Documentation/DocBook/media/v4l/vidioc-queryctrl.xml

   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 it 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>