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1 <refentry id="vidioc-queryctrl">
2 <refmeta>
3 <refentrytitle>ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU</refentrytitle>
4 &manvol;
5 </refmeta>
7 <refnamediv>
8 <refname>VIDIOC_QUERYCTRL</refname>
9 <refname>VIDIOC_QUERYMENU</refname>
10 <refpurpose>Enumerate controls and menu control items</refpurpose>
11 </refnamediv>
13 <refsynopsisdiv>
14 <funcsynopsis>
15 <funcprototype>
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>
20 </funcprototype>
21 </funcsynopsis>
22 <funcsynopsis>
23 <funcprototype>
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>
28 </funcprototype>
29 </funcsynopsis>
30 </refsynopsisdiv>
32 <refsect1>
33 <title>Arguments</title>
35 <variablelist>
36 <varlistentry>
37 <term><parameter>fd</parameter></term>
38 <listitem>
39 <para>&fd;</para>
40 </listitem>
41 </varlistentry>
42 <varlistentry>
43 <term><parameter>request</parameter></term>
44 <listitem>
45 <para>VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU</para>
46 </listitem>
47 </varlistentry>
48 <varlistentry>
49 <term><parameter>argp</parameter></term>
50 <listitem>
51 <para></para>
52 </listitem>
53 </varlistentry>
54 </variablelist>
55 </refsect1>
57 <refsect1>
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>
117 <tgroup cols="3">
118 &cs-str;
119 <tbody valign="top">
120 <row>
121 <entry>__u32</entry>
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>
128 </row>
129 <row>
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>
134 </row>
135 <row>
136 <entry>__u8</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>
140 </row>
141 <row>
142 <entry>__s32</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>
152 </row>
153 <row>
154 <entry>__s32</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
160 set of usable bits.
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>
166 </row>
167 <row>
168 <entry>__s32</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>
189 </row>
190 <row>
191 <entry>__s32</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>
199 </row>
200 <row>
201 <entry>__u32</entry>
202 <entry><structfield>flags</structfield></entry>
203 <entry>Control flags, see <xref
204 linkend="control-flags" />.</entry>
205 </row>
206 <row>
207 <entry>__u32</entry>
208 <entry><structfield>reserved</structfield>[2]</entry>
209 <entry>Reserved for future extensions. Drivers must set
210 the array to zero.</entry>
211 </row>
212 </tbody>
213 </tgroup>
214 </table>
216 <table pgwide="1" frame="none" id="v4l2-querymenu">
217 <title>struct <structname>v4l2_querymenu</structname></title>
218 <tgroup cols="3">
219 &cs-str;
220 <tbody valign="top">
221 <row>
222 <entry>__u32</entry>
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>
227 </row>
228 <row>
229 <entry>__u32</entry>
230 <entry><structfield>index</structfield></entry>
231 <entry>Index of the menu item, starting at zero, set by
232 the application.</entry>
233 </row>
234 <row>
235 <entry>__u8</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>
239 </row>
240 <row>
241 <entry>__u32</entry>
242 <entry><structfield>reserved</structfield></entry>
243 <entry>Reserved for future extensions. Drivers must set
244 the array to zero.</entry>
245 </row>
246 </tbody>
247 </tgroup>
248 </table>
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*" />
258 <thead>
259 <row>
260 <entry>Type</entry>
261 <entry><structfield>minimum</structfield></entry>
262 <entry><structfield>step</structfield></entry>
263 <entry><structfield>maximum</structfield></entry>
264 <entry>Description</entry>
265 </row>
266 </thead>
267 <tbody valign="top">
268 <row>
269 <entry><constant>V4L2_CTRL_TYPE_INTEGER</constant></entry>
270 <entry>any</entry>
271 <entry>any</entry>
272 <entry>any</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>
276 </row>
277 <row>
278 <entry><constant>V4L2_CTRL_TYPE_BOOLEAN</constant></entry>
279 <entry>0</entry>
280 <entry>1</entry>
281 <entry>1</entry>
282 <entry>A boolean-valued control. Zero corresponds to
283 "disabled", and one means "enabled".</entry>
284 </row>
285 <row>
286 <entry><constant>V4L2_CTRL_TYPE_MENU</constant></entry>
287 <entry>≥ 0</entry>
288 <entry>1</entry>
289 <entry>N-1</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>
293 </row>
294 <row>
295 <entry><constant>V4L2_CTRL_TYPE_BITMASK</constant></entry>
296 <entry>0</entry>
297 <entry>n/a</entry>
298 <entry>any</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>
302 </row>
303 <row>
304 <entry><constant>V4L2_CTRL_TYPE_BUTTON</constant></entry>
305 <entry>0</entry>
306 <entry>0</entry>
307 <entry>0</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>
312 </row>
313 <row>
314 <entry><constant>V4L2_CTRL_TYPE_INTEGER64</constant></entry>
315 <entry>n/a</entry>
316 <entry>n/a</entry>
317 <entry>n/a</entry>
318 <entry>A 64-bit integer valued control. Minimum, maximum
319 and step size cannot be queried.</entry>
320 </row>
321 <row>
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>
334 </row>
335 <row>
336 <entry><constant>V4L2_CTRL_TYPE_CTRL_CLASS</constant></entry>
337 <entry>n/a</entry>
338 <entry>n/a</entry>
339 <entry>n/a</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
345 &EINVAL;.</entry>
346 </row>
347 </tbody>
348 </tgroup>
349 </table>
351 <table pgwide="1" frame="none" id="control-flags">
352 <title>Control Flags</title>
353 <tgroup cols="3">
354 &cs-def;
355 <tbody valign="top">
356 <row>
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>
362 </row>
363 <row>
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
370 &EBUSY;.</entry>
371 </row>
372 <row>
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>
377 </row>
378 <row>
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>
384 </row>
385 <row>
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
392 control.</entry>
393 </row>
394 <row>
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>
399 </row>
400 <row>
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>
408 </row>
409 <row>
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>
417 </row>
418 </tbody>
419 </tgroup>
420 </table>
421 </refsect1>
423 <refsect1>
424 &return-value;
426 <variablelist>
427 <varlistentry>
428 <term><errorcode>EINVAL</errorcode></term>
429 <listitem>
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>
435 </listitem>
436 </varlistentry>
437 <varlistentry>
438 <term><errorcode>EACCES</errorcode></term>
439 <listitem>
440 <para>An attempt was made to read a write-only control.</para>
441 </listitem>
442 </varlistentry>
443 </variablelist>
444 </refsect1>
445 </refentry>
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