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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>
-</refentry>