| <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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