Based on kernel version 4.7.2. Page generated on 2016-08-22 22:45 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 ≥ <constant>V4L2_CTRL_COMPOUND_TYPES</constant> and/or array 105 control, in other words controls that contain more than one value). 106 Specify both <constant>V4L2_CTRL_FLAG_NEXT_CTRL</constant> and 107 <constant>V4L2_CTRL_FLAG_NEXT_COMPOUND</constant> in order to enumerate 108 all controls, compound or not. Drivers which do not support these flags yet 109 always return <errorcode>EINVAL</errorcode>.</para> 110 111 <para>The <constant>VIDIOC_QUERY_EXT_CTRL</constant> ioctl was 112 introduced in order to better support controls that can use compound 113 types, and to expose additional control information that cannot be 114 returned in &v4l2-queryctrl; since that structure is full.</para> 115 116 <para><constant>VIDIOC_QUERY_EXT_CTRL</constant> is used in the 117 same way as <constant>VIDIOC_QUERYCTRL</constant>, except that the 118 <structfield>reserved</structfield> array must be zeroed as well.</para> 119 120 <para>Additional information is required for menu controls: the 121 names of the menu items. To query them applications set the 122 <structfield>id</structfield> and <structfield>index</structfield> 123 fields of &v4l2-querymenu; and call the 124 <constant>VIDIOC_QUERYMENU</constant> ioctl with a pointer to this 125 structure. The driver fills the rest of the structure or returns an 126 &EINVAL; when the <structfield>id</structfield> or 127 <structfield>index</structfield> is invalid. Menu items are enumerated 128 by calling <constant>VIDIOC_QUERYMENU</constant> with successive 129 <structfield>index</structfield> values from &v4l2-queryctrl; 130 <structfield>minimum</structfield> to 131 <structfield>maximum</structfield>, inclusive. Note that it is possible 132 for <constant>VIDIOC_QUERYMENU</constant> to return an &EINVAL; for some 133 indices between <structfield>minimum</structfield> and <structfield>maximum</structfield>. 134 In that case that particular menu item is not supported by this driver. Also note that 135 the <structfield>minimum</structfield> value is not necessarily 0.</para> 136 137 <para>See also the examples in <xref linkend="control" />.</para> 138 139 <table pgwide="1" frame="none" id="v4l2-queryctrl"> 140 <title>struct <structname>v4l2_queryctrl</structname></title> 141 <tgroup cols="3"> 142 &cs-str; 143 <tbody valign="top"> 144 <row> 145 <entry>__u32</entry> 146 <entry><structfield>id</structfield></entry> 147 <entry>Identifies the control, set by the application. See 148 <xref linkend="control-id" /> for predefined IDs. When the ID is ORed 149 with V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returns 150 the first control with a higher ID. Drivers which do not support this 151 flag yet always return an &EINVAL;.</entry> 152 </row> 153 <row> 154 <entry>__u32</entry> 155 <entry><structfield>type</structfield></entry> 156 <entry>Type of control, see <xref 157 linkend="v4l2-ctrl-type" />.</entry> 158 </row> 159 <row> 160 <entry>__u8</entry> 161 <entry><structfield>name</structfield>[32]</entry> 162 <entry>Name of the control, a NUL-terminated ASCII 163 string. This information is intended for the user.</entry> 164 </row> 165 <row> 166 <entry>__s32</entry> 167 <entry><structfield>minimum</structfield></entry> 168 <entry>Minimum value, inclusive. This field gives a lower 169 bound for the control. See &v4l2-ctrl-type; how the minimum value is to 170 be used for each possible control type. Note that this a signed 32-bit value.</entry> 171 </row> 172 <row> 173 <entry>__s32</entry> 174 <entry><structfield>maximum</structfield></entry> 175 <entry>Maximum value, inclusive. This field gives an upper 176 bound for the control. See &v4l2-ctrl-type; how the maximum value is to 177 be used for each possible control type. Note that this a signed 32-bit value.