7.49. ioctls VIDIOC_QUERYCTRL, VIDIOC_QUERY_EXT_CTRL and VIDIOC_QUERYMENU

7.49.1. Name

VIDIOC_QUERYCTRL - VIDIOC_QUERY_EXT_CTRL - VIDIOC_QUERYMENU - Enumerate controls and menu control items

7.49.2. Synopsis

int ioctl(int fd, int VIDIOC_QUERYCTRL, struct v4l2_queryctrl *argp)
int ioctl(int fd, VIDIOC_QUERY_EXT_CTRL, struct v4l2_query_ext_ctrl *argp)
int ioctl(int fd, VIDIOC_QUERYMENU, struct v4l2_querymenu *argp)

7.49.3. Arguments

fd
File descriptor returned by open().
argp
Pointer to struct v4l2_queryctrl, v4l2_query_ext_ctrl or v4l2_querymenu (depending on the ioctl).

7.49.4. Description

To query the attributes of a control applications set the id field of a struct 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. [1]

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 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 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 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 User Controls.

struct v4l2_queryctrl
__u32 id Identifies the control, set by the application. See Control IDs 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 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 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 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 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 Control Flags.
__u32 reserved[2] Reserved for future extensions. Drivers must set the array to zero.
struct v4l2_query_ext_ctrl
__u32 id Identifies the control, set by the application. See Control IDs 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 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 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 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 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 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.
struct v4l2_querymenu
__u32   id Identifies the control, set by the application from the respective struct v4l2_queryctrl id.
__u32   index Index of the menu item, starting at zero, set by the application.
union      
  __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.
v4l2_ctrl_type
enum v4l2_ctrl_type
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 EINVAL 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 VIDIOC_S_EXT_CTRLS you need to set the size field of struct v4l2_ext_control to 9. For 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 Control classes) + 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 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 v4l2_ctrl_mpeg2_quantization, containing MPEG-2 quantization matrices for stateless video decoders.
V4L2_CTRL_TYPE_H264_SPS n/a n/a n/a A struct 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 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 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 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 v4l2_ctrl_h264_decode_params, containing H264 decode parameters for stateless video decoders.
Control Flags
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 V4L2_CTRL_FLAG_EXECUTE_ON_WRITE is also set. Setting a new value for a volatile control will never trigger a V4L2_EVENT_CTRL_CH_VALUE 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 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.
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.

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.

7.49.5. 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 Generic Error Codes chapter.

EINVAL
The struct v4l2_queryctrl id is invalid. The struct 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.
[1]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).