81 lines
2.5 KiB
ReStructuredText
81 lines
2.5 KiB
ReStructuredText
.. -*- coding: utf-8; mode: rst -*-
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.. _VIDIOC_QUERYBUF:
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*********************
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ioctl VIDIOC_QUERYBUF
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*********************
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Name
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====
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VIDIOC_QUERYBUF - Query the status of a buffer
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Synopsis
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========
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.. c:function:: int ioctl( int fd, VIDIOC_QUERYBUF, struct v4l2_buffer *argp )
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:name: VIDIOC_QUERYBUF
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Arguments
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=========
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``fd``
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File descriptor returned by :ref:`open() <func-open>`.
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``argp``
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Pointer to struct :c:type:`v4l2_buffer`.
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Description
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===========
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This ioctl is part of the :ref:`streaming <mmap>` I/O method. It can
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be used to query the status of a buffer at any time after buffers have
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been allocated with the :ref:`VIDIOC_REQBUFS` ioctl.
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Applications set the ``type`` field of a struct
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:c:type:`v4l2_buffer` to the same buffer type as was
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previously used with struct :c:type:`v4l2_format` ``type``
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and struct :c:type:`v4l2_requestbuffers` ``type``,
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and the ``index`` field. Valid index numbers range from zero to the
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number of buffers allocated with
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:ref:`VIDIOC_REQBUFS` (struct
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:c:type:`v4l2_requestbuffers` ``count``) minus
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one. The ``reserved`` and ``reserved2`` fields must be set to 0. When
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using the :ref:`multi-planar API <planar-apis>`, the ``m.planes``
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field must contain a userspace pointer to an array of struct
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:c:type:`v4l2_plane` and the ``length`` field has to be set
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to the number of elements in that array. After calling
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:ref:`VIDIOC_QUERYBUF` with a pointer to this structure drivers return an
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error code or fill the rest of the structure.
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In the ``flags`` field the ``V4L2_BUF_FLAG_MAPPED``,
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``V4L2_BUF_FLAG_PREPARED``, ``V4L2_BUF_FLAG_QUEUED`` and
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``V4L2_BUF_FLAG_DONE`` flags will be valid. The ``memory`` field will be
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set to the current I/O method. For the single-planar API, the
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``m.offset`` contains the offset of the buffer from the start of the
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device memory, the ``length`` field its size. For the multi-planar API,
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fields ``m.mem_offset`` and ``length`` in the ``m.planes`` array
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elements will be used instead and the ``length`` field of struct
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:c:type:`v4l2_buffer` is set to the number of filled-in
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array elements. The driver may or may not set the remaining fields and
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flags, they are meaningless in this context.
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The struct :c:type:`v4l2_buffer` structure is specified in
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:ref:`buffer`.
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Return Value
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============
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On success 0 is returned, on error -1 and the ``errno`` variable is set
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appropriately. The generic error codes are described at the
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:ref:`Generic Error Codes <gen-errors>` chapter.
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EINVAL
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The buffer ``type`` is not supported, or the ``index`` is out of
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bounds.
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