1 .. SPDX-License-Identifier: GFDL-1.1-no-invari 1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2 .. c:namespace:: V4L 2 .. c:namespace:: V4L
3 3
4 .. _VIDIOC_DQEVENT: 4 .. _VIDIOC_DQEVENT:
5 5
6 ******************** 6 ********************
7 ioctl VIDIOC_DQEVENT 7 ioctl VIDIOC_DQEVENT
8 ******************** 8 ********************
9 9
10 Name 10 Name
11 ==== 11 ====
12 12
13 VIDIOC_DQEVENT - Dequeue event 13 VIDIOC_DQEVENT - Dequeue event
14 14
15 Synopsis 15 Synopsis
16 ======== 16 ========
17 17
18 .. c:macro:: VIDIOC_DQEVENT 18 .. c:macro:: VIDIOC_DQEVENT
19 19
20 ``int ioctl(int fd, VIDIOC_DQEVENT, struct v4l 20 ``int ioctl(int fd, VIDIOC_DQEVENT, struct v4l2_event *argp)``
21 21
22 Arguments 22 Arguments
23 ========= 23 =========
24 24
25 ``fd`` 25 ``fd``
26 File descriptor returned by :c:func:`open( 26 File descriptor returned by :c:func:`open()`.
27 27
28 ``argp`` 28 ``argp``
29 Pointer to struct :c:type:`v4l2_event`. 29 Pointer to struct :c:type:`v4l2_event`.
30 30
31 Description 31 Description
32 =========== 32 ===========
33 33
34 Dequeue an event from a video device. No input 34 Dequeue an event from a video device. No input is required for this
35 ioctl. All the fields of the struct :c:type:`v 35 ioctl. All the fields of the struct :c:type:`v4l2_event`
36 structure are filled by the driver. The file h 36 structure are filled by the driver. The file handle will also receive
37 exceptions which the application may get by e. 37 exceptions which the application may get by e.g. using the select system
38 call. 38 call.
39 39
40 .. c:type:: v4l2_event 40 .. c:type:: v4l2_event
41 41
42 .. tabularcolumns:: |p{3.0cm}|p{3.4cm}|p{10.9c 42 .. tabularcolumns:: |p{3.0cm}|p{3.4cm}|p{10.9cm}|
43 43
44 44
45 .. flat-table:: struct v4l2_event 45 .. flat-table:: struct v4l2_event
46 :header-rows: 0 46 :header-rows: 0
47 :stub-columns: 0 47 :stub-columns: 0
48 :widths: 1 1 2 48 :widths: 1 1 2
49 49
50 * - __u32 50 * - __u32
51 - ``type`` 51 - ``type``
52 - Type of the event, see :ref:`event-typ 52 - Type of the event, see :ref:`event-type`.
53 * - union { 53 * - union {
54 - ``u`` 54 - ``u``
55 * - struct :c:type:`v4l2_event_vsync` 55 * - struct :c:type:`v4l2_event_vsync`
56 - ``vsync`` 56 - ``vsync``
57 - Event data for event ``V4L2_EVENT_VSYN 57 - Event data for event ``V4L2_EVENT_VSYNC``.
58 * - struct :c:type:`v4l2_event_ctrl` 58 * - struct :c:type:`v4l2_event_ctrl`
59 - ``ctrl`` 59 - ``ctrl``
60 - Event data for event ``V4L2_EVENT_CTRL 60 - Event data for event ``V4L2_EVENT_CTRL``.
61 * - struct :c:type:`v4l2_event_frame_sync` 61 * - struct :c:type:`v4l2_event_frame_sync`
62 - ``frame_sync`` 62 - ``frame_sync``
63 - Event data for event ``V4L2_EVENT_FRAM 63 - Event data for event ``V4L2_EVENT_FRAME_SYNC``.
64 * - struct :c:type:`v4l2_event_motion_det` 64 * - struct :c:type:`v4l2_event_motion_det`
65 - ``motion_det`` 65 - ``motion_det``
66 - Event data for event V4L2_EVENT_MOTION 66 - Event data for event V4L2_EVENT_MOTION_DET.
