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1<refentry id="vidioc-queryctrl">
2 <refmeta>
3 <refentrytitle>ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU</refentrytitle>
4 &manvol;
5 </refmeta>
6
7 <refnamediv>
8 <refname>VIDIOC_QUERYCTRL</refname>
9 <refname>VIDIOC_QUERYMENU</refname>
10 <refpurpose>Enumerate controls and menu control items</refpurpose>
11 </refnamediv>
12
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>
31
32 <refsect1>
33 <title>Arguments</title>
34
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>
56
57 <refsect1>
58 <title>Description</title>
59
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
63structure. The driver fills the rest of the structure or returns an
64&EINVAL; when the <structfield>id</structfield> is invalid.</para>
65
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
72supported. Further applications can enumerate private controls, which
73are 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>
77
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
81disabled and should be ignored by the application.<footnote>
82 <para><constant>V4L2_CTRL_FLAG_DISABLED</constant> was
83intended for two purposes: Drivers can skip predefined controls not
84supported by the hardware (although returning EINVAL would do as
85well), or disable predefined and private controls after hardware
86detection without the trouble of reordering control arrays and indices
87(EINVAL cannot be used to skip private controls because it would
88prematurely end the enumeration).</para></footnote></para>
89
90 <para>When the application ORs <structfield>id</structfield> with
91<constant>V4L2_CTRL_FLAG_NEXT_CTRL</constant> the driver returns the
92next supported control, or <errorcode>EINVAL</errorcode> if there is
93none. Drivers which do not support this flag yet always return
94<errorcode>EINVAL</errorcode>.</para>
95
96 <para>Additional information is required for menu controls: the
97names of the menu items. To query them applications set the
98<structfield>id</structfield> and <structfield>index</structfield>
99fields of &v4l2-querymenu; and call the
100<constant>VIDIOC_QUERYMENU</constant> ioctl with a pointer to this
101structure. 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
104by 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
108for <constant>VIDIOC_QUERYMENU</constant> to return an &EINVAL; for some
109indices between <structfield>minimum</structfield> and <structfield>maximum</structfield>.
110In that case that particular menu item is not supported by this driver. Also note that
111the <structfield>minimum</structfield> value is not necessarily 0.</para>
112
113 <para>See also the examples in <xref linkend="control" />.</para>
114
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
125with V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returns
126the first control with a higher ID. Drivers which do not support this
127flag 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
139string. 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
145bound for <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls and the
146lowest valid index for <constant>V4L2_CTRL_TYPE_MENU</constant> controls.
147For <constant>V4L2_CTRL_TYPE_STRING</constant> controls the minimum value
148gives the minimum length of the string. This length <emphasis>does not include the terminating
149zero</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
151signed 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
157bound for <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls and the
158highest valid index for <constant>V4L2_CTRL_TYPE_MENU</constant>
159controls.
160For <constant>V4L2_CTRL_TYPE_STRING</constant> controls the maximum value
161gives the maximum length of the string. This length <emphasis>does not include the terminating
162zero</emphasis>. It may not be valid for any other type of control, including
163<constant>V4L2_CTRL_TYPE_INTEGER64</constant> controls. Note that this is a
164signed value.</entry>
165 </row>
166 <row>
167 <entry>__s32</entry>
168 <entry><structfield>step</structfield></entry>
169 <entry><para>This field gives a step size for
170<constant>V4L2_CTRL_TYPE_INTEGER</constant> controls. For
171<constant>V4L2_CTRL_TYPE_STRING</constant> controls this field refers to
172the string length that has to be a multiple of this step size.
173It may not be valid for any other type of control, including
174<constant>V4L2_CTRL_TYPE_INTEGER64</constant>
175controls.</para><para>Generally drivers should not scale hardware
176control values. It may be necessary for example when the
177<structfield>name</structfield> or <structfield>id</structfield> imply
178a particular unit and the hardware actually accepts only multiples of
179said unit. If so, drivers must take care values are properly rounded
180when scaling, such that errors will not accumulate on repeated
181read-write cycles.</para><para>This field gives the smallest change of
182an integer control actually affecting hardware. Often the information
183is needed when the user can change controls by keyboard or GUI
184buttons, rather than a slider. When for example a hardware register
185accepts values 0-511 and the driver reports 0-65535, step should be
186128.</para><para>Note that although signed, the step value is supposed to
187be always positive.</para></entry>
188 </row>
189 <row>
190 <entry>__s32</entry>
191 <entry><structfield>default_value</structfield></entry>
192 <entry>The default value of a
193<constant>V4L2_CTRL_TYPE_INTEGER</constant>,
194<constant>_BOOLEAN</constant> or <constant>_MENU</constant> control.
