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- .. -*- coding: utf-8; mode: rst -*-
- .. _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
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