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Based on kernel version 4.1. Page generated on 2015-06-28 12:12 EST.

1	<refentry id="vidioc-queryctrl">
2	  <refmeta>
3	    <refentrytitle>ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERY_EXT_CTRL, VIDIOC_QUERYMENU</refentrytitle>
4	    &manvol;
5	  </refmeta>
6	
7	  <refnamediv>
8	    <refname>VIDIOC_QUERYCTRL</refname>
9	    <refname>VIDIOC_QUERY_EXT_CTRL</refname>
10	    <refname>VIDIOC_QUERYMENU</refname>
11	    <refpurpose>Enumerate controls and menu control items</refpurpose>
12	  </refnamediv>
13	
14	  <refsynopsisdiv>
15	    <funcsynopsis>
16	      <funcprototype>
17		<funcdef>int <function>ioctl</function></funcdef>
18		<paramdef>int <parameter>fd</parameter></paramdef>
19		<paramdef>int <parameter>request</parameter></paramdef>
20		<paramdef>struct v4l2_queryctrl *<parameter>argp</parameter></paramdef>
21	      </funcprototype>
22	    </funcsynopsis>
23	    <funcsynopsis>
24	      <funcprototype>
25		<funcdef>int <function>ioctl</function></funcdef>
26		<paramdef>int <parameter>fd</parameter></paramdef>
27		<paramdef>int <parameter>request</parameter></paramdef>
28		<paramdef>struct v4l2_query_ext_ctrl *<parameter>argp</parameter></paramdef>
29	      </funcprototype>
30	    </funcsynopsis>
31	    <funcsynopsis>
32	      <funcprototype>
33		<funcdef>int <function>ioctl</function></funcdef>
34		<paramdef>int <parameter>fd</parameter></paramdef>
35		<paramdef>int <parameter>request</parameter></paramdef>
36		<paramdef>struct v4l2_querymenu *<parameter>argp</parameter></paramdef>
37	      </funcprototype>
38	    </funcsynopsis>
39	  </refsynopsisdiv>
40	
41	  <refsect1>
42	    <title>Arguments</title>
43	
44	    <variablelist>
45	      <varlistentry>
46		<term><parameter>fd</parameter></term>
47		<listitem>
48		  <para>&fd;</para>
49		</listitem>
50	      </varlistentry>
51	      <varlistentry>
52		<term><parameter>request</parameter></term>
53		<listitem>
54		  <para>VIDIOC_QUERYCTRL, VIDIOC_QUERY_EXT_CTRL, VIDIOC_QUERYMENU</para>
55		</listitem>
56	      </varlistentry>
57	      <varlistentry>
58		<term><parameter>argp</parameter></term>
59		<listitem>
60		  <para></para>
61		</listitem>
62	      </varlistentry>
63	    </variablelist>
64	  </refsect1>
65	
66	  <refsect1>
67	    <title>Description</title>
68	
69	    <para>To query the attributes of a control applications set the
70	<structfield>id</structfield> field of a &v4l2-queryctrl; and call the
71	<constant>VIDIOC_QUERYCTRL</constant> ioctl with a pointer to this
72	structure. The driver fills the rest of the structure or returns an
73	&EINVAL; when the <structfield>id</structfield> is invalid.</para>
74	
75	    <para>It is possible to enumerate controls by calling
76	<constant>VIDIOC_QUERYCTRL</constant> with successive
77	<structfield>id</structfield> values starting from
78	<constant>V4L2_CID_BASE</constant> up to and exclusive
79	<constant>V4L2_CID_LASTP1</constant>. Drivers may return
80	<errorcode>EINVAL</errorcode> if a control in this range is not
81	supported. Further applications can enumerate private controls, which
82	are not defined in this specification, by starting at
83	<constant>V4L2_CID_PRIVATE_BASE</constant> and incrementing
84	<structfield>id</structfield> until the driver returns
85	<errorcode>EINVAL</errorcode>.</para>
86	
87	    <para>In both cases, when the driver sets the
88	<constant>V4L2_CTRL_FLAG_DISABLED</constant> flag in the
89	<structfield>flags</structfield> field this control is permanently
90	disabled and should be ignored by the application.<footnote>
91		<para><constant>V4L2_CTRL_FLAG_DISABLED</constant> was
92	intended for two purposes: Drivers can skip predefined controls not
93	supported by the hardware (although returning EINVAL would do as
94	well), or disable predefined and private controls after hardware
95	detection without the trouble of reordering control arrays and indices
96	(EINVAL cannot be used to skip private controls because it would
97	prematurely end the enumeration).</para></footnote></para>
98	
99	    <para>When the application ORs <structfield>id</structfield> with
100	<constant>V4L2_CTRL_FLAG_NEXT_CTRL</constant> the driver returns the
