![Mauro Carvalho Chehab](/assets/img/avatar_default.png)
The V4L and DVB API's are there for a long time. however, up to now, no efforts were done to merge them to kernel DocBook. This patch adds the current versions of the specs as an unique compendium. Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
139 lines
4.8 KiB
XML
139 lines
4.8 KiB
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 <sys/time.h>
|
|
#include <sys/types.h>
|
|
#include <unistd.h></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>
|
|
|
|
<!--
|
|
Local Variables:
|
|
mode: sgml
|
|
sgml-parent-document: "v4l2.sgml"
|
|
indent-tabs-mode: nil
|
|
End:
|
|
-->
|