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-<refentry id="func-read">
- <refmeta>
- <refentrytitle>V4L2 read()</refentrytitle>
- &manvol;
- </refmeta>
-
- <refnamediv>
- <refname>v4l2-read</refname>
- <refpurpose>Read from a V4L2 device</refpurpose>
- </refnamediv>
-
- <refsynopsisdiv>
- <funcsynopsis>
- <funcsynopsisinfo>#include &lt;unistd.h&gt;</funcsynopsisinfo>
- <funcprototype>
- <funcdef>ssize_t <function>read</function></funcdef>
- <paramdef>int <parameter>fd</parameter></paramdef>
- <paramdef>void *<parameter>buf</parameter></paramdef>
- <paramdef>size_t <parameter>count</parameter></paramdef>
- </funcprototype>
- </funcsynopsis>
- </refsynopsisdiv>
-
- <refsect1>
- <title>Arguments</title>
-
- <variablelist>
- <varlistentry>
- <term><parameter>fd</parameter></term>
- <listitem>
- <para>&fd;</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><parameter>buf</parameter></term>
- <listitem>
- <para></para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><parameter>count</parameter></term>
- <listitem>
- <para></para>
- </listitem>
- </varlistentry>
- </variablelist>
- </refsect1>
-
- <refsect1>
- <title>Description</title>
-
- <para><function>read()</function> attempts to read up to
-<parameter>count</parameter> bytes from file descriptor
-<parameter>fd</parameter> into the buffer starting at
-<parameter>buf</parameter>. The layout of the data in the buffer is
-discussed in the respective device interface section, see ##. If <parameter>count</parameter> is zero,
-<function>read()</function> returns zero and has no other results. If
-<parameter>count</parameter> is greater than
-<constant>SSIZE_MAX</constant>, the result is unspecified. Regardless
-of the <parameter>count</parameter> value each
-<function>read()</function> call will provide at most one frame (two
-fields) worth of data.</para>
-
- <para>By default <function>read()</function> blocks until data
-becomes available. When the <constant>O_NONBLOCK</constant> flag was
-given to the &func-open; function it
-returns immediately with an &EAGAIN; when no data is available. The
-&func-select; or &func-poll; functions
-can always be used to suspend execution until data becomes available. All
-drivers supporting the <function>read()</function> function must also
-support <function>select()</function> and
-<function>poll()</function>.</para>
-
- <para>Drivers can implement read functionality in different
-ways, using a single or multiple buffers and discarding the oldest or
-newest frames once the internal buffers are filled.</para>
-
- <para><function>read()</function> never returns a "snapshot" of a
-buffer being filled. Using a single buffer the driver will stop
-capturing when the application starts reading the buffer until the
-read is finished. Thus only the period of the vertical blanking
-interval is available for reading, or the capture rate must fall below
-the nominal frame rate of the video standard.</para>
-
-<para>The behavior of
-<function>read()</function> when called during the active picture
-period or the vertical blanking separating the top and bottom field
-depends on the discarding policy. A driver discarding the oldest
-frames keeps capturing into an internal buffer, continuously
-overwriting the previously, not read frame, and returns the frame
-being received at the time of the <function>read()</function> call as
-soon as it is complete.</para>
-
- <para>A driver discarding the newest frames stops capturing until
-the next <function>read()</function> call. The frame being received at
-<function>read()</function> time is discarded, returning the following
-frame instead. Again this implies a reduction of the capture rate to
-one half or less of the nominal frame rate. An example of this model
-is the video read mode of the bttv driver, initiating a DMA to user
-memory when <function>read()</function> is called and returning when
-the DMA finished.</para>
-
- <para>In the multiple buffer model drivers maintain a ring of
-internal buffers, automatically advancing to the next free buffer.
-This allows continuous capturing when the application can empty the
-buffers fast enough. Again, the behavior when the driver runs out of
-free buffers depends on the discarding policy.</para>
-
- <para>Applications can get and set the number of buffers used
-internally by the driver with the &VIDIOC-G-PARM; and &VIDIOC-S-PARM;
-ioctls. They are optional, however. The discarding policy is not
-reported and cannot be changed. For minimum requirements see <xref
- linkend="devices" />.</para>
- </refsect1>
-
- <refsect1>
- <title>Return Value</title>
-
- <para>On success, the number of bytes read is returned. It is not
-an error if this number is smaller than the number of bytes requested,
-or the amount of data required for one frame. This may happen for
-example because <function>read()</function> was interrupted by a
-signal. On error, -1 is returned, and the <varname>errno</varname>
-variable is set appropriately. In this case the next read will start
-at the beginning of a new frame. Possible error codes are:</para>
-
- <variablelist>
- <varlistentry>
- <term><errorcode>EAGAIN</errorcode></term>
- <listitem>
- <para>Non-blocking I/O has been selected using
-O_NONBLOCK and no data was immediately available for reading.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><errorcode>EBADF</errorcode></term>
- <listitem>
- <para><parameter>fd</parameter> is not a valid file
-descriptor or is not open for reading, or the process already has the
-maximum number of files open.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><errorcode>EBUSY</errorcode></term>
- <listitem>
- <para>The driver does not support multiple read streams and the
-device is already in use.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><errorcode>EFAULT</errorcode></term>
- <listitem>
- <para><parameter>buf</parameter> references an inaccessible
-memory area.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><errorcode>EINTR</errorcode></term>
- <listitem>
- <para>The call was interrupted by a signal before any
-data was read.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><errorcode>EIO</errorcode></term>
- <listitem>
- <para>I/O error. This indicates some hardware problem or a
-failure to communicate with a remote device (USB camera etc.).</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><errorcode>EINVAL</errorcode></term>
- <listitem>
- <para>The <function>read()</function> function is not
-supported by this driver, not on this device, or generally not on this
-type of device.</para>
- </listitem>
- </varlistentry>
- </variablelist>
- </refsect1>
-</refentry>