101	next supported non-compound control, or <errorcode>EINVAL</errorcode>
102	if there is none. In addition, the <constant>V4L2_CTRL_FLAG_NEXT_COMPOUND</constant>
103	flag can be specified to enumerate all compound controls (i.e. controls
104	with type &ge; <constant>V4L2_CTRL_COMPOUND_TYPES</constant>). Specify both
105	<constant>V4L2_CTRL_FLAG_NEXT_CTRL</constant> and
106	<constant>V4L2_CTRL_FLAG_NEXT_COMPOUND</constant> in order to enumerate
107	all controls, compound or not. Drivers which do not support these flags yet
108	always return <errorcode>EINVAL</errorcode>.</para>
109	
110	    <para>The <constant>VIDIOC_QUERY_EXT_CTRL</constant> ioctl was
111	introduced in order to better support controls that can use compound
112	types, and to expose additional control information that cannot be
113	returned in &v4l2-queryctrl; since that structure is full.</para>
114	
115	    <para><constant>VIDIOC_QUERY_EXT_CTRL</constant> is used in the
116	same way as <constant>VIDIOC_QUERYCTRL</constant>, except that the
117	<structfield>reserved</structfield> array must be zeroed as well.</para>
118	
119	    <para>Additional information is required for menu controls: the
120	names of the menu items. To query them applications set the
121	<structfield>id</structfield> and <structfield>index</structfield>
122	fields of &v4l2-querymenu; and call the
123	<constant>VIDIOC_QUERYMENU</constant> ioctl with a pointer to this
124	structure. The driver fills the rest of the structure or returns an
125	&EINVAL; when the <structfield>id</structfield> or
126	<structfield>index</structfield> is invalid. Menu items are enumerated
127	by calling <constant>VIDIOC_QUERYMENU</constant> with successive
128	<structfield>index</structfield> values from &v4l2-queryctrl;
129	<structfield>minimum</structfield> to
130	<structfield>maximum</structfield>, inclusive. Note that it is possible
131	for <constant>VIDIOC_QUERYMENU</constant> to return an &EINVAL; for some
132	indices between <structfield>minimum</structfield> and <structfield>maximum</structfield>.
133	In that case that particular menu item is not supported by this driver. Also note that
134	the <structfield>minimum</structfield> value is not necessarily 0.</para>
135	
136	    <para>See also the examples in <xref linkend="control" />.</para>
137	
138	    <table pgwide="1" frame="none" id="v4l2-queryctrl">
139	      <title>struct <structname>v4l2_queryctrl</structname></title>
140	      <tgroup cols="3">
141		&cs-str;
142		<tbody valign="top">
143		  <row>
144		    <entry>__u32</entry>
145		    <entry><structfield>id</structfield></entry>
146		    <entry>Identifies the control, set by the application. See
147	<xref linkend="control-id" /> for predefined IDs. When the ID is ORed
148	with V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returns
149	the first control with a higher ID. Drivers which do not support this
150	flag yet always return an &EINVAL;.</entry>
151		  </row>
152		  <row>
153		    <entry>__u32</entry>
154		    <entry><structfield>type</structfield></entry>
155		    <entry>Type of control, see <xref
156			linkend="v4l2-ctrl-type" />.</entry>
157		  </row>
158		  <row>
159		    <entry>__u8</entry>
160		    <entry><structfield>name</structfield>[32]</entry>
161		    <entry>Name of the control, a NUL-terminated ASCII
162	string. This information is intended for the user.</entry>
163		  </row>
164		  <row>
165		    <entry>__s32</entry>
166		    <entry><structfield>minimum</structfield></entry>
167		    <entry>Minimum value, inclusive. This field gives a lower
168	bound for the control. See &v4l2-ctrl-type; how the minimum value is to
169	be used for each possible control type. Note that this a signed 32-bit value.</entry>
170		  </row>
171		  <row>
172		    <entry>__s32</entry>
173		    <entry><structfield>maximum</structfield></entry>
174		    <entry>Maximum value, inclusive. This field gives an upper
175	bound for the control. See &v4l2-ctrl-type; how the maximum value is to
176	be used for each possible control type. Note that this a signed 32-bit value.</entry>
177		  </row>
178		  <row>
179		    <entry>__s32</entry>
180		    <entry><structfield>step</structfield></entry>
181		    <entry><para>This field gives a step size for the control.