</entry> 178 </row> 179 <row> 180 <entry>__s32</entry> 181 <entry><structfield>step</structfield></entry> 182 <entry><para>This field gives a step size for the control. 183 See &v4l2-ctrl-type; how the step value is to be used for each possible 184 control type. Note that this an unsigned 32-bit value. 185 </para><para>Generally drivers should not scale hardware 186 control values. It may be necessary for example when the 187 <structfield>name</structfield> or <structfield>id</structfield> imply 188 a particular unit and the hardware actually accepts only multiples of 189 said unit. If so, drivers must take care values are properly rounded 190 when scaling, such that errors will not accumulate on repeated 191 read-write cycles.</para><para>This field gives the smallest change of 192 an integer control actually affecting hardware. Often the information 193 is needed when the user can change controls by keyboard or GUI 194 buttons, rather than a slider. When for example a hardware register 195 accepts values 0-511 and the driver reports 0-65535, step should be 196 128.</para><para>Note that although signed, the step value is supposed to 197 be always positive.</para></entry> 198 </row> 199 <row> 200 <entry>__s32</entry> 201 <entry><structfield>default_value</structfield></entry> 202 <entry>The default value of a 203 <constant>V4L2_CTRL_TYPE_INTEGER</constant>, 204 <constant>_BOOLEAN</constant>, <constant>_BITMASK</constant>, 205 <constant>_MENU</constant> or <constant>_INTEGER_MENU</constant> control. 206 Not valid for other types of controls. 207 Note that drivers reset controls to their default value only when the 208 driver is first loaded, never afterwards.</entry> 209 </row> 210 <row> 211 <entry>__u32</entry> 212 <entry><structfield>flags</structfield></entry> 213 <entry>Control flags, see <xref 214 linkend="control-flags" />.</entry> 215 </row> 216 <row> 217 <entry>__u32</entry> 218 <entry><structfield>reserved</structfield>[2]</entry> 219 <entry>Reserved for future extensions. Drivers must set 220 the array to zero.</entry> 221 </row> 222 </tbody> 223 </tgroup> 224 </table> 225 226 <table pgwide="1" frame="none" id="v4l2-query-ext-ctrl"> 227 <title>struct <structname>v4l2_query_ext_ctrl</structname></title> 228 <tgroup cols="3"> 229 &cs-str; 230 <tbody valign="top"> 231 <row> 232 <entry>__u32</entry> 233 <entry><structfield>id</structfield></entry> 234 <entry>Identifies the control, set by the application. See 235 <xref linkend="control-id" /> for predefined IDs. When the ID is ORed 236 with <constant>V4L2_CTRL_FLAG_NEXT_CTRL</constant> the driver clears the 237 flag and returns the first non-compound control with a higher ID. When the 238 ID is ORed with <constant>V4L2_CTRL_FLAG_NEXT_COMPOUND</constant> the driver 239 clears the flag and returns the first compound control with a higher ID. 240 Set both to get the first control (compound or not) with a higher ID.</entry> 241 </row> 242 <row> 243 <entry>__u32</entry> 244 <entry><structfield>type</structfield></entry> 245 <entry>Type of control, see <xref 246 linkend="v4l2-ctrl-type" />.</entry> 247 </row> 248 <row> 249 <entry>char</entry> 250 <entry><structfield>name</structfield>[32]</entry> 251 <entry>Name of the control, a NUL-terminated ASCII 252 string. This information is intended for the user.</entry> 253 </row> 254 <row> 255 <entry>__s64</entry> 256 <entry><structfield>minimum</structfield></entry> 257 <entry>Minimum value, inclusive. This field gives a lower 258 bound for the control. See &v4l2-ctrl-type; how the minimum value is to 259 be used for each possible control type. Note that this a signed 64-bit value.</entry> 260 </row> 261 <row> 262 <entry>__s64</entry> 263 <entry><structfield>maximum</structfield></entry> 264 <entry>Maximum value, inclusive. This field gives an upper 265 bound for the control. See &v4l2-ctrl-type; how the maximum value is to 266 be used for each possible control type. Note that this a signed 64-bit value.