ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU
&manvol;
VIDIOC_QUERYCTRL
VIDIOC_QUERYMENU
Enumerate controls and menu control items
int ioctl
int fd
int request
struct v4l2_queryctrl *argp
int ioctl
int fd
int request
struct v4l2_querymenu *argp
Arguments
fd
&fd;
request
VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU
argp
Description
To query the attributes of a control applications set the
id field of a &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; 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.
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).
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
names of the menu items. To query them applications set the
id and index
fields of &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; when the id or
index is invalid. Menu items are enumerated
by calling VIDIOC_QUERYMENU with successive
index values from &v4l2-queryctrl;
minimum to
maximum, inclusive. Note that it is possible
for VIDIOC_QUERYMENU to return an &EINVAL; 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 .
struct <structname>v4l2_queryctrl</structname>
&cs-str;
__u32
id
Identifies the control, set by the application. See
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;.
&v4l2-ctrl-type;
type
Type of control, see .
__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 and the
lowest valid index for V4L2_CTRL_TYPE_MENU controls.
For V4L2_CTRL_TYPE_STRING controls the minimum value
gives the minimum length of the string. This length does not include the terminating
zero. It may not be valid for any other type of control, including
V4L2_CTRL_TYPE_INTEGER64 controls. Note that 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. For V4L2_CTRL_TYPE_BITMASK controls it is the
set of usable bits.
For V4L2_CTRL_TYPE_STRING controls the maximum value
gives the maximum length of the string. This length does not include the terminating
zero. It may not be valid for any other type of control, including
V4L2_CTRL_TYPE_INTEGER64 controls. Note that this is a
signed value.
__s32
step
This field gives a step size for
V4L2_CTRL_TYPE_INTEGER controls. For
V4L2_CTRL_TYPE_STRING controls this field refers to
the string length that has to be a multiple of this step size.
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 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 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 .
__u32
reserved[2]
Reserved for future extensions. Drivers must set
the array to zero.
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_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; 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_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 &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 ) + 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;.
Control Flags
&cs-def;
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;.
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;.
V4L2_CTRL_FLAG_READ_ONLY
0x0004
This control is permanently readable only. Any
attempt to change the control will result in an &EINVAL;.
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. 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
(⪚ 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 that setting a new value for
a volatile control will have no effect. The new value will just be ignored.
&return-value;
EINVAL
The &v4l2-queryctrl; id
is invalid. The &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.