Based on kernel version 4.2. Page generated on 2015-09-09 12:12 EST.
1 <title>Event Interface</title> 2 3 <para>The V4L2 event interface provides a means for a 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. Changes in the value or state of a V4L2 control can also be 7 reported through events. 8 </para> 9 10 <para>To receive events, the events the user is interested in first must 11 be subscribed using the &VIDIOC-SUBSCRIBE-EVENT; ioctl. Once an event is 12 subscribed, the events of subscribed types are dequeueable using the 13 &VIDIOC-DQEVENT; ioctl. Events may be unsubscribed using 14 VIDIOC_UNSUBSCRIBE_EVENT ioctl. The special event type V4L2_EVENT_ALL may 15 be used to unsubscribe all the events the driver supports.</para> 16 17 <para>The event subscriptions and event queues are specific to file 18 handles. Subscribing an event on one file handle does not affect 19 other file handles.</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 25 <para>Starting with kernel 3.1 certain guarantees can be given with 26 regards to events:<orderedlist> 27 <listitem> 28 <para>Each subscribed event has its own internal dedicated event queue. 29 This means that flooding of one event type will not interfere with other 30 event types.</para> 31 </listitem> 32 <listitem> 33 <para>If the internal event queue for a particular subscribed event 34 becomes full, then the oldest event in that queue will be dropped.</para> 35 </listitem> 36 <listitem> 37 <para>Where applicable, certain event types can ensure that the payload 38 of the oldest event that is about to be dropped will be merged with the payload 39 of the next oldest event. Thus ensuring that no information is lost, but only an 40 intermediate step leading up to that information. See the documentation for the 41 event you want to subscribe to whether this is applicable for that event or not.</para> 42 </listitem> 43 </orderedlist></para>