Documentation/media/uapi/v4l/io.rst

.. 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

.. _io:

############
Input/Output
############
The V4L2 API defines several different methods to read from or write to
a device. All drivers exchanging data with applications must support at
least one of them.

The classic I/O method using the :ref:`read() <func-read>` and
:ref:`write() <func-write>` function is automatically selected after opening a
V4L2 device. When the driver does not support this method attempts to
read or write will fail at any time.

Other methods must be negotiated. To select the streaming I/O method
with memory mapped or user buffers applications call the
:ref:`VIDIOC_REQBUFS` ioctl. The asynchronous I/O
method is not defined yet.

Video overlay can be considered another I/O method, although the
application does not directly receive the image data. It is selected by
initiating video overlay with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
ioctl. For more information see :ref:`overlay`.

Generally exactly one I/O method, including overlay, is associated with
each file descriptor. The only exceptions are applications not
exchanging data with a driver ("panel applications", see :ref:`open`)
and drivers permitting simultaneous video capturing and overlay using
the same file descriptor, for compatibility with V4L and earlier
versions of V4L2.

:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_REQBUFS` would permit this to some
degree, but for simplicity drivers need not support switching the I/O
method (after first switching away from read/write) other than by
closing and reopening the device.

The following sections describe the various I/O methods in more detail.


.. toctree::
    :maxdepth: 1

    rw
    mmap
    userp
    dmabuf
    async
    buffer
    field-order