67 * - struct :c:type:`v4l2_event_src_change` 67 * - struct :c:type:`v4l2_event_src_change`
68 - ``src_change`` 68 - ``src_change``
69 - Event data for event V4L2_EVENT_SOURCE 69 - Event data for event V4L2_EVENT_SOURCE_CHANGE.
70 * - __u8 70 * - __u8
71 - ``data``\ [64] 71 - ``data``\ [64]
72 - Event data. Defined by the event type. 72 - Event data. Defined by the event type. The union should be used to
73 define easily accessible type for even 73 define easily accessible type for events.
74 * - } 74 * - }
75 - 75 -
76 * - __u32 76 * - __u32
77 - ``pending`` 77 - ``pending``
78 - Number of pending events excluding thi 78 - Number of pending events excluding this one.
79 * - __u32 79 * - __u32
80 - ``sequence`` 80 - ``sequence``
81 - Event sequence number. The sequence nu 81 - Event sequence number. The sequence number is incremented for
82 every subscribed event that takes plac 82 every subscribed event that takes place. If sequence numbers are
83 not contiguous it means that events ha 83 not contiguous it means that events have been lost.
84 * - struct timespec 84 * - struct timespec
85 - ``timestamp`` 85 - ``timestamp``
86 - Event timestamp. The timestamp has bee 86 - Event timestamp. The timestamp has been taken from the
87 ``CLOCK_MONOTONIC`` clock. To access t 87 ``CLOCK_MONOTONIC`` clock. To access the same clock outside V4L2,
88 use :c:func:`clock_gettime`. 88 use :c:func:`clock_gettime`.
89 * - u32 89 * - u32
90 - ``id`` 90 - ``id``
91 - The ID associated with the event sourc 91 - The ID associated with the event source. If the event does not
92 have an associated ID (this depends on 92 have an associated ID (this depends on the event type), then this
93 is 0. 93 is 0.
94 * - __u32 94 * - __u32
95 - ``reserved``\ [8] 95 - ``reserved``\ [8]
96 - Reserved for future extensions. Driver 96 - Reserved for future extensions. Drivers must set the array to
97 zero. 97 zero.
98 98
99 99
100 .. tabularcolumns:: |p{6.2cm}|p{2.6cm}|p{8.5cm 100 .. tabularcolumns:: |p{6.2cm}|p{2.6cm}|p{8.5cm}|
101 101
102 .. cssclass:: longtable 102 .. cssclass:: longtable
103 103
104 .. _event-type: 104 .. _event-type:
105 105
106 .. flat-table:: Event Types 106 .. flat-table:: Event Types
107 :header-rows: 0 107 :header-rows: 0
108 :stub-columns: 0 108 :stub-columns: 0
109 :widths: 3 1 4 109 :widths: 3 1 4
110 110
111 * - ``V4L2_EVENT_ALL`` 111 * - ``V4L2_EVENT_ALL``
112 - 0 112 - 0
113 - All events. V4L2_EVENT_ALL is valid on 113 - All events. V4L2_EVENT_ALL is valid only for
114 VIDIOC_UNSUBSCRIBE_EVENT for unsubscri 114 VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
115 * - ``V4L2_EVENT_VSYNC`` 115 * - ``V4L2_EVENT_VSYNC``
116 - 1 116 - 1
117 - This event is triggered on the vertica 117 - This event is triggered on the vertical sync. This event has a
118 struct :c:type:`v4l2_event_vsync` asso 118 struct :c:type:`v4l2_event_vsync` associated
119 with it. 119 with it.
120 * - ``V4L2_EVENT_EOS`` 120 * - ``V4L2_EVENT_EOS``
121 - 2 121 - 2
122 - This event is triggered when the end o 122 - This event is triggered when the end of a stream is reached. This
123 is typically used with MPEG decoders t 123 is typically used with MPEG decoders to report to the application
124 when the last of the MPEG stream has b 124 when the last of the MPEG stream has been decoded.
