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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: V4L
+
+.. _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 :c:func:`read()` and
+:c: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