1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2.. c:namespace:: V4L
3
4.. _VIDIOC_DQEVENT:
5
6********************
7ioctl VIDIOC_DQEVENT
8********************
9
10Name
11====
12
13VIDIOC_DQEVENT - Dequeue event
14
15Synopsis
16========
17
18.. c:macro:: VIDIOC_DQEVENT
19
20``int ioctl(int fd, VIDIOC_DQEVENT, struct v4l2_event *argp)``
21
22Arguments
23=========
24
25``fd``
26    File descriptor returned by :c:func:`open()`.
27
28``argp``
29    Pointer to struct :c:type:`v4l2_event`.
30
31Description
32===========
33
34Dequeue an event from a video device. No input is required for this
35ioctl. All the fields of the struct :c:type:`v4l2_event`
36structure are filled by the driver. The file handle will also receive
37exceptions which the application may get by e.g. using the select system
38call.
39
40.. tabularcolumns:: |p{3.0cm}|p{4.4cm}|p{2.4cm}|p{7.7cm}|
41
42.. c:type:: v4l2_event
43
44.. cssclass: longtable
45
46.. flat-table:: struct v4l2_event
47    :header-rows:  0
48    :stub-columns: 0
49    :widths:       1 1 2
50
51    * - __u32
52      - ``type``
53      - Type of the event, see :ref:`event-type`.
54    * - union {
55      - ``u``
56    * - struct :c:type:`v4l2_event_vsync`
57      - ``vsync``
58      - Event data for event ``V4L2_EVENT_VSYNC``.
59    * - struct :c:type:`v4l2_event_ctrl`
60      - ``ctrl``
61      - Event data for event ``V4L2_EVENT_CTRL``.
62    * - struct :c:type:`v4l2_event_frame_sync`
63      - ``frame_sync``
64      - Event data for event ``V4L2_EVENT_FRAME_SYNC``.
65    * - struct :c:type:`v4l2_event_motion_det`
66      - ``motion_det``
67      - Event data for event V4L2_EVENT_MOTION_DET.
68    * - struct :c:type:`v4l2_event_src_change`
69      - ``src_change``
70      - Event data for event V4L2_EVENT_SOURCE_CHANGE.
71    * - __u8
72      - ``data``\ [64]
73      - Event data. Defined by the event type. The union should be used to
74	define easily accessible type for events.
75    * - }
76      -
77    * - __u32
78      - ``pending``
79      - Number of pending events excluding this one.
80    * - __u32
81      - ``sequence``
82      - Event sequence number. The sequence number is incremented for
83	every subscribed event that takes place. If sequence numbers are
84	not contiguous it means that events have been lost.
85    * - struct timespec
86      - ``timestamp``
87      - Event timestamp. The timestamp has been taken from the
88	``CLOCK_MONOTONIC`` clock. To access the same clock outside V4L2,
89	use :c:func:`clock_gettime`.
90    * - u32
91      - ``id``
92      - The ID associated with the event source. If the event does not
93	have an associated ID (this depends on the event type), then this
94	is 0.
95    * - __u32
96      - ``reserved``\ [8]
97      - Reserved for future extensions. Drivers must set the array to
98	zero.
99
100
101.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
102
103.. cssclass:: longtable
104
105.. _event-type:
106
107.. flat-table:: Event Types
108    :header-rows:  0
109    :stub-columns: 0
110    :widths:       3 1 4
111
112    * - ``V4L2_EVENT_ALL``
113      - 0
114      - All events. V4L2_EVENT_ALL is valid only for
115	VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
116    * - ``V4L2_EVENT_VSYNC``
117      - 1
118      - This event is triggered on the vertical sync. This event has a
119	struct :c:type:`v4l2_event_vsync` associated
120	with it.
121    * - ``V4L2_EVENT_EOS``
122      - 2
123      - This event is triggered when the end of a stream is reached. This
124	is typically used with MPEG decoders to report to the application
125	when the last of the MPEG stream has been decoded.
126    * - ``V4L2_EVENT_CTRL``
127      - 3
128      - This event requires that the ``id`` matches the control ID from
129	which you want to receive events. This event is triggered if the
130	control's value changes, if a button control is pressed or if the
131	control's flags change. This event has a struct
132	:c:type:`v4l2_event_ctrl` associated with it.
