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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/userspace-api/media/fdl-appendix.rst.
+..
+.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _VIDIOC_QUERYCTRL:
+
+*******************************************************************
+ioctls VIDIOC_QUERYCTRL, VIDIOC_QUERY_EXT_CTRL and VIDIOC_QUERYMENU
+*******************************************************************
+
+Name
+====
+
+VIDIOC_QUERYCTRL - VIDIOC_QUERY_EXT_CTRL - VIDIOC_QUERYMENU - Enumerate controls and menu control items
+
+
+Synopsis
+========
+
+.. c:function:: int ioctl( int fd, int VIDIOC_QUERYCTRL, struct v4l2_queryctrl *argp )
+ :name: VIDIOC_QUERYCTRL
+
+.. c:function:: int ioctl( int fd, VIDIOC_QUERY_EXT_CTRL, struct v4l2_query_ext_ctrl *argp )
+ :name: VIDIOC_QUERY_EXT_CTRL
+
+.. c:function:: int ioctl( int fd, VIDIOC_QUERYMENU, struct v4l2_querymenu *argp )
+ :name: VIDIOC_QUERYMENU
+
+
+Arguments
+=========
+
+``fd``
+ File descriptor returned by :ref:`open() <func-open>`.
+
+``argp``
+ Pointer to struct :c:type:`v4l2_queryctrl`, :c:type:`v4l2_query_ext_ctrl`
+ or :c:type:`v4l2_querymenu` (depending on the ioctl).
+
+
+Description
+===========
+
+To query the attributes of a control applications set the ``id`` field
+of a struct :ref:`v4l2_queryctrl <v4l2-queryctrl>` and call the
+``VIDIOC_QUERYCTRL`` ioctl with a pointer to this structure. The driver
+fills the rest of the structure or returns an ``EINVAL`` error code when the
+``id`` is invalid.
+
+It is possible to enumerate controls by calling ``VIDIOC_QUERYCTRL``
+with successive ``id`` values starting from ``V4L2_CID_BASE`` up to and
+exclusive ``V4L2_CID_LASTP1``. Drivers may return ``EINVAL`` if a control in
+this range is not supported. Further applications can enumerate private
+controls, which are not defined in this specification, by starting at
+``V4L2_CID_PRIVATE_BASE`` and incrementing ``id`` until the driver
+returns ``EINVAL``.
+
+In both cases, when the driver sets the ``V4L2_CTRL_FLAG_DISABLED`` flag
+in the ``flags`` field this control is permanently disabled and should
+be ignored by the application. [#f1]_
+
+When the application ORs ``id`` with ``V4L2_CTRL_FLAG_NEXT_CTRL`` the
+driver returns the next supported non-compound control, or ``EINVAL`` if
+there is none. In addition, the ``V4L2_CTRL_FLAG_NEXT_COMPOUND`` flag
+can be specified to enumerate all compound controls (i.e. controls with
+type ≥ ``V4L2_CTRL_COMPOUND_TYPES`` and/or array control, in other words
+controls that contain more than one value). Specify both
+``V4L2_CTRL_FLAG_NEXT_CTRL`` and ``V4L2_CTRL_FLAG_NEXT_COMPOUND`` in
+order to enumerate all controls, compound or not. Drivers which do not
+support these flags yet always return ``EINVAL``.
+
+The ``VIDIOC_QUERY_EXT_CTRL`` ioctl was introduced in order to better
+support controls that can use compound types, and to expose additional
+control information that cannot be returned in struct
+:ref:`v4l2_queryctrl <v4l2-queryctrl>` since that structure is full.
+
+``VIDIOC_QUERY_EXT_CTRL`` is used in the same way as
+``VIDIOC_QUERYCTRL``, except that the ``reserved`` array must be zeroed
+as well.
+
+Additional information is required for menu controls: the names of the
+menu items. To query them applications set the ``id`` and ``index``
+fields of struct :ref:`v4l2_querymenu <v4l2-querymenu>` and call the
+``VIDIOC_QUERYMENU`` ioctl with a pointer to this structure. The driver
+fills the rest of the structure or returns an ``EINVAL`` error code when the
+``id`` or ``index`` is invalid. Menu items are enumerated by calling
+``VIDIOC_QUERYMENU`` with successive ``index`` values from struct
+:ref:`v4l2_queryctrl <v4l2-queryctrl>` ``minimum`` to ``maximum``,
+inclusive.
