func-read.xml

<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>

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