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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

1	<refentry id="vidioc-subscribe-event">
2	  <refmeta>
3	    <refentrytitle>ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</refentrytitle>
4	    &manvol;
5	  </refmeta>
6	
7	  <refnamediv>
8	    <refname>VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</refname>
9	    <refpurpose>Subscribe or unsubscribe event</refpurpose>
10	  </refnamediv>
11	
12	  <refsynopsisdiv>
13	    <funcsynopsis>
14	      <funcprototype>
15		<funcdef>int <function>ioctl</function></funcdef>
16		<paramdef>int <parameter>fd</parameter></paramdef>
17		<paramdef>int <parameter>request</parameter></paramdef>
18		<paramdef>struct v4l2_event_subscription
19	*<parameter>argp</parameter></paramdef>
20	      </funcprototype>
21	    </funcsynopsis>
22	  </refsynopsisdiv>
23	
24	  <refsect1>
25	    <title>Arguments</title>
26	
27	    <variablelist>
28	      <varlistentry>
29		<term><parameter>fd</parameter></term>
30		<listitem>
31		  <para>&fd;</para>
32		</listitem>
33	      </varlistentry>
34	      <varlistentry>
35		<term><parameter>request</parameter></term>
36		<listitem>
37		  <para>VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</para>
38		</listitem>
39	      </varlistentry>
40	      <varlistentry>
41		<term><parameter>argp</parameter></term>
42		<listitem>
43		  <para></para>
44		</listitem>
45	      </varlistentry>
46	    </variablelist>
47	  </refsect1>
48	
49	  <refsect1>
50	    <title>Description</title>
51	
52	    <para>Subscribe or unsubscribe V4L2 event. Subscribed events are
53	    dequeued by using the &VIDIOC-DQEVENT; ioctl.</para>
54	
55	    <table frame="none" pgwide="1" id="v4l2-event-subscription">
56	      <title>struct <structname>v4l2_event_subscription</structname></title>
57	      <tgroup cols="3">
58		&cs-str;
59		<tbody valign="top">
60		  <row>
61		    <entry>__u32</entry>
62		    <entry><structfield>type</structfield></entry>
63		    <entry>Type of the event.</entry>
64		  </row>
65		  <row>
66		    <entry>__u32</entry>
67		    <entry><structfield>id</structfield></entry>
68		    <entry>ID of the event source. If there is no ID associated with
69			the event source, then set this to 0. Whether or not an event
70			needs an ID depends on the event type.</entry>
71		  </row>
72		  <row>
73		    <entry>__u32</entry>
74		    <entry><structfield>flags</structfield></entry>
75		    <entry>Event flags, see <xref linkend="event-flags" />.</entry>
76		  </row>
77		  <row>
78		    <entry>__u32</entry>
79		    <entry><structfield>reserved</structfield>[5]</entry>
80		    <entry>Reserved for future extensions. Drivers and applications
81		    must set the array to zero.</entry>
82		  </row>
83		</tbody>
84	      </tgroup>
85	    </table>
86	
87	    <table frame="none" pgwide="1" id="event-type">
88	      <title>Event Types</title>
89	      <tgroup cols="3">
90		&cs-def;
91		<tbody valign="top">
92		  <row>
93		    <entry><constant>V4L2_EVENT_ALL</constant></entry>
94		    <entry>0</entry>
95		    <entry>All events. V4L2_EVENT_ALL is valid only for
96		    VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
97		    </entry>
98		  </row>
99		  <row>
100		    <entry><constant>V4L2_EVENT_VSYNC</constant></entry>
101		    <entry>1</entry>
102		    <entry>This event is triggered on the vertical sync.
103		    This event has a &v4l2-event-vsync; associated with it.
104		    </entry>
105		  </row>
106		  <row>
107		    <entry><constant>V4L2_EVENT_EOS</constant></entry>
108		    <entry>2</entry>
109		    <entry>This event is triggered when the end of a stream is reached.
110		    This is typically used with MPEG decoders to report to the application
111		    when the last of the MPEG stream has been decoded.
