.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L
.. _VIDIOC_DQEVENT:
********************
ioctl VIDIOC_DQEVENT
********************
Name
====
VIDIOC_DQEVENT - Dequeue event
Synopsis
========
.. c:macro:: VIDIOC_DQEVENT
``int ioctl(int fd, VIDIOC_DQEVENT, struct v4l2_event *argp)``
Arguments
=========
``fd``
File descriptor returned by :c:func:`open()`.
``argp``
Pointer to struct :c:type:`v4l2_event`.
Description
===========
Dequeue an event from a video device. No input is required for this
ioctl. All the fields of the struct :c:type:`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.
.. c:type:: v4l2_event
.. tabularcolumns:: |p{3.0cm}|p{3.4cm}|p{10.9cm}|
.. flat-table:: struct v4l2_event
:header-rows: 0
:stub-columns: 0
:widths: 1 1 2
* - __u32
- ``type``
- Type of the event, see :ref:`event-type`.
* - union {
- ``u``
* - struct :c:type:`v4l2_event_vsync`
- ``vsync``
- Event data for event ``V4L2_EVENT_VSYNC``.
* - struct :c:type:`v4l2_event_ctrl`
- ``ctrl``
- Event data for event ``V4L2_EVENT_CTRL``.
* - struct :c:type:`v4l2_event_frame_sync`
- ``frame_sync``
- Event data for event ``V4L2_EVENT_FRAME_SYNC``.
* - struct :c:type:`v4l2_event_motion_det`
- ``motion_det``
- Event data for event V4L2_EVENT_MOTION_DET.
* - struct :c:type:`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 :c:func:`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.
.. tabularcolumns:: |p{6.2cm}|p{2.6cm}|p{8.5cm}|
.. cssclass:: longtable
.. _event-type:
.. flat-table:: Event Types
:header-rows: 0
:stub-columns: 0
:widths: 3 1 4
* - ``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 :c:type:`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
:c:type:`v4l2_event_ctrl` associated with it.
This struct contains much of the same information as struct
:ref:`v4l2_queryctrl <v4l2-queryctrl>` and struct
:c:type:`v4l2_control`.
If the event is generated due to a call to
:ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` or
:ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_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 :c:type:`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
:c:type:`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
:c:type:`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
:c:type:`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
:c:type:`v4l2_event_motion_det`
associated with it.
* - ``V4L2_EVENT_PRIVATE_START``
- 0x08000000
- Base event number for driver-private events.
.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
.. c:type:: v4l2_event_vsync
.. flat-table:: struct v4l2_event_vsync
:header-rows: 0
:stub-columns: 0
:widths: 1 1 2
* - __u8
- ``field``
- The upcoming field. See enum :c:type:`v4l2_field`.
.. tabularcolumns:: |p{3.5cm}|p{3.0cm}|p{10.8cm}|
.. c:type:: v4l2_event_ctrl
.. flat-table:: struct v4l2_event_ctrl
:header-rows: 0
:stub-columns: 0
:widths: 1 1 2
* - __u32
- ``changes``
- A bitmask that tells what has changed. See
:ref:`ctrl-changes-flags`.
* - __u32
- ``type``
- The type of the control. See enum
:c:type:`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 :ref:`VIDIOC_DQEVENT`.
* - __s64
- ``value64``
- The 64-bit value of the control for 64-bit control types.
* - }
-
* - __u32
- ``flags``
- The control flags. See :ref:`control-flags`.
* - __s32
- ``minimum``
- The minimum value of the control. See struct
:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
* - __s32
- ``maximum``
- The maximum value of the control. See struct
:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
* - __s32
- ``step``
- The step value of the control. See struct
:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
* - __s32
- ``default_value``
- The default value of the control. See struct
:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
.. c:type:: v4l2_event_frame_sync
.. flat-table:: struct v4l2_event_frame_sync
:header-rows: 0
:stub-columns: 0
:widths: 1 1 2
* - __u32
- ``frame_sequence``
- The sequence number of the frame being received.
.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
.. c:type:: v4l2_event_src_change
.. flat-table:: struct v4l2_event_src_change
:header-rows: 0
:stub-columns: 0
:widths: 1 1 2
* - __u32
- ``changes``
- A bitmask that tells what has changed. See
:ref:`src-changes-flags`.
.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
.. c:type:: v4l2_event_motion_det
.. flat-table:: struct v4l2_event_motion_det
:header-rows: 0
:stub-columns: 0
:widths: 1 1 2
* - __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 :ref:`detect-controls`) 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.
.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm}|
.. _ctrl-changes-flags:
.. flat-table:: Control Changes
:header-rows: 0
:stub-columns: 0
:widths: 3 1 4
* - ``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_CTRL_CH_DIMENSIONS``
- 0x0008
- This control event was triggered because the dimensions of the
control changed. Note that the number of dimensions remains the
same.
.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm}|
.. _src-changes-flags:
.. flat-table:: Source Changes
:header-rows: 0
:stub-columns: 0
:widths: 3 1 4
* - ``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. Applications will have to query the new resolution (if
any, the signal may also have been lost).
For stateful decoders follow the guidelines in :ref:`decoder`.
Video Capture devices have to query the new timings using
:ref:`VIDIOC_QUERY_DV_TIMINGS` or
:ref:`VIDIOC_QUERYSTD <VIDIOC_QUERYSTD>`.
*Important*: even if the new video timings appear identical to the old
ones, receiving this event indicates that there was an issue with the
video signal and you must stop and restart streaming
(:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
followed by :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`). The reason is
that many Video Capture devices are not able to recover from a temporary
loss of signal and so restarting streaming I/O is required in order for
the hardware to synchronize to the video signal.
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
:ref:`Generic Error Codes <gen-errors>` chapter.
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