</entry> 267 </row> 268 <row> 269 <entry>__u64</entry> 270 <entry><structfield>step</structfield></entry> 271 <entry><para>This field gives a step size for the control. 272 See &v4l2-ctrl-type; how the step value is to be used for each possible 273 control type. Note that this an unsigned 64-bit value. 274 </para><para>Generally drivers should not scale hardware 275 control values. It may be necessary for example when the 276 <structfield>name</structfield> or <structfield>id</structfield> imply 277 a particular unit and the hardware actually accepts only multiples of 278 said unit. If so, drivers must take care values are properly rounded 279 when scaling, such that errors will not accumulate on repeated 280 read-write cycles.</para><para>This field gives the smallest change of 281 an integer control actually affecting hardware. Often the information 282 is needed when the user can change controls by keyboard or GUI 283 buttons, rather than a slider. When for example a hardware register 284 accepts values 0-511 and the driver reports 0-65535, step should be 285 128.</para></entry> 286 </row> 287 <row> 288 <entry>__s64</entry> 289 <entry><structfield>default_value</structfield></entry> 290 <entry>The default value of a 291 <constant>V4L2_CTRL_TYPE_INTEGER</constant>, <constant>_INTEGER64</constant>, 292 <constant>_BOOLEAN</constant>, <constant>_BITMASK</constant>, 293 <constant>_MENU</constant>, <constant>_INTEGER_MENU</constant>, 294 <constant>_U8</constant> or <constant>_U16</constant> control. 295 Not valid for other types of controls. 296 Note that drivers reset controls to their default value only when the 297 driver is first loaded, never afterwards. 298 </entry> 299 </row> 300 <row> 301 <entry>__u32</entry> 302 <entry><structfield>flags</structfield></entry> 303 <entry>Control flags, see <xref 304 linkend="control-flags" />.</entry> 305 </row> 306 <row> 307 <entry>__u32</entry> 308 <entry><structfield>elem_size</structfield></entry> 309 <entry>The size in bytes of a single element of the array. 310 Given a char pointer <constant>p</constant> to a 3-dimensional array you can find the 311 position of cell <constant>(z, y, x)</constant> as follows: 312 <constant>p + ((z * dims[1] + y) * dims[0] + x) * elem_size</constant>. <structfield>elem_size</structfield> 313 is always valid, also when the control isn't an array. For string controls 314 <structfield>elem_size</structfield> is equal to <structfield>maximum + 1</structfield>. 315 </entry> 316 </row> 317 <row> 318 <entry>__u32</entry> 319 <entry><structfield>elems</structfield></entry> 320 <entry>The number of elements in the N-dimensional array. If this control 321 is not an array, then <structfield>elems</structfield> is 1. The <structfield>elems</structfield> 322 field can never be 0.</entry> 323 </row> 324 <row> 325 <entry>__u32</entry> 326 <entry><structfield>nr_of_dims</structfield></entry> 327 <entry>The number of dimension in the N-dimensional array. If this control 328 is not an array, then this field is 0.</entry> 329 </row> 330 <row> 331 <entry>__u32</entry> 332 <entry><structfield>dims[V4L2_CTRL_MAX_DIMS]</structfield></entry> 333 <entry>The size of each dimension. The first <structfield>nr_of_dims</structfield> 334 elements of this array must be non-zero, all remaining elements must be zero.</entry> 335 </row> 336 <row> 337 <entry>__u32</entry> 338 <entry><structfield>reserved</structfield>[32]</entry> 339 <entry>Reserved for future extensions. Applications and drivers 340 must set the array to zero.</entry> 341 </row> 342 </tbody> 343 </tgroup> 344 </table> 345 346 <table pgwide="1" frame="none" id="v4l2-querymenu"> 347 <title>struct <structname>v4l2_querymenu</structname></title> 348 <tgroup cols="4"> 349 &cs-str; 350 <tbody valign="top"> 351 <row> 352 <entry>__u32</entry> 353 <entry></entry> 354 <entry><structfield>id</structfield></entry> 355 <entry>Identifies the control, set by the application 356 from the respective &v4l2-queryctrl; 357 <structfield>id</structfield>.