1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3.. _rw:
4
5**********
6Read/Write
7**********
8
9Input and output devices support the :ref:`read() <func-read>` and
10:ref:`write() <func-write>` function, respectively, when the
11``V4L2_CAP_READWRITE`` flag in the ``capabilities`` field of struct
12:c:type:`v4l2_capability` returned by the
13:ref:`VIDIOC_QUERYCAP` ioctl is set.
14
15Drivers may need the CPU to copy the data, but they may also support DMA
16to or from user memory, so this I/O method is not necessarily less
17efficient than other methods merely exchanging buffer pointers. It is
18considered inferior though because no meta-information like frame
19counters or timestamps are passed. This information is necessary to
20recognize frame dropping and to synchronize with other data streams.
21However this is also the simplest I/O method, requiring little or no
22setup to exchange data. It permits command line stunts like this (the
23vidctrl tool is fictitious):
24
25
26.. code-block:: none
27
28    $ vidctrl /dev/video --input=0 --format=YUYV --size=352x288
29    $ dd if=/dev/video of=myimage.422 bs=202752 count=1
30
31To read from the device applications use the :ref:`read() <func-read>`
32function, to write the :ref:`write() <func-write>` function. Drivers
33must implement one I/O method if they exchange data with applications,
34but it need not be this. [#f1]_ When reading or writing is supported, the
35driver must also support the :ref:`select() <func-select>` and
36:ref:`poll() <func-poll>` function. [#f2]_
37
38.. [#f1]
39   It would be desirable if applications could depend on drivers
40   supporting all I/O interfaces, but as much as the complex memory
41   mapping I/O can be inadequate for some devices we have no reason to
42   require this interface, which is most useful for simple applications
43   capturing still images.
44
45.. [#f2]
46   At the driver level :ref:`select() <func-select>` and :ref:`poll() <func-poll>` are
47   the same, and :ref:`select() <func-select>` is too important to be optional.
48