182	See &v4l2-ctrl-type; how the step value is to be used for each possible
183	control type. Note that this an unsigned 32-bit value.
184	</para><para>Generally drivers should not scale hardware
185	control values. It may be necessary for example when the
186	<structfield>name</structfield> or <structfield>id</structfield> imply
187	a particular unit and the hardware actually accepts only multiples of
188	said unit. If so, drivers must take care values are properly rounded
189	when scaling, such that errors will not accumulate on repeated
190	read-write cycles.</para><para>This field gives the smallest change of
191	an integer control actually affecting hardware. Often the information
192	is needed when the user can change controls by keyboard or GUI
193	buttons, rather than a slider. When for example a hardware register
194	accepts values 0-511 and the driver reports 0-65535, step should be
195	128.</para><para>Note that although signed, the step value is supposed to
196	be always positive.</para></entry>
197		  </row>
198		  <row>
199		    <entry>__s32</entry>
200		    <entry><structfield>default_value</structfield></entry>
201		    <entry>The default value of a
202	<constant>V4L2_CTRL_TYPE_INTEGER</constant>,
203	<constant>_BOOLEAN</constant>, <constant>_BITMASK</constant>,
204	<constant>_MENU</constant> or <constant>_INTEGER_MENU</constant> control.
205	Not valid for other types of controls.
206	Note that drivers reset controls to their default value only when the
207	driver is first loaded, never afterwards.</entry>
208		  </row>
209		  <row>
210		    <entry>__u32</entry>
211		    <entry><structfield>flags</structfield></entry>
212		    <entry>Control flags, see <xref
213			linkend="control-flags" />.</entry>
214		  </row>
215		  <row>
216		    <entry>__u32</entry>
217		    <entry><structfield>reserved</structfield>[2]</entry>
218		    <entry>Reserved for future extensions. Drivers must set
219	the array to zero.</entry>
220		  </row>
221		</tbody>
222	      </tgroup>
223	    </table>
224	
225	    <table pgwide="1" frame="none" id="v4l2-query-ext-ctrl">
226	      <title>struct <structname>v4l2_query_ext_ctrl</structname></title>
227	      <tgroup cols="3">
228		&cs-str;
229		<tbody valign="top">
230		  <row>
231		    <entry>__u32</entry>
232		    <entry><structfield>id</structfield></entry>
233		    <entry>Identifies the control, set by the application. See
234	<xref linkend="control-id" /> for predefined IDs. When the ID is ORed
235	with <constant>V4L2_CTRL_FLAG_NEXT_CTRL</constant> the driver clears the
236	flag and returns the first non-compound control with a higher ID. When the
237	ID is ORed with <constant>V4L2_CTRL_FLAG_NEXT_COMPOUND</constant> the driver
238	clears the flag and returns the first compound control with a higher ID.
239	Set both to get the first control (compound or not) with a higher ID.</entry>
240		  </row>
241		  <row>
242		    <entry>__u32</entry>
243		    <entry><structfield>type</structfield></entry>
244		    <entry>Type of control, see <xref
245			linkend="v4l2-ctrl-type" />.</entry>
246		  </row>
247		  <row>
248		    <entry>char</entry>
249		    <entry><structfield>name</structfield>[32]</entry>
250		    <entry>Name of the control, a NUL-terminated ASCII
251	string. This information is intended for the user.</entry>
252		  </row>
253		  <row>
254		    <entry>__s64</entry>
255		    <entry><structfield>minimum</structfield></entry>
256		    <entry>Minimum value, inclusive. This field gives a lower
257	bound for the control. See &v4l2-ctrl-type; how the minimum value is to
258	be used for each possible control type. Note that this a signed 64-bit value.</entry>
259		  </row>
260		  <row>
261		    <entry>__s64</entry>
262		    <entry><structfield>maximum</structfield></entry>
263		    <entry>Maximum value, inclusive. This field gives an upper
264	bound for the control. See &v4l2-ctrl-type; how the maximum value is to
265	be used for each possible control type. Note that this a signed 64-bit value.</entry>
266		  </row>
267		  <row>
268		    <entry>__u64</entry>
269		    <entry><structfield>step</structfield></entry>
270		    <entry><para>This field gives a step size for the control.