</entry> 358 </row> 359 <row> 360 <entry>__u32</entry> 361 <entry></entry> 362 <entry><structfield>index</structfield></entry> 363 <entry>Index of the menu item, starting at zero, set by 364 the application.</entry> 365 </row> 366 <row> 367 <entry>union</entry> 368 <entry></entry> 369 <entry></entry> 370 <entry></entry> 371 </row> 372 <row> 373 <entry></entry> 374 <entry>__u8</entry> 375 <entry><structfield>name</structfield>[32]</entry> 376 <entry>Name of the menu item, a NUL-terminated ASCII 377 string. This information is intended for the user. This field is valid 378 for <constant>V4L2_CTRL_FLAG_MENU</constant> type controls.</entry> 379 </row> 380 <row> 381 <entry></entry> 382 <entry>__s64</entry> 383 <entry><structfield>value</structfield></entry> 384 <entry> 385 Value of the integer menu item. This field is valid for 386 <constant>V4L2_CTRL_FLAG_INTEGER_MENU</constant> type 387 controls. 388 </entry> 389 </row> 390 <row> 391 <entry>__u32</entry> 392 <entry></entry> 393 <entry><structfield>reserved</structfield></entry> 394 <entry>Reserved for future extensions. Drivers must set 395 the array to zero.</entry> 396 </row> 397 </tbody> 398 </tgroup> 399 </table> 400 401 <table pgwide="1" frame="none" id="v4l2-ctrl-type"> 402 <title>enum v4l2_ctrl_type</title> 403 <tgroup cols="5" align="left"> 404 <colspec colwidth="30*" /> 405 <colspec colwidth="5*" align="center" /> 406 <colspec colwidth="5*" align="center" /> 407 <colspec colwidth="5*" align="center" /> 408 <colspec colwidth="55*" /> 409 <thead> 410 <row> 411 <entry>Type</entry> 412 <entry><structfield>minimum</structfield></entry> 413 <entry><structfield>step</structfield></entry> 414 <entry><structfield>maximum</structfield></entry> 415 <entry>Description</entry> 416 </row> 417 </thead> 418 <tbody valign="top"> 419 <row> 420 <entry><constant>V4L2_CTRL_TYPE_INTEGER</constant></entry> 421 <entry>any</entry> 422 <entry>any</entry> 423 <entry>any</entry> 424 <entry>An integer-valued control ranging from minimum to 425 maximum inclusive. The step value indicates the increment between 426 values.</entry> 427 </row> 428 <row> 429 <entry><constant>V4L2_CTRL_TYPE_BOOLEAN</constant></entry> 430 <entry>0</entry> 431 <entry>1</entry> 432 <entry>1</entry> 433 <entry>A boolean-valued control. Zero corresponds to 434 "disabled", and one means "enabled".</entry> 435 </row> 436 <row> 437 <entry><constant>V4L2_CTRL_TYPE_MENU</constant></entry> 438 <entry>≥ 0</entry> 439 <entry>1</entry> 440 <entry>N-1</entry> 441 <entry>The control has a menu of N choices. The names of 442 the menu items can be enumerated with the 443 <constant>VIDIOC_QUERYMENU</constant> ioctl.</entry> 444 </row> 445 <row> 446 <entry><constant>V4L2_CTRL_TYPE_INTEGER_MENU</constant></entry> 447 <entry>≥ 0</entry> 448 <entry>1</entry> 449 <entry>N-1</entry> 450 <entry> 451 The control has a menu of N choices. The values of the 452 menu items can be enumerated with the 453 <constant>VIDIOC_QUERYMENU</constant> ioctl. This is 454 similar to <constant>V4L2_CTRL_TYPE_MENU</constant> 455 except that instead of strings, the menu items are 456 signed 64-bit integers. 457 </entry> 458 </row> 459 <row> 460 <entry><constant>V4L2_CTRL_TYPE_BITMASK</constant></entry> 461 <entry>0</entry> 462 <entry>n/a</entry> 463 <entry>any</entry> 464 <entry>A bitmask field. The maximum value is the set of bits that can 465 be used, all other bits are to be 0. The maximum value is interpreted as a __u32, 466 allowing the use of bit 31 in the bitmask.</entry> 467 </row> 468 <row> 469 <entry><constant>V4L2_CTRL_TYPE_BUTTON</constant></entry> 470 <entry>0</entry> 471 <entry>0</entry> 472 <entry>0</entry> 473 <entry>A control which performs an action when set. 474 Drivers must ignore the value passed with 475 <constant>VIDIOC_S_CTRL</constant> and return an &EINVAL; on a 476 <constant>VIDIOC_G_CTRL</constant> attempt.