.. -*- 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.
