1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2 .. c:namespace:: V4L 3 4 .. _rw: 5 6 ********** 7 Read/Write 8 ********** 9 10 Input 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 16 Drivers may need the CPU to copy the data, but they may also support DMA 17 to or from user memory, so this I/O method is not necessarily less 18 efficient than other methods merely exchanging buffer pointers. It is 19 considered inferior though because no meta-information like frame 20 counters or timestamps are passed. This information is necessary to 21 recognize frame dropping and to synchronize with other data streams. 22 However this is also the simplest I/O method, requiring little or no 23 setup to exchange data. It permits command line stunts like this (the 24 vidctrl 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 31 To read from the device applications use the :c:func:`read()` 32 function, to write the :c:func:`write()` function. Drivers 33 must implement one I/O method if they exchange data with applications, 34 but it need not be this. [#f1]_ When reading or writing is supported, the 35 driver 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.
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