1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3.. _VIDIOC_DQEVENT:
4
5********************
6ioctl VIDIOC_DQEVENT
7********************
8
9Name
10====
11
12VIDIOC_DQEVENT - Dequeue event
13
14
15Synopsis
16========
17
18.. c:function:: int ioctl( int fd, VIDIOC_DQEVENT, struct v4l2_event *argp )
19    :name: VIDIOC_DQEVENT
20
21
22Arguments
23=========
24
25``fd``
26    File descriptor returned by :ref:`open() <func-open>`.
27
28``argp``
29    Pointer to struct :c:type:`v4l2_event`.
30
31
32Description
33===========
34
35Dequeue an event from a video device. No input is required for this
36ioctl. All the fields of the struct :c:type:`v4l2_event`
37structure are filled by the driver. The file handle will also receive
38exceptions which the application may get by e.g. using the select system
39call.
40
41
42.. tabularcolumns:: |p{3.0cm}|p{4.4cm}|p{2.4cm}|p{7.7cm}|
43
44.. c:type:: v4l2_event
45
46.. cssclass: longtable
47
48.. flat-table:: struct v4l2_event
49    :header-rows:  0
50    :stub-columns: 0
51    :widths:       1 1 2
52
53    * - __u32
54      - ``type``
55      - Type of the event, see :ref:`event-type`.
56    * - union {
57      - ``u``
58    * - struct :c:type:`v4l2_event_vsync`
59      - ``vsync``
60      - Event data for event ``V4L2_EVENT_VSYNC``.
61    * - struct :c:type:`v4l2_event_ctrl`
62      - ``ctrl``
63      - Event data for event ``V4L2_EVENT_CTRL``.
64    * - struct :c:type:`v4l2_event_frame_sync`
65      - ``frame_sync``
66      - Event data for event ``V4L2_EVENT_FRAME_SYNC``.
67    * - struct :c:type:`v4l2_event_motion_det`
68      - ``motion_det``
69      - Event data for event V4L2_EVENT_MOTION_DET.
70    * - struct :c:type:`v4l2_event_src_change`
71      - ``src_change``
72      - Event data for event V4L2_EVENT_SOURCE_CHANGE.
73    * - __u8
74      - ``data``\ [64]
75      - Event data. Defined by the event type. The union should be used to
76	define easily accessible type for events.
77    * - }
78      -
79    * - __u32
80      - ``pending``
81      - Number of pending events excluding this one.
82    * - __u32
83      - ``sequence``
84      - Event sequence number. The sequence number is incremented for
85	every subscribed event that takes place. If sequence numbers are
86	not contiguous it means that events have been lost.
87    * - struct timespec
88      - ``timestamp``
89      - Event timestamp. The timestamp has been taken from the
90	``CLOCK_MONOTONIC`` clock. To access the same clock outside V4L2,
91	use :c:func:`clock_gettime`.
92    * - u32
93      - ``id``
94      - The ID associated with the event source. If the event does not
95	have an associated ID (this depends on the event type), then this
96	is 0.
97    * - __u32
98      - ``reserved``\ [8]
99      - Reserved for future extensions. Drivers must set the array to
100	zero.
101
102
103
104.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
105
106.. cssclass:: longtable
107
108.. _event-type:
109
110.. flat-table:: Event Types
111    :header-rows:  0
112    :stub-columns: 0
113    :widths:       3 1 4
114
115    * - ``V4L2_EVENT_ALL``
116      - 0
117      - All events. V4L2_EVENT_ALL is valid only for
118	VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
119    * - ``V4L2_EVENT_VSYNC``
120      - 1
121      - This event is triggered on the vertical sync. This event has a
122	struct :c:type:`v4l2_event_vsync` associated
123	with it.
124    * - ``V4L2_EVENT_EOS``
125      - 2
126      - This event is triggered when the end of a stream is reached. This
127	is typically used with MPEG decoders to report to the application
128	when the last of the MPEG stream has been decoded.
129    * - ``V4L2_EVENT_CTRL``
130      - 3
131      - This event requires that the ``id`` matches the control ID from
132	which you want to receive events. This event is triggered if the
133	control's value changes, if a button control is pressed or if the
134	control's flags change. This event has a struct
135	:c:type:`v4l2_event_ctrl` associated with it.
