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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _rw:
+
+**********
+Read/Write
+**********
+
+Input and output devices support the :ref:`read() <func-read>` and
+:ref:`write() <func-write>` function, respectively, when the
+``V4L2_CAP_READWRITE`` flag in the ``capabilities`` field of struct
+:ref:`v4l2_capability <v4l2-capability>` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl is set.
+
+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
+vidctrl tool is fictitious):
+
+
+.. code-block:: none
+
+    $ vidctrl /dev/video --input=0 --format=YUYV --size=352x288
+    $ dd if=/dev/video of=myimage.422 bs=202752 count=1
+
+To read from the device applications use the :ref:`read() <func-read>`
+function, to write the :ref:`write() <func-write>` function. Drivers
+must implement one I/O method if they exchange data with applications,
+but it need not be this. [#f1]_ When reading or writing is supported, the
+driver must also support the :ref:`select() <func-select>` and
+:ref:`poll() <func-poll>` function. [#f2]_
+
+.. [#f1]
+   It would be desirable if applications could depend on drivers
+   supporting all I/O interfaces, but as much as the complex memory
+   mapping I/O can be inadequate for some devices we have no reason to
+   require this interface, which is most useful for simple applications
+   capturing still images.
+
+.. [#f2]
+   At the driver level :ref:`select() <func-select>` and :ref:`poll() <func-poll>` are
+   the same, and :ref:`select() <func-select>` is too important to be optional.