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+<refentry id="func-poll">
+ <refmeta>
+ <refentrytitle>V4L2 poll()</refentrytitle>
+ &manvol;
+ </refmeta>
+ <refnamediv>
+ <refname>v4l2-poll</refname>
+ <refpurpose>Wait for some event on a file descriptor</refpurpose>
+ </refnamediv>
+ <refsynopsisdiv>
+ <funcsynopsis>
+ <funcsynopsisinfo>#include &lt;sys/poll.h&gt;</funcsynopsisinfo>
+ <funcprototype>
+ <funcdef>int <function>poll</function></funcdef>
+ <paramdef>struct pollfd *<parameter>ufds</parameter></paramdef>
+ <paramdef>unsigned int <parameter>nfds</parameter></paramdef>
+ <paramdef>int <parameter>timeout</parameter></paramdef>
+ </funcprototype>
+ </funcsynopsis>
+ </refsynopsisdiv>
+ <refsect1>
+ <title>Description</title>
+ <para>With the <function>poll()</function> function applications
+can suspend execution until the driver has captured data or is ready
+to accept data for output.</para>
+ <para>When streaming I/O has been negotiated this function waits
+until a buffer has been filled or displayed and can be dequeued with
+the &VIDIOC-DQBUF; ioctl. When buffers are already in the outgoing
+queue of the driver the function returns immediately.</para>
+ <para>On success <function>poll()</function> returns the number of
+file descriptors that have been selected (that is, file descriptors
+for which the <structfield>revents</structfield> field of the
+respective <structname>pollfd</structname> structure is non-zero).
+Capture devices set the <constant>POLLIN</constant> and
+<constant>POLLRDNORM</constant> flags in the
+<structfield>revents</structfield> field, output devices the
+<constant>POLLOUT</constant> and <constant>POLLWRNORM</constant>
+flags. When the function timed out it returns a value of zero, on
+failure it returns <returnvalue>-1</returnvalue> and the
+<varname>errno</varname> variable is set appropriately. When the
+application did not call &VIDIOC-QBUF; or &VIDIOC-STREAMON; yet the
+<function>poll()</function> function succeeds, but sets the
+<constant>POLLERR</constant> flag in the
+<structfield>revents</structfield> field.</para>
+ <para>When use of the <function>read()</function> function has
+been negotiated and the driver does not capture yet, the
+<function>poll</function> function starts capturing. When that fails
+it returns a <constant>POLLERR</constant> as above. Otherwise it waits
+until data has been captured and can be read. When the driver captures
+continuously (as opposed to, for example, still images) the function
+may return immediately.</para>
+ <para>When use of the <function>write()</function> function has
+been negotiated the <function>poll</function> function just waits
+until the driver is ready for a non-blocking
+<function>write()</function> call.</para>
+ <para>All drivers implementing the <function>read()</function> or
+<function>write()</function> function or streaming I/O must also
+support the <function>poll()</function> function.</para>
+ <para>For more details see the
+<function>poll()</function> manual page.</para>
+ </refsect1>
+ <refsect1>
+ <title>Return Value</title>
+ <para>On success, <function>poll()</function> returns the number
+structures which have non-zero <structfield>revents</structfield>
+fields, or zero if the call timed out. On error
+<returnvalue>-1</returnvalue> is returned, and the
+<varname>errno</varname> variable is set appropriately:</para>
+ <variablelist>
+ <varlistentry>
+ <term><errorcode>EBADF</errorcode></term>
+ <listitem>
+ <para>One or more of the <parameter>ufds</parameter> members
+specify an invalid file descriptor.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term><errorcode>EBUSY</errorcode></term>
+ <listitem>
+ <para>The driver does not support multiple read or write
+streams and the device is already in use.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term><errorcode>EFAULT</errorcode></term>
+ <listitem>
+ <para><parameter>ufds</parameter> references an inaccessible
+memory area.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term><errorcode>EINTR</errorcode></term>
+ <listitem>
+ <para>The call was interrupted by a signal.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term><errorcode>EINVAL</errorcode></term>
+ <listitem>
+ <para>The <parameter>nfds</parameter> argument is greater
+than <constant>OPEN_MAX</constant>.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </refsect1>