136	This struct contains much of the same information as struct
137	:ref:`v4l2_queryctrl <v4l2-queryctrl>` and struct
138	:c:type:`v4l2_control`.
139
140	If the event is generated due to a call to
141	:ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` or
142	:ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`, then the
143	event will *not* be sent to the file handle that called the ioctl
144	function. This prevents nasty feedback loops. If you *do* want to
145	get the event, then set the ``V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK``
146	flag.
147
148	This event type will ensure that no information is lost when more
149	events are raised than there is room internally. In that case the
150	struct :c:type:`v4l2_event_ctrl` of the
151	second-oldest event is kept, but the ``changes`` field of the
152	second-oldest event is ORed with the ``changes`` field of the
153	oldest event.
154    * - ``V4L2_EVENT_FRAME_SYNC``
155      - 4
156      - Triggered immediately when the reception of a frame has begun.
157	This event has a struct
158	:c:type:`v4l2_event_frame_sync`
159	associated with it.
160
161	If the hardware needs to be stopped in the case of a buffer
162	underrun it might not be able to generate this event. In such
163	cases the ``frame_sequence`` field in struct
164	:c:type:`v4l2_event_frame_sync` will not
165	be incremented. This causes two consecutive frame sequence numbers
166	to have n times frame interval in between them.
167    * - ``V4L2_EVENT_SOURCE_CHANGE``
168      - 5
169      - This event is triggered when a source parameter change is detected
170	during runtime by the video device. It can be a runtime resolution
171	change triggered by a video decoder or the format change happening
172	on an input connector. This event requires that the ``id`` matches
173	the input index (when used with a video device node) or the pad
174	index (when used with a subdevice node) from which you want to
175	receive events.
176
177	This event has a struct
178	:c:type:`v4l2_event_src_change`
179	associated with it. The ``changes`` bitfield denotes what has
180	changed for the subscribed pad. If multiple events occurred before
181	application could dequeue them, then the changes will have the
182	ORed value of all the events generated.
183    * - ``V4L2_EVENT_MOTION_DET``
184      - 6
185      - Triggered whenever the motion detection state for one or more of
186	the regions changes. This event has a struct
187	:c:type:`v4l2_event_motion_det`
188	associated with it.
189    * - ``V4L2_EVENT_PRIVATE_START``
190      - 0x08000000
191      - Base event number for driver-private events.
192
193
194
195.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
196
197.. c:type:: v4l2_event_vsync
198
199.. flat-table:: struct v4l2_event_vsync
200    :header-rows:  0
201    :stub-columns: 0
202    :widths:       1 1 2
203
204    * - __u8
205      - ``field``
206      - The upcoming field. See enum :c:type:`v4l2_field`.
207
208
209
210.. tabularcolumns:: |p{3.5cm}|p{3.0cm}|p{1.8cm}|p{8.5cm}|
211
212.. c:type:: v4l2_event_ctrl
213
214.. flat-table:: struct v4l2_event_ctrl
215    :header-rows:  0
216    :stub-columns: 0
217    :widths:       1 1 2
218
219    * - __u32
220      - ``changes``
221      - A bitmask that tells what has changed. See
222	:ref:`ctrl-changes-flags`.
223    * - __u32
224      - ``type``
225      - The type of the control. See enum
226	:c:type:`v4l2_ctrl_type`.
227    * - union {
228      - (anonymous)
229    * - __s32
230      - ``value``
231      - The 32-bit value of the control for 32-bit control types. This is
232	0 for string controls since the value of a string cannot be passed
233	using :ref:`VIDIOC_DQEVENT`.
234    * - __s64
235      - ``value64``
236      - The 64-bit value of the control for 64-bit control types.
237    * - }
238      -
239    * - __u32
240      - ``flags``
241      - The control flags. See :ref:`control-flags`.
242    * - __s32
243      - ``minimum``
244      - The minimum value of the control. See struct
245	:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
246    * - __s32
247      - ``maximum``
248      - The maximum value of the control. See struct
249	:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
250    * - __s32
251      - ``step``
252      - The step value of the control. See struct
253	:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
254    * - __s32
255      - ``default_value``
256      - The default value of the control. See struct
257	:ref:`v4l2_queryctrl <v4l2-queryctrl>`.
