1
<refentry id="vidioc-queryctrl">
3
<refentrytitle>ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU</refentrytitle>
8
<refname>VIDIOC_QUERYCTRL</refname>
9
<refname>VIDIOC_QUERYMENU</refname>
10
<refpurpose>Enumerate controls and menu control items</refpurpose>
16
<funcdef>int <function>ioctl</function></funcdef>
17
<paramdef>int <parameter>fd</parameter></paramdef>
18
<paramdef>int <parameter>request</parameter></paramdef>
19
<paramdef>struct v4l2_queryctrl *<parameter>argp</parameter></paramdef>
24
<funcdef>int <function>ioctl</function></funcdef>
25
<paramdef>int <parameter>fd</parameter></paramdef>
26
<paramdef>int <parameter>request</parameter></paramdef>
27
<paramdef>struct v4l2_querymenu *<parameter>argp</parameter></paramdef>
33
<title>Arguments</title>
37
<term><parameter>fd</parameter></term>
43
<term><parameter>request</parameter></term>
45
<para>VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU</para>
49
<term><parameter>argp</parameter></term>
58
<title>Description</title>
60
<para>To query the attributes of a control applications set the
61
<structfield>id</structfield> field of a &v4l2-queryctrl; and call the
62
<constant>VIDIOC_QUERYCTRL</constant> ioctl with a pointer to this
63
structure. The driver fills the rest of the structure or returns an
64
&EINVAL; when the <structfield>id</structfield> is invalid.</para>
66
<para>It is possible to enumerate controls by calling
67
<constant>VIDIOC_QUERYCTRL</constant> with successive
68
<structfield>id</structfield> values starting from
69
<constant>V4L2_CID_BASE</constant> up to and exclusive
70
<constant>V4L2_CID_BASE_LASTP1</constant>. Drivers may return
71
<errorcode>EINVAL</errorcode> if a control in this range is not
72
supported. Further applications can enumerate private controls, which
73
are not defined in this specification, by starting at
74
<constant>V4L2_CID_PRIVATE_BASE</constant> and incrementing
75
<structfield>id</structfield> until the driver returns
76
<errorcode>EINVAL</errorcode>.</para>
78
<para>In both cases, when the driver sets the
79
<constant>V4L2_CTRL_FLAG_DISABLED</constant> flag in the
80
<structfield>flags</structfield> field this control is permanently
81
disabled and should be ignored by the application.<footnote>
82
<para><constant>V4L2_CTRL_FLAG_DISABLED</constant> was
83
intended for two purposes: Drivers can skip predefined controls not
84
supported by the hardware (although returning EINVAL would do as
85
well), or disable predefined and private controls after hardware
86
detection without the trouble of reordering control arrays and indices
87
(EINVAL cannot be used to skip private controls because it would
88
prematurely end the enumeration).</para></footnote></para>
90
<para>When the application ORs <structfield>id</structfield> with
91
<constant>V4L2_CTRL_FLAG_NEXT_CTRL</constant> the driver returns the
92
next supported control, or <errorcode>EINVAL</errorcode> if there is
93
none. Drivers which do not support this flag yet always return
94
<errorcode>EINVAL</errorcode>.</para>
96
<para>Additional information is required for menu controls: the
97
names of the menu items. To query them applications set the
98
<structfield>id</structfield> and <structfield>index</structfield>
99
fields of &v4l2-querymenu; and call the
100
<constant>VIDIOC_QUERYMENU</constant> ioctl with a pointer to this
101
structure. The driver fills the rest of the structure or returns an
102
&EINVAL; when the <structfield>id</structfield> or
103
<structfield>index</structfield> is invalid. Menu items are enumerated
104
by calling <constant>VIDIOC_QUERYMENU</constant> with successive
105
<structfield>index</structfield> values from &v4l2-queryctrl;
106
<structfield>minimum</structfield> to
107
<structfield>maximum</structfield>, inclusive. Note that it is possible
108
for <constant>VIDIOC_QUERYMENU</constant> to return an &EINVAL; for some
109
indices between <structfield>minimum</structfield> and <structfield>maximum</structfield>.