+
+.. note::
+
+ It is possible for ``VIDIOC_QUERYMENU`` to return
+ an ``EINVAL`` error code for some indices between ``minimum`` and
+ ``maximum``. In that case that particular menu item is not supported by
+ this driver. Also note that the ``minimum`` value is not necessarily 0.
+
+See also the examples in :ref:`control`.
+
+
+.. tabularcolumns:: |p{1.2cm}|p{3.6cm}|p{12.7cm}|
+
+.. _v4l2-queryctrl:
+
+.. cssclass:: longtable
+
+.. flat-table:: struct v4l2_queryctrl
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 1 1 2
+
+ * - __u32
+ - ``id``
+ - Identifies the control, set by the application. See
+ :ref:`control-id` for predefined IDs. When the ID is ORed with
+ V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and
+ returns the first control with a higher ID. Drivers which do not
+ support this flag yet always return an ``EINVAL`` error code.
+ * - __u32
+ - ``type``
+ - Type of control, see :c:type:`v4l2_ctrl_type`.
+ * - __u8
+ - ``name``\ [32]
+ - Name of the control, a NUL-terminated ASCII string. This
+ information is intended for the user.
+ * - __s32
+ - ``minimum``
+ - Minimum value, inclusive. This field gives a lower bound for the
+ control. See enum :c:type:`v4l2_ctrl_type` how
+ the minimum value is to be used for each possible control type.
+ Note that this a signed 32-bit value.
+ * - __s32
+ - ``maximum``
+ - Maximum value, inclusive. This field gives an upper bound for the
+ control. See enum :c:type:`v4l2_ctrl_type` how
+ the maximum value is to be used for each possible control type.
+ Note that this a signed 32-bit value.
+ * - __s32
+ - ``step``
+ - This field gives a step size for the control. See enum
+ :c:type:`v4l2_ctrl_type` how the step value is
+ to be used for each possible control type. Note that this an
+ unsigned 32-bit value.
+
+ Generally drivers should not scale hardware control values. It may
+ be necessary for example when the ``name`` or ``id`` imply a
+ particular unit and the hardware actually accepts only multiples
+ of said unit. If so, drivers must take care values are properly
+ rounded when scaling, such that errors will not accumulate on
+ repeated read-write cycles.
+
+ This field gives the smallest change of an integer control
+ actually affecting hardware. Often the information is needed when
+ the user can change controls by keyboard or GUI buttons, rather
+ than a slider. When for example a hardware register accepts values
+ 0-511 and the driver reports 0-65535, step should be 128.
+
+ Note that although signed, the step value is supposed to be always
+ positive.
+ * - __s32
+ - ``default_value``
+ - The default value of a ``V4L2_CTRL_TYPE_INTEGER``, ``_BOOLEAN``,
+ ``_BITMASK``, ``_MENU`` or ``_INTEGER_MENU`` control. Not valid
+ for other types of controls.
+
+ .. note::
+
+ Drivers reset controls to their default value only when
+ the driver is first loaded, never afterwards.
+ * - __u32
+ - ``flags``
+ - Control flags, see :ref:`control-flags`.
+ * - __u32
+ - ``reserved``\ [2]
+ - Reserved for future extensions. Drivers must set the array to
+ zero.
+
+
+
+.. tabularcolumns:: |p{1.2cm}|p{5.0cm}|p{11.3cm}|
+
+.. _v4l2-query-ext-ctrl:
+
+.. cssclass:: longtable
+
+.. flat-table:: struct v4l2_query_ext_ctrl
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 1 1 2
+
+ * - __u32
+ - ``id``
+ - Identifies the control, set by the application. See
+ :ref:`control-id` for predefined IDs. When the ID is ORed with
+ ``V4L2_CTRL_FLAG_NEXT_CTRL`` the driver clears the flag and
+ returns the first non-compound control with a higher ID. When the
+ ID is ORed with ``V4L2_CTRL_FLAG_NEXT_COMPOUND`` the driver clears
+ the flag and returns the first compound control with a higher ID.
+ Set both to get the first control (compound or not) with a higher
+ ID.
