1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2 .. c:namespace:: V4L
3
4 .. _VIDIOC_DQEVENT:
5
6 ********************
7 ioctl VIDIOC_DQEVENT
8 ********************
9
10 Name
11 ====
12
13 VIDIOC_DQEVENT - Dequeue event
14
15 Synopsis
16 ========
17
18 .. c:macro:: VIDIOC_DQEVENT
19
20 ``int ioctl(int fd, VIDIOC_DQEVENT, struct v4l2_event *argp)``
21
22 Arguments
23 =========
24
25 ``fd``
26 File descriptor returned by :c:func:`open()`.
27
28 ``argp``
29 Pointer to struct :c:type:`v4l2_event`.
30
31 Description
32 ===========
33
34 Dequeue an event from a video device. No input is required for this
35 ioctl. All the fields of the struct :c:type:`v4l2_event`
36 structure are filled by the driver. The file handle will also receive
37 exceptions which the application may get by e.g. using the select system
38 call.
39
40 .. c:type:: v4l2_event
41
42 .. tabularcolumns:: |p{3.0cm}|p{3.4cm}|p{10.9cm}|
43
44
45 .. flat-table:: struct v4l2_event
46 :header-rows: 0
47 :stub-columns: 0
48 :widths: 1 1 2
49
50 * - __u32
51 - ``type``
52 - Type of the event, see :ref:`event-type`.
53 * - union {
54 - ``u``
55 * - struct :c:type:`v4l2_event_vsync`
56 - ``vsync``
57 - Event data for event ``V4L2_EVENT_VSYNC``.
58 * - struct :c:type:`v4l2_event_ctrl`
59 - ``ctrl``
60 - Event data for event ``V4L2_EVENT_CTRL``.
61 * - struct :c:type:`v4l2_event_frame_sync`
62 - ``frame_sync``
63 - Event data for event ``V4L2_EVENT_FRAME_SYNC``.
64 * - struct :c:type:`v4l2_event_motion_det`
65 - ``motion_det``
66 - Event data for event V4L2_EVENT_MOTION_DET.
67 * - struct :c:type:`v4l2_event_src_change`
68 - ``src_change``
69 - Event data for event V4L2_EVENT_SOURCE_CHANGE.
70 * - __u8
71 - ``data``\ [64]
72 - Event data. Defined by the event type. The union should be used to
73 define easily accessible type for events.
74 * - }
75 -
76 * - __u32
77 - ``pending``
78 - Number of pending events excluding this one.
79 * - __u32
80 - ``sequence``
81 - Event sequence number. The sequence number is incremented for
82 every subscribed event that takes place. If sequence numbers are
83 not contiguous it means that events have been lost.
84 * - struct timespec
85 - ``timestamp``
86 - Event timestamp. The timestamp has been taken from the
87 ``CLOCK_MONOTONIC`` clock. To access the same clock outside V4L2,
88 use :c:func:`clock_gettime`.
89 * - u32
90 - ``id``
91 - The ID associated with the event source. If the event does not
92 have an associated ID (this depends on the event type), then this
93 is 0.
94 * - __u32
95 - ``reserved``\ [8]
96 - Reserved for future extensions. Drivers must set the array to
97 zero.
98
99
100 .. tabularcolumns:: |p{6.2cm}|p{2.6cm}|p{8.5cm}|
101
102 .. cssclass:: longtable
103
104 .. _event-type:
105
106 .. flat-table:: Event Types
107 :header-rows: 0
108 :stub-columns: 0
109 :widths: 3 1 4
110
111 * - ``V4L2_EVENT_ALL``
112 - 0
113 - All events. V4L2_EVENT_ALL is valid only for
114 VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
115 * - ``V4L2_EVENT_VSYNC``
116 - 1
117 - This event is triggered on the vertical sync. This event has a
118 struct :c:type:`v4l2_event_vsync` associated
119 with it.
120 * - ``V4L2_EVENT_EOS``
121 - 2
122 - This event is triggered when the end of a stream is reached. This
123 is typically used with MPEG decoders to report to the application
124 when the last of the MPEG stream has been decoded.
125 * - ``V4L2_EVENT_CTRL``
126 - 3
127 - This event requires that the ``id`` matches the control ID from
128 which you want to receive events. This event is triggered if the
129 control's value changes, if a button control is pressed or if the
130 control's flags change. This event has a struct
131 :c:type:`v4l2_event_ctrl` associated with it.