133	This struct contains much of the same information as struct
134	:ref:`v4l2_queryctrl <v4l2-queryctrl>` and struct
135	:c:type:`v4l2_control`.
136
137	If the event is generated due to a call to
138	:ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` or
139	:ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`, then the
140	event will *not* be sent to the file handle that called the ioctl
141	function. This prevents nasty feedback loops. If you *do* want to
142	get the event, then set the ``V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK``
143	flag.
144
145	This event type will ensure that no information is lost when more
146	events are raised than there is room internally. In that case the
147	struct :c:type:`v4l2_event_ctrl` of the
148	second-oldest event is kept, but the ``changes`` field of the
149	second-oldest event is ORed with the ``changes`` field of the
150	oldest event.
151    * - ``V4L2_EVENT_FRAME_SYNC``
152      - 4
153      - Triggered immediately when the reception of a frame has begun.
154	This event has a struct
155	:c:type:`v4l2_event_frame_sync`
156	associated with it.
157
158	If the hardware needs to be stopped in the case of a buffer
159	underrun it might not be able to generate this event. In such
160	cases the ``frame_sequence`` field in struct
161	:c:type:`v4l2_event_frame_sync` will not
162	be incremented. This causes two consecutive frame sequence numbers
163	to have n times frame interval in between them.
164    * - ``V4L2_EVENT_SOURCE_CHANGE``
165      - 5
166      - This event is triggered when a source parameter change is detected
167	during runtime by the video device. It can be a runtime resolution
168	change triggered by a video decoder or the format change happening
169	on an input connector. This event requires that the ``id`` matches
170	the input index (when used with a video device node) or the pad
171	index (when used with a subdevice node) from which you want to
172	receive events.
173
174	This event has a struct
175	:c:type:`v4l2_event_src_change`
176	associated with it. The ``changes`` bitfield denotes what has
177	changed for the subscribed pad. If multiple events occurred before
178	application could dequeue them, then the changes will have the
179	ORed value of all the events generated.
180    * - ``V4L2_EVENT_MOTION_DET``
181      - 6
182      - Triggered whenever the motion detection state for one or more of
183	the regions changes. This event has a struct
184	:c:type:`v4l2_event_motion_det`
185	associated with it.
186    * - ``V4L2_EVENT_PRIVATE_START``
187      - 0x08000000
188      - Base event number for driver-private events.
189
190
191.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
192
193.. c:type:: v4l2_event_vsync
194
195.. flat-table:: struct v4l2_event_vsync
196    :header-rows:  0
197    :stub-columns: 0
198    :widths:       1 1 2
199
200    * - __u8
201      - ``field``
202      - The upcoming field. See enum :c:type:`v4l2_field`.
203
204
205.. tabularcolumns:: |p{3.5cm}|p{3.0cm}|p{1.8cm}|p{8.5cm}|
206
207.. c:type:: v4l2_event_ctrl
208
209.. flat-table:: struct v4l2_event_ctrl
210    :header-rows:  0
211    :stub-columns: 0
212    :widths:       1 1 2
213
214    * - __u32
215      - ``changes``
216      - A bitmask that tells what has changed. See
217	:ref:`ctrl-changes-flags`.
218    * - __u32
219      - ``type``
220      - The type of the control. See enum
221	:c:type:`v4l2_ctrl_type`.
222    * - union {
223      - (anonymous)
224    * - __s32
225      - ``value``
226      - The 32-bit value of the control for 32-bit control types. This is
227	0 for string controls since the value of a string cannot be passed
228	using :ref:`VIDIOC_DQEVENT`.
229    * - __s64
230      - ``value64``
231      - The 64-bit value of the control for 64-bit control types.
232    * - }
233      -
234    * - __u32
235      - ``flags``
236      - The control flags. See :ref:`control-flags`.