+ * - __u32
+ - ``type``
+ - Type of control, see :c:type:`v4l2_ctrl_type`.
+ * - char
+ - ``name``\ [32]
+ - Name of the control, a NUL-terminated ASCII string. This
+ information is intended for the user.
+ * - __s64
+ - ``minimum``
+ - Minimum value, inclusive. This field gives a lower bound for the
+ control. See enum :c:type:`v4l2_ctrl_type` how
+ the minimum value is to be used for each possible control type.
+ Note that this a signed 64-bit value.
+ * - __s64
+ - ``maximum``
+ - Maximum value, inclusive. This field gives an upper bound for the
+ control. See enum :c:type:`v4l2_ctrl_type` how
+ the maximum value is to be used for each possible control type.
+ Note that this a signed 64-bit value.
+ * - __u64
+ - ``step``
+ - This field gives a step size for the control. See enum
+ :c:type:`v4l2_ctrl_type` how the step value is
+ to be used for each possible control type. Note that this an
+ unsigned 64-bit value.
+
+ Generally drivers should not scale hardware control values. It may
+ be necessary for example when the ``name`` or ``id`` imply a
+ particular unit and the hardware actually accepts only multiples
+ of said unit. If so, drivers must take care values are properly
+ rounded when scaling, such that errors will not accumulate on
+ repeated read-write cycles.
+
+ This field gives the smallest change of an integer control
+ actually affecting hardware. Often the information is needed when
+ the user can change controls by keyboard or GUI buttons, rather
+ than a slider. When for example a hardware register accepts values
+ 0-511 and the driver reports 0-65535, step should be 128.
+ * - __s64
+ - ``default_value``
+ - The default value of a ``V4L2_CTRL_TYPE_INTEGER``, ``_INTEGER64``,
+ ``_BOOLEAN``, ``_BITMASK``, ``_MENU``, ``_INTEGER_MENU``, ``_U8``
+ or ``_U16`` control. Not valid for other types of controls.
+
+ .. note::
+
+ Drivers reset controls to their default value only when
+ the driver is first loaded, never afterwards.
+ * - __u32
+ - ``flags``
+ - Control flags, see :ref:`control-flags`.
+ * - __u32
+ - ``elem_size``
+ - The size in bytes of a single element of the array. Given a char
+ pointer ``p`` to a 3-dimensional array you can find the position
+ of cell ``(z, y, x)`` as follows:
+ ``p + ((z * dims[1] + y) * dims[0] + x) * elem_size``.
+ ``elem_size`` is always valid, also when the control isn't an
+ array. For string controls ``elem_size`` is equal to
+ ``maximum + 1``.
+ * - __u32
+ - ``elems``
+ - The number of elements in the N-dimensional array. If this control
+ is not an array, then ``elems`` is 1. The ``elems`` field can
+ never be 0.
+ * - __u32
+ - ``nr_of_dims``
+ - The number of dimension in the N-dimensional array. If this
+ control is not an array, then this field is 0.
+ * - __u32
+ - ``dims[V4L2_CTRL_MAX_DIMS]``
+ - The size of each dimension. The first ``nr_of_dims`` elements of
+ this array must be non-zero, all remaining elements must be zero.
+ * - __u32
+ - ``reserved``\ [32]
+ - Reserved for future extensions. Applications and drivers must set
+ the array to zero.
+
+
+
+.. tabularcolumns:: |p{1.2cm}|p{1.0cm}|p{1.7cm}|p{13.0cm}|
+
+.. _v4l2-querymenu:
+
+.. flat-table:: struct v4l2_querymenu
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 1 1 2
+
+ * - __u32
+ - ``id``
+ - Identifies the control, set by the application from the respective
+ struct :ref:`v4l2_queryctrl <v4l2-queryctrl>` ``id``.
+ * - __u32
+ - ``index``
+ - Index of the menu item, starting at zero, set by the application.
+ * - union {
+ - (anonymous)
+ * - __u8
+ - ``name``\ [32]
+ - Name of the menu item, a NUL-terminated ASCII string. This
+ information is intended for the user. This field is valid for
+ ``V4L2_CTRL_TYPE_MENU`` type controls.
+ * - __s64
+ - ``value``
+ - Value of the integer menu item. This field is valid for
+ ``V4L2_CTRL_TYPE_INTEGER_MENU`` type controls.
