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 72structure. 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 81supported. Further applications can enumerate private controls, which 82are 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 90disabled and should be ignored by the application.<footnote> 91 <para><constant>V4L2_CTRL_FLAG_DISABLED</constant> was 92intended for two purposes: Drivers can skip predefined controls not 93supported by the hardware (although returning EINVAL would do as 94well), or disable predefined and private controls after hardware 95detection without the trouble of reordering control arrays and indices 96(EINVAL cannot be used to skip private controls because it would 97prematurely 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 101next supported non-compound control, or <errorcode>EINVAL</errorcode> 102if there is none. In addition, the <constant>V4L2_CTRL_FLAG_NEXT_COMPOUND</constant> 103flag can be specified to enumerate all compound controls (i.e. controls 104with type ≥ <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 107all controls, compound or not. Drivers which do not support these flags yet 108always return <errorcode>EINVAL</errorcode>.</para> 109 110 <para>The <constant>VIDIOC_QUERY_EXT_CTRL</constant> ioctl was 111introduced in order to better support controls that can use compound 112types, and to expose additional control information that cannot be 113returned in &v4l2-queryctrl; since that structure is full.</para> 114 115 <para><constant>VIDIOC_QUERY_EXT_CTRL</constant> is used in the 116same 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 120names of the menu items. To query them applications set the 121<structfield>id</structfield> and <structfield>index</structfield> 122fields of &v4l2-querymenu; and call the 123<constant>VIDIOC_QUERYMENU</constant> ioctl with a pointer to this 124structure. 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 127by 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 131for <constant>VIDIOC_QUERYMENU</constant> to return an &EINVAL; for some 132indices between <structfield>minimum</structfield> and <structfield>maximum</structfield>. 133In that case that particular menu item is not supported by this driver. Also note that 134the <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 148with V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returns 149the first control with a higher ID. Drivers which do not support this 150flag 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 162string. 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 168bound for the control. See &v4l2-ctrl-type; how the minimum value is to 169be 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 175bound for the control. See &v4l2-ctrl-type; how the maximum value is to 176be 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. 182See &v4l2-ctrl-type; how the step value is to be used for each possible 183control type. Note that this an unsigned 32-bit value. 184</para><para>Generally drivers should not scale hardware 185control values. It may be necessary for example when the 186<structfield>name</structfield> or <structfield>id</structfield> imply 187a particular unit and the hardware actually accepts only multiples of 188said unit. If so, drivers must take care values are properly rounded 189when scaling, such that errors will not accumulate on repeated 190read-write cycles.</para><para>This field gives the smallest change of 191an integer control actually affecting hardware. Often the information 192is needed when the user can change controls by keyboard or GUI 193buttons, rather than a slider. When for example a hardware register 194accepts values 0-511 and the driver reports 0-65535, step should be 195128.</para><para>Note that although signed, the step value is supposed to 196be 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. 205Not valid for other types of controls. 206Note that drivers reset controls to their default value only when the 207driver 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 219the 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 235with <constant>V4L2_CTRL_FLAG_NEXT_CTRL</constant> the driver clears the 236flag and returns the first non-compound control with a higher ID. When the 237ID is ORed with <constant>V4L2_CTRL_FLAG_NEXT_COMPOUND</constant> the driver 238clears the flag and returns the first compound control with a higher ID. 239Set 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 251string. 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 257bound for the control. See &v4l2-ctrl-type; how the minimum value is to 258be 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 264bound for the control. See &v4l2-ctrl-type; how the maximum value is to 265be 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. 271See &v4l2-ctrl-type; how the step value is to be used for each possible 272control type. Note that this an unsigned 64-bit value. 273</para><para>Generally drivers should not scale hardware 274control values. It may be necessary for example when the 275<structfield>name</structfield> or <structfield>id</structfield> imply 276a particular unit and the hardware actually accepts only multiples of 277said unit. If so, drivers must take care values are properly rounded 278when scaling, such that errors will not accumulate on repeated 279read-write cycles.</para><para>This field gives the smallest change of 280an integer control actually affecting hardware. Often the information 281is needed when the user can change controls by keyboard or GUI 282buttons, rather than a slider. When for example a hardware register 283accepts values 0-511 and the driver reports 0-65535, step should be 284128.</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. 294Not valid for other types of controls. 295Note that drivers reset controls to their default value only when the 296driver 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. 309Given a char pointer <constant>p</constant> to a 3-dimensional array you can find the 310position 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> 312is 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 320is not an array, then <structfield>elems</structfield> is 1. The <structfield>elems</structfield> 321field 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 327is 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> 333elements 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 339must 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 355from 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 376string. This information is intended for the user. This field is valid 377for <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 394the 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 424maximum inclusive. The step value indicates the increment between 425values 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>≥ 0</entry> 438 <entry>1</entry> 439 <entry>N-1</entry> 440 <entry>The control has a menu of N choices. The names of 441the 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>≥ 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 464be used, all other bits are to be 0. The maximum value is interpreted as a __u32, 465allowing 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. 473Drivers 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 483and step size cannot be queried using <constant>VIDIOC_QUERYCTRL</constant>. 484Only <constant>VIDIOC_QUERY_EXT_CTRL</constant> can retrieve the 64-bit 485min/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>≥ 0</entry> 491 <entry>≥ 1</entry> 492 <entry>≥ 0</entry> 493 <entry>The minimum and maximum string lengths. The step size 494means that the string must be (minimum + N * step) characters long for 495N ≥ 0. These lengths do not include the terminating zero, so in order to 496pass 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 498set the <structfield>size</structfield> field to <structfield>maximum</structfield> + 1. 499Which character encoding is used will depend on the string control itself and 500should 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 509equal to a control class code (see <xref linkend="ctrl-class" />) + 1, the 510ioctl returns the name of the control class and this control type. 511Older 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 520maximum inclusive. The step value indicates the increment between 521values 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 530maximum inclusive. The step value indicates the increment between 531values 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 547ignored by the application. Any attempt to change the control will 548result 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 554example because another application took over control of the 555respective resource. Such controls may be displayed specially in a 556user 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 563attempt 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 569value of other controls within the same control class. Applications 570should 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 576configuration and should be displayed accordingly in a user interface. 577For example the flag may be set on a MPEG audio level 2 bitrate 578control when MPEG audio encoding level 1 was selected with another 579control.</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 585slider-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 591attempt to read the control will result in an &EACCES; error code. This 592flag is typically present for relative controls or action controls where 593writing a value will cause the device to carry out a given action 594(⪚ 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 600changes continuously. A typical example would be the current gain value if the device 601is in auto-gain mode. In such a case the hardware calculates the gain value based on 602the lighting conditions which can change over time. Note that setting a new value for 603a volatile control will have no effect and no <constant>V4L2_EVENT_CTRL_CH_VALUE</constant> 604will 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 611using one of the pointer fields of &v4l2-ext-control;. This flag is set for controls 612that are an array, string, or have a compound type. In all cases you have to set a 613pointer 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 619even if remains constant. This is required when the control represents an action 620on the hardware. For example: clearing an error flag or triggering the flash. All the 621controls 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> 636is invalid. The &v4l2-querymenu; <structfield>id</structfield> is 637invalid or <structfield>index</structfield> is out of range (less than 638<structfield>minimum</structfield> or greater than <structfield>maximum</structfield>) 639or 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> 651