110
In that case that particular menu item is not supported by this driver. Also note that
111
the <structfield>minimum</structfield> value is not necessarily 0.</para>
113
<para>See also the examples in <xref linkend="control" />.</para>
115
<table pgwide="1" frame="none" id="v4l2-queryctrl">
116
<title>struct <structname>v4l2_queryctrl</structname></title>
122
<entry><structfield>id</structfield></entry>
123
<entry>Identifies the control, set by the application. See
124
<xref linkend="control-id" /> for predefined IDs. When the ID is ORed
125
with V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returns
126
the first control with a higher ID. Drivers which do not support this
127
flag yet always return an &EINVAL;.</entry>
130
<entry>&v4l2-ctrl-type;</entry>
131
<entry><structfield>type</structfield></entry>
132
<entry>Type of control, see <xref
133
linkend="v4l2-ctrl-type" />.</entry>
137
<entry><structfield>name</structfield>[32]</entry>
138
<entry>Name of the control, a NUL-terminated ASCII
139
string. This information is intended for the user.</entry>
143
<entry><structfield>minimum</structfield></entry>
144
<entry>Minimum value, inclusive. This field gives a lower
145
bound for <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls and the
146
lowest valid index for <constant>V4L2_CTRL_TYPE_MENU</constant> controls.
147
For <constant>V4L2_CTRL_TYPE_STRING</constant> controls the minimum value
148
gives the minimum length of the string. This length <emphasis>does not include the terminating
149
zero</emphasis>. It may not be valid for any other type of control, including
150
<constant>V4L2_CTRL_TYPE_INTEGER64</constant> controls. Note that this is a
151
signed value.</entry>
155
<entry><structfield>maximum</structfield></entry>
156
<entry>Maximum value, inclusive. This field gives an upper
157
bound for <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls and the
158
highest valid index for <constant>V4L2_CTRL_TYPE_MENU</constant>
159
controls. For <constant>V4L2_CTRL_TYPE_BITMASK</constant> controls it is the
161
For <constant>V4L2_CTRL_TYPE_STRING</constant> controls the maximum value
162
gives the maximum length of the string. This length <emphasis>does not include the terminating
163
zero</emphasis>. It may not be valid for any other type of control, including
164
<constant>V4L2_CTRL_TYPE_INTEGER64</constant> controls. Note that this is a
165
signed value.</entry>
169
<entry><structfield>step</structfield></entry>
170
<entry><para>This field gives a step size for
171
<constant>V4L2_CTRL_TYPE_INTEGER</constant> controls. For
172
<constant>V4L2_CTRL_TYPE_STRING</constant> controls this field refers to
173
the string length that has to be a multiple of this step size.
174
It may not be valid for any other type of control, including
175
<constant>V4L2_CTRL_TYPE_INTEGER64</constant>
176
controls.</para><para>Generally drivers should not scale hardware
177
control values. It may be necessary for example when the
178
<structfield>name</structfield> or <structfield>id</structfield> imply
179
a particular unit and the hardware actually accepts only multiples of
180
said unit. If so, drivers must take care values are properly rounded
181
when scaling, such that errors will not accumulate on repeated
182
read-write cycles.</para><para>This field gives the smallest change of
183
an integer control actually affecting hardware. Often the information
184
is needed when the user can change controls by keyboard or GUI
185
buttons, rather than a slider. When for example a hardware register
186
accepts values 0-511 and the driver reports 0-65535, step should be
187
128.</para><para>Note that although signed, the step value is supposed to
188
be always positive.</para></entry>
192
<entry><structfield>default_value</structfield></entry>
193
<entry>The default value of a
194
<constant>V4L2_CTRL_TYPE_INTEGER</constant>,
195
<constant>_BOOLEAN</constant> or <constant>_MENU</constant> control.
