2024-09-09 08:52:07 +00:00
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<refentry id="vidioc-subscribe-event">
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<refmeta>
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<refentrytitle>ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</refentrytitle>
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&manvol;
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</refmeta>
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<refnamediv>
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<refname>VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</refname>
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<refpurpose>Subscribe or unsubscribe event</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>ioctl</function></funcdef>
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<paramdef>int <parameter>fd</parameter></paramdef>
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<paramdef>int <parameter>request</parameter></paramdef>
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<paramdef>struct v4l2_event_subscription
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*<parameter>argp</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Arguments</title>
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<variablelist>
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<varlistentry>
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<term><parameter>fd</parameter></term>
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<listitem>
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<para>&fd;</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>request</parameter></term>
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<listitem>
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<para>VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>argp</parameter></term>
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<listitem>
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<para></para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Description</title>
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<para>Subscribe or unsubscribe V4L2 event. Subscribed events are
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dequeued by using the &VIDIOC-DQEVENT; ioctl.</para>
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<table frame="none" pgwide="1" id="v4l2-event-subscription">
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<title>struct <structname>v4l2_event_subscription</structname></title>
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<tgroup cols="3">
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&cs-str;
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<tbody valign="top">
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<row>
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<entry>__u32</entry>
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<entry><structfield>type</structfield></entry>
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<entry>Type of the event.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>id</structfield></entry>
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<entry>ID of the event source. If there is no ID associated with
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the event source, then set this to 0. Whether or not an event
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needs an ID depends on the event type.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>flags</structfield></entry>
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<entry>Event flags, see <xref linkend="event-flags" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>reserved</structfield>[5]</entry>
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<entry>Reserved for future extensions. Drivers and applications
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must set the array to zero.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<table frame="none" pgwide="1" id="event-type">
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<title>Event Types</title>
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<tgroup cols="3">
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&cs-def;
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<tbody valign="top">
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<row>
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<entry><constant>V4L2_EVENT_ALL</constant></entry>
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<entry>0</entry>
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<entry>All events. V4L2_EVENT_ALL is valid only for
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VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
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</entry>
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</row>
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<row>
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<entry><constant>V4L2_EVENT_VSYNC</constant></entry>
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<entry>1</entry>
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<entry>This event is triggered on the vertical sync.
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This event has a &v4l2-event-vsync; associated with it.
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</entry>
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</row>
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<row>
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<entry><constant>V4L2_EVENT_EOS</constant></entry>
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<entry>2</entry>
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<entry>This event is triggered when the end of a stream is reached.
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This is typically used with MPEG decoders to report to the application
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when the last of the MPEG stream has been decoded.
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</entry>
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</row>
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<row>
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<entry><constant>V4L2_EVENT_CTRL</constant></entry>
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<entry>3</entry>
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<entry><para>This event requires that the <structfield>id</structfield>
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matches the control ID from which you want to receive events.
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This event is triggered if the control's value changes, if a
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button control is pressed or if the control's flags change.
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This event has a &v4l2-event-ctrl; associated with it. This struct
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contains much of the same information as &v4l2-queryctrl; and
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&v4l2-control;.</para>
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<para>If the event is generated due to a call to &VIDIOC-S-CTRL; or
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&VIDIOC-S-EXT-CTRLS;, then the event will <emphasis>not</emphasis> be sent to
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the file handle that called the ioctl function. This prevents
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nasty feedback loops. If you <emphasis>do</emphasis> want to get the
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event, then set the <constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant>
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flag.
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</para>
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<para>This event type will ensure that no information is lost when
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more events are raised than there is room internally. In that
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case the &v4l2-event-ctrl; of the second-oldest event is kept,
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but the <structfield>changes</structfield> field of the
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second-oldest event is ORed with the <structfield>changes</structfield>
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field of the oldest event.</para>
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</entry>
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</row>
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<row>
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<entry><constant>V4L2_EVENT_FRAME_SYNC</constant></entry>
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<entry>4</entry>
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<entry>
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<para>Triggered immediately when the reception of a
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frame has begun. This event has a
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&v4l2-event-frame-sync; associated with it.</para>
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<para>If the hardware needs to be stopped in the case of a
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buffer underrun it might not be able to generate this event.
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In such cases the <structfield>frame_sequence</structfield>
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field in &v4l2-event-frame-sync; will not be incremented. This
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causes two consecutive frame sequence numbers to have n times
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frame interval in between them.</para>
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</entry>
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</row>
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2024-09-09 08:57:42 +00:00
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<row>
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<entry><constant>V4L2_EVENT_SOURCE_CHANGE</constant></entry>
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<entry>5</entry>
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<entry>
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<para>This event is triggered when a source parameter change is
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detected during runtime by the video device. It can be a
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runtime resolution change triggered by a video decoder or the
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format change happening on an input connector.
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This event requires that the <structfield>id</structfield>
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matches the input index (when used with a video device node)
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or the pad index (when used with a subdevice node) from which
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you want to receive events.</para>
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<para>This event has a &v4l2-event-src-change; associated
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with it. The <structfield>changes</structfield> bitfield denotes
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what has changed for the subscribed pad. If multiple events
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occurred before application could dequeue them, then the changes
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will have the ORed value of all the events generated.</para>
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</entry>
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</row>
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<row>
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<entry><constant>V4L2_EVENT_MOTION_DET</constant></entry>
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<entry>6</entry>
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<entry>
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<para>Triggered whenever the motion detection state for one or more of the regions
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changes. This event has a &v4l2-event-motion-det; associated with it.</para>
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</entry>
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</row>
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2024-09-09 08:52:07 +00:00
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<row>
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<entry><constant>V4L2_EVENT_PRIVATE_START</constant></entry>
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<entry>0x08000000</entry>
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<entry>Base event number for driver-private events.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<table pgwide="1" frame="none" id="event-flags">
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<title>Event Flags</title>
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<tgroup cols="3">
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&cs-def;
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<tbody valign="top">
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<row>
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<entry><constant>V4L2_EVENT_SUB_FL_SEND_INITIAL</constant></entry>
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<entry>0x0001</entry>
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<entry>When this event is subscribed an initial event will be sent
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containing the current status. This only makes sense for events
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that are triggered by a status change such as <constant>V4L2_EVENT_CTRL</constant>.
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Other events will ignore this flag.</entry>
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</row>
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<row>
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<entry><constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant></entry>
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<entry>0x0002</entry>
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<entry><para>If set, then events directly caused by an ioctl will also be sent to
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the filehandle that called that ioctl. For example, changing a control using
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&VIDIOC-S-CTRL; will cause a V4L2_EVENT_CTRL to be sent back to that same
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filehandle. Normally such events are suppressed to prevent feedback loops
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where an application changes a control to a one value and then another, and
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then receives an event telling it that that control has changed to the first
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value.</para>
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<para>Since it can't tell whether that event was caused by another application
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or by the &VIDIOC-S-CTRL; call it is hard to decide whether to set the
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control to the value in the event, or ignore it.</para>
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<para>Think carefully when you set this flag so you won't get into situations
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like that.</para>
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</refsect1>
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<refsect1>
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&return-value;
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</refsect1>
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</refentry>
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