1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2 .. c:namespace:: V4L
3
4 .. _io:
5
6 ############
7 Input/Output
8 ############
9 The V4L2 API defines several different methods to read from or write to
10 a device. All drivers exchanging data with applications must support at
11 least one of them.
12
13 The classic I/O method using the :c:func:`read()` and
14 :c:func:`write()` function is automatically selected after opening a
15 V4L2 device. When the driver does not support this method attempts to
16 read or write will fail at any time.
17
18 Other methods must be negotiated. To select the streaming I/O method
19 with memory mapped or user buffers applications call the
20 :ref:`VIDIOC_REQBUFS` ioctl.
21
22 Video overlay can be considered another I/O method, although the
23 application does not directly receive the image data. It is selected by
24 initiating video overlay with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
25 ioctl. For more information see :ref:`overlay`.
26
27 Generally exactly one I/O method, including overlay, is associated with
28 each file descriptor. The only exceptions are applications not
29 exchanging data with a driver ("panel applications", see :ref:`open`)
30 and drivers permitting simultaneous video capturing and overlay using
31 the same file descriptor, for compatibility with V4L and earlier
32 versions of V4L2.
33
34 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_REQBUFS` would permit this to some
35 degree, but for simplicity drivers need not support switching the I/O
36 method (after first switching away from read/write) other than by
37 closing and reopening the device.
38
39 The following sections describe the various I/O methods in more detail.
40
41 .. toctree::
42 :maxdepth: 1
43
44 rw
45 mmap
46 userp
47 dmabuf
48 buffer
49 field-order
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