196
Not valid for other types of controls. Drivers reset controls only
197
when the driver is loaded, not later, in particular not when the
198
func-open; is called.</entry>
202
<entry><structfield>flags</structfield></entry>
203
<entry>Control flags, see <xref
204
linkend="control-flags" />.</entry>
208
<entry><structfield>reserved</structfield>[2]</entry>
209
<entry>Reserved for future extensions. Drivers must set
210
the array to zero.</entry>
216
<table pgwide="1" frame="none" id="v4l2-querymenu">
217
<title>struct <structname>v4l2_querymenu</structname></title>
223
<entry><structfield>id</structfield></entry>
224
<entry>Identifies the control, set by the application
225
from the respective &v4l2-queryctrl;
226
<structfield>id</structfield>.</entry>
230
<entry><structfield>index</structfield></entry>
231
<entry>Index of the menu item, starting at zero, set by
232
the application.</entry>
236
<entry><structfield>name</structfield>[32]</entry>
237
<entry>Name of the menu item, a NUL-terminated ASCII
238
string. This information is intended for the user.</entry>
242
<entry><structfield>reserved</structfield></entry>
243
<entry>Reserved for future extensions. Drivers must set
244
the array to zero.</entry>
250
<table pgwide="1" frame="none" id="v4l2-ctrl-type">
251
<title>enum v4l2_ctrl_type</title>
252
<tgroup cols="5" align="left">
253
<colspec colwidth="30*" />
254
<colspec colwidth="5*" align="center" />
255
<colspec colwidth="5*" align="center" />
256
<colspec colwidth="5*" align="center" />
257
<colspec colwidth="55*" />
261
<entry><structfield>minimum</structfield></entry>
262
<entry><structfield>step</structfield></entry>
263
<entry><structfield>maximum</structfield></entry>
264
<entry>Description</entry>
269
<entry><constant>V4L2_CTRL_TYPE_INTEGER</constant></entry>
273
<entry>An integer-valued control ranging from minimum to
274
maximum inclusive. The step value indicates the increment between
275
values which are actually different on the hardware.</entry>
278
<entry><constant>V4L2_CTRL_TYPE_BOOLEAN</constant></entry>
282
<entry>A boolean-valued control. Zero corresponds to
283
"disabled", and one means "enabled".</entry>
286
<entry><constant>V4L2_CTRL_TYPE_MENU</constant></entry>
287
<entry>≥ 0</entry>
290
<entry>The control has a menu of N choices. The names of
291
the menu items can be enumerated with the
292
<constant>VIDIOC_QUERYMENU</constant> ioctl.</entry>
295
<entry><constant>V4L2_CTRL_TYPE_BITMASK</constant></entry>
299
<entry>A bitmask field. The maximum value is the set of bits that can
300
be used, all other bits are to be 0. The maximum value is interpreted as a __u32,
301
allowing the use of bit 31 in the bitmask.</entry>
304
<entry><constant>V4L2_CTRL_TYPE_BUTTON</constant></entry>
308
<entry>A control which performs an action when set.
309
Drivers must ignore the value passed with
310
<constant>VIDIOC_S_CTRL</constant> and return an &EINVAL; on a
311
<constant>VIDIOC_G_CTRL</constant> attempt.</entry>
314
<entry><constant>V4L2_CTRL_TYPE_INTEGER64</constant></entry>
318
<entry>A 64-bit integer valued control. Minimum, maximum
319
and step size cannot be queried.</entry>
322
<entry><constant>V4L2_CTRL_TYPE_STRING</constant></entry>
323
<entry>≥ 0</entry>
324
<entry>≥ 1</entry>
325
<entry>≥ 0</entry>
326
<entry>The minimum and maximum string lengths. The step size
327
means that the string must be (minimum + N * step) characters long for
328
N ≥ 0. These lengths do not include the terminating zero, so in order to
329
pass a string of length 8 to &VIDIOC-S-EXT-CTRLS; you need to set the
330
<structfield>size</structfield> field of &v4l2-ext-control; to 9. For &VIDIOC-G-EXT-CTRLS; you can
331
set the <structfield>size</structfield> field to <structfield>maximum</structfield> + 1.