</entry> 477 </row> 478 <row> 479 <entry><constant>V4L2_CTRL_TYPE_INTEGER64</constant></entry> 480 <entry>any</entry> 481 <entry>any</entry> 482 <entry>any</entry> 483 <entry>A 64-bit integer valued control. Minimum, maximum 484 and step size cannot be queried using <constant>VIDIOC_QUERYCTRL</constant>. 485 Only <constant>VIDIOC_QUERY_EXT_CTRL</constant> can retrieve the 64-bit 486 min/max/step values, they should be interpreted as n/a when using 487 <constant>VIDIOC_QUERYCTRL</constant>.</entry> 488 </row> 489 <row> 490 <entry><constant>V4L2_CTRL_TYPE_STRING</constant></entry> 491 <entry>≥ 0</entry> 492 <entry>≥ 1</entry> 493 <entry>≥ 0</entry> 494 <entry>The minimum and maximum string lengths. The step size 495 means that the string must be (minimum + N * step) characters long for 496 N ≥ 0. These lengths do not include the terminating zero, so in order to 497 pass a string of length 8 to &VIDIOC-S-EXT-CTRLS; you need to set the 498 <structfield>size</structfield> field of &v4l2-ext-control; to 9. For &VIDIOC-G-EXT-CTRLS; you can 499 set the <structfield>size</structfield> field to <structfield>maximum</structfield> + 1. 500 Which character encoding is used will depend on the string control itself and 501 should be part of the control documentation.</entry> 502 </row> 503 <row> 504 <entry><constant>V4L2_CTRL_TYPE_CTRL_CLASS</constant></entry> 505 <entry>n/a</entry> 506 <entry>n/a</entry> 507 <entry>n/a</entry> 508 <entry>This is not a control. When 509 <constant>VIDIOC_QUERYCTRL</constant> is called with a control ID 510 equal to a control class code (see <xref linkend="ctrl-class" />) + 1, the 511 ioctl returns the name of the control class and this control type. 512 Older drivers which do not support this feature return an 513 &EINVAL;.</entry> 514 </row> 515 <row> 516 <entry><constant>V4L2_CTRL_TYPE_U8</constant></entry> 517 <entry>any</entry> 518 <entry>any</entry> 519 <entry>any</entry> 520 <entry>An unsigned 8-bit valued control ranging from minimum to 521 maximum inclusive. The step value indicates the increment between 522 values. 523 </entry> 524 </row> 525 <row> 526 <entry><constant>V4L2_CTRL_TYPE_U16</constant></entry> 527 <entry>any</entry> 528 <entry>any</entry> 529 <entry>any</entry> 530 <entry>An unsigned 16-bit valued control ranging from minimum to 531 maximum inclusive. The step value indicates the increment between 532 values. 533 </entry> 534 </row> 535 <row> 536 <entry><constant>V4L2_CTRL_TYPE_U32</constant></entry> 537 <entry>any</entry> 538 <entry>any</entry> 539 <entry>any</entry> 540 <entry>An unsigned 32-bit valued control ranging from minimum to 541 maximum inclusive. The step value indicates the increment between 542 values. 543 </entry> 544 </row> 545 </tbody> 546 </tgroup> 547 </table> 548 549 <table pgwide="1" frame="none" id="control-flags"> 550 <title>Control Flags</title> 551 <tgroup cols="3"> 552 &cs-def; 553 <tbody valign="top"> 554 <row> 555 <entry><constant>V4L2_CTRL_FLAG_DISABLED</constant></entry> 556 <entry>0x0001</entry> 557 <entry>This control is permanently disabled and should be 558 ignored by the application. Any attempt to change the control will 559 result in an &EINVAL;.</entry> 560 </row> 561 <row> 562 <entry><constant>V4L2_CTRL_FLAG_GRABBED</constant></entry> 563 <entry>0x0002</entry> 564 <entry>This control is temporarily unchangeable, for 565 example because another application took over control of the 566 respective resource. Such controls may be displayed specially in a 567 user interface. Attempts to change the control may result in an 568 &EBUSY;.</entry> 569 </row> 570 <row> 571 <entry><constant>V4L2_CTRL_FLAG_READ_ONLY</constant></entry> 572 <entry>0x0004</entry> 573 <entry>This control is permanently readable only. Any 574 attempt to change the control will result in an &EINVAL;.