271	See &v4l2-ctrl-type; how the step value is to be used for each possible
272	control type. Note that this an unsigned 64-bit value.
273	</para><para>Generally drivers should not scale hardware
274	control values. It may be necessary for example when the
275	<structfield>name</structfield> or <structfield>id</structfield> imply
276	a particular unit and the hardware actually accepts only multiples of
277	said unit. If so, drivers must take care values are properly rounded
278	when scaling, such that errors will not accumulate on repeated
279	read-write cycles.</para><para>This field gives the smallest change of
280	an integer control actually affecting hardware. Often the information
281	is needed when the user can change controls by keyboard or GUI
282	buttons, rather than a slider. When for example a hardware register
283	accepts values 0-511 and the driver reports 0-65535, step should be
284	128.</para></entry>
285		  </row>
286		  <row>
287		    <entry>__s64</entry>
288		    <entry><structfield>default_value</structfield></entry>
289		    <entry>The default value of a
290	<constant>V4L2_CTRL_TYPE_INTEGER</constant>, <constant>_INTEGER64</constant>,
291	<constant>_BOOLEAN</constant>, <constant>_BITMASK</constant>,
292	<constant>_MENU</constant>, <constant>_INTEGER_MENU</constant>,
293	<constant>_U8</constant> or <constant>_U16</constant> control.
294	Not valid for other types of controls.
295	Note that drivers reset controls to their default value only when the
296	driver is first loaded, never afterwards.
297	</entry>
298		  </row>
299		  <row>
300		    <entry>__u32</entry>
301		    <entry><structfield>flags</structfield></entry>
302		    <entry>Control flags, see <xref
303			linkend="control-flags" />.</entry>
304		  </row>
305		  <row>
306		    <entry>__u32</entry>
307		    <entry><structfield>elem_size</structfield></entry>
308		    <entry>The size in bytes of a single element of the array.
309	Given a char pointer <constant>p</constant> to a 3-dimensional array you can find the
310	position of cell <constant>(z, y, x)</constant> as follows:
311	<constant>p + ((z * dims[1] + y) * dims[0] + x) * elem_size</constant>. <structfield>elem_size</structfield>
312	is always valid, also when the control isn't an array. For string controls
313	<structfield>elem_size</structfield> is equal to <structfield>maximum + 1</structfield>.
314	</entry>
315		  </row>
316		  <row>
317		    <entry>__u32</entry>
318		    <entry><structfield>elems</structfield></entry>
319		    <entry>The number of elements in the N-dimensional array. If this control
320	is not an array, then <structfield>elems</structfield> is 1. The <structfield>elems</structfield>
321	field can never be 0.</entry>
322		  </row>
323		  <row>
324		    <entry>__u32</entry>
325		    <entry><structfield>nr_of_dims</structfield></entry>
326		    <entry>The number of dimension in the N-dimensional array. If this control
327	is not an array, then this field is 0.</entry>
328		  </row>
329		  <row>
330		    <entry>__u32</entry>
331		    <entry><structfield>dims[V4L2_CTRL_MAX_DIMS]</structfield></entry>
332		    <entry>The size of each dimension. The first <structfield>nr_of_dims</structfield>
333	elements of this array must be non-zero, all remaining elements must be zero.</entry>
334		  </row>
335		  <row>
336		    <entry>__u32</entry>
337		    <entry><structfield>reserved</structfield>[32]</entry>
338		    <entry>Reserved for future extensions. Applications and drivers
339	must set the array to zero.</entry>
340		  </row>
341		</tbody>
342	      </tgroup>
343	    </table>
344	
345	    <table pgwide="1" frame="none" id="v4l2-querymenu">
346	      <title>struct <structname>v4l2_querymenu</structname></title>
347	      <tgroup cols="4">
348		&cs-str;
349		<tbody valign="top">
350		  <row>
351		    <entry>__u32</entry>
352		    <entry></entry>
353		    <entry><structfield>id</structfield></entry>
354		    <entry>Identifies the control, set by the application
355	from the respective &v4l2-queryctrl;
356	<structfield>id</structfield>.</entry>
357		  </row>
358		  <row>
359		    <entry>__u32</entry>
360		    <entry></entry>
361		    <entry><structfield>index</structfield></entry>
362		    <entry>Index of the menu item, starting at zero, set by
363		    the application.</entry>
364		  </row>
365		  <row>
366		    <entry>union</entry>
367		    <entry></entry>
368		    <entry></entry>
369		    <entry></entry>
370		  </row>
371		  <row>
372		    <entry></entry>
373		    <entry>__u8</entry>
374		    <entry><structfield>name</structfield>[32]</entry>
375		    <entry>Name of the menu item, a NUL-terminated ASCII
376	string. This information is intended for the user. This field is valid
377	for <constant>V4L2_CTRL_FLAG_MENU</constant> type controls.</entry>
378		  </row>
379		  <row>
380		    <entry></entry>
381		    <entry>__s64</entry>
382		    <entry><structfield>value</structfield></entry>
383		    <entry>
384	              Value of the integer menu item. This field is valid for
385	              <constant>V4L2_CTRL_FLAG_INTEGER_MENU</constant> type
386	              controls.