195Not valid for other types of controls. Drivers reset controls only
196when the driver is loaded, not later, in particular not when the
197func-open; is called.</entry>
198 </row>
199 <row>
200 <entry>__u32</entry>
201 <entry><structfield>flags</structfield></entry>
202 <entry>Control flags, see <xref
203 linkend="control-flags" />.</entry>
204 </row>
205 <row>
206 <entry>__u32</entry>
207 <entry><structfield>reserved</structfield>[2]</entry>
208 <entry>Reserved for future extensions. Drivers must set
209the array to zero.</entry>
210 </row>
211 </tbody>
212 </tgroup>
213 </table>
214
215 <table pgwide="1" frame="none" id="v4l2-querymenu">
216 <title>struct <structname>v4l2_querymenu</structname></title>
217 <tgroup cols="3">
218 &cs-str;
219 <tbody valign="top">
220 <row>
221 <entry>__u32</entry>
222 <entry><structfield>id</structfield></entry>
223 <entry>Identifies the control, set by the application
224from the respective &v4l2-queryctrl;
225<structfield>id</structfield>.</entry>
226 </row>
227 <row>
228 <entry>__u32</entry>
229 <entry><structfield>index</structfield></entry>
230 <entry>Index of the menu item, starting at zero, set by
231 the application.</entry>
232 </row>
233 <row>
234 <entry>__u8</entry>
235 <entry><structfield>name</structfield>[32]</entry>
236 <entry>Name of the menu item, a NUL-terminated ASCII
237string. This information is intended for the user.</entry>
238 </row>
239 <row>
240 <entry>__u32</entry>
241 <entry><structfield>reserved</structfield></entry>
242 <entry>Reserved for future extensions. Drivers must set
243the array to zero.</entry>
244 </row>
245 </tbody>
246 </tgroup>
247 </table>
248
249 <table pgwide="1" frame="none" id="v4l2-ctrl-type">
250 <title>enum v4l2_ctrl_type</title>
251 <tgroup cols="5" align="left">
252 <colspec colwidth="30*" />
253 <colspec colwidth="5*" align="center" />
254 <colspec colwidth="5*" align="center" />
255 <colspec colwidth="5*" align="center" />
256 <colspec colwidth="55*" />
257 <thead>
258 <row>
259 <entry>Type</entry>
260 <entry><structfield>minimum</structfield></entry>
261 <entry><structfield>step</structfield></entry>
262 <entry><structfield>maximum</structfield></entry>
263 <entry>Description</entry>
264 </row>
265 </thead>
266 <tbody valign="top">
267 <row>
268 <entry><constant>V4L2_CTRL_TYPE_INTEGER</constant></entry>
269 <entry>any</entry>
270 <entry>any</entry>
271 <entry>any</entry>
272 <entry>An integer-valued control ranging from minimum to
273maximum inclusive. The step value indicates the increment between
274values which are actually different on the hardware.</entry>
275 </row>
276 <row>
277 <entry><constant>V4L2_CTRL_TYPE_BOOLEAN</constant></entry>
278 <entry>0</entry>
279 <entry>1</entry>
280 <entry>1</entry>
281 <entry>A boolean-valued control. Zero corresponds to
282"disabled", and one means "enabled".</entry>
283 </row>
284 <row>
285 <entry><constant>V4L2_CTRL_TYPE_MENU</constant></entry>
286 <entry>≥ 0</entry>
287 <entry>1</entry>
288 <entry>N-1</entry>
289 <entry>The control has a menu of N choices. The names of
290the menu items can be enumerated with the
291<constant>VIDIOC_QUERYMENU</constant> ioctl.</entry>
292 </row>
293 <row>
294 <entry><constant>V4L2_CTRL_TYPE_BUTTON</constant></entry>
295 <entry>0</entry>
296 <entry>0</entry>
297 <entry>0</entry>
298 <entry>A control which performs an action when set.
299Drivers must ignore the value passed with
300<constant>VIDIOC_S_CTRL</constant> and return an &EINVAL; on a
301<constant>VIDIOC_G_CTRL</constant> attempt.</entry>
302 </row>
303 <row>
304 <entry><constant>V4L2_CTRL_TYPE_INTEGER64</constant></entry>
305 <entry>n/a</entry>
306 <entry>n/a</entry>
307 <entry>n/a</entry>
308 <entry>A 64-bit integer valued control. Minimum, maximum
309and step size cannot be queried.</entry>
310 </row>
311 <row>
312 <entry><constant>V4L2_CTRL_TYPE_STRING</constant></entry>
313 <entry>≥ 0</entry>
314 <entry>≥ 1</entry>
315 <entry>≥ 0</entry>
316 <entry>The minimum and maximum string lengths. The step size
317means that the string must be (minimum + N * step) characters long for
318N ≥ 0. These lengths do not include the terminating zero, so in order to
319pass a string of length 8 to &VIDIOC-S-EXT-CTRLS; you need to set the
320<structfield>size</structfield> field of &v4l2-ext-control; to 9. For &VIDIOC-G-EXT-CTRLS; you can
321set the <structfield>size</structfield> field to <structfield>maximum</structfield> + 1.
