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6. vidioc-subscribe-event.xml

<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>