kernel/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, see <xref linkend="event-type" />. Note that
  64 <constant>V4L2_EVENT_ALL</constant> can be used with VIDIOC_UNSUBSCRIBE_EVENT
  65 for unsubscribing all events at once.</entry>
  66           </row>
  67           <row>
  68             <entry>__u32</entry>
  69             <entry><structfield>id</structfield></entry>
  70             <entry>ID of the event source. If there is no ID associated with
  71                 the event source, then set this to 0. Whether or not an event
  72                 needs an ID depends on the event type.</entry>
  73           </row>
  74           <row>
  75             <entry>__u32</entry>
  76             <entry><structfield>flags</structfield></entry>
  77             <entry>Event flags, see <xref linkend="event-flags" />.</entry>
  78           </row>
  79           <row>
  80             <entry>__u32</entry>
  81             <entry><structfield>reserved</structfield>[5]</entry>
  82             <entry>Reserved for future extensions. Drivers and applications
  83             must set the array to zero.</entry>
  84           </row>
  85         </tbody>
  86       </tgroup>
  87     </table>
  88
  89     <table pgwide="1" frame="none" id="event-flags">
  90       <title>Event Flags</title>
  91       <tgroup cols="3">
  92         &cs-def;
  93         <tbody valign="top">
  94           <row>
  95             <entry><constant>V4L2_EVENT_SUB_FL_SEND_INITIAL</constant></entry>
  96             <entry>0x0001</entry>
  97             <entry>When this event is subscribed an initial event will be sent
  98                 containing the current status. This only makes sense for events
  99                 that are triggered by a status change such as <constant>V4L2_EVENT_CTRL</constant>.
 100                 Other events will ignore this flag.</entry>
 101           </row>
 102           <row>
 103             <entry><constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant></entry>
 104             <entry>0x0002</entry>
 105             <entry><para>If set, then events directly caused by an ioctl will also be sent to
 106                 the filehandle that called that ioctl. For example, changing a control using
 107                 &VIDIOC-S-CTRL; will cause a V4L2_EVENT_CTRL to be sent back to that same
 108                 filehandle. Normally such events are suppressed to prevent feedback loops
 109                 where an application changes a control to a one value and then another, and
 110                 then receives an event telling it that that control has changed to the first
 111                 value.</para>
 112
 113                 <para>Since it can't tell whether that event was caused by another application
 114                 or by the &VIDIOC-S-CTRL; call it is hard to decide whether to set the
 115                 control to the value in the event, or ignore it.</para>
 116
 117                 <para>Think carefully when you set this flag so you won't get into situations
 118                 like that.</para>
 119             </entry>
 120           </row>
 121         </tbody>
 122       </tgroup>
 123     </table>
 124
 125   </refsect1>
 126   <refsect1>
 127     &return-value;
 128   </refsect1>
 129 </refentry>