+ * - }
+ -
+ * - __u32
+ - ``reserved``
+ - Reserved for future extensions. Drivers must set the array to
+ zero.
+
+
+
+.. tabularcolumns:: |p{5.8cm}|p{1.4cm}|p{1.0cm}|p{1.4cm}|p{6.9cm}|
+
+.. c:type:: v4l2_ctrl_type
+
+.. cssclass:: longtable
+
+.. flat-table:: enum v4l2_ctrl_type
+ :header-rows: 1
+ :stub-columns: 0
+ :widths: 30 5 5 5 55
+
+ * - Type
+ - ``minimum``
+ - ``step``
+ - ``maximum``
+ - Description
+ * - ``V4L2_CTRL_TYPE_INTEGER``
+ - any
+ - any
+ - any
+ - An integer-valued control ranging from minimum to maximum
+ inclusive. The step value indicates the increment between values.
+ * - ``V4L2_CTRL_TYPE_BOOLEAN``
+ - 0
+ - 1
+ - 1
+ - A boolean-valued control. Zero corresponds to "disabled", and one
+ means "enabled".
+ * - ``V4L2_CTRL_TYPE_MENU``
+ - ≥ 0
+ - 1
+ - N-1
+ - The control has a menu of N choices. The names of the menu items
+ can be enumerated with the ``VIDIOC_QUERYMENU`` ioctl.
+ * - ``V4L2_CTRL_TYPE_INTEGER_MENU``
+ - ≥ 0
+ - 1
+ - N-1
+ - The control has a menu of N choices. The values of the menu items
+ can be enumerated with the ``VIDIOC_QUERYMENU`` ioctl. This is
+ similar to ``V4L2_CTRL_TYPE_MENU`` except that instead of strings,
+ the menu items are signed 64-bit integers.
+ * - ``V4L2_CTRL_TYPE_BITMASK``
+ - 0
+ - n/a
+ - any
+ - A bitmask field. The maximum value is the set of bits that can be
+ used, all other bits are to be 0. The maximum value is interpreted
+ as a __u32, allowing the use of bit 31 in the bitmask.
+ * - ``V4L2_CTRL_TYPE_BUTTON``
+ - 0
+ - 0
+ - 0
+ - A control which performs an action when set. Drivers must ignore
+ the value passed with ``VIDIOC_S_CTRL`` and return an ``EACCES`` error
+ code on a ``VIDIOC_G_CTRL`` attempt.
+ * - ``V4L2_CTRL_TYPE_INTEGER64``
+ - any
+ - any
+ - any
+ - A 64-bit integer valued control. Minimum, maximum and step size
+ cannot be queried using ``VIDIOC_QUERYCTRL``. Only
+ ``VIDIOC_QUERY_EXT_CTRL`` can retrieve the 64-bit min/max/step
+ values, they should be interpreted as n/a when using
+ ``VIDIOC_QUERYCTRL``.
+ * - ``V4L2_CTRL_TYPE_STRING``
+ - ≥ 0
+ - ≥ 1
+ - ≥ 0
+ - The minimum and maximum string lengths. The step size means that
+ the string must be (minimum + N * step) characters long for N ≥ 0.
+ These lengths do not include the terminating zero, so in order to
+ pass a string of length 8 to
+ :ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` you need to
+ set the ``size`` field of struct
+ :c:type:`v4l2_ext_control` to 9. For
+ :ref:`VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` you can set
+ the ``size`` field to ``maximum`` + 1. Which character encoding is
+ used will depend on the string control itself and should be part
+ of the control documentation.
+ * - ``V4L2_CTRL_TYPE_CTRL_CLASS``
+ - n/a
+ - n/a
+ - n/a
+ - This is not a control. When ``VIDIOC_QUERYCTRL`` is called with a
+ control ID equal to a control class code (see :ref:`ctrl-class`)
+ + 1, the ioctl returns the name of the control class and this
+ control type. Older drivers which do not support this feature
+ return an ``EINVAL`` error code.
+ * - ``V4L2_CTRL_TYPE_U8``
+ - any
+ - any
+ - any
+ - An unsigned 8-bit valued control ranging from minimum to maximum
+ inclusive. The step value indicates the increment between values.
