mirror of https://gitee.com/openkylin/linux.git
84 lines
2.0 KiB
ReStructuredText
84 lines
2.0 KiB
ReStructuredText
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.. -*- coding: utf-8; mode: rst -*-
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.. _func-open:
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***********
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V4L2 open()
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***********
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Name
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====
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v4l2-open - Open a V4L2 device
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Synopsis
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========
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.. code-block:: c
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#include <fcntl.h>
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.. cpp:function:: int open( const char *device_name, int flags )
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Arguments
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=========
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``device_name``
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Device to be opened.
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``flags``
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Open flags. Access mode must be ``O_RDWR``. This is just a
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technicality, input devices still support only reading and output
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devices only writing.
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When the ``O_NONBLOCK`` flag is given, the :ref:`read() <func-read>`
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function and the :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl will
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return the ``EAGAIN`` error code when no data is available or no
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buffer is in the driver outgoing queue, otherwise these functions
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block until data becomes available. All V4L2 drivers exchanging data
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with applications must support the ``O_NONBLOCK`` flag.
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Other flags have no effect.
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Description
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===========
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To open a V4L2 device applications call :ref:`open() <func-open>` with the
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desired device name. This function has no side effects; all data format
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parameters, current input or output, control values or other properties
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remain unchanged. At the first :ref:`open() <func-open>` call after loading the
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driver they will be reset to default values, drivers are never in an
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undefined state.
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Return Value
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============
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On success :ref:`open() <func-open>` returns the new file descriptor. On error
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-1 is returned, and the ``errno`` variable is set appropriately.
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Possible error codes are:
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EACCES
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The caller has no permission to access the device.
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EBUSY
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The driver does not support multiple opens and the device is already
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in use.
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ENXIO
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No device corresponding to this device special file exists.
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ENOMEM
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Not enough kernel memory was available to complete the request.
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EMFILE
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The process already has the maximum number of files open.
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ENFILE
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The limit on the total number of files open on the system has been
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reached.
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