fdReadwrite

(typename $event
  (record
    ;;; User-provided value that got attached to `subscription::userdata`.
    (field $userdata $userdata)
    ;;; If non-zero, an error that occurred while processing the subscription request.
    (field $error $errno)
    ;;; The type of event that occured
    (field $type $eventtype)
    ;;; The contents of the event, if it is an `eventtype::fd_read` or
    ;;; `eventtype::fd_write`. `eventtype::clock` events ignore this field.
    (field $fd_readwrite $event_fd_readwrite)
  )
)