125 * - ``V4L2_EVENT_CTRL`` 125 * - ``V4L2_EVENT_CTRL``
126 - 3 126 - 3
127 - This event requires that the ``id`` ma 127 - This event requires that the ``id`` matches the control ID from
128 which you want to receive events. This 128 which you want to receive events. This event is triggered if the
129 control's value changes, if a button c 129 control's value changes, if a button control is pressed or if the
130 control's flags change. This event has 130 control's flags change. This event has a struct
131 :c:type:`v4l2_event_ctrl` associated w 131 :c:type:`v4l2_event_ctrl` associated with it.
132 This struct contains much of the same 132 This struct contains much of the same information as struct
133 :ref:`v4l2_queryctrl <v4l2-queryctrl>` 133 :ref:`v4l2_queryctrl <v4l2-queryctrl>` and struct
134 :c:type:`v4l2_control`. 134 :c:type:`v4l2_control`.
135 135
136 If the event is generated due to a cal 136 If the event is generated due to a call to
137 :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` o 137 :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` or
138 :ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT 138 :ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`, then the
139 event will *not* be sent to the file h 139 event will *not* be sent to the file handle that called the ioctl
140 function. This prevents nasty feedback 140 function. This prevents nasty feedback loops. If you *do* want to
141 get the event, then set the ``V4L2_EVE 141 get the event, then set the ``V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK``
142 flag. 142 flag.
143 143
144 This event type will ensure that no in 144 This event type will ensure that no information is lost when more
145 events are raised than there is room i 145 events are raised than there is room internally. In that case the
146 struct :c:type:`v4l2_event_ctrl` of th 146 struct :c:type:`v4l2_event_ctrl` of the
147 second-oldest event is kept, but the ` 147 second-oldest event is kept, but the ``changes`` field of the
148 second-oldest event is ORed with the ` 148 second-oldest event is ORed with the ``changes`` field of the
149 oldest event. 149 oldest event.
150 * - ``V4L2_EVENT_FRAME_SYNC`` 150 * - ``V4L2_EVENT_FRAME_SYNC``
151 - 4 151 - 4
152 - Triggered immediately when the recepti 152 - Triggered immediately when the reception of a frame has begun.
153 This event has a struct 153 This event has a struct
154 :c:type:`v4l2_event_frame_sync` 154 :c:type:`v4l2_event_frame_sync`
155 associated with it. 155 associated with it.
156 156
157 If the hardware needs to be stopped in 157 If the hardware needs to be stopped in the case of a buffer
158 underrun it might not be able to gener 158 underrun it might not be able to generate this event. In such
159 cases the ``frame_sequence`` field in 159 cases the ``frame_sequence`` field in struct
160 :c:type:`v4l2_event_frame_sync` will n 160 :c:type:`v4l2_event_frame_sync` will not
161 be incremented. This causes two consec 161 be incremented. This causes two consecutive frame sequence numbers
162 to have n times frame interval in betw 162 to have n times frame interval in between them.
163 * - ``V4L2_EVENT_SOURCE_CHANGE`` 163 * - ``V4L2_EVENT_SOURCE_CHANGE``
164 - 5 164 - 5
165 - This event is triggered when a source 165 - This event is triggered when a source parameter change is detected
166 during runtime by the video device. It 166 during runtime by the video device. It can be a runtime resolution
167 change triggered by a video decoder or 167 change triggered by a video decoder or the format change happening
168 on an input connector. This event requ 168 on an input connector. This event requires that the ``id`` matches
169 the input index (when used with a vide 169 the input index (when used with a video device node) or the pad
170 index (when used with a subdevice node 170 index (when used with a subdevice node) from which you want to
171 receive events. 171 receive events.
172 172
173 This event has a struct 173 This event has a struct
174 :c:type:`v4l2_event_src_change` 174 :c:type:`v4l2_event_src_change`
175 associated with it. The ``changes`` bi 175 associated with it. The ``changes`` bitfield denotes what has
176 changed for the subscribed pad. If mul 176 changed for the subscribed pad. If multiple events occurred before
177 application could dequeue them, then t 177 application could dequeue them, then the changes will have the
178 ORed value of all the events generated 178 ORed value of all the events generated.
179 * - ``V4L2_EVENT_MOTION_DET`` 179 * - ``V4L2_EVENT_MOTION_DET``
180 - 6 180 - 6
181 - Triggered whenever the motion detectio 181 - Triggered whenever the motion detection state for one or more of
182 the regions changes. This event has a 182 the regions changes. This event has a struct
183 :c:type:`v4l2_event_motion_det` 183 :c:type:`v4l2_event_motion_det`
184 associated with it. 184 associated with it.
