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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 by the capture device and can be dequeued
with the &VIDIOC-DQBUF; ioctl. For output devices this function waits
until the device is ready to accept a new buffer to be queued up with
the &VIDIOC-QBUF; ioctl for display. When buffers are already in the outgoing
queue of the driver (capture) or the incoming queue isn't full (display)
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-STREAMON; the
<function>poll()</function> function succeeds, but sets the
<constant>POLLERR</constant> flag in the
<structfield>revents</structfield> field. When the
application has called &VIDIOC-STREAMON; for a capture device but hasn't
yet called &VIDIOC-QBUF;, the <function>poll()</function> function
succeeds and sets the <constant>POLLERR</constant> flag in the
<structfield>revents</structfield> field. For output devices this
same situation will cause <function>poll()</function> to succeed
as well, but it sets the <constant>POLLOUT</constant> and
<constant>POLLWRNORM</constant> flags in the <structfield>revents</structfield>
field.</para>

    <para>If an event occurred (see &VIDIOC-DQEVENT;) then
<constant>POLLPRI</constant> will be set in the <structfield>revents</structfield>
field and <function>poll()</function> will return.</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 and the driver does not stream yet, the
<function>poll</function> function starts streaming. When that fails
it returns a <constant>POLLERR</constant> as above. Otherwise it waits
until the driver is ready for a non-blocking
<function>write()</function> call.</para>

    <para>If the caller is only interested in events (just
<constant>POLLPRI</constant> is set in the <structfield>events</structfield>
field), then <function>poll()</function> will <emphasis>not</emphasis>
start streaming if the driver does not stream yet. This makes it
possible to just poll for events and not for buffers.</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>