+ * - ``V4L2_CTRL_TYPE_U16``
+ - any
+ - any
+ - any
+ - An unsigned 16-bit valued control ranging from minimum to maximum
+ inclusive. The step value indicates the increment between values.
+ * - ``V4L2_CTRL_TYPE_U32``
+ - any
+ - any
+ - any
+ - An unsigned 32-bit valued control ranging from minimum to maximum
+ inclusive. The step value indicates the increment between values.
+ * - ``V4L2_CTRL_TYPE_MPEG2_SLICE_PARAMS``
+ - n/a
+ - n/a
+ - n/a
+ - A struct :c:type:`v4l2_ctrl_mpeg2_slice_params`, containing MPEG-2
+ slice parameters for stateless video decoders.
+ * - ``V4L2_CTRL_TYPE_MPEG2_QUANTIZATION``
+ - n/a
+ - n/a
+ - n/a
+ - A struct :c:type:`v4l2_ctrl_mpeg2_quantization`, containing MPEG-2
+ quantization matrices for stateless video decoders.
+ * - ``V4L2_CTRL_TYPE_AREA``
+ - n/a
+ - n/a
+ - n/a
+ - A struct :c:type:`v4l2_area`, containing the width and the height
+ of a rectangular area. Units depend on the use case.
+ * - ``V4L2_CTRL_TYPE_H264_SPS``
+ - n/a
+ - n/a
+ - n/a
+ - A struct :c:type:`v4l2_ctrl_h264_sps`, containing H264
+ sequence parameters for stateless video decoders.
+ * - ``V4L2_CTRL_TYPE_H264_PPS``
+ - n/a
+ - n/a
+ - n/a
+ - A struct :c:type:`v4l2_ctrl_h264_pps`, containing H264
+ picture parameters for stateless video decoders.
+ * - ``V4L2_CTRL_TYPE_H264_SCALING_MATRIX``
+ - n/a
+ - n/a
+ - n/a
+ - A struct :c:type:`v4l2_ctrl_h264_scaling_matrix`, containing H264
+ scaling matrices for stateless video decoders.
+ * - ``V4L2_CTRL_TYPE_H264_SLICE_PARAMS``
+ - n/a
+ - n/a
+ - n/a
+ - A struct :c:type:`v4l2_ctrl_h264_slice_params`, containing H264
+ slice parameters for stateless video decoders.
+ * - ``V4L2_CTRL_TYPE_H264_DECODE_PARAMS``
+ - n/a
+ - n/a
+ - n/a
+ - A struct :c:type:`v4l2_ctrl_h264_decode_params`, containing H264
+ decode parameters for stateless video decoders.
+ * - ``V4L2_CTRL_TYPE_HEVC_SPS``
+ - n/a
+ - n/a
+ - n/a
+ - A struct :c:type:`v4l2_ctrl_hevc_sps`, containing HEVC Sequence
+ Parameter Set for stateless video decoders.
+ * - ``V4L2_CTRL_TYPE_HEVC_PPS``
+ - n/a
+ - n/a
+ - n/a
+ - A struct :c:type:`v4l2_ctrl_hevc_pps`, containing HEVC Picture
+ Parameter Set for stateless video decoders.
+ * - ``V4L2_CTRL_TYPE_HEVC_SLICE_PARAMS``
+ - n/a
+ - n/a
+ - n/a
+ - A struct :c:type:`v4l2_ctrl_hevc_slice_params`, containing HEVC
+ slice parameters for stateless video decoders.
+
+.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
+
+.. _control-flags:
+
+.. cssclass:: longtable
+
+.. flat-table:: Control Flags
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 3 1 4
+
+ * - ``V4L2_CTRL_FLAG_DISABLED``
+ - 0x0001
+ - This control is permanently disabled and should be ignored by the
+ application. Any attempt to change the control will result in an
+ ``EINVAL`` error code.
+ * - ``V4L2_CTRL_FLAG_GRABBED``
+ - 0x0002
+ - This control is temporarily unchangeable, for example because
+ another application took over control of the respective resource.
+ Such controls may be displayed specially in a user interface.
+ Attempts to change the control may result in an ``EBUSY`` error code.
