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[deliverable/linux.git] / Documentation / DocBook / v4l / func-select.xml

<refentry id="func-select">
  <refmeta>
    <refentrytitle>V4L2 select()</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>v4l2-select</refname>
    <refpurpose>Synchronous I/O multiplexing</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>
#include &lt;sys/time.h&gt;
#include &lt;sys/types.h&gt;
#include &lt;unistd.h&gt;</funcsynopsisinfo>
      <funcprototype>
	<funcdef>int <function>select</function></funcdef>
	<paramdef>int <parameter>nfds</parameter></paramdef>
	<paramdef>fd_set *<parameter>readfds</parameter></paramdef>
	<paramdef>fd_set *<parameter>writefds</parameter></paramdef>
	<paramdef>fd_set *<parameter>exceptfds</parameter></paramdef>
	<paramdef>struct timeval *<parameter>timeout</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>With the <function>select()</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>select()</function> returns the total
number of bits set in the <structname>fd_set</structname>s. 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>select()</function> function succeeds, setting the bit of
the file descriptor in <parameter>readfds</parameter> or
<parameter>writefds</parameter>, but subsequent &VIDIOC-DQBUF; calls
will fail.<footnote><para>The Linux kernel implements
<function>select()</function> like the &func-poll; function, but
<function>select()</function> cannot return a
<constant>POLLERR</constant>.</para>
      </footnote></para>

    <para>When use of the <function>read()</function> function has
been negotiated and the driver does not capture yet, the
<function>select()</function> function starts capturing. When that
fails, <function>select()</function> returns successful and a
subsequent <function>read()</function> call, which also attempts to
start capturing, will return an appropriate error code. When the
driver captures continuously (as opposed to, for example, still
images) and data is already available the
<function>select()</function> function returns immediately.</para>

    <para>When use of the <function>write()</function> function has
been negotiated the <function>select()</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>select()</function> function.</para>

    <para>For more details see the <function>select()</function>
manual page.</para>

  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>On success, <function>select()</function> returns the number
of descriptors contained in the three returned descriptor sets, which
will be zero if the timeout expired. On error
<returnvalue>-1</returnvalue> is returned, and the
<varname>errno</varname> variable is set appropriately; the sets and
<parameter>timeout</parameter> are undefined. Possible error codes
are:</para>

    <variablelist>
      <varlistentry>
	<term><errorcode>EBADF</errorcode></term>
	<listitem>
	  <para>One or more of the file descriptor sets specified a
file descriptor that is not open.</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>The <parameter>readfds</parameter>,
<parameter>writefds</parameter>, <parameter>exceptfds</parameter> or
<parameter>timeout</parameter> pointer 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 less than
zero or greater than <constant>FD_SETSIZE</constant>.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>
</refentry>

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