185 * - ``V4L2_EVENT_PRIVATE_START`` 185 * - ``V4L2_EVENT_PRIVATE_START``
186 - 0x08000000 186 - 0x08000000
187 - Base event number for driver-private e 187 - Base event number for driver-private events.
188 188
189 189
190 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm 190 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
191 191
192 .. c:type:: v4l2_event_vsync 192 .. c:type:: v4l2_event_vsync
193 193
194 .. flat-table:: struct v4l2_event_vsync 194 .. flat-table:: struct v4l2_event_vsync
195 :header-rows: 0 195 :header-rows: 0
196 :stub-columns: 0 196 :stub-columns: 0
197 :widths: 1 1 2 197 :widths: 1 1 2
198 198
199 * - __u8 199 * - __u8
200 - ``field`` 200 - ``field``
201 - The upcoming field. See enum :c:type:` 201 - The upcoming field. See enum :c:type:`v4l2_field`.
202 202
203 203
204 .. tabularcolumns:: |p{3.5cm}|p{3.0cm}|p{10.8c 204 .. tabularcolumns:: |p{3.5cm}|p{3.0cm}|p{10.8cm}|
205 205
206 .. c:type:: v4l2_event_ctrl 206 .. c:type:: v4l2_event_ctrl
207 207
208 .. flat-table:: struct v4l2_event_ctrl 208 .. flat-table:: struct v4l2_event_ctrl
209 :header-rows: 0 209 :header-rows: 0
210 :stub-columns: 0 210 :stub-columns: 0
211 :widths: 1 1 2 211 :widths: 1 1 2
212 212
213 * - __u32 213 * - __u32
214 - ``changes`` 214 - ``changes``
215 - A bitmask that tells what has changed. 215 - A bitmask that tells what has changed. See
216 :ref:`ctrl-changes-flags`. 216 :ref:`ctrl-changes-flags`.
217 * - __u32 217 * - __u32
218 - ``type`` 218 - ``type``
219 - The type of the control. See enum 219 - The type of the control. See enum
220 :c:type:`v4l2_ctrl_type`. 220 :c:type:`v4l2_ctrl_type`.
221 * - union { 221 * - union {
222 - (anonymous) 222 - (anonymous)
223 * - __s32 223 * - __s32
224 - ``value`` 224 - ``value``
225 - The 32-bit value of the control for 32 225 - The 32-bit value of the control for 32-bit control types. This is
226 0 for string controls since the value 226 0 for string controls since the value of a string cannot be passed
227 using :ref:`VIDIOC_DQEVENT`. 227 using :ref:`VIDIOC_DQEVENT`.
228 * - __s64 228 * - __s64
229 - ``value64`` 229 - ``value64``
230 - The 64-bit value of the control for 64 230 - The 64-bit value of the control for 64-bit control types.
231 * - } 231 * - }
232 - 232 -
233 * - __u32 233 * - __u32
234 - ``flags`` 234 - ``flags``
235 - The control flags. See :ref:`control-f 235 - The control flags. See :ref:`control-flags`.
236 * - __s32 236 * - __s32
237 - ``minimum`` 237 - ``minimum``
238 - The minimum value of the control. See 238 - The minimum value of the control. See struct
239 :ref:`v4l2_queryctrl <v4l2-queryctrl>` 239 :ref:`v4l2_queryctrl <v4l2-queryctrl>`.
240 * - __s32 240 * - __s32
241 - ``maximum`` 241 - ``maximum``
242 - The maximum value of the control. See 242 - The maximum value of the control. See struct
243 :ref:`v4l2_queryctrl <v4l2-queryctrl>` 243 :ref:`v4l2_queryctrl <v4l2-queryctrl>`.