387	            </entry>
388		  </row>
389		  <row>
390		    <entry>__u32</entry>
391		    <entry></entry>
392		    <entry><structfield>reserved</structfield></entry>
393		    <entry>Reserved for future extensions. Drivers must set
394	the array to zero.</entry>
395		  </row>
396		</tbody>
397	      </tgroup>
398	    </table>
399	
400	    <table pgwide="1" frame="none" id="v4l2-ctrl-type">
401	      <title>enum v4l2_ctrl_type</title>
402	      <tgroup cols="5" align="left">
403		<colspec colwidth="30*" />
404		<colspec colwidth="5*" align="center" />
405		<colspec colwidth="5*" align="center" />
406		<colspec colwidth="5*" align="center" />
407		<colspec colwidth="55*" />
408		<thead>
409		  <row>
410		    <entry>Type</entry>
411		    <entry><structfield>minimum</structfield></entry>
412		    <entry><structfield>step</structfield></entry>
413		    <entry><structfield>maximum</structfield></entry>
414		    <entry>Description</entry>
415		  </row>
416		</thead>
417		<tbody valign="top">
418		  <row>
419		    <entry><constant>V4L2_CTRL_TYPE_INTEGER</constant></entry>
420		    <entry>any</entry>
421		    <entry>any</entry>
422		    <entry>any</entry>
423		    <entry>An integer-valued control ranging from minimum to
424	maximum inclusive. The step value indicates the increment between
425	values which are actually different on the hardware.</entry>
426		  </row>
427		  <row>
428		    <entry><constant>V4L2_CTRL_TYPE_BOOLEAN</constant></entry>
429		    <entry>0</entry>
430		    <entry>1</entry>
431		    <entry>1</entry>
432		    <entry>A boolean-valued control. Zero corresponds to
433	"disabled", and one means "enabled".</entry>
434		  </row>
435		  <row>
436		    <entry><constant>V4L2_CTRL_TYPE_MENU</constant></entry>
437		    <entry>&ge; 0</entry>
438		    <entry>1</entry>
439		    <entry>N-1</entry>
440		    <entry>The control has a menu of N choices. The names of
441	the menu items can be enumerated with the
442	<constant>VIDIOC_QUERYMENU</constant> ioctl.</entry>
443		  </row>
444		  <row>
445		    <entry><constant>V4L2_CTRL_TYPE_INTEGER_MENU</constant></entry>
446		    <entry>&ge; 0</entry>
447		    <entry>1</entry>
448		    <entry>N-1</entry>
449		    <entry>
450	              The control has a menu of N choices. The values of the
451	              menu items can be enumerated with the
452	              <constant>VIDIOC_QUERYMENU</constant> ioctl. This is
453	              similar to <constant>V4L2_CTRL_TYPE_MENU</constant>
454	              except that instead of strings, the menu items are
455	              signed 64-bit integers.
456	            </entry>
457		  </row>
458		  <row>
459		    <entry><constant>V4L2_CTRL_TYPE_BITMASK</constant></entry>
460		    <entry>0</entry>
461		    <entry>n/a</entry>
462		    <entry>any</entry>
463		    <entry>A bitmask field. The maximum value is the set of bits that can
464	be used, all other bits are to be 0. The maximum value is interpreted as a __u32,
465	allowing the use of bit 31 in the bitmask.</entry>
466		  </row>
467		  <row>
468		    <entry><constant>V4L2_CTRL_TYPE_BUTTON</constant></entry>
469		    <entry>0</entry>
470		    <entry>0</entry>
471		    <entry>0</entry>
472		    <entry>A control which performs an action when set.
