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Diffstat (limited to 'Documentation/DocBook/media/v4l/func-poll.xml')
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diff --git a/Documentation/DocBook/media/v4l/func-poll.xml b/Documentation/DocBook/media/v4l/func-poll.xml new file mode 100644 index 000000000..4c73f1152 --- /dev/null +++ b/Documentation/DocBook/media/v4l/func-poll.xml @@ -0,0 +1,142 @@ +<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 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> |