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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.</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 frame="none" pgwide="1" id="event-type">
+ <title>Event Types</title>
+ <tgroup cols="3">
+ &cs-def;
+ <tbody valign="top">
+ <row>
+ <entry><constant>V4L2_EVENT_ALL</constant></entry>
+ <entry>0</entry>
+ <entry>All events. V4L2_EVENT_ALL is valid only for
+ VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
+ </entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_EVENT_VSYNC</constant></entry>
+ <entry>1</entry>
+ <entry>This event is triggered on the vertical sync.
+ This event has a &v4l2-event-vsync; associated with it.
+ </entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_EVENT_EOS</constant></entry>
+ <entry>2</entry>
+ <entry>This event is triggered when the end of a stream is reached.
+ This is typically used with MPEG decoders to report to the application
+ when the last of the MPEG stream has been decoded.
+ </entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_EVENT_CTRL</constant></entry>
+ <entry>3</entry>
+ <entry><para>This event requires that the <structfield>id</structfield>
+ matches the control ID from which you want to receive events.
+ This event is triggered if the control's value changes, if a
+ button control is pressed or if the control's flags change.
+ This event has a &v4l2-event-ctrl; associated with it. This struct
+ contains much of the same information as &v4l2-queryctrl; and
+ &v4l2-control;.</para>
+
+ <para>If the event is generated due to a call to &VIDIOC-S-CTRL; or
+ &VIDIOC-S-EXT-CTRLS;, then the event will <emphasis>not</emphasis> be sent to
+ the file handle that called the ioctl function. This prevents
+ nasty feedback loops. If you <emphasis>do</emphasis> want to get the
+ event, then set the <constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant>
+ flag.
+ </para>
+
+ <para>This event type will ensure that no information is lost when
+ more events are raised than there is room internally. In that
+ case the &v4l2-event-ctrl; of the second-oldest event is kept,
+ but the <structfield>changes</structfield> field of the
+ second-oldest event is ORed with the <structfield>changes</structfield>
+ field of the oldest event.</para>
+ </entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_EVENT_FRAME_SYNC</constant></entry>
+ <entry>4</entry>
+ <entry>
+ <para>Triggered immediately when the reception of a
+ frame has begun. This event has a
+ &v4l2-event-frame-sync; associated with it.</para>
+
+ <para>If the hardware needs to be stopped in the case of a
+ buffer underrun it might not be able to generate this event.
+ In such cases the <structfield>frame_sequence</structfield>
+ field in &v4l2-event-frame-sync; will not be incremented. This
+ causes two consecutive frame sequence numbers to have n times
+ frame interval in between them.</para>
+ </entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_EVENT_PRIVATE_START</constant></entry>
+ <entry>0x08000000</entry>
+ <entry>Base event number for driver-private events.</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>