473	Drivers must ignore the value passed with
474	<constant>VIDIOC_S_CTRL</constant> and return an &EINVAL; on a
475	<constant>VIDIOC_G_CTRL</constant> attempt.</entry>
476		  </row>
477		  <row>
478		    <entry><constant>V4L2_CTRL_TYPE_INTEGER64</constant></entry>
479		    <entry>any</entry>
480		    <entry>any</entry>
481		    <entry>any</entry>
482		    <entry>A 64-bit integer valued control. Minimum, maximum
483	and step size cannot be queried using <constant>VIDIOC_QUERYCTRL</constant>.
484	Only <constant>VIDIOC_QUERY_EXT_CTRL</constant> can retrieve the 64-bit
485	min/max/step values, they should be interpreted as n/a when using
486	<constant>VIDIOC_QUERYCTRL</constant>.</entry>
487		  </row>
488		  <row>
489		    <entry><constant>V4L2_CTRL_TYPE_STRING</constant></entry>
490		    <entry>&ge; 0</entry>
491		    <entry>&ge; 1</entry>
492		    <entry>&ge; 0</entry>
493		    <entry>The minimum and maximum string lengths. The step size
494	means that the string must be (minimum + N * step) characters long for
495	N &ge; 0. These lengths do not include the terminating zero, so in order to
496	pass a string of length 8 to &VIDIOC-S-EXT-CTRLS; you need to set the
497	<structfield>size</structfield> field of &v4l2-ext-control; to 9. For &VIDIOC-G-EXT-CTRLS; you can
498	set the <structfield>size</structfield> field to <structfield>maximum</structfield> + 1.
499	Which character encoding is used will depend on the string control itself and
500	should be part of the control documentation.</entry>
501		  </row>
502		  <row>
503		    <entry><constant>V4L2_CTRL_TYPE_CTRL_CLASS</constant></entry>
504		    <entry>n/a</entry>
505		    <entry>n/a</entry>
506		    <entry>n/a</entry>
507		    <entry>This is not a control. When
508	<constant>VIDIOC_QUERYCTRL</constant> is called with a control ID
509	equal to a control class code (see <xref linkend="ctrl-class" />) + 1, the
510	ioctl returns the name of the control class and this control type.
511	Older drivers which do not support this feature return an
512	&EINVAL;.</entry>
513		  </row>
514		  <row>
515		    <entry><constant>V4L2_CTRL_TYPE_U8</constant></entry>
516		    <entry>any</entry>
517		    <entry>any</entry>
518		    <entry>any</entry>
519		    <entry>An unsigned 8-bit valued control ranging from minimum to
520	maximum inclusive. The step value indicates the increment between
521	values which are actually different on the hardware.
522	</entry>
523		  </row>
524		  <row>
525		    <entry><constant>V4L2_CTRL_TYPE_U16</constant></entry>
526		    <entry>any</entry>
527		    <entry>any</entry>
528		    <entry>any</entry>
529		    <entry>An unsigned 16-bit valued control ranging from minimum to
530	maximum inclusive. The step value indicates the increment between
531	values which are actually different on the hardware.
532	</entry>
533		  </row>
534		</tbody>
535	      </tgroup>
536	    </table>
537	
538	    <table pgwide="1" frame="none" id="control-flags">
539	      <title>Control Flags</title>
540	      <tgroup cols="3">
541		&cs-def;
542		<tbody valign="top">
543		  <row>
544		    <entry><constant>V4L2_CTRL_FLAG_DISABLED</constant></entry>
545		    <entry>0x0001</entry>
546		    <entry>This control is permanently disabled and should be
547	ignored by the application. Any attempt to change the control will
548	result in an &EINVAL;.</entry>
549		  </row>
550		  <row>
551		    <entry><constant>V4L2_CTRL_FLAG_GRABBED</constant></entry>
552		    <entry>0x0002</entry>
553		    <entry>This control is temporarily unchangeable, for
554	example because another application took over control of the
555	respective resource. Such controls may be displayed specially in a
556	user interface. Attempts to change the control may result in an
557	&EBUSY;.</entry>
558		  </row>
559		  <row>
560		    <entry><constant>V4L2_CTRL_FLAG_READ_ONLY</constant></entry>
561		    <entry>0x0004</entry>
562		    <entry>This control is permanently readable only. Any
563	attempt to change the control will result in an &EINVAL;.</entry>
564		  </row>
565		  <row>
566		    <entry><constant>V4L2_CTRL_FLAG_UPDATE</constant></entry>
567		    <entry>0x0008</entry>
568		    <entry>A hint that changing this control may affect the
569	value of other controls within the same control class. Applications
570	should update their user interface accordingly.</entry>
571		  </row>
572		  <row>
573		    <entry><constant>V4L2_CTRL_FLAG_INACTIVE</constant></entry>
574		    <entry>0x0010</entry>
575		    <entry>This control is not applicable to the current
576	configuration and should be displayed accordingly in a user interface.
