diff options
Diffstat (limited to 'Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst')
| -rw-r--r-- | Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst | 660 |
1 files changed, 660 insertions, 0 deletions
diff --git a/Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst b/Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst new file mode 100644 index 000000000..a20dfa2a9 --- /dev/null +++ b/Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst @@ -0,0 +1,660 @@ +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later +.. c:namespace:: V4L + +.. _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 +======== + +``int ioctl(int fd, int VIDIOC_QUERYCTRL, struct v4l2_queryctrl *argp)`` + +.. c:macro:: VIDIOC_QUERY_EXT_CTRL + +``int ioctl(int fd, VIDIOC_QUERY_EXT_CTRL, struct v4l2_query_ext_ctrl *argp)`` + +.. c:macro:: VIDIOC_QUERYMENU + +``int ioctl(int fd, VIDIOC_QUERYMENU, struct v4l2_querymenu *argp)`` + +Arguments +========= + +``fd`` + File descriptor returned by :c: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{12.5cm}| + +.. _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{5.5cm}|p{10.6cm}| + +.. _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{3.0cm}|p{13.1cm}| + +.. _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. + +.. c:type:: v4l2_ctrl_type + +.. raw:: latex + + \footnotesize + +.. tabularcolumns:: |p{6.5cm}|p{1.5cm}|p{1.1cm}|p{1.5cm}|p{6.8cm}| + +.. 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_QUANTISATION`` + - n/a + - n/a + - n/a + - A struct :c:type:`v4l2_ctrl_mpeg2_quantisation`, containing MPEG-2 + quantisation matrices for stateless video decoders. + * - ``V4L2_CTRL_TYPE_MPEG2_SEQUENCE`` + - n/a + - n/a + - n/a + - A struct :c:type:`v4l2_ctrl_mpeg2_sequence`, containing MPEG-2 + sequence parameters for stateless video decoders. + * - ``V4L2_CTRL_TYPE_MPEG2_PICTURE`` + - n/a + - n/a + - n/a + - A struct :c:type:`v4l2_ctrl_mpeg2_picture`, containing MPEG-2 + picture parameters 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_FWHT_PARAMS`` + - n/a + - n/a + - n/a + - A struct :c:type:`v4l2_ctrl_fwht_params`, containing FWHT + 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. + * - ``V4L2_CTRL_TYPE_HEVC_SCALING_MATRIX`` + - n/a + - n/a + - n/a + - A struct :c:type:`v4l2_ctrl_hevc_scaling_matrix`, containing HEVC + scaling matrix for stateless video decoders. + * - ``V4L2_CTRL_TYPE_VP8_FRAME`` + - n/a + - n/a + - n/a + - A struct :c:type:`v4l2_ctrl_vp8_frame`, containing VP8 + frame parameters for stateless video decoders. + * - ``V4L2_CTRL_TYPE_HEVC_DECODE_PARAMS`` + - n/a + - n/a + - n/a + - A struct :c:type:`v4l2_ctrl_hevc_decode_params`, containing HEVC + decoding parameters for stateless video decoders. + * - ``V4L2_CTRL_TYPE_VP9_COMPRESSED_HDR`` + - n/a + - n/a + - n/a + - A struct :c:type:`v4l2_ctrl_vp9_compressed_hdr`, containing VP9 + probabilities updates for stateless video decoders. + * - ``V4L2_CTRL_TYPE_VP9_FRAME`` + - n/a + - n/a + - n/a + - A struct :c:type:`v4l2_ctrl_vp9_frame`, containing VP9 + frame decode parameters for stateless video decoders. + +.. raw:: latex + + \normalsize + +.. tabularcolumns:: |p{7.3cm}|p{1.8cm}|p{8.2cm}| + +.. cssclass:: longtable + +.. _control-flags: + +.. 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. + * - ``V4L2_CTRL_FLAG_DYNAMIC_ARRAY`` + - 0x0800 + - This control is a dynamically sized 1-dimensional array. It + behaves the same as a regular array, except that the number + of elements as reported by the ``elems`` field is between 1 and + ``dims[0]``. So setting the control with a differently sized + array will change the ``elems`` field when the control is + queried afterwards. + +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). |