CooCenter-System-kernel/Documentation/DocBook/media/v4l/func-write.xml

<refentry id="func-write">
  <refmeta>
    <refentrytitle>V4L2 write()</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>v4l2-write</refname>
    <refpurpose>Write to a V4L2 device</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;unistd.h&gt;</funcsynopsisinfo>
      <funcprototype>
	<funcdef>ssize_t <function>write</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>write()</function> writes up to
<parameter>count</parameter> bytes to the device referenced by the
file descriptor <parameter>fd</parameter> from the buffer starting at
<parameter>buf</parameter>. When the hardware outputs are not active
yet, this function enables them. When <parameter>count</parameter> is
zero, <function>write()</function> returns
<returnvalue>0</returnvalue> without any other effect.</para>

    <para>When the application does not provide more data in time, the
previous video frame, raw VBI image, sliced VPS or WSS data is
displayed again. Sliced Teletext or Closed Caption data is not
repeated, the driver inserts a blank line instead.</para>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>On success, the number of bytes written are returned. Zero
indicates nothing was written. On error, <returnvalue>-1</returnvalue>
is returned, and the <varname>errno</varname> variable is set
appropriately. In this case the next write 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 the <link
linkend="func-open"><constant>O_NONBLOCK</constant></link> flag and no
buffer space was available to write the data immediately.</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 writing.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EBUSY</errorcode></term>
	<listitem>
	  <para>The driver does not support multiple write 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 written.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EIO</errorcode></term>
	<listitem>
	  <para>I/O error. This indicates some hardware problem.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EINVAL</errorcode></term>
	<listitem>
	  <para>The <function>write()</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>