577	For example the flag may be set on a MPEG audio level 2 bitrate
578	control when MPEG audio encoding level 1 was selected with another
579	control.</entry>
580		  </row>
581		  <row>
582		    <entry><constant>V4L2_CTRL_FLAG_SLIDER</constant></entry>
583		    <entry>0x0020</entry>
584		    <entry>A hint that this control is best represented as a
585	slider-like element in a user interface.</entry>
586		  </row>
587		  <row>
588		    <entry><constant>V4L2_CTRL_FLAG_WRITE_ONLY</constant></entry>
589		    <entry>0x0040</entry>
590		    <entry>This control is permanently writable only. Any
591	attempt to read the control will result in an &EACCES; error code. This
592	flag is typically present for relative controls or action controls where
593	writing a value will cause the device to carry out a given action
594	(&eg; motor control) but no meaningful value can be returned.</entry>
595		  </row>
596		  <row>
597		    <entry><constant>V4L2_CTRL_FLAG_VOLATILE</constant></entry>
598		    <entry>0x0080</entry>
599		    <entry>This control is volatile, which means that the value of the control
600	changes continuously. A typical example would be the current gain value if the device
601	is in auto-gain mode. In such a case the hardware calculates the gain value based on
602	the lighting conditions which can change over time. Note that setting a new value for
603	a volatile control will have no effect and no <constant>V4L2_EVENT_CTRL_CH_VALUE</constant>
604	will be sent, unless the <constant>V4L2_CTRL_FLAG_EXECUTE_ON_WRITE</constant> flag
605	(see below) is also set. Otherwise the new value will just be ignored.</entry>
606		  </row>
607		  <row>
608		    <entry><constant>V4L2_CTRL_FLAG_HAS_PAYLOAD</constant></entry>
609		    <entry>0x0100</entry>
610		    <entry>This control has a pointer type, so its value has to be accessed
611	using one of the pointer fields of &v4l2-ext-control;. This flag is set for controls
612	that are an array, string, or have a compound type. In all cases you have to set a
613	pointer to memory containing the payload of the control.</entry>
614		  </row>
615		  <row>
616		    <entry><constant>V4L2_CTRL_FLAG_EXECUTE_ON_WRITE</constant></entry>
617		    <entry>0x0200</entry>
618		    <entry>The value provided to the control will be propagated to the driver
619	even if remains constant. This is required when the control represents an action
620	on the hardware. For example: clearing an error flag or triggering the flash. All the
621	controls of the type <constant>V4L2_CTRL_TYPE_BUTTON</constant> have this flag set.</entry>
622		  </row>
623		</tbody>
624	      </tgroup>
625	    </table>
626	  </refsect1>
627	
628	  <refsect1>
629	    &return-value;
630	
631	    <variablelist>
632	      <varlistentry>
633		<term><errorcode>EINVAL</errorcode></term>
634		<listitem>
635		  <para>The &v4l2-queryctrl; <structfield>id</structfield>
636	is invalid. The &v4l2-querymenu; <structfield>id</structfield> is
637	invalid or <structfield>index</structfield> is out of range (less than
638	<structfield>minimum</structfield> or greater than <structfield>maximum</structfield>)
639	or this particular menu item is not supported by the driver.</para>
640		</listitem>
641	      </varlistentry>
642	      <varlistentry>
643		<term><errorcode>EACCES</errorcode></term>
644		<listitem>
645		  <para>An attempt was made to read a write-only control.</para>
646		</listitem>
647	      </varlistentry>
648	    </variablelist>
649	  </refsect1>
650	</refentry>
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