322Which character encoding is used will depend on the string control itself and
323should be part of the control documentation.</entry>
324 </row>
325 <row>
326 <entry><constant>V4L2_CTRL_TYPE_CTRL_CLASS</constant></entry>
327 <entry>n/a</entry>
328 <entry>n/a</entry>
329 <entry>n/a</entry>
330 <entry>This is not a control. When
331<constant>VIDIOC_QUERYCTRL</constant> is called with a control ID
332equal to a control class code (see <xref linkend="ctrl-class" />) + 1, the
333ioctl returns the name of the control class and this control type.
334Older drivers which do not support this feature return an
335&EINVAL;.</entry>
336 </row>
337 </tbody>
338 </tgroup>
339 </table>
340
341 <table pgwide="1" frame="none" id="control-flags">
342 <title>Control Flags</title>
343 <tgroup cols="3">
344 &cs-def;
345 <tbody valign="top">
346 <row>
347 <entry><constant>V4L2_CTRL_FLAG_DISABLED</constant></entry>
348 <entry>0x0001</entry>
349 <entry>This control is permanently disabled and should be
350ignored by the application. Any attempt to change the control will
351result in an &EINVAL;.</entry>
352 </row>
353 <row>
354 <entry><constant>V4L2_CTRL_FLAG_GRABBED</constant></entry>
355 <entry>0x0002</entry>
356 <entry>This control is temporarily unchangeable, for
357example because another application took over control of the
358respective resource. Such controls may be displayed specially in a
359user interface. Attempts to change the control may result in an
360&EBUSY;.</entry>
361 </row>
362 <row>
363 <entry><constant>V4L2_CTRL_FLAG_READ_ONLY</constant></entry>
364 <entry>0x0004</entry>
365 <entry>This control is permanently readable only. Any
366attempt to change the control will result in an &EINVAL;.</entry>
367 </row>
368 <row>
369 <entry><constant>V4L2_CTRL_FLAG_UPDATE</constant></entry>
370 <entry>0x0008</entry>
371 <entry>A hint that changing this control may affect the
372value of other controls within the same control class. Applications
373should update their user interface accordingly.</entry>
374 </row>
375 <row>
376 <entry><constant>V4L2_CTRL_FLAG_INACTIVE</constant></entry>
377 <entry>0x0010</entry>
378 <entry>This control is not applicable to the current
379configuration and should be displayed accordingly in a user interface.
380For example the flag may be set on a MPEG audio level 2 bitrate
381control when MPEG audio encoding level 1 was selected with another
382control.</entry>
383 </row>
384 <row>
385 <entry><constant>V4L2_CTRL_FLAG_SLIDER</constant></entry>
386 <entry>0x0020</entry>
387 <entry>A hint that this control is best represented as a
388slider-like element in a user interface.</entry>
389 </row>
390 <row>
391 <entry><constant>V4L2_CTRL_FLAG_WRITE_ONLY</constant></entry>
392 <entry>0x0040</entry>
393 <entry>This control is permanently writable only. Any
394attempt to read the control will result in an &EACCES; error code. This
395flag is typically present for relative controls or action controls where
396writing a value will cause the device to carry out a given action
397(⪚ motor control) but no meaningful value can be returned.</entry>
398 </row>
399 </tbody>
400 </tgroup>
401 </table>
402 </refsect1>
403
404 <refsect1>
405 &return-value;
406
407 <variablelist>
408 <varlistentry>
409 <term><errorcode>EINVAL</errorcode></term>
410 <listitem>
411 <para>The &v4l2-queryctrl; <structfield>id</structfield>
412is invalid. The &v4l2-querymenu; <structfield>id</structfield> is
413invalid or <structfield>index</structfield> is out of range (less than
414<structfield>minimum</structfield> or greater than <structfield>maximum</structfield>)
415or this particular menu item is not supported by the driver.</para>
416 </listitem>
417 </varlistentry>
418 <varlistentry>
419 <term><errorcode>EACCES</errorcode></term>
420 <listitem>
421 <para>An attempt was made to read a write-only control.</para>
422 </listitem>
423 </varlistentry>
424 </variablelist>
425 </refsect1>
426</refentry>
427
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