132 This struct contains much of the same information as struct
133 :ref:`v4l2_queryctrl <v4l2-queryctrl>` and struct
134 :c:type:`v4l2_control`.
135
136 If the event is generated due to a call to
137 :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` or
138 :ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`, then the
139 event will *not* be sent to the file handle that called the ioctl
140 function. This prevents nasty feedback loops. If you *do* want to
141 get the event, then set the ``V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK``
142 flag.
143
144 This event type will ensure that no information is lost when more
145 events are raised than there is room internally. In that case the
146 struct :c:type:`v4l2_event_ctrl` of the
147 second-oldest event is kept, but the ``changes`` field of the
148 second-oldest event is ORed with the ``changes`` field of the
149 oldest event.
150 * - ``V4L2_EVENT_FRAME_SYNC``
151 - 4
152 - Triggered immediately when the reception of a frame has begun.
153 This event has a struct
154 :c:type:`v4l2_event_frame_sync`
155 associated with it.
156
157 If the hardware needs to be stopped in the case of a buffer
158 underrun it might not be able to generate this event. In such
159 cases the ``frame_sequence`` field in struct
160 :c:type:`v4l2_event_frame_sync` will not
161 be incremented. This causes two consecutive frame sequence numbers
162 to have n times frame interval in between them.
163 * - ``V4L2_EVENT_SOURCE_CHANGE``
164 - 5
165 - This event is triggered when a source parameter change is detected
166 during runtime by the video device. It can be a runtime resolution
167 change triggered by a video decoder or the format change happening
168 on an input connector. This event requires that the ``id`` matches
169 the input index (when used with a video device node) or the pad
170 index (when used with a subdevice node) from which you want to
171 receive events.
172
173 This event has a struct
174 :c:type:`v4l2_event_src_change`
175 associated with it. The ``changes`` bitfield denotes what has
176 changed for the subscribed pad. If multiple events occurred before
177 application could dequeue them, then the changes will have the
178 ORed value of all the events generated.
179 * - ``V4L2_EVENT_MOTION_DET``
180 - 6
181 - Triggered whenever the motion detection state for one or more of
182 the regions changes. This event has a struct
183 :c:type:`v4l2_event_motion_det`
184 associated with it.
185 * - ``V4L2_EVENT_PRIVATE_START``
186 - 0x08000000
187 - Base event number for driver-private events.
188
189
190 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
191
192 .. c:type:: v4l2_event_vsync
193
194 .. flat-table:: struct v4l2_event_vsync
195 :header-rows: 0
196 :stub-columns: 0
197 :widths: 1 1 2
198
199 * - __u8
200 - ``field``
201 - The upcoming field. See enum :c:type:`v4l2_field`.
202
203
204 .. tabularcolumns:: |p{3.5cm}|p{3.0cm}|p{10.8cm}|
205
206 .. c:type:: v4l2_event_ctrl
207
208 .. flat-table:: struct v4l2_event_ctrl
209 :header-rows: 0
210 :stub-columns: 0
211 :widths: 1 1 2
212
213 * - __u32
214 - ``changes``
215 - A bitmask that tells what has changed. See
216 :ref:`ctrl-changes-flags`.
217 * - __u32
218 - ``type``
219 - The type of the control. See enum
220 :c:type:`v4l2_ctrl_type`.
221 * - union {
222 - (anonymous)
223 * - __s32
224 - ``value``
225 - The 32-bit value of the control for 32-bit control types. This is
226 0 for string controls since the value of a string cannot be passed
227 using :ref:`VIDIOC_DQEVENT`.
228 * - __s64
229 - ``value64``
230 - The 64-bit value of the control for 64-bit control types.
231 * - }
232 -
233 * - __u32
234 - ``flags``
235 - The control flags. See :ref:`control-flags`.
236 * - __s32
237 - ``minimum``
238 - The minimum value of the control. See struct
239 :ref:`v4l2_queryctrl <v4l2-queryctrl>`.
240 * - __s32
241 - ``maximum``
242 - The maximum value of the control. See struct
243 :ref:`v4l2_queryctrl <v4l2-queryctrl>`.
244 * - __s32
245 - ``step``
246 - The step value of the control. See struct
247 :ref:`v4l2_queryctrl <v4l2-queryctrl>`.
