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- <title>Event Interface</title>
- <para>The V4L2 event interface provides a means for a 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. Changes in the value or state of a V4L2 control can also be
- reported through events.
- </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>
- <para>Starting with kernel 3.1 certain guarantees can be given with
- regards to events:<orderedlist>
- <listitem>
- <para>Each subscribed event has its own internal dedicated event queue.
-This means that flooding of one event type will not interfere with other
-event types.</para>
- </listitem>
- <listitem>
- <para>If the internal event queue for a particular subscribed event
-becomes full, then the oldest event in that queue will be dropped.</para>
- </listitem>
- <listitem>
- <para>Where applicable, certain event types can ensure that the payload
-of the oldest event that is about to be dropped will be merged with the payload
-of the next oldest event. Thus ensuring that no information is lost, but only an
-intermediate step leading up to that information. See the documentation for the
-event you want to subscribe to whether this is applicable for that event or not.</para>
- </listitem>
- </orderedlist></para>