mirror of https://gitee.com/openkylin/linux.git
131 lines
4.1 KiB
XML
131 lines
4.1 KiB
XML
<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</refname>
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<refname>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, see <xref linkend="event-type" />. Note that
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<constant>V4L2_EVENT_ALL</constant> can be used with VIDIOC_UNSUBSCRIBE_EVENT
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for unsubscribing all events at once.</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 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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