244 * - __s32 244 * - __s32
245 - ``step`` 245 - ``step``
246 - The step value of the control. See str 246 - The step value of the control. See struct
247 :ref:`v4l2_queryctrl <v4l2-queryctrl>` 247 :ref:`v4l2_queryctrl <v4l2-queryctrl>`.
248 * - __s32 248 * - __s32
249 - ``default_value`` 249 - ``default_value``
250 - The default value of the control. See 250 - The default value of the control. See struct
251 :ref:`v4l2_queryctrl <v4l2-queryctrl>` 251 :ref:`v4l2_queryctrl <v4l2-queryctrl>`.
252 252
253 253
254 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm 254 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
255 255
256 .. c:type:: v4l2_event_frame_sync 256 .. c:type:: v4l2_event_frame_sync
257 257
258 .. flat-table:: struct v4l2_event_frame_sync 258 .. flat-table:: struct v4l2_event_frame_sync
259 :header-rows: 0 259 :header-rows: 0
260 :stub-columns: 0 260 :stub-columns: 0
261 :widths: 1 1 2 261 :widths: 1 1 2
262 262
263 * - __u32 263 * - __u32
264 - ``frame_sequence`` 264 - ``frame_sequence``
265 - The sequence number of the frame being 265 - The sequence number of the frame being received.
266 266
267 267
268 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm 268 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
269 269
270 .. c:type:: v4l2_event_src_change 270 .. c:type:: v4l2_event_src_change
271 271
272 .. flat-table:: struct v4l2_event_src_change 272 .. flat-table:: struct v4l2_event_src_change
273 :header-rows: 0 273 :header-rows: 0
274 :stub-columns: 0 274 :stub-columns: 0
275 :widths: 1 1 2 275 :widths: 1 1 2
276 276
277 * - __u32 277 * - __u32
278 - ``changes`` 278 - ``changes``
279 - A bitmask that tells what has changed. 279 - A bitmask that tells what has changed. See
280 :ref:`src-changes-flags`. 280 :ref:`src-changes-flags`.
281 281
282 282
283 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm 283 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
284 284
285 .. c:type:: v4l2_event_motion_det 285 .. c:type:: v4l2_event_motion_det
286 286
287 .. flat-table:: struct v4l2_event_motion_det 287 .. flat-table:: struct v4l2_event_motion_det
288 :header-rows: 0 288 :header-rows: 0
289 :stub-columns: 0 289 :stub-columns: 0
290 :widths: 1 1 2 290 :widths: 1 1 2
291 291
292 * - __u32 292 * - __u32
293 - ``flags`` 293 - ``flags``
294 - Currently only one flag is available: 294 - Currently only one flag is available: if
295 ``V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ`` is 295 ``V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ`` is set, then the
296 ``frame_sequence`` field is valid, oth 296 ``frame_sequence`` field is valid, otherwise that field should be
297 ignored. 297 ignored.
298 * - __u32 298 * - __u32
299 - ``frame_sequence`` 299 - ``frame_sequence``
300 - The sequence number of the frame being 300 - The sequence number of the frame being received. Only valid if the
301 ``V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ`` fl 301 ``V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ`` flag was set.
302 * - __u32 302 * - __u32
303 - ``region_mask`` 303 - ``region_mask``
304 - The bitmask of the regions that report 304 - The bitmask of the regions that reported motion. There is at least
305 one region. If this field is 0, then n 305 one region. If this field is 0, then no motion was detected at
306 all. If there is no ``V4L2_CID_DETECT_ 306 all. If there is no ``V4L2_CID_DETECT_MD_REGION_GRID`` control
307 (see :ref:`detect-controls`) to assign 307 (see :ref:`detect-controls`) to assign a different region to
308 each cell in the motion detection grid 308 each cell in the motion detection grid, then that all cells are
309 automatically assigned to the default 309 automatically assigned to the default region 0.
