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1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later2.. c:namespace:: V4L3 4.. _rw:5 6**********7Read/Write8**********9 10Input and output devices support the :c:func:`read()` and11:c:func:`write()` function, respectively, when the12``V4L2_CAP_READWRITE`` flag in the ``capabilities`` field of struct13:c:type:`v4l2_capability` returned by the14:ref:`VIDIOC_QUERYCAP` ioctl is set.15 16Drivers may need the CPU to copy the data, but they may also support DMA17to or from user memory, so this I/O method is not necessarily less18efficient than other methods merely exchanging buffer pointers. It is19considered inferior though because no meta-information like frame20counters or timestamps are passed. This information is necessary to21recognize frame dropping and to synchronize with other data streams.22However this is also the simplest I/O method, requiring little or no23setup to exchange data. It permits command line stunts like this (the24vidctrl tool is fictitious):25 26.. code-block:: none27 28 $ vidctrl /dev/video --input=0 --format=YUYV --size=352x28829 $ dd if=/dev/video of=myimage.422 bs=202752 count=130 31To read from the device applications use the :c:func:`read()`32function, to write the :c:func:`write()` function. Drivers33must implement one I/O method if they exchange data with applications,34but it need not be this. [#f1]_ When reading or writing is supported, the35driver must also support the :c:func:`select()` and36:c:func:`poll()` function. [#f2]_37 38.. [#f1]39 It would be desirable if applications could depend on drivers40 supporting all I/O interfaces, but as much as the complex memory41 mapping I/O can be inadequate for some devices we have no reason to42 require this interface, which is most useful for simple applications43 capturing still images.44 45.. [#f2]46 At the driver level :c:func:`select()` and :c:func:`poll()` are47 the same, and :c:func:`select()` is too important to be optional.48