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>
207