| Sakari Ailus | dd96608 | 2010-03-27 10:58:24 -0300 | [diff] [blame] | 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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