+ * - ``V4L2_CTRL_FLAG_READ_ONLY``
+ - 0x0004
+ - This control is permanently readable only. Any attempt to change
+ the control will result in an ``EINVAL`` error code.
+ * - ``V4L2_CTRL_FLAG_UPDATE``
+ - 0x0008
+ - A hint that changing this control may affect the value of other
+ controls within the same control class. Applications should update
+ their user interface accordingly.
+ * - ``V4L2_CTRL_FLAG_INACTIVE``
+ - 0x0010
+ - This control is not applicable to the current configuration and
+ should be displayed accordingly in a user interface. For example
+ the flag may be set on a MPEG audio level 2 bitrate control when
+ MPEG audio encoding level 1 was selected with another control.
+ * - ``V4L2_CTRL_FLAG_SLIDER``
+ - 0x0020
+ - A hint that this control is best represented as a slider-like
+ element in a user interface.
+ * - ``V4L2_CTRL_FLAG_WRITE_ONLY``
+ - 0x0040
+ - This control is permanently writable only. Any attempt to read the
+ control will result in an ``EACCES`` error code error code. This flag
+ is typically present for relative controls or action controls
+ where writing a value will cause the device to carry out a given
+ action (e. g. motor control) but no meaningful value can be
+ returned.
+ * - ``V4L2_CTRL_FLAG_VOLATILE``
+ - 0x0080
+ - This control is volatile, which means that the value of the
+ control changes continuously. A typical example would be the
+ current gain value if the device is in auto-gain mode. In such a
+ case the hardware calculates the gain value based on the lighting
+ conditions which can change over time.
+
+ .. note::
+
+ Setting a new value for a volatile control will be ignored
+ unless
+ :ref:`V4L2_CTRL_FLAG_EXECUTE_ON_WRITE <FLAG_EXECUTE_ON_WRITE>`
+ is also set.
+ Setting a new value for a volatile control will *never* trigger a
+ :ref:`V4L2_EVENT_CTRL_CH_VALUE <ctrl-changes-flags>` event.
+ * - ``V4L2_CTRL_FLAG_HAS_PAYLOAD``
+ - 0x0100
+ - This control has a pointer type, so its value has to be accessed
+ using one of the pointer fields of struct
+ :c:type:`v4l2_ext_control`. This flag is set
+ for controls that are an array, string, or have a compound type.
+ In all cases you have to set a pointer to memory containing the
+ payload of the control.
+ * .. _FLAG_EXECUTE_ON_WRITE:
+
+ - ``V4L2_CTRL_FLAG_EXECUTE_ON_WRITE``
+ - 0x0200
+ - The value provided to the control will be propagated to the driver
+ even if it remains constant. This is required when the control
+ represents an action on the hardware. For example: clearing an
+ error flag or triggering the flash. All the controls of the type
+ ``V4L2_CTRL_TYPE_BUTTON`` have this flag set.
+ * .. _FLAG_MODIFY_LAYOUT:
+
+ - ``V4L2_CTRL_FLAG_MODIFY_LAYOUT``
+ - 0x0400
+ - Changing this control value may modify the layout of the
+ buffer (for video devices) or the media bus format (for sub-devices).
+
+ A typical example would be the ``V4L2_CID_ROTATE`` control.
+
+ Note that typically controls with this flag will also set the
+ ``V4L2_CTRL_FLAG_GRABBED`` flag when buffers are allocated or
+ streaming is in progress since most drivers do not support changing
+ the format in that case.
+
+
+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 :ref:`v4l2_queryctrl <v4l2-queryctrl>` ``id`` is
+ invalid. The struct :ref:`v4l2_querymenu <v4l2-querymenu>` ``id``
+ is invalid or ``index`` is out of range (less than ``minimum`` or
+ greater than ``maximum``) or this particular menu item is not
+ supported by the driver.
+
+EACCES
+ An attempt was made to read a write-only control.
+
+.. [#f1]
+ ``V4L2_CTRL_FLAG_DISABLED`` was intended for two purposes: Drivers
+ can skip predefined controls not supported by the hardware (although
+ returning ``EINVAL`` would do as well), or disable predefined and private
+ controls after hardware detection without the trouble of reordering
+ control arrays and indices (``EINVAL`` cannot be used to skip private
+ controls because it would prematurely end the enumeration).