path: root/Documentation/DocBook/v4l/io.xml

  <title>Input/Output</title>

  <para>The V4L2 API defines several different methods to read from or
write to a device. All drivers exchanging data with applications must
support at least one of them.</para>

  <para>The classic I/O method using the <function>read()</function>
and <function>write()</function> function is automatically selected
after opening a V4L2 device. When the driver does not support this
method attempts to read or write will fail at any time.</para>

  <para>Other methods must be negotiated. To select the streaming I/O
method with memory mapped or user buffers applications call the
&VIDIOC-REQBUFS; ioctl. The asynchronous I/O method is not defined
yet.</para>

  <para>Video overlay can be considered another I/O method, although
the application does not directly receive the image data. It is
selected by initiating video overlay with the &VIDIOC-S-FMT; ioctl.
For more information see <xref linkend="overlay" />.</para>

  <para>Generally exactly one I/O method, including overlay, is
associated with each file descriptor. The only exceptions are
applications not exchanging data with a driver ("panel applications",
see <xref linkend="open" />) and drivers permitting simultaneous video capturing
and overlay using the same file descriptor, for compatibility with V4L
and earlier versions of V4L2.</para>

  <para><constant>VIDIOC_S_FMT</constant> and
<constant>VIDIOC_REQBUFS</constant> would permit this to some degree,
but for simplicity drivers need not support switching the I/O method
(after first switching away from read/write) other than by closing
and reopening the device.</para>

  <para>The following sections describe the various I/O methods in
more detail.</para>

  <section id="rw">
    <title>Read/Write</title>

    <para>Input and output devices support the
<function>read()</function> and <function>write()</function> function,
respectively, when the <constant>V4L2_CAP_READWRITE</constant> flag in
the <structfield>capabilities</structfield> field of &v4l2-capability;
returned by the &VIDIOC-QUERYCAP; ioctl is set.</para>

    <para>Drivers may need the CPU to copy the data, but they may also
support DMA to or from user memory, so this I/O method is not
necessarily less efficient than other methods merely exchanging buffer
pointers. It is considered inferior though because no meta-information
like frame counters or timestamps are passed. This information is
necessary to recognize frame dropping and to synchronize with other
data streams. However this is also the simplest I/O method, requiring
little or no setup to exchange data. It permits command line stunts
like this (the <application>vidctrl</application> tool is
fictitious):</para>

    <informalexample>
      <screen>
&gt; vidctrl /dev/video --input=0 --format=YUYV --size=352x288
&gt; dd if=/dev/video of=myimage.422 bs=202752 count=1
</screen>
    </informalexample>

    <para>To read from the device applications use the
&func-read; function, to write the &func-write; function.
Drivers must implement one I/O method if they
exchange data with applications,