332
Which character encoding is used will depend on the string control itself and
333
should be part of the control documentation.</entry>
336
<entry><constant>V4L2_CTRL_TYPE_CTRL_CLASS</constant></entry>
340
<entry>This is not a control. When
341
<constant>VIDIOC_QUERYCTRL</constant> is called with a control ID
342
equal to a control class code (see <xref linkend="ctrl-class" />) + 1, the
343
ioctl returns the name of the control class and this control type.
344
Older drivers which do not support this feature return an
351
<table pgwide="1" frame="none" id="control-flags">
352
<title>Control Flags</title>
357
<entry><constant>V4L2_CTRL_FLAG_DISABLED</constant></entry>
358
<entry>0x0001</entry>
359
<entry>This control is permanently disabled and should be
360
ignored by the application. Any attempt to change the control will
361
result in an &EINVAL;.</entry>
364
<entry><constant>V4L2_CTRL_FLAG_GRABBED</constant></entry>
365
<entry>0x0002</entry>
366
<entry>This control is temporarily unchangeable, for
367
example because another application took over control of the
368
respective resource. Such controls may be displayed specially in a
369
user interface. Attempts to change the control may result in an
373
<entry><constant>V4L2_CTRL_FLAG_READ_ONLY</constant></entry>
374
<entry>0x0004</entry>
375
<entry>This control is permanently readable only. Any
376
attempt to change the control will result in an &EINVAL;.</entry>
379
<entry><constant>V4L2_CTRL_FLAG_UPDATE</constant></entry>
380
<entry>0x0008</entry>
381
<entry>A hint that changing this control may affect the
382
value of other controls within the same control class. Applications
383
should update their user interface accordingly.</entry>
386
<entry><constant>V4L2_CTRL_FLAG_INACTIVE</constant></entry>
387
<entry>0x0010</entry>
388
<entry>This control is not applicable to the current
389
configuration and should be displayed accordingly in a user interface.
390
For example the flag may be set on a MPEG audio level 2 bitrate
391
control when MPEG audio encoding level 1 was selected with another
395
<entry><constant>V4L2_CTRL_FLAG_SLIDER</constant></entry>
396
<entry>0x0020</entry>
397
<entry>A hint that this control is best represented as a
398
slider-like element in a user interface.</entry>
401
<entry><constant>V4L2_CTRL_FLAG_WRITE_ONLY</constant></entry>
402
<entry>0x0040</entry>
403
<entry>This control is permanently writable only. Any
404
attempt to read the control will result in an &EACCES; error code. This
405
flag is typically present for relative controls or action controls where
406
writing a value will cause the device to carry out a given action
407
(⪚ motor control) but no meaningful value can be returned.</entry>
410
<entry><constant>V4L2_CTRL_FLAG_VOLATILE</constant></entry>
411
<entry>0x0080</entry>
412
<entry>This control is volatile, which means that the value of the control
413
changes continuously. A typical example would be the current gain value if the device
414
is in auto-gain mode. In such a case the hardware calculates the gain value based on
415
the lighting conditions which can change over time. Note that setting a new value for
416
a volatile control will have no effect. The new value will just be ignored.</entry>
428
<term><errorcode>EINVAL</errorcode></term>
430
<para>The &v4l2-queryctrl; <structfield>id</structfield>
431
is invalid. The &v4l2-querymenu; <structfield>id</structfield> is
432
invalid or <structfield>index</structfield> is out of range (less than
433
<structfield>minimum</structfield> or greater than <structfield>maximum</structfield>)
434
or this particular menu item is not supported by the driver.</para>
438
<term><errorcode>EACCES</errorcode></term>
440
<para>An attempt was made to read a write-only control.</para>
450
sgml-parent-document: "v4l2.sgml"
451
indent-tabs-mode: nil