VIDIOC_DQEVENT — Dequeue event


int ioctl(int fd,
 int request,
 struct v4l2_event *argp);



File descriptor returned by open().





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.

Table A.12. struct v4l2_event

__u32type Type of the event, see Table A.13, “Event Types”.
 struct v4l2_event_vsyncvsyncEvent data for event V4L2_EVENT_VSYNC.
 struct v4l2_event_ctrlctrlEvent data for event V4L2_EVENT_CTRL.
 struct v4l2_event_frame_syncframe_syncEvent data for event V4L2_EVENT_FRAME_SYNC.
 struct v4l2_event_motion_detmotion_detEvent data for event V4L2_EVENT_MOTION_DET.
 struct v4l2_event_src_changesrc_changeEvent data for event V4L2_EVENT_SOURCE_CHANGE.
 __u8data[64]Event data. Defined by the event type. The union should be used to define easily accessible type for events.
__u32pending Number of pending events excluding this one.
__u32sequence 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 timespectimestamp Event timestamp. The timestamp has been taken from the CLOCK_MONOTONIC clock. To access the same clock outside V4L2, use clock_gettime(2).
u32id 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.
__u32reserved[8] Reserved for future extensions. Drivers must set the array to zero.

Table A.13. Event Types

V4L2_EVENT_ALL0All events. V4L2_EVENT_ALL is valid only for VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
V4L2_EVENT_VSYNC1This event is triggered on the vertical sync. This event has a struct v4l2_event_vsync associated with it.
V4L2_EVENT_EOS2This 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.

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.


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.


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.


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_START0x08000000Base event number for driver-private events.

Table A.14. struct v4l2_event_vsync

__u8fieldThe upcoming field. See enum v4l2_field.

Table A.15. struct v4l2_event_ctrl

__u32changes A bitmask that tells what has changed. See Table A.19, “Control Changes”.
__u32type The type of the control. See enum v4l2_ctrl_type.
union (anonymous)   
 __s32valueThe 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 VIDIOC_DQEVENT.
 __s64value64The 64-bit value of the control for 64-bit control types.
__u32flags The control flags. See Table A.99, “Control Flags”.
__s32minimum The minimum value of the control. See struct v4l2_queryctrl.
__s32maximum The maximum value of the control. See struct v4l2_queryctrl.
__s32step The step value of the control. See struct v4l2_queryctrl.
__s32default_value The default value value of the control. See struct v4l2_queryctrl.

Table A.16. struct v4l2_event_frame_sync

__u32frame_sequence The sequence number of the frame being received.

Table A.17. struct v4l2_event_src_change

__u32changes A bitmask that tells what has changed. See Table A.20, “Source Changes”.

Table A.18. struct v4l2_event_motion_det

__u32flags 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.
__u32frame_sequence The sequence number of the frame being received. Only valid if the V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ flag was set.
__u32region_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 the section called “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.

Table A.19. Control Changes

V4L2_EVENT_CTRL_CH_VALUE0x0001This 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_FLAGS0x0002This control event was triggered because the control flags changed.
V4L2_EVENT_CTRL_CH_RANGE0x0004This control event was triggered because the minimum, maximum, step or the default value of the control changed.

Table A.20. Source Changes

V4L2_EVENT_SRC_CH_RESOLUTION0x0001This event gets triggered when a resolution change is detected at an input. This can come from an input connector or from a video decoder.

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.