310 310
311 311
312 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm 312 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm}|
313 313
314 .. _ctrl-changes-flags: 314 .. _ctrl-changes-flags:
315 315
316 .. flat-table:: Control Changes 316 .. flat-table:: Control Changes
317 :header-rows: 0 317 :header-rows: 0
318 :stub-columns: 0 318 :stub-columns: 0
319 :widths: 3 1 4 319 :widths: 3 1 4
320 320
321 * - ``V4L2_EVENT_CTRL_CH_VALUE`` 321 * - ``V4L2_EVENT_CTRL_CH_VALUE``
322 - 0x0001 322 - 0x0001
323 - This control event was triggered becau 323 - This control event was triggered because the value of the control
324 changed. Special cases: Volatile contr 324 changed. Special cases: Volatile controls do no generate this
325 event; If a control has the ``V4L2_CTR 325 event; If a control has the ``V4L2_CTRL_FLAG_EXECUTE_ON_WRITE``
326 flag set, then this event is sent as w 326 flag set, then this event is sent as well, regardless its value.
327 * - ``V4L2_EVENT_CTRL_CH_FLAGS`` 327 * - ``V4L2_EVENT_CTRL_CH_FLAGS``
328 - 0x0002 328 - 0x0002
329 - This control event was triggered becau 329 - This control event was triggered because the control flags
330 changed. 330 changed.
331 * - ``V4L2_EVENT_CTRL_CH_RANGE`` 331 * - ``V4L2_EVENT_CTRL_CH_RANGE``
332 - 0x0004 332 - 0x0004
333 - This control event was triggered becau 333 - This control event was triggered because the minimum, maximum,
334 step or the default value of the contr 334 step or the default value of the control changed.
335 * - ``V4L2_EVENT_CTRL_CH_DIMENSIONS`` <<
336 - 0x0008 <<
337 - This control event was triggered becau <<
338 control changed. Note that the number <<
339 same. <<
340 335
341 336
342 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm 337 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm}|
343 338
344 .. _src-changes-flags: 339 .. _src-changes-flags:
345 340
346 .. flat-table:: Source Changes 341 .. flat-table:: Source Changes
347 :header-rows: 0 342 :header-rows: 0
348 :stub-columns: 0 343 :stub-columns: 0
349 :widths: 3 1 4 344 :widths: 3 1 4
350 345
351 * - ``V4L2_EVENT_SRC_CH_RESOLUTION`` 346 * - ``V4L2_EVENT_SRC_CH_RESOLUTION``
352 - 0x0001 347 - 0x0001
353 - This event gets triggered when a resol 348 - This event gets triggered when a resolution change is detected at
354 an input. This can come from an input 349 an input. This can come from an input connector or from a video
355 decoder. Applications will have to que 350 decoder. Applications will have to query the new resolution (if
356 any, the signal may also have been los 351 any, the signal may also have been lost).
357 352
358 For stateful decoders follow the guide 353 For stateful decoders follow the guidelines in :ref:`decoder`.
359 Video Capture devices have to query th 354 Video Capture devices have to query the new timings using
360 :ref:`VIDIOC_QUERY_DV_TIMINGS` or 355 :ref:`VIDIOC_QUERY_DV_TIMINGS` or
361 :ref:`VIDIOC_QUERYSTD <VIDIOC_QUERYSTD 356 :ref:`VIDIOC_QUERYSTD <VIDIOC_QUERYSTD>`.
362 357
363 *Important*: even if the new video tim 358 *Important*: even if the new video timings appear identical to the old
364 ones, receiving this event indicates t 359 ones, receiving this event indicates that there was an issue with the
365 video signal and you must stop and res 360 video signal and you must stop and restart streaming
366 (:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAM 361 (:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
367 followed by :ref:`VIDIOC_STREAMON <VID 362 followed by :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`). The reason is
368 that many Video Capture devices are no 363 that many Video Capture devices are not able to recover from a temporary
369 loss of signal and so restarting strea 364 loss of signal and so restarting streaming I/O is required in order for
370 the hardware to synchronize to the vid 365 the hardware to synchronize to the video signal.
371 366
372 Return Value 367 Return Value
373 ============ 368 ============
374 369
375 On success 0 is returned, on error -1 and the 370 On success 0 is returned, on error -1 and the ``errno`` variable is set
376 appropriately. The generic error codes are des 371 appropriately. The generic error codes are described at the
377 :ref:`Generic Error Codes <gen-errors>` chapte 372 :ref:`Generic Error Codes <gen-errors>` chapter.
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