| <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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