143 строки
5.6 KiB
XML
143 строки
5.6 KiB
XML
<refentry id="func-poll">
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<refmeta>
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<refentrytitle>V4L2 poll()</refentrytitle>
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&manvol;
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</refmeta>
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<refnamediv>
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<refname>v4l2-poll</refname>
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<refpurpose>Wait for some event on a file descriptor</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <sys/poll.h></funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>poll</function></funcdef>
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<paramdef>struct pollfd *<parameter>ufds</parameter></paramdef>
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<paramdef>unsigned int <parameter>nfds</parameter></paramdef>
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<paramdef>int <parameter>timeout</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>With the <function>poll()</function> function applications
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can suspend execution until the driver has captured data or is ready
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to accept data for output.</para>
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<para>When streaming I/O has been negotiated this function waits
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until a buffer has been filled by the capture device and can be dequeued
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with the &VIDIOC-DQBUF; ioctl. For output devices this function waits
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until the device is ready to accept a new buffer to be queued up with
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the &VIDIOC-QBUF; ioctl for display. When buffers are already in the outgoing
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queue of the driver (capture) or the incoming queue isn't full (display)
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the function returns immediately.</para>
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<para>On success <function>poll()</function> returns the number of
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file descriptors that have been selected (that is, file descriptors
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for which the <structfield>revents</structfield> field of the
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respective <structname>pollfd</structname> structure is non-zero).
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Capture devices set the <constant>POLLIN</constant> and
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<constant>POLLRDNORM</constant> flags in the
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<structfield>revents</structfield> field, output devices the
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<constant>POLLOUT</constant> and <constant>POLLWRNORM</constant>
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flags. When the function timed out it returns a value of zero, on
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failure it returns <returnvalue>-1</returnvalue> and the
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<varname>errno</varname> variable is set appropriately. When the
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application did not call &VIDIOC-STREAMON; the
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<function>poll()</function> function succeeds, but sets the
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<constant>POLLERR</constant> flag in the
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<structfield>revents</structfield> field. When the
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application has called &VIDIOC-STREAMON; for a capture device but hasn't
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yet called &VIDIOC-QBUF;, the <function>poll()</function> function
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succeeds and sets the <constant>POLLERR</constant> flag in the
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<structfield>revents</structfield> field. For output devices this
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same situation will cause <function>poll()</function> to succeed
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as well, but it sets the <constant>POLLOUT</constant> and
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<constant>POLLWRNORM</constant> flags in the <structfield>revents</structfield>
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field.</para>
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<para>If an event occurred (see &VIDIOC-DQEVENT;) then
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<constant>POLLPRI</constant> will be set in the <structfield>revents</structfield>
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field and <function>poll()</function> will return.</para>
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<para>When use of the <function>read()</function> function has
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been negotiated and the driver does not capture yet, the
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<function>poll</function> function starts capturing. When that fails
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it returns a <constant>POLLERR</constant> as above. Otherwise it waits
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until data has been captured and can be read. When the driver captures
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continuously (as opposed to, for example, still images) the function
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may return immediately.</para>
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<para>When use of the <function>write()</function> function has
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been negotiated and the driver does not stream yet, the
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<function>poll</function> function starts streaming. When that fails
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it returns a <constant>POLLERR</constant> as above. Otherwise it waits
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until the driver is ready for a non-blocking
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<function>write()</function> call.</para>
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<para>If the caller is only interested in events (just
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<constant>POLLPRI</constant> is set in the <structfield>events</structfield>
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field), then <function>poll()</function> will <emphasis>not</emphasis>
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start streaming if the driver does not stream yet. This makes it
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possible to just poll for events and not for buffers.</para>
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<para>All drivers implementing the <function>read()</function> or
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<function>write()</function> function or streaming I/O must also
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support the <function>poll()</function> function.</para>
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<para>For more details see the
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<function>poll()</function> manual page.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On success, <function>poll()</function> returns the number
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structures which have non-zero <structfield>revents</structfield>
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fields, or zero if the call timed out. On error
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<returnvalue>-1</returnvalue> is returned, and the
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<varname>errno</varname> variable is set appropriately:</para>
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<variablelist>
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<varlistentry>
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<term><errorcode>EBADF</errorcode></term>
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<listitem>
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<para>One or more of the <parameter>ufds</parameter> members
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specify an invalid file descriptor.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>EBUSY</errorcode></term>
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<listitem>
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<para>The driver does not support multiple read or write
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streams and the device is already in use.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>EFAULT</errorcode></term>
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<listitem>
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<para><parameter>ufds</parameter> references an inaccessible
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memory area.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>EINTR</errorcode></term>
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<listitem>
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<para>The call was interrupted by a signal.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>EINVAL</errorcode></term>
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<listitem>
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<para>The <parameter>nfds</parameter> argument is greater
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than <constant>OPEN_MAX</constant>.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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</refentry>
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