.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L

.. _rw:

**********
Read/Write
**********

Input and output devices support the :c:func:`read()` and
:c:func:`write()` function, respectively, when the
``V4L2_CAP_READWRITE`` flag in the ``capabilities`` field of struct
:c:type:`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 :c:func:`read()`
function, to write the :c: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 :c:func:`select()` and
:c: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 :c:func:`select()` and :c:func:`poll()` are
   the same, and :c:func:`select()` is too important to be optional.
