7.63. ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT

7.63.1. Name

VIDIOC_SUBSCRIBE_EVENT - VIDIOC_UNSUBSCRIBE_EVENT - Subscribe or unsubscribe event

7.63.2. Synopsis

int ioctl(int fd, VIDIOC_SUBSCRIBE_EVENT, struct v4l2_event_subscription *argp)
int ioctl(int fd, VIDIOC_UNSUBSCRIBE_EVENT, struct v4l2_event_subscription *argp)

7.63.3. Arguments

fd

File descriptor returned by open().

argp

Pointer to struct v4l2_event_subscription.

7.63.4. Description

Subscribe or unsubscribe V4L2 event. Subscribed events are dequeued by using the ioctl VIDIOC_DQEVENT ioctl.

v4l2_event_subscription
struct v4l2_event_subscription

__u32

type

Type of the event, see Event Types.

Note

V4L2_EVENT_ALL can be used with VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.

__u32

id

ID of the event source. If there is no ID associated with the event source, then set this to 0. Whether or not an event needs an ID depends on the event type.

__u32

flags

Event flags, see Event Flags.

__u32

reserved[5]

Reserved for future extensions. Drivers and applications must set the array to zero.

Event Flags

V4L2_EVENT_SUB_FL_SEND_INITIAL

0x0001

When this event is subscribed an initial event will be sent containing the current status. This only makes sense for events that are triggered by a status change such as V4L2_EVENT_CTRL. Other events will ignore this flag.

V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK

0x0002

If set, then events directly caused by an ioctl will also be sent to the filehandle that called that ioctl. For example, changing a control using VIDIOC_S_CTRL will cause a V4L2_EVENT_CTRL to be sent back to that same filehandle. Normally such events are suppressed to prevent feedback loops where an application changes a control to a one value and then another, and then receives an event telling it that that control has changed to the first value.

Since it can’t tell whether that event was caused by another application or by the VIDIOC_S_CTRL call it is hard to decide whether to set the control to the value in the event, or ignore it.

Think carefully when you set this flag so you won’t get into situations like that.

7.63.5. Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.