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.13. struct v4l2_event

__u32type Type of the event.
 struct v4l2_event_vsyncvsyncEvent data for event V4L2_EVENT_VSYNC.
 struct v4l2_event_ctrlctrlEvent data for event V4L2_EVENT_CTRL.
 struct v4l2_event_frame_syncframeEvent data for event V4L2_EVENT_FRAME_SYNC.
 __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.
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.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.17, “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.90, “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. Changes

V4L2_EVENT_CTRL_CH_VALUE0x0001This control event was triggered because the value of the control changed. Special case: if a button control is pressed, then this event is sent as well, even though there is not explicit value associated with a button control.
V4L2_EVENT_CTRL_CH_FLAGS0x0002This control event was triggered because the control flags changed.

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.