About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / DocBook / media / v4l / vidioc-subscribe-event.xml




Custom Search

Based on kernel version 4.1. Page generated on 2015-06-28 12:12 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, see <xref linkend="event-type" />. Note that
64	<constant>V4L2_EVENT_ALL</constant> can be used with VIDIOC_UNSUBSCRIBE_EVENT
65	for unsubscribing all events at once.</entry>
66		  </row>
67		  <row>
68		    <entry>__u32</entry>
69		    <entry><structfield>id</structfield></entry>
70		    <entry>ID of the event source. If there is no ID associated with
71			the event source, then set this to 0. Whether or not an event
72			needs an ID depends on the event type.</entry>
73		  </row>
74		  <row>
75		    <entry>__u32</entry>
76		    <entry><structfield>flags</structfield></entry>
77		    <entry>Event flags, see <xref linkend="event-flags" />.</entry>
78		  </row>
79		  <row>
80		    <entry>__u32</entry>
81		    <entry><structfield>reserved</structfield>[5]</entry>
82		    <entry>Reserved for future extensions. Drivers and applications
83		    must set the array to zero.</entry>
84		  </row>
85		</tbody>
86	      </tgroup>
87	    </table>
88	
89	    <table pgwide="1" frame="none" id="event-flags">
90	      <title>Event Flags</title>
91	      <tgroup cols="3">
92		&cs-def;
93		<tbody valign="top">
94		  <row>
95		    <entry><constant>V4L2_EVENT_SUB_FL_SEND_INITIAL</constant></entry>
96		    <entry>0x0001</entry>
97		    <entry>When this event is subscribed an initial event will be sent
98			containing the current status. This only makes sense for events
99			that are triggered by a status change such as <constant>V4L2_EVENT_CTRL</constant>.
100			Other events will ignore this flag.</entry>
101		  </row>
102		  <row>
103		    <entry><constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant></entry>
104		    <entry>0x0002</entry>
105		    <entry><para>If set, then events directly caused by an ioctl will also be sent to
106			the filehandle that called that ioctl. For example, changing a control using
107			&VIDIOC-S-CTRL; will cause a V4L2_EVENT_CTRL to be sent back to that same
108			filehandle. Normally such events are suppressed to prevent feedback loops
109			where an application changes a control to a one value and then another, and
110			then receives an event telling it that that control has changed to the first
111			value.</para>
112	
113			<para>Since it can't tell whether that event was caused by another application
114			or by the &VIDIOC-S-CTRL; call it is hard to decide whether to set the
115			control to the value in the event, or ignore it.</para>
116	
117			<para>Think carefully when you set this flag so you won't get into situations
118			like that.</para>
119		    </entry>
120		  </row>
121		</tbody>
122	      </tgroup>
123	    </table>
124	
125	  </refsect1>
126	  <refsect1>
127	    &return-value;
128	  </refsect1>
129	</refentry>
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.