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Open Menu /Documentation/media/uapi/v4l/vidioc-g-ctrl.rst 
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.. Permission is granted to copy, distribute and/or modify this
.. document under the terms of the GNU Free Documentation License,
.. Version 1.1 or any later version published by the Free Software
.. Foundation, with no Invariant Sections, no Front-Cover Texts
.. and no Back-Cover Texts. A copy of the license is included at
.. Documentation/media/uapi/fdl-appendix.rst.
..
.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections

.. _VIDIOC_G_CTRL:

**********************************
ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL
**********************************

Name
====

VIDIOC_G_CTRL - VIDIOC_S_CTRL - Get or set the value of a control


Synopsis
========

.. c:function:: int ioctl( int fd, VIDIOC_G_CTRL, struct v4l2_control *argp )
    :name: VIDIOC_G_CTRL

.. c:function:: int ioctl( int fd, VIDIOC_S_CTRL, struct v4l2_control *argp )
    :name: VIDIOC_S_CTRL


Arguments
=========

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

``argp``
    Pointer to struct :c:type:`v4l2_control`.


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

To get the current value of a control applications initialize the ``id``
field of a struct :c:type:`v4l2_control` and call the
:ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` ioctl with a pointer to this structure. To change the
value of a control applications initialize the ``id`` and ``value``
fields of a struct :c:type:`v4l2_control` and call the
:ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctl.

When the ``id`` is invalid drivers return an ``EINVAL`` error code. When the
``value`` is out of bounds drivers can choose to take the closest valid
value or return an ``ERANGE`` error code, whatever seems more appropriate.
However, :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` is a write-only ioctl, it does not return the
actual new value. If the ``value`` is inappropriate for the control
(e.g. if it refers to an unsupported menu index of a menu control), then
EINVAL error code is returned as well.

These ioctls work only with user controls. For other control classes the
:ref:`VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`,
:ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` or
:ref:`VIDIOC_TRY_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` must be used.


.. c:type:: v4l2_control

.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|

.. flat-table:: struct v4l2_control
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2

    * - __u32
      - ``id``
      - Identifies the control, set by the application.
    * - __s32
      - ``value``
      - New value or current value.


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

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 :c:type:`v4l2_control` ``id`` is invalid
    or the ``value`` is inappropriate for the given control (i.e. if a
    menu item is selected that is not supported by the driver according
    to :ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>`).

ERANGE
    The struct :c:type:`v4l2_control` ``value`` is out of
    bounds.

EBUSY
    The control is temporarily not changeable, possibly because another
    applications took over control of the device function this control
    belongs to.

EACCES
    Attempt to set a read-only control or to get a write-only control.