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1 <title>Event Interface</title>
2
3 <para>The V4L2 event interface provides means for 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.
7 </para>
8
9 <para>To receive events, the events the user is interested in first must
10 be subscribed using the &VIDIOC-SUBSCRIBE-EVENT; ioctl. Once an event is
11 subscribed, the events of subscribed types are dequeueable using the
12 &VIDIOC-DQEVENT; ioctl. Events may be unsubscribed using
13 VIDIOC_UNSUBSCRIBE_EVENT ioctl. The special event type V4L2_EVENT_ALL may
14 be used to unsubscribe all the events the driver supports.</para>
15
16 <para>The event subscriptions and event queues are specific to file
17 handles. Subscribing an event on one file handle does not affect
18 other file handles.
19 </para>
20
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>
24
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