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Based on kernel version 3.19. Page generated on 2015-02-13 21:20 EST.

1	  <title>Event Interface</title>
3	  <para>The V4L2 event interface provides a means for a user to get
4	  immediately notified on certain conditions taking place on a device.
5	  This might include start of frame or loss of signal events, for
6	  example. Changes in the value or state of a V4L2 control can also be
7	  reported through events.
8	  </para>
10	  <para>To receive events, the events the user is interested in first must
11	  be subscribed using the &VIDIOC-SUBSCRIBE-EVENT; ioctl. Once an event is
12	  subscribed, the events of subscribed types are dequeueable using the
13	  &VIDIOC-DQEVENT; ioctl. Events may be unsubscribed using
14	  VIDIOC_UNSUBSCRIBE_EVENT ioctl. The special event type V4L2_EVENT_ALL may
15	  be used to unsubscribe all the events the driver supports.</para>
17	  <para>The event subscriptions and event queues are specific to file
18	  handles. Subscribing an event on one file handle does not affect
19	  other file handles.</para>
21	  <para>The information on dequeueable events is obtained by using select or
22	  poll system calls on video devices. The V4L2 events use POLLPRI events on
23	  poll system call and exceptions on select system call.</para>
25	  <para>Starting with kernel 3.1 certain guarantees can be given with
26	  regards to events:<orderedlist>
27		<listitem>
28		  <para>Each subscribed event has its own internal dedicated event queue.
29	This means that flooding of one event type will not interfere with other
30	event types.</para>
31		</listitem>
32		<listitem>
33		  <para>If the internal event queue for a particular subscribed event
34	becomes full, then the oldest event in that queue will be dropped.</para>
35		</listitem>
36		<listitem>
37		  <para>Where applicable, certain event types can ensure that the payload
38	of the oldest event that is about to be dropped will be merged with the payload
39	of the next oldest event. Thus ensuring that no information is lost, but only an
40	intermediate step leading up to that information. See the documentation for the
41	event you want to subscribe to whether this is applicable for that event or not.</para>
42		</listitem>
43	      </orderedlist></para>
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