<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