237    * - __s32
238      - ``minimum``
239      - The minimum value of the control. See struct
240	:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
241    * - __s32
242      - ``maximum``
243      - The maximum value of the control. See struct
244	:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
245    * - __s32
246      - ``step``
247      - The step value of the control. See struct
248	:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
249    * - __s32
250      - ``default_value``
251      - The default value of the control. See struct
252	:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
253
254
255.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
256
257.. c:type:: v4l2_event_frame_sync
258
259.. flat-table:: struct v4l2_event_frame_sync
260    :header-rows:  0
261    :stub-columns: 0
262    :widths:       1 1 2
263
264    * - __u32
265      - ``frame_sequence``
266      - The sequence number of the frame being received.
267
268
269.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
270
271.. c:type:: v4l2_event_src_change
272
273.. flat-table:: struct v4l2_event_src_change
274    :header-rows:  0
275    :stub-columns: 0
276    :widths:       1 1 2
277
278    * - __u32
279      - ``changes``
280      - A bitmask that tells what has changed. See
281	:ref:`src-changes-flags`.
282
283
284.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
285
286.. c:type:: v4l2_event_motion_det
287
288.. flat-table:: struct v4l2_event_motion_det
289    :header-rows:  0
290    :stub-columns: 0
291    :widths:       1 1 2
292
293    * - __u32
294      - ``flags``
295      - Currently only one flag is available: if
296	``V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ`` is set, then the
297	``frame_sequence`` field is valid, otherwise that field should be
298	ignored.
299    * - __u32
300      - ``frame_sequence``
301      - The sequence number of the frame being received. Only valid if the
302	``V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ`` flag was set.
303    * - __u32
304      - ``region_mask``
305      - The bitmask of the regions that reported motion. There is at least
306	one region. If this field is 0, then no motion was detected at
307	all. If there is no ``V4L2_CID_DETECT_MD_REGION_GRID`` control
308	(see :ref:`detect-controls`) to assign a different region to
309	each cell in the motion detection grid, then that all cells are
310	automatically assigned to the default region 0.
311
312
313.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
314
315.. _ctrl-changes-flags:
316
317.. flat-table:: Control Changes
318    :header-rows:  0
319    :stub-columns: 0
320    :widths:       3 1 4
321
322    * - ``V4L2_EVENT_CTRL_CH_VALUE``
323      - 0x0001
324      - This control event was triggered because the value of the control
325	changed. Special cases: Volatile controls do no generate this
326	event; If a control has the ``V4L2_CTRL_FLAG_EXECUTE_ON_WRITE``
327	flag set, then this event is sent as well, regardless its value.
328    * - ``V4L2_EVENT_CTRL_CH_FLAGS``
329      - 0x0002
330      - This control event was triggered because the control flags
331	changed.
332    * - ``V4L2_EVENT_CTRL_CH_RANGE``
333      - 0x0004
334      - This control event was triggered because the minimum, maximum,
335	step or the default value of the control changed.
336
337
338.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
339
340.. _src-changes-flags:
341
342.. flat-table:: Source Changes
343    :header-rows:  0
344    :stub-columns: 0
345    :widths:       3 1 4
346
347    * - ``V4L2_EVENT_SRC_CH_RESOLUTION``
348      - 0x0001
349      - This event gets triggered when a resolution change is detected at
350	an input. This can come from an input connector or from a video
351	decoder. Applications will have to query the new resolution (if
352	any, the signal may also have been lost).
353
354	For stateful decoders follow the guidelines in :ref:`decoder`.
355	Video Capture devices have to query the new timings using
356	:ref:`VIDIOC_QUERY_DV_TIMINGS` or
357	:ref:`VIDIOC_QUERYSTD <VIDIOC_QUERYSTD>`.
358
359	*Important*: even if the new video timings appear identical to the old
360	ones, receiving this event indicates that there was an issue with the
361	video signal and you must stop and restart streaming
362	(:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
363	followed by :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`). The reason is
364	that many Video Capture devices are not able to recover from a temporary
365	loss of signal and so restarting streaming I/O is required in order for
366	the hardware to synchronize to the video signal.
367
368Return Value
369============
370
371On success 0 is returned, on error -1 and the ``errno`` variable is set
372appropriately. The generic error codes are described at the
373:ref:`Generic Error Codes <gen-errors>` chapter.
374