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+<refentry id="vidioc-subscribe-event">
+ <refmeta>
+ <refentrytitle>ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</refentrytitle>
+ &manvol;
+ </refmeta>
+
+ <refnamediv>
+ <refname>VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</refname>
+ <refpurpose>Subscribe or unsubscribe event</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+ <funcsynopsis>
+ <funcprototype>
+ <funcdef>int <function>ioctl</function></funcdef>
+ <paramdef>int <parameter>fd</parameter></paramdef>
+ <paramdef>int <parameter>request</parameter></paramdef>
+ <paramdef>struct v4l2_event_subscription
+*<parameter>argp</parameter></paramdef>
+ </funcprototype>
+ </funcsynopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+ <title>Arguments</title>
+
+ <variablelist>
+ <varlistentry>
+ <term><parameter>fd</parameter></term>
+ <listitem>
+ <para>&fd;</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term><parameter>request</parameter></term>
+ <listitem>
+ <para>VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term><parameter>argp</parameter></term>
+ <listitem>
+ <para></para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </refsect1>
+
+ <refsect1>
+ <title>Description</title>
+
+ <para>Subscribe or unsubscribe V4L2 event. Subscribed events are
+ dequeued by using the &VIDIOC-DQEVENT; ioctl.</para>
+
+ <table frame="none" pgwide="1" id="v4l2-event-subscription">
+ <title>struct <structname>v4l2_event_subscription</structname></title>
+ <tgroup cols="3">
+ &cs-str;
+ <tbody valign="top">
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>type</structfield></entry>
+ <entry>Type of the event, see <xref linkend="event-type" />. Note that
+<constant>V4L2_EVENT_ALL</constant> can be used with VIDIOC_UNSUBSCRIBE_EVENT
+for unsubscribing all events at once.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>id</structfield></entry>
+ <entry>ID of the event source. If there is no ID associated with
+ the event source, then set this to 0. Whether or not an event
+ needs an ID depends on the event type.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>flags</structfield></entry>
+ <entry>Event flags, see <xref linkend="event-flags" />.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>reserved</structfield>[5]</entry>
+ <entry>Reserved for future extensions. Drivers and applications
+ must set the array to zero.</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <table pgwide="1" frame="none" id="event-flags">
+ <title>Event Flags</title>
+ <tgroup cols="3">
+ &cs-def;
+ <tbody valign="top">
+ <row>
+ <entry><constant>V4L2_EVENT_SUB_FL_SEND_INITIAL</constant></entry>
+ <entry>0x0001</entry>
+ <entry>When this event is subscribed an initial event will be sent
+ containing the current status. This only makes sense for events
+ that are triggered by a status change such as <constant>V4L2_EVENT_CTRL</constant>.
+ Other events will ignore this flag.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant></entry>
+ <entry>0x0002</entry>
+ <entry><para>If set, then events directly caused by an ioctl will also be sent to
+ the filehandle that called that ioctl. For example, changing a control using
+ &VIDIOC-S-CTRL; will cause a V4L2_EVENT_CTRL to be sent back to that same
+ filehandle. Normally such events are suppressed to prevent feedback loops
+ where an application changes a control to a one value and then another, and
+ then receives an event telling it that that control has changed to the first
+ value.</para>
+
+ <para>Since it can't tell whether that event was caused by another application
+ or by the &VIDIOC-S-CTRL; call it is hard to decide whether to set the
+ control to the value in the event, or ignore it.</para>
+
+ <para>Think carefully when you set this flag so you won't get into situations
+ like that.</para>
+ </entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ </refsect1>
+ <refsect1>
+ &return-value;
+ </refsect1>
+</refentry>