vidioc-g-fmt.rst 5.8 KB
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.. -*- coding: utf-8; mode: rst -*-

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.. _VIDIOC_G_FMT:
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************************************************
ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT
************************************************

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Name
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====
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VIDIOC_G_FMT - VIDIOC_S_FMT - VIDIOC_TRY_FMT - Get or set the data format, try a format
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Synopsis
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========

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.. c:function:: int ioctl( int fd, VIDIOC_G_FMT, struct v4l2_format *argp )
    :name: VIDIOC_G_FMT
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.. c:function:: int ioctl( int fd, VIDIOC_S_FMT, struct v4l2_format *argp )
    :name: VIDIOC_S_FMT

.. c:function:: int ioctl( int fd, VIDIOC_TRY_FMT, struct v4l2_format *argp )
    :name: VIDIOC_TRY_FMT
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Arguments
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=========

``fd``
    File descriptor returned by :ref:`open() <func-open>`.

``argp``


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Description
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===========

These ioctls are used to negotiate the format of data (typically image
format) exchanged between driver and application.

To query the current parameters applications set the ``type`` field of a
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struct :ref:`struct v4l2_format <v4l2-format>` to the respective buffer (stream)
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type. For example video capture devices use
``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``. When the application calls the
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:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer to this structure the driver fills
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the respective member of the ``fmt`` union. In case of video capture
devices that is either the struct
:ref:`v4l2_pix_format <v4l2-pix-format>` ``pix`` or the struct
:ref:`v4l2_pix_format_mplane <v4l2-pix-format-mplane>` ``pix_mp``
member. When the requested buffer type is not supported drivers return
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an ``EINVAL`` error code.
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To change the current format parameters applications initialize the
``type`` field and all fields of the respective ``fmt`` union member.
For details see the documentation of the various devices types in
:ref:`devices`. Good practice is to query the current parameters
first, and to modify only those parameters not suitable for the
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application. When the application calls the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl with
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a pointer to a :ref:`struct v4l2_format <v4l2-format>` structure the driver
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checks and adjusts the parameters against hardware abilities. Drivers
should not return an error code unless the ``type`` field is invalid,
this is a mechanism to fathom device capabilities and to approach
parameters acceptable for both the application and driver. On success
the driver may program the hardware, allocate resources and generally
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prepare for data exchange. Finally the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl returns
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the current format parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. Very simple,
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inflexible devices may even ignore all input and always return the
default parameters. However all V4L2 devices exchanging data with the
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application must implement the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
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ioctl. When the requested buffer type is not supported drivers return an
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EINVAL error code on a :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` attempt. When I/O is already in
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progress or the resource is not available for other reasons drivers
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return the ``EBUSY`` error code.
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The :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl is equivalent to :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` with one
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exception: it does not change driver state. It can also be called at any
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time, never returning ``EBUSY``. This function is provided to negotiate
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parameters, to learn about hardware limitations, without disabling I/O
or possibly time consuming hardware preparations. Although strongly
recommended drivers are not required to implement this ioctl.

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The format as returned by :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` must be identical to what
:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` returns for the same input or output.
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.. _v4l2-format:

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.. tabularcolumns::  |p{1.2cm}|p{4.3cm}|p{3.0cm}|p{9.0cm}|

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.. flat-table:: struct v4l2_format
    :header-rows:  0
    :stub-columns: 0


    -  .. row 1

       -  __u32

       -  ``type``

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       -
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       -  Type of the data stream, see :ref:`v4l2-buf-type`.

    -  .. row 2

       -  union

       -  ``fmt``

    -  .. row 3

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       -
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       -  struct :ref:`v4l2_pix_format <v4l2-pix-format>`

       -  ``pix``

       -  Definition of an image format, see :ref:`pixfmt`, used by video
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	  capture and output devices.
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    -  .. row 4

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       -
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       -  struct :ref:`v4l2_pix_format_mplane <v4l2-pix-format-mplane>`

       -  ``pix_mp``

       -  Definition of an image format, see :ref:`pixfmt`, used by video
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	  capture and output devices that support the
	  :ref:`multi-planar version of the API <planar-apis>`.
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    -  .. row 5

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       -
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       -  struct :ref:`v4l2_window <v4l2-window>`

       -  ``win``

       -  Definition of an overlaid image, see :ref:`overlay`, used by
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	  video overlay devices.
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    -  .. row 6

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       -
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       -  struct :ref:`v4l2_vbi_format <v4l2-vbi-format>`

       -  ``vbi``

       -  Raw VBI capture or output parameters. This is discussed in more
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	  detail in :ref:`raw-vbi`. Used by raw VBI capture and output
	  devices.
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    -  .. row 7

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       -
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       -  struct :ref:`v4l2_sliced_vbi_format <v4l2-sliced-vbi-format>`

       -  ``sliced``

       -  Sliced VBI capture or output parameters. See :ref:`sliced` for
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	  details. Used by sliced VBI capture and output devices.
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    -  .. row 8

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       -
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       -  struct :ref:`v4l2_sdr_format <v4l2-sdr-format>`

       -  ``sdr``

       -  Definition of a data format, see :ref:`pixfmt`, used by SDR
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	  capture and output devices.
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    -  .. row 9

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       -
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       -  __u8

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       -  ``raw_data``\ [200]
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       -  Place holder for future extensions.


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Return Value
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============

On success 0 is returned, on error -1 and the ``errno`` variable is set
appropriately. The generic error codes are described at the
:ref:`Generic Error Codes <gen-errors>` chapter.

EINVAL
    The struct :ref:`v4l2_format <v4l2-format>` ``type`` field is
    invalid or the requested buffer type not supported.