Video for Linux Two API Specification: Revision 0.24
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ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU

Name

VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU -- Enumerate controls and menu control items

Synopsis

int ioctl(int fd, int request, struct v4l2_queryctrl *argp);

int ioctl(int fd, int request, struct v4l2_querymenu *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU

argp

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_BASE_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 control, or EINVAL if there is none. Drivers which do not support this flag yet always return EINVAL.

Additional information is required for menu controls, the name of 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 (0) to maximum, inclusive.

See also the examples in Section 1.8.

Table 1. struct v4l2_queryctrl

__u32idIdentifies the control, set by the application. See Table 1-1 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.
enum v4l2_ctrl_typetypeType of control, see Table 3.
__u8name[32]Name of the control, a NUL-terminated ASCII string. This information is intended for the user.
__s32minimumMinimum value, inclusive. This field gives a lower bound for V4L2_CTRL_TYPE_INTEGER controls. It may not be valid for any other type of control, including V4L2_CTRL_TYPE_INTEGER64 controls. Note this is a signed value.
__s32maximumMaximum value, inclusive. This field gives an upper bound for V4L2_CTRL_TYPE_INTEGER controls and the highest valid index for V4L2_CTRL_TYPE_MENU controls. It may not be valid for any other type of control, including V4L2_CTRL_TYPE_INTEGER64 controls. Note this is a signed value.
__s32step

This field gives a step size for V4L2_CTRL_TYPE_INTEGER controls. It may not be valid for any other type of control, including V4L2_CTRL_TYPE_INTEGER64 controls.

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 although signed, the step value is supposed to be always positive.

__s32default_valueThe default value of a V4L2_CTRL_TYPE_INTEGER, _BOOLEAN or _MENU control. Not valid for other types of controls. Drivers reset controls only when the driver is loaded, not later, in particular not when the func-open; is called.
__u32flagsControl flags, see Table 4.
__u32reserved[2]Reserved for future extensions. Drivers must set the array to zero.

Table 2. struct v4l2_querymenu

__u32idIdentifies the control, set by the application from the respective struct v4l2_queryctrl id.
__u32indexIndex of the menu item, starting at zero, set by the application.
__u8name[32]Name of the menu item, a NUL-terminated ASCII string. This information is intended for the user.
__u32reservedReserved for future extensions. Drivers must set the array to zero.

Table 3. enum v4l2_ctrl_type

TypeminimumstepmaximumDescription
V4L2_CTRL_TYPE_INTEGERanyanyanyAn integer-valued control ranging from minimum to maximum inclusive. The step value indicates the increment between values which are actually different on the hardware.
V4L2_CTRL_TYPE_BOOLEAN011A boolean-valued control. Zero corresponds to "disabled", and one means "enabled".
V4L2_CTRL_TYPE_MENU01N-1The control has a menu of N choices. The names of the menu items can be enumerated with the VIDIOC_QUERYMENU ioctl.
V4L2_CTRL_TYPE_BUTTON000A 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_INTEGER64n/an/an/aA 64-bit integer valued control. Minimum, maximum and step size cannot be queried.
V4L2_CTRL_TYPE_CTRL_CLASSn/an/an/aThis is not a control. When VIDIOC_QUERYCTRL is called with a control ID equal to a control class code (see Table 3), 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.

Table 4. Control Flags

V4L2_CTRL_FLAG_DISABLED0x0001This 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_GRABBED0x0002This 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_ONLY0x0004This control is permanently readable only. Any attempt to change the control will result in an EINVAL error code.
V4L2_CTRL_FLAG_UPDATE0x0008A 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_INACTIVE0x0010This 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_SLIDER0x0020A hint that this control is best represented as a slider-like element in a user interface.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_queryctrl id is invalid. The struct v4l2_querymenu id or index is invalid.

Notes

[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).

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