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> 207