7.8. ioctl VIDIOC_DQEVENT¶
7.8.1. Name¶
VIDIOC_DQEVENT - Dequeue event
7.8.2. Synopsis¶
-
int
ioctl
(int fd, VIDIOC_DQEVENT, struct v4l2_event *argp)¶
7.8.3. Arguments¶
fd
- File descriptor returned by open().
argp
- Pointer to struct
v4l2_event
.
7.8.4. 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
¶
__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. |
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 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 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_FRAME_SYNC |
4 | Triggered immediately when the reception of a frame has begun.
This event has a struct
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 |
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 This event has a struct
|
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
¶
__u8 | field |
The upcoming field. See enum v4l2_field . |
-
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
¶
__u32 | frame_sequence |
The sequence number of the frame being received. |
-
v4l2_event_src_change
¶
__u32 | changes |
A bitmask that tells what has changed. See Source Changes. |
-
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. |
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. |
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. |
7.8.5. 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.