dev-event.xml

  <title>Event Interface</title>

  <para>The V4L2 event interface provides means for user to get
  immediately notified on certain conditions taking place on a device.
  This might include start of frame or loss of signal events, for
  example.
  </para>

  <para>To receive events, the events the user is interested in first must
  be subscribed using the &VIDIOC-SUBSCRIBE-EVENT; ioctl. Once an event is
  subscribed, the events of subscribed types are dequeueable using the
  &VIDIOC-DQEVENT; ioctl. Events may be unsubscribed using
  VIDIOC_UNSUBSCRIBE_EVENT ioctl. The special event type V4L2_EVENT_ALL may
  be used to unsubscribe all the events the driver supports.</para>

  <para>The event subscriptions and event queues are specific to file
  handles. Subscribing an event on one file handle does not affect
  other file handles.
  </para>

  <para>The information on dequeueable events is obtained by using select or
  poll system calls on video devices. The V4L2 events use POLLPRI events on
  poll system call and exceptions on select system call.  </para>

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