8.8. ioctl VIDIOC_DQEVENT

8.8.1. Name

VIDIOC_DQEVENT - Dequeue event

8.8.2. Synopsis

int ioctl(int fd, VIDIOC_DQEVENT, struct v4l2_event *argp)

8.8.3. Arguments

fd

File descriptor returned by open().

argp

8.8.4. Description

Dequeue an event from a video device. No input is required for this ioctl. All the fields of the struct v4l2_event structure are filled by the driver. The file handle will also receive exceptions which the application may get by e.g. using the select system call.

v4l2_event

struct v4l2_event

__u32	type		Type of the event, see Event Types.
union	u
	struct v4l2_event_vsync	vsync	Event data for event V4L2_EVENT_VSYNC.
	struct v4l2_event_ctrl	ctrl	Event data for event V4L2_EVENT_CTRL.
	struct v4l2_event_frame_sync	frame_sync	Event data for event V4L2_EVENT_FRAME_SYNC.
	struct v4l2_event_motion_det	motion_det	Event data for event V4L2_EVENT_MOTION_DET.
	struct v4l2_event_src_change	src_change	Event data for event V4L2_EVENT_SOURCE_CHANGE.
	__u8	data[64]	Event data. Defined by the event type. The union should be used to define easily accessible type for events.
__u32	pending		Number of pending events excluding this one.
__u32	sequence		Event sequence number. The sequence number is incremented for every subscribed event that takes place. If sequence numbers are not contiguous it means that events have been lost.
struct timespec	timestamp		Event timestamp. The timestamp has been taken from the CLOCK_MONOTONIC clock. To access the same clock outside V4L2, use clock_gettime().
u32	id		The ID associated with the event source. If the event does not have an associated ID (this depends on the event type), then this is 0.
__u32	reserved[8]		Reserved for future extensions. Drivers must set the array to zero.

Event Types

V4L2_EVENT_ALL	0	All events. V4L2_EVENT_ALL is valid only for VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
V4L2_EVENT_VSYNC	1	This event is triggered on the vertical sync. This event has a struct v4l2_event_vsync associated with it.
V4L2_EVENT_EOS	2	This event is triggered when the end of a stream is reached. This is typically used with MPEG decoders to report to the application when the last of the MPEG stream has been decoded.
V4L2_EVENT_CTRL	3	This event requires that the id matches the control ID from which you want to receive events. This event is triggered if the control’s value changes, if a button control is pressed or if the control’s flags change. This event has a struct v4l2_event_ctrl associated with it. This struct contains much of the same information as struct v4l2_queryctrl and struct v4l2_control. If the event is generated due to a call to VIDIOC_S_CTRL or VIDIOC_S_EXT_CTRLS, then the event will not be sent to the file handle that called the ioctl function. This prevents nasty feedback loops. If you do want to get the event, then set the V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK flag. This event type will ensure that no information is lost when more events are raised than there is room internally. In that case the struct v4l2_event_ctrl of the second-oldest event is kept, but the changes field of the second-oldest event is ORed with the changes field of the oldest event.
V4L2_EVENT_FRAME_SYNC	4	Triggered immediately when the reception of a frame has begun. This event has a struct v4l2_event_frame_sync associated with it. If the hardware needs to be stopped in the case of a buffer underrun it might not be able to generate this event. In such cases the frame_sequence field in struct v4l2_event_frame_sync will not be incremented. This causes two consecutive frame sequence numbers to have n times frame interval in between them.
V4L2_EVENT_SOURCE_CHANGE	5	This event is triggered when a source parameter change is detected during runtime by the video device. It can be a runtime resolution change triggered by a video decoder or the format change happening on an input connector. This event requires that the id matches the input index (when used with a video device node) or the pad index (when used with a subdevice node) from which you want to receive events. This event has a struct v4l2_event_src_change associated with it. The changes bitfield denotes what has changed for the subscribed pad. If multiple events occurred before application could dequeue them, then the changes will have the ORed value of all the events generated.
V4L2_EVENT_MOTION_DET	6	Triggered whenever the motion detection state for one or more of the regions changes. This event has a struct v4l2_event_motion_det associated with it.
V4L2_EVENT_PRIVATE_START	0x08000000	Base event number for driver-private events.

v4l2_event_vsync

struct v4l2_event_vsync

__u8

field

The upcoming field. See enum v4l2_field.

v4l2_event_ctrl

struct v4l2_event_ctrl

__u32	changes		A bitmask that tells what has changed. See Control Changes.
__u32	type		The type of the control. See enum v4l2_ctrl_type.
union (anonymous)
	__s32	value	The 32-bit value of the control for 32-bit control types. This is 0 for string controls since the value of a string cannot be passed using ioctl VIDIOC_DQEVENT.
	__s64	value64	The 64-bit value of the control for 64-bit control types.
__u32	flags		The control flags. See Control Flags.
__s32	minimum		The minimum value of the control. See struct v4l2_queryctrl.
__s32	maximum		The maximum value of the control. See struct v4l2_queryctrl.
__s32	step		The step value of the control. See struct v4l2_queryctrl.
__s32	default_value		The default value value of the control. See struct v4l2_queryctrl.

v4l2_event_frame_sync

struct v4l2_event_frame_sync

__u32

frame_sequence

The sequence number of the frame being received.

v4l2_event_src_change

struct v4l2_event_src_change

__u32

changes

A bitmask that tells what has changed. See Source Changes.

v4l2_event_motion_det

struct v4l2_event_motion_det

__u32	flags	Currently only one flag is available: if V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ is set, then the frame_sequence field is valid, otherwise that field should be ignored.
__u32	frame_sequence	The sequence number of the frame being received. Only valid if the V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ flag was set.
__u32	region_mask	The bitmask of the regions that reported motion. There is at least one region. If this field is 0, then no motion was detected at all. If there is no V4L2_CID_DETECT_MD_REGION_GRID control (see Detect Control Reference) to assign a different region to each cell in the motion detection grid, then that all cells are automatically assigned to the default region 0.

Control Changes

V4L2_EVENT_CTRL_CH_VALUE	0x0001	This control event was triggered because the value of the control changed. Special cases: Volatile controls do no generate this event; If a control has the V4L2_CTRL_FLAG_EXECUTE_ON_WRITE flag set, then this event is sent as well, regardless its value.
V4L2_EVENT_CTRL_CH_FLAGS	0x0002	This control event was triggered because the control flags changed.
V4L2_EVENT_CTRL_CH_RANGE	0x0004	This control event was triggered because the minimum, maximum, step or the default value of the control changed.

Source Changes

V4L2_EVENT_SRC_CH_RESOLUTION

0x0001

This event gets triggered when a resolution change is detected at an input. This can come from an input connector or from a video decoder.

8.8.5. 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.