.. Permission is granted to copy, distribute and/or modify this .. document under the terms of the GNU Free Documentation License, .. Version 1.1 or any later version published by the Free Software .. Foundation, with no Invariant Sections, no Front-Cover Texts .. and no Back-Cover Texts. A copy of the license is included at .. Documentation/media/uapi/fdl-appendix.rst. .. .. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections .. _rw: ********** Read/Write ********** Input and output devices support the :ref:`read() ` and :ref:`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 :ref:`read() ` function, to write the :ref:`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 :ref:`select() ` and :ref:`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 :ref:`select() ` and :ref:`poll() ` are the same, and :ref:`select() ` is too important to be optional.