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

   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.</entry>
  64           </row>
  65           <row>
  66             <entry>__u32</entry>
  67             <entry><structfield>id</structfield></entry>
  68             <entry>ID of the event source. If there is no ID associated with
  69                 the event source, then set this to 0. Whether or not an event
  70                 needs an ID depends on the event type.</entry>
  71           </row>
  72           <row>
  73             <entry>__u32</entry>
  74             <entry><structfield>flags</structfield></entry>
  75             <entry>Event flags, see <xref linkend="event-flags" />.</entry>
  76           </row>
  77           <row>
  78             <entry>__u32</entry>
  79             <entry><structfield>reserved</structfield>[5]</entry>
  80             <entry>Reserved for future extensions. Drivers and applications
  81             must set the array to zero.</entry>
  82           </row>
  83         </tbody>
  84       </tgroup>
  85     </table>
  86
  87     <table frame="none" pgwide="1" id="event-type">
  88       <title>Event Types</title>
  89       <tgroup cols="3">
  90         &cs-def;
  91         <tbody valign="top">
  92           <row>
  93             <entry><constant>V4L2_EVENT_ALL</constant></entry>
  94             <entry>0</entry>
  95             <entry>All events. V4L2_EVENT_ALL is valid only for
  96             VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
  97             </entry>
  98           </row>
  99           <row>
 100             <entry><constant>V4L2_EVENT_VSYNC</constant></entry>
 101             <entry>1</entry>
 102             <entry>This event is triggered on the vertical sync.
 103             This event has a &v4l2-event-vsync; associated with it.
 104             </entry>
 105           </row>
 106           <row>
 107             <entry><constant>V4L2_EVENT_EOS</constant></entry>
 108             <entry>2</entry>
 109             <entry>This event is triggered when the end of a stream is reached.
 110             This is typically used with MPEG decoders to report to the application
 111             when the last of the MPEG stream has been decoded.
 112             </entry>
 113           </row>
 114           <row>
 115             <entry><constant>V4L2_EVENT_CTRL</constant></entry>
 116             <entry>3</entry>
 117             <entry><para>This event requires that the <structfield>id</structfield>
 118                 matches the control ID from which you want to receive events.
 119                 This event is triggered if the control's value changes, if a
 120                 button control is pressed or if the control's flags change.
 121                 This event has a &v4l2-event-ctrl; associated with it. This struct
 122                 contains much of the same information as &v4l2-queryctrl; and
 123                 &v4l2-control;.</para>
 124
 125                 <para>If the event is generated due to a call to &VIDIOC-S-CTRL; or
 126                 &VIDIOC-S-EXT-CTRLS;, then the event will <emphasis>not</emphasis> be sent to
 127                 the file handle that called the ioctl function. This prevents
 128                 nasty feedback loops. If you <emphasis>do</emphasis> want to get the
 129                 event, then set the <constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant>
 130                 flag.
 131                 </para>
 132
 133                 <para>This event type will ensure that no information is lost when
 134                 more events are raised than there is room internally. In that
 135                 case the &v4l2-event-ctrl; of the second-oldest event is kept,
 136                 but the <structfield>changes</structfield> field of the
 137                 second-oldest event is ORed with the <structfield>changes</structfield>
 138                 field of the oldest event.</para>
 139             </entry>
 140           </row>
 141           <row>
 142             <entry><constant>V4L2_EVENT_FRAME_SYNC</constant></entry>
 143             <entry>4</entry>
 144             <entry>
 145               <para>Triggered immediately when the reception of a
 146               frame has begun. This event has a
 147               &v4l2-event-frame-sync; associated with it.</para>
 148
 149               <para>If the hardware needs to be stopped in the case of a
 150               buffer underrun it might not be able to generate this event.
 151               In such cases the <structfield>frame_sequence</structfield>
 152               field in &v4l2-event-frame-sync; will not be incremented. This
 153               causes two consecutive frame sequence numbers to have n times
 154               frame interval in between them.</para>
 155             </entry>
 156           </row>
 157           <row>
 158             <entry><constant>V4L2_EVENT_PRIVATE_START</constant></entry>
 159             <entry>0x08000000</entry>
 160             <entry>Base event number for driver-private events.</entry>
 161           </row>
 162         </tbody>
 163       </tgroup>
 164     </table>
 165
 166     <table pgwide="1" frame="none" id="event-flags">
 167       <title>Event Flags</title>
 168       <tgroup cols="3">
 169         &cs-def;
 170         <tbody valign="top">
 171           <row>
 172             <entry><constant>V4L2_EVENT_SUB_FL_SEND_INITIAL</constant></entry>
 173             <entry>0x0001</entry>
 174             <entry>When this event is subscribed an initial event will be sent
 175                 containing the current status. This only makes sense for events
 176                 that are triggered by a status change such as <constant>V4L2_EVENT_CTRL</constant>.
 177                 Other events will ignore this flag.</entry>
 178           </row>
 179           <row>
 180             <entry><constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant></entry>
 181             <entry>0x0002</entry>
 182             <entry><para>If set, then events directly caused by an ioctl will also be sent to
 183                 the filehandle that called that ioctl. For example, changing a control using
 184                 &VIDIOC-S-CTRL; will cause a V4L2_EVENT_CTRL to be sent back to that same
 185                 filehandle. Normally such events are suppressed to prevent feedback loops
 186                 where an application changes a control to a one value and then another, and
 187                 then receives an event telling it that that control has changed to the first
 188                 value.</para>
 189
 190                 <para>Since it can't tell whether that event was caused by another application
 191                 or by the &VIDIOC-S-CTRL; call it is hard to decide whether to set the
 192                 control to the value in the event, or ignore it.</para>
 193
 194                 <para>Think carefully when you set this flag so you won't get into situations
 195                 like that.</para>
 196             </entry>
 197           </row>
 198         </tbody>
 199       </tgroup>
 200     </table>
 201
 202   </refsect1>
 203   <refsect1>
 204     &return-value;
 205   </refsect1>
 206 </refentry>