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_PRIVATE_START</constant></entry>
 143             <entry>0x08000000</entry>
 144             <entry>Base event number for driver-private events.</entry>
 145           </row>
 146         </tbody>
 147       </tgroup>
 148     </table>
 149
 150     <table pgwide="1" frame="none" id="event-flags">
 151       <title>Event Flags</title>
 152       <tgroup cols="3">
 153         &cs-def;
 154         <tbody valign="top">
 155           <row>
 156             <entry><constant>V4L2_EVENT_SUB_FL_SEND_INITIAL</constant></entry>
 157             <entry>0x0001</entry>
 158             <entry>When this event is subscribed an initial event will be sent
 159                 containing the current status. This only makes sense for events
 160                 that are triggered by a status change such as <constant>V4L2_EVENT_CTRL</constant>.
 161                 Other events will ignore this flag.</entry>
 162           </row>
 163           <row>
 164             <entry><constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant></entry>
 165             <entry>0x0002</entry>
 166             <entry><para>If set, then events directly caused by an ioctl will also be sent to
 167                 the filehandle that called that ioctl. For example, changing a control using
 168                 &VIDIOC-S-CTRL; will cause a V4L2_EVENT_CTRL to be sent back to that same
 169                 filehandle. Normally such events are suppressed to prevent feedback loops
 170                 where an application changes a control to a one value and then another, and
 171                 then receives an event telling it that that control has changed to the first
 172                 value.</para>
 173
 174                 <para>Since it can't tell whether that event was caused by another application
 175                 or by the &VIDIOC-S-CTRL; call it is hard to decide whether to set the
 176                 control to the value in the event, or ignore it.</para>
 177
 178                 <para>Think carefully when you set this flag so you won't get into situations
 179                 like that.</para>
 180             </entry>
 181           </row>
 182         </tbody>
 183       </tgroup>
 184     </table>
 185
 186     <table frame="none" pgwide="1" id="v4l2-event-vsync">
 187       <title>struct <structname>v4l2_event_vsync</structname></title>
 188       <tgroup cols="3">
 189         &cs-str;
 190         <tbody valign="top">
 191           <row>
 192             <entry>__u8</entry>
 193             <entry><structfield>field</structfield></entry>
 194             <entry>The upcoming field. See &v4l2-field;.</entry>
 195           </row>
 196         </tbody>
 197       </tgroup>
 198     </table>
 199
 200     <table frame="none" pgwide="1" id="v4l2-event-ctrl">
 201       <title>struct <structname>v4l2_event_ctrl</structname></title>
 202       <tgroup cols="4">
 203         &cs-str;
 204         <tbody valign="top">
 205           <row>
 206             <entry>__u32</entry>
 207             <entry><structfield>changes</structfield></entry>
 208             <entry></entry>
 209             <entry>A bitmask that tells what has changed. See <xref linkend="changes-flags" />.</entry>
 210           </row>
 211           <row>
 212             <entry>__u32</entry>
 213             <entry><structfield>type</structfield></entry>
 214             <entry></entry>
 215             <entry>The type of the control. See &v4l2-ctrl-type;.</entry>
 216           </row>
 217           <row>
 218             <entry>union (anonymous)</entry>
 219             <entry></entry>
 220             <entry></entry>
 221             <entry></entry>
 222           </row>
 223           <row>
 224             <entry></entry>
 225             <entry>__s32</entry>
 226             <entry><structfield>value</structfield></entry>
 227             <entry>The 32-bit value of the control for 32-bit control types.
 228                 This is 0 for string controls since the value of a string
 229                 cannot be passed using &VIDIOC-DQEVENT;.</entry>
 230           </row>
 231           <row>
 232             <entry></entry>
 233             <entry>__s64</entry>
 234             <entry><structfield>value64</structfield></entry>
 235             <entry>The 64-bit value of the control for 64-bit control types.</entry>
 236           </row>
 237           <row>
 238             <entry>__u32</entry>
 239             <entry><structfield>flags</structfield></entry>
 240             <entry></entry>
 241             <entry>The control flags. See <xref linkend="control-flags" />.</entry>
 242           </row>
 243           <row>
 244             <entry>__s32</entry>
 245             <entry><structfield>minimum</structfield></entry>
 246             <entry></entry>
 247             <entry>The minimum value of the control. See &v4l2-queryctrl;.</entry>
 248           </row>
 249           <row>
 250             <entry>__s32</entry>
 251             <entry><structfield>maximum</structfield></entry>
 252             <entry></entry>
 253             <entry>The maximum value of the control. See &v4l2-queryctrl;.</entry>
 254           </row>
 255           <row>
 256             <entry>__s32</entry>
 257             <entry><structfield>step</structfield></entry>
 258             <entry></entry>
 259             <entry>The step value of the control. See &v4l2-queryctrl;.</entry>
 260           </row>
 261           <row>
 262             <entry>__s32</entry>
 263             <entry><structfield>default_value</structfield></entry>
 264             <entry></entry>
 265             <entry>The default value value of the control. See &v4l2-queryctrl;.</entry>
 266           </row>
 267         </tbody>
 268       </tgroup>
 269     </table>
 270
 271     <table pgwide="1" frame="none" id="changes-flags">
 272       <title>Changes</title>
 273       <tgroup cols="3">
 274         &cs-def;
 275         <tbody valign="top">
 276           <row>
 277             <entry><constant>V4L2_EVENT_CTRL_CH_VALUE</constant></entry>
 278             <entry>0x0001</entry>
 279             <entry>This control event was triggered because the value of the control
 280                 changed. Special case: if a button control is pressed, then this
 281                 event is sent as well, even though there is not explicit value
 282                 associated with a button control.</entry>
 283           </row>
 284           <row>
 285             <entry><constant>V4L2_EVENT_CTRL_CH_FLAGS</constant></entry>
 286             <entry>0x0002</entry>
 287             <entry>This control event was triggered because the control flags
 288                 changed.</entry>
 289           </row>
 290         </tbody>
 291       </tgroup>
 292     </table>
 293   </refsect1>
 294   <refsect1>
 295     &return-value;
 296   </refsect1>
 297 </refentry>