,
    $SYNCH uses only the low-order byte.

 iosb

    OpenVMS usage:io_status_block
    type:         quadword (unsigned)
    access:       read only
    mechanism:    by 32- or 64-bit reference (Alpha and Integrity
                  servers)
    I/O status block specified in the call to the system service
    whose completion status is to be checked by $SYNCH. The iosb
    argument is the address of this quadword I/O status block.