# Event Interface

The V4L2 event interface provides a means for a 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. Changes in the value or state of a V4L2 control can also be reported through events.

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.

The event subscriptions and event queues are specific to file handles. Subscribing an event on one file handle does not affect other file handles.

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.

Starting with kernel 3.1 certain guarantees can be given with regards to events:

1. Each subscribed event has its own internal dedicated event queue. This means that flooding of one event type will not interfere with other event types.

2. If the internal event queue for a particular subscribed event becomes full, then the oldest event in that queue will be dropped.

3. Where applicable, certain event types can ensure that the payload of the oldest event that is about to be dropped will be merged with the payload of the next oldest event. Thus ensuring that no information is lost, but only an intermediate step leading up to that information. See the documentation for the event you want to subscribe to whether this is applicable for that event or not.