50 lines
1.7 KiB
ReStructuredText
50 lines
1.7 KiB
ReStructuredText
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.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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.. c:namespace:: V4L
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.. _io:
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############
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Input/Output
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############
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The V4L2 API defines several different methods to read from or write to
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a device. All drivers exchanging data with applications must support at
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least one of them.
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The classic I/O method using the :c:func:`read()` and
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:c:func:`write()` function is automatically selected after opening a
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V4L2 device. When the driver does not support this method attempts to
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read or write will fail at any time.
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Other methods must be negotiated. To select the streaming I/O method
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with memory mapped or user buffers applications call the
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:ref:`VIDIOC_REQBUFS` ioctl.
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Video overlay can be considered another I/O method, although the
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application does not directly receive the image data. It is selected by
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initiating video overlay with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
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ioctl. For more information see :ref:`overlay`.
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Generally exactly one I/O method, including overlay, is associated with
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each file descriptor. The only exceptions are applications not
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exchanging data with a driver ("panel applications", see :ref:`open`)
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and drivers permitting simultaneous video capturing and overlay using
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the same file descriptor, for compatibility with V4L and earlier
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versions of V4L2.
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_REQBUFS` would permit this to some
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degree, but for simplicity drivers need not support switching the I/O
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method (after first switching away from read/write) other than by
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closing and reopening the device.
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The following sections describe the various I/O methods in more detail.
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.. toctree::
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:maxdepth: 1
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rw
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mmap
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userp
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dmabuf
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buffer
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field-order
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