112		    </entry>
113		  </row>
114		  <row>
115		    <entry><constant>V4L2_EVENT_CTRL</constant></entry>
116		    <entry>3</entry>
117		    <entry><para>This event requires that the <structfield>id</structfield>
118			matches the control ID from which you want to receive events.
119			This event is triggered if the control's value changes, if a
120			button control is pressed or if the control's flags change.
121		    	This event has a &v4l2-event-ctrl; associated with it. This struct
122			contains much of the same information as &v4l2-queryctrl; and
123			&v4l2-control;.</para>
124	
125			<para>If the event is generated due to a call to &VIDIOC-S-CTRL; or
126			&VIDIOC-S-EXT-CTRLS;, then the event will <emphasis>not</emphasis> be sent to
127			the file handle that called the ioctl function. This prevents
128			nasty feedback loops. If you <emphasis>do</emphasis> want to get the
129			event, then set the <constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant>
130			flag.
131			</para>
132	
133			<para>This event type will ensure that no information is lost when
134			more events are raised than there is room internally. In that
135			case the &v4l2-event-ctrl; of the second-oldest event is kept,
136			but the <structfield>changes</structfield> field of the
137			second-oldest event is ORed with the <structfield>changes</structfield>
138			field of the oldest event.</para>
139		    </entry>
140		  </row>
141		  <row>
142		    <entry><constant>V4L2_EVENT_FRAME_SYNC</constant></entry>
143		    <entry>4</entry>
144		    <entry>
145		      <para>Triggered immediately when the reception of a
146		      frame has begun. This event has a
147		      &v4l2-event-frame-sync; associated with it.</para>
148	
149		      <para>If the hardware needs to be stopped in the case of a
150		      buffer underrun it might not be able to generate this event.
151		      In such cases the <structfield>frame_sequence</structfield>
152		      field in &v4l2-event-frame-sync; will not be incremented. This
153		      causes two consecutive frame sequence numbers to have n times
154		      frame interval in between them.</para>
155		    </entry>
156		  </row>
157		  <row>
158		    <entry><constant>V4L2_EVENT_PRIVATE_START</constant></entry>
159		    <entry>0x08000000</entry>
160		    <entry>Base event number for driver-private events.</entry>
161		  </row>
162		</tbody>
163	      </tgroup>
164	    </table>
165	
166	    <table pgwide="1" frame="none" id="event-flags">
167	      <title>Event Flags</title>
168	      <tgroup cols="3">
169		&cs-def;
170		<tbody valign="top">
171		  <row>
172		    <entry><constant>V4L2_EVENT_SUB_FL_SEND_INITIAL</constant></entry>
173		    <entry>0x0001</entry>
174		    <entry>When this event is subscribed an initial event will be sent
175			containing the current status. This only makes sense for events
176			that are triggered by a status change such as <constant>V4L2_EVENT_CTRL</constant>.
177			Other events will ignore this flag.</entry>
178		  </row>
179		  <row>
180		    <entry><constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant></entry>
181		    <entry>0x0002</entry>
182		    <entry><para>If set, then events directly caused by an ioctl will also be sent to
183			the filehandle that called that ioctl. For example, changing a control using
184			&VIDIOC-S-CTRL; will cause a V4L2_EVENT_CTRL to be sent back to that same
185			filehandle. Normally such events are suppressed to prevent feedback loops
186			where an application changes a control to a one value and then another, and
187			then receives an event telling it that that control has changed to the first
188			value.</para>
189	
190			<para>Since it can't tell whether that event was caused by another application
191			or by the &VIDIOC-S-CTRL; call it is hard to decide whether to set the
192			control to the value in the event, or ignore it.</para>
193	
194			<para>Think carefully when you set this flag so you won't get into situations
195			like that.</para>
196		    </entry>
197		  </row>
198		</tbody>
199	      </tgroup>
200	    </table>
201	
202	  </refsect1>
203	  <refsect1>
204	    &return-value;
205	  </refsect1>
206	</refentry>
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