1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2.. c:namespace:: V4L
3
4.. _rw:
5
6**********
7Read/Write
8**********
9
10Input and output devices support the :c:func:`read()` and
11:c:func:`write()` function, respectively, when the
12``V4L2_CAP_READWRITE`` flag in the ``capabilities`` field of struct
13:c:type:`v4l2_capability` returned by the
14:ref:`VIDIOC_QUERYCAP` ioctl is set.
15
16Drivers may need the CPU to copy the data, but they may also support DMA
17to or from user memory, so this I/O method is not necessarily less
18efficient than other methods merely exchanging buffer pointers. It is
19considered inferior though because no meta-information like frame
20counters or timestamps are passed. This information is necessary to
21recognize frame dropping and to synchronize with other data streams.
22However this is also the simplest I/O method, requiring little or no
23setup to exchange data. It permits command line stunts like this (the
24vidctrl tool is fictitious):
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 :c:func:`read()`
32function, to write the :c: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 :c:func:`select()` and
36:c: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 :c:func:`select()` and :c:func:`poll()` are
47   the same, and :c:func:`select()` is too important to be optional.
48