.. 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.