248 * - __s32
249 - ``default_value``
250 - The default value of the control. See struct
251 :ref:`v4l2_queryctrl <v4l2-queryctrl>`.
252
253
254 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
255
256 .. c:type:: v4l2_event_frame_sync
257
258 .. flat-table:: struct v4l2_event_frame_sync
259 :header-rows: 0
260 :stub-columns: 0
261 :widths: 1 1 2
262
263 * - __u32
264 - ``frame_sequence``
265 - The sequence number of the frame being received.
266
267
268 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
269
270 .. c:type:: v4l2_event_src_change
271
272 .. flat-table:: struct v4l2_event_src_change
273 :header-rows: 0
274 :stub-columns: 0
275 :widths: 1 1 2
276
277 * - __u32
278 - ``changes``
279 - A bitmask that tells what has changed. See
280 :ref:`src-changes-flags`.
281
282
283 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
284
285 .. c:type:: v4l2_event_motion_det
286
287 .. flat-table:: struct v4l2_event_motion_det
288 :header-rows: 0
289 :stub-columns: 0
290 :widths: 1 1 2
291
292 * - __u32
293 - ``flags``
294 - Currently only one flag is available: if
295 ``V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ`` is set, then the
296 ``frame_sequence`` field is valid, otherwise that field should be
297 ignored.
298 * - __u32
299 - ``frame_sequence``
300 - The sequence number of the frame being received. Only valid if the
301 ``V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ`` flag was set.
302 * - __u32
303 - ``region_mask``
304 - The bitmask of the regions that reported motion. There is at least
305 one region. If this field is 0, then no motion was detected at
306 all. If there is no ``V4L2_CID_DETECT_MD_REGION_GRID`` control
307 (see :ref:`detect-controls`) to assign a different region to
308 each cell in the motion detection grid, then that all cells are
309 automatically assigned to the default region 0.
310
311
312 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm}|
313
314 .. _ctrl-changes-flags:
315
316 .. flat-table:: Control Changes
317 :header-rows: 0
318 :stub-columns: 0
319 :widths: 3 1 4
320
321 * - ``V4L2_EVENT_CTRL_CH_VALUE``
322 - 0x0001
323 - This control event was triggered because the value of the control
324 changed. Special cases: Volatile controls do no generate this
325 event; If a control has the ``V4L2_CTRL_FLAG_EXECUTE_ON_WRITE``
326 flag set, then this event is sent as well, regardless its value.
327 * - ``V4L2_EVENT_CTRL_CH_FLAGS``
328 - 0x0002
329 - This control event was triggered because the control flags
330 changed.
331 * - ``V4L2_EVENT_CTRL_CH_RANGE``
332 - 0x0004
333 - This control event was triggered because the minimum, maximum,
334 step or the default value of the control changed.
335 * - ``V4L2_EVENT_CTRL_CH_DIMENSIONS``
336 - 0x0008
337 - This control event was triggered because the dimensions of the
338 control changed. Note that the number of dimensions remains the
339 same.
340
341
342 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm}|
343
344 .. _src-changes-flags:
345
346 .. flat-table:: Source Changes
347 :header-rows: 0
348 :stub-columns: 0
349 :widths: 3 1 4
350
351 * - ``V4L2_EVENT_SRC_CH_RESOLUTION``
352 - 0x0001
353 - This event gets triggered when a resolution change is detected at
354 an input. This can come from an input connector or from a video
355 decoder. Applications will have to query the new resolution (if
356 any, the signal may also have been lost).
357
358 For stateful decoders follow the guidelines in :ref:`decoder`.
359 Video Capture devices have to query the new timings using
360 :ref:`VIDIOC_QUERY_DV_TIMINGS` or
361 :ref:`VIDIOC_QUERYSTD <VIDIOC_QUERYSTD>`.
362
363 *Important*: even if the new video timings appear identical to the old
364 ones, receiving this event indicates that there was an issue with the
365 video signal and you must stop and restart streaming
366 (:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
367 followed by :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`). The reason is
368 that many Video Capture devices are not able to recover from a temporary
369 loss of signal and so restarting streaming I/O is required in order for
370 the hardware to synchronize to the video signal.
371
372 Return Value
373 ============
374
375 On success 0 is returned, on error -1 and the ``errno`` variable is set
376 appropriately. The generic error codes are described at the
377 :ref:`Generic Error Codes <gen-errors>` chapter.
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