</entry> 575 </row> 576 <row> 577 <entry><constant>V4L2_CTRL_FLAG_UPDATE</constant></entry> 578 <entry>0x0008</entry> 579 <entry>A hint that changing this control may affect the 580 value of other controls within the same control class. Applications 581 should update their user interface accordingly.</entry> 582 </row> 583 <row> 584 <entry><constant>V4L2_CTRL_FLAG_INACTIVE</constant></entry> 585 <entry>0x0010</entry> 586 <entry>This control is not applicable to the current 587 configuration and should be displayed accordingly in a user interface. 588 For example the flag may be set on a MPEG audio level 2 bitrate 589 control when MPEG audio encoding level 1 was selected with another 590 control.</entry> 591 </row> 592 <row> 593 <entry><constant>V4L2_CTRL_FLAG_SLIDER</constant></entry> 594 <entry>0x0020</entry> 595 <entry>A hint that this control is best represented as a 596 slider-like element in a user interface.</entry> 597 </row> 598 <row> 599 <entry><constant>V4L2_CTRL_FLAG_WRITE_ONLY</constant></entry> 600 <entry>0x0040</entry> 601 <entry>This control is permanently writable only. Any 602 attempt to read the control will result in an &EACCES; error code. This 603 flag is typically present for relative controls or action controls where 604 writing a value will cause the device to carry out a given action 605 (⪚ motor control) but no meaningful value can be returned.</entry> 606 </row> 607 <row> 608 <entry><constant>V4L2_CTRL_FLAG_VOLATILE</constant></entry> 609 <entry>0x0080</entry> 610 <entry>This control is volatile, which means that the value of the control 611 changes continuously. A typical example would be the current gain value if the device 612 is in auto-gain mode. In such a case the hardware calculates the gain value based on 613 the lighting conditions which can change over time. Note that setting a new value for 614 a volatile control will have no effect and no <constant>V4L2_EVENT_CTRL_CH_VALUE</constant> 615 will be sent, unless the <constant>V4L2_CTRL_FLAG_EXECUTE_ON_WRITE</constant> flag 616 (see below) is also set. Otherwise the new value will just be ignored.</entry> 617 </row> 618 <row> 619 <entry><constant>V4L2_CTRL_FLAG_HAS_PAYLOAD</constant></entry> 620 <entry>0x0100</entry> 621 <entry>This control has a pointer type, so its value has to be accessed 622 using one of the pointer fields of &v4l2-ext-control;. This flag is set for controls 623 that are an array, string, or have a compound type. In all cases you have to set a 624 pointer to memory containing the payload of the control.</entry> 625 </row> 626 <row> 627 <entry><constant>V4L2_CTRL_FLAG_EXECUTE_ON_WRITE</constant></entry> 628 <entry>0x0200</entry> 629 <entry>The value provided to the control will be propagated to the driver 630 even if it remains constant. This is required when the control represents an action 631 on the hardware. For example: clearing an error flag or triggering the flash. All the 632 controls of the type <constant>V4L2_CTRL_TYPE_BUTTON</constant> have this flag set.</entry> 633 </row> 634 </tbody> 635 </tgroup> 636 </table> 637 </refsect1> 638 639 <refsect1> 640 &return-value; 641 642 <variablelist> 643 <varlistentry> 644 <term><errorcode>EINVAL</errorcode></term> 645 <listitem> 646 <para>The &v4l2-queryctrl; <structfield>id</structfield> 647 is invalid. The &v4l2-querymenu; <structfield>id</structfield> is 648 invalid or <structfield>index</structfield> is out of range (less than 649 <structfield>minimum</structfield> or greater than <structfield>maximum</structfield>) 650 or this particular menu item is not supported by the driver.</para> 651 </listitem> 652 </varlistentry> 653 <varlistentry> 654 <term><errorcode>EACCES</errorcode></term> 655 <listitem> 656 <para>An attempt was made to read a write-only control.</para> 657 </listitem> 658 </varlistentry> 659 </variablelist> 660 </refsect1> 661 </refentry>
