1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2.. c:namespace:: V4L
3
4.. _io:
5
6############
7Input/Output
8############
9The V4L2 API defines several different methods to read from or write to
10a device. All drivers exchanging data with applications must support at
11least one of them.
12
13The classic I/O method using the :c:func:`read()` and
14:c:func:`write()` function is automatically selected after opening a
15V4L2 device. When the driver does not support this method attempts to
16read or write will fail at any time.
17
18Other methods must be negotiated. To select the streaming I/O method
19with memory mapped or user buffers applications call the
20:ref:`VIDIOC_REQBUFS` ioctl.
21
22Video overlay can be considered another I/O method, although the
23application does not directly receive the image data. It is selected by
24initiating video overlay with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
25ioctl. For more information see :ref:`overlay`.
26
27Generally exactly one I/O method, including overlay, is associated with
28each file descriptor. The only exceptions are applications not
29exchanging data with a driver ("panel applications", see :ref:`open`)
30and drivers permitting simultaneous video capturing and overlay using
31the same file descriptor, for compatibility with V4L and earlier
32versions of V4L2.
33
34:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_REQBUFS` would permit this to some
35degree, but for simplicity drivers need not support switching the I/O
36method (after first switching away from read/write) other than by
37closing and reopening the device.
38
39The 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
50