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`

__u32 | `id` | Identifies 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_type | `type` | Type of control, see Table 3.
__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 `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.
__s32 | `maximum` | Maximum 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.

__s32

`step`

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.

__s32 | `default_value` | The 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.
__u32 | `flags` | Control flags, see Table 4.
__u32 | `reserved`[2] | Reserved for future extensions. Drivers must set the array to zero.

Table 2. 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.
__u8 | `name`[32] | Name of the menu item, a NUL-terminated ASCII string. This information is intended for the user.
__u32 | `reserved` | Reserved for future extensions. Drivers must set the array to zero.

Table 3. 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 which are actually different on the hardware.
`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_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` | n/a | n/a | n/a | A 64-bit integer valued control. Minimum, maximum and step size cannot be queried.
`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 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_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.

## 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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