258
259
260
261.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
262
263.. c:type:: v4l2_event_frame_sync
264
265.. flat-table:: struct v4l2_event_frame_sync
266    :header-rows:  0
267    :stub-columns: 0
268    :widths:       1 1 2
269
270    * - __u32
271      - ``frame_sequence``
272      - The sequence number of the frame being received.
273
274
275
276.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
277
278.. c:type:: v4l2_event_src_change
279
280.. flat-table:: struct v4l2_event_src_change
281    :header-rows:  0
282    :stub-columns: 0
283    :widths:       1 1 2
284
285    * - __u32
286      - ``changes``
287      - A bitmask that tells what has changed. See
288	:ref:`src-changes-flags`.
289
290
291
292.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
293
294.. c:type:: v4l2_event_motion_det
295
296.. flat-table:: struct v4l2_event_motion_det
297    :header-rows:  0
298    :stub-columns: 0
299    :widths:       1 1 2
300
301    * - __u32
302      - ``flags``
303      - Currently only one flag is available: if
304	``V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ`` is set, then the
305	``frame_sequence`` field is valid, otherwise that field should be
306	ignored.
307    * - __u32
308      - ``frame_sequence``
309      - The sequence number of the frame being received. Only valid if the
310	``V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ`` flag was set.
311    * - __u32
312      - ``region_mask``
313      - The bitmask of the regions that reported motion. There is at least
314	one region. If this field is 0, then no motion was detected at
315	all. If there is no ``V4L2_CID_DETECT_MD_REGION_GRID`` control
316	(see :ref:`detect-controls`) to assign a different region to
317	each cell in the motion detection grid, then that all cells are
318	automatically assigned to the default region 0.
319
320
321
322.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
323
324.. _ctrl-changes-flags:
325
326.. flat-table:: Control Changes
327    :header-rows:  0
328    :stub-columns: 0
329    :widths:       3 1 4
330
331    * - ``V4L2_EVENT_CTRL_CH_VALUE``
332      - 0x0001
333      - This control event was triggered because the value of the control
334	changed. Special cases: Volatile controls do no generate this
335	event; If a control has the ``V4L2_CTRL_FLAG_EXECUTE_ON_WRITE``
336	flag set, then this event is sent as well, regardless its value.
337    * - ``V4L2_EVENT_CTRL_CH_FLAGS``
338      - 0x0002
339      - This control event was triggered because the control flags
340	changed.
341    * - ``V4L2_EVENT_CTRL_CH_RANGE``
342      - 0x0004
343      - This control event was triggered because the minimum, maximum,
344	step or the default value of the control changed.
345
346
347
348.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
349
350.. _src-changes-flags:
351
352.. flat-table:: Source Changes
353    :header-rows:  0
354    :stub-columns: 0
355    :widths:       3 1 4
356
357    * - ``V4L2_EVENT_SRC_CH_RESOLUTION``
358      - 0x0001
359      - This event gets triggered when a resolution change is detected at
360	an input. This can come from an input connector or from a video
361	decoder. Applications will have to query the new resolution (if
362	any, the signal may also have been lost).
363
364	For stateful decoders follow the guidelines in :ref:`decoder`.
365	Video Capture devices have to query the new timings using
366	:ref:`VIDIOC_QUERY_DV_TIMINGS` or
367	:ref:`VIDIOC_QUERYSTD <VIDIOC_QUERYSTD>`.
368
369	*Important*: even if the new video timings appear identical to the old
370	ones, receiving this event indicates that there was an issue with the
371	video signal and you must stop and restart streaming
372	(:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
373	followed by :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`). The reason is
374	that many Video Capture devices are not able to recover from a temporary
375	loss of signal and so restarting streaming I/O is required in order for
376	the hardware to synchronize to the video signal.
377
378
379Return Value
380============
381
382On success 0 is returned, on error -1 and the ``errno`` variable is set
383appropriately. The generic error codes are described at the
384:ref:`Generic Error Codes <gen-errors>` chapter.
385