128 lines
4.4 KiB
XML
128 lines
4.4 KiB
XML
<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 <sys/poll.h></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>
|
|
|
|
<!--
|
|
Local Variables:
|
|
mode: sgml
|
|
sgml-parent-document: "v4l2.sgml"
|
|
indent-tabs-mode: nil
|
|
End:
|
|
-->
|