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-<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>