Documentation/DocBook/v4l/vidioc-queryctrl.xml

   1 <refentry id="vidioc-queryctrl">
   2   <refmeta>
   3     <refentrytitle>ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU</refentrytitle>
   4     &manvol;
   5   </refmeta>
   6
   7   <refnamediv>
   8     <refname>VIDIOC_QUERYCTRL</refname>
   9     <refname>VIDIOC_QUERYMENU</refname>
  10     <refpurpose>Enumerate controls and menu control items</refpurpose>
  11   </refnamediv>
  12
  13   <refsynopsisdiv>
  14     <funcsynopsis>
  15       <funcprototype>
  16         <funcdef>int <function>ioctl</function></funcdef>
  17         <paramdef>int <parameter>fd</parameter></paramdef>
  18         <paramdef>int <parameter>request</parameter></paramdef>
  19         <paramdef>struct v4l2_queryctrl *<parameter>argp</parameter></paramdef>
  20       </funcprototype>
  21     </funcsynopsis>
  22     <funcsynopsis>
  23       <funcprototype>
  24         <funcdef>int <function>ioctl</function></funcdef>
  25         <paramdef>int <parameter>fd</parameter></paramdef>
  26         <paramdef>int <parameter>request</parameter></paramdef>
  27         <paramdef>struct v4l2_querymenu *<parameter>argp</parameter></paramdef>
  28       </funcprototype>
  29     </funcsynopsis>
  30   </refsynopsisdiv>
  31
  32   <refsect1>
  33     <title>Arguments</title>
  34
  35     <variablelist>
  36       <varlistentry>
  37         <term><parameter>fd</parameter></term>
  38         <listitem>
  39           <para>&fd;</para>
  40         </listitem>
  41       </varlistentry>
  42       <varlistentry>
  43         <term><parameter>request</parameter></term>
  44         <listitem>
  45           <para>VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU</para>
  46         </listitem>
  47       </varlistentry>
  48       <varlistentry>
  49         <term><parameter>argp</parameter></term>
  50         <listitem>
  51           <para></para>
  52         </listitem>
  53       </varlistentry>
  54     </variablelist>
  55   </refsect1>
  56
  57   <refsect1>
  58     <title>Description</title>
  59
  60     <para>To query the attributes of a control applications set the
  61 <structfield>id</structfield> field of a &v4l2-queryctrl; and call the
  62 <constant>VIDIOC_QUERYCTRL</constant> ioctl with a pointer to this
  63 structure. The driver fills the rest of the structure or returns an
  64 &EINVAL; when the <structfield>id</structfield> is invalid.</para>
  65
  66     <para>It is possible to enumerate controls by calling
  67 <constant>VIDIOC_QUERYCTRL</constant> with successive
  68 <structfield>id</structfield> values starting from
  69 <constant>V4L2_CID_BASE</constant> up to and exclusive
  70 <constant>V4L2_CID_BASE_LASTP1</constant>. Drivers may return
  71 <errorcode>EINVAL</errorcode> if a control in this range is not
  72 supported. Further applications can enumerate private controls, which
  73 are not defined in this specification, by starting at
  74 <constant>V4L2_CID_PRIVATE_BASE</constant> and incrementing
  75 <structfield>id</structfield> until the driver returns
  76 <errorcode>EINVAL</errorcode>.</para>
  77
  78     <para>In both cases, when the driver sets the
  79 <constant>V4L2_CTRL_FLAG_DISABLED</constant> flag in the
  80 <structfield>flags</structfield> field this control is permanently
  81 disabled and should be ignored by the application.<footnote>
  82         <para><constant>V4L2_CTRL_FLAG_DISABLED</constant> was
  83 intended for two purposes: Drivers can skip predefined controls not
  84 supported by the hardware (although returning EINVAL would do as
  85 well), or disable predefined and private controls after hardware
  86 detection without the trouble of reordering control arrays and indices
  87 (EINVAL cannot be used to skip private controls because it would
  88 prematurely end the enumeration).</para></footnote></para>
  89
  90     <para>When the application ORs <structfield>id</structfield> with
  91 <constant>V4L2_CTRL_FLAG_NEXT_CTRL</constant> the driver returns the
  92 next supported control, or <errorcode>EINVAL</errorcode> if there is
  93 none. Drivers which do not support this flag yet always return
  94 <errorcode>EINVAL</errorcode>.</para>
  95
  96     <para>Additional information is required for menu controls: the
  97 names of the menu items. To query them applications set the
  98 <structfield>id</structfield> and <structfield>index</structfield>
  99 fields of &v4l2-querymenu; and call the
 100 <constant>VIDIOC_QUERYMENU</constant> ioctl with a pointer to this
 101 structure. The driver fills the rest of the structure or returns an
 102 &EINVAL; when the <structfield>id</structfield> or
 103 <structfield>index</structfield> is invalid. Menu items are enumerated
 104 by calling <constant>VIDIOC_QUERYMENU</constant> with successive
 105 <structfield>index</structfield> values from &v4l2-queryctrl;
 106 <structfield>minimum</structfield> to
 107 <structfield>maximum</structfield>, inclusive. Note that it is possible
 108 for <constant>VIDIOC_QUERYMENU</constant> to return an &EINVAL; for some
 109 indices between <structfield>minimum</structfield> and <structfield>maximum</structfield>.
 110 In that case that particular menu item is not supported by this driver. Also note that
 111 the <structfield>minimum</structfield> value is not necessarily 0.</para>
 112
 113     <para>See also the examples in <xref linkend="control" />.</para>
 114
 115     <table pgwide="1" frame="none" id="v4l2-queryctrl">
 116       <title>struct <structname>v4l2_queryctrl</structname></title>
 117       <tgroup cols="3">
 118         &cs-str;
 119         <tbody valign="top">
 120           <row>
 121             <entry>__u32</entry>
 122             <entry><structfield>id</structfield></entry>
 123             <entry>Identifies the control, set by the application. See
 124 <xref linkend="control-id" /> for predefined IDs. When the ID is ORed
 125 with V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returns
 126 the first control with a higher ID. Drivers which do not support this
 127 flag yet always return an &EINVAL;.</entry>
 128           </row>
 129           <row>
 130             <entry>&v4l2-ctrl-type;</entry>
 131             <entry><structfield>type</structfield></entry>
 132             <entry>Type of control, see <xref
 133                 linkend="v4l2-ctrl-type" />.</entry>
 134           </row>
 135           <row>
 136             <entry>__u8</entry>
 137             <entry><structfield>name</structfield>[32]</entry>
 138             <entry>Name of the control, a NUL-terminated ASCII
 139 string. This information is intended for the user.</entry>
 140           </row>
 141           <row>
 142             <entry>__s32</entry>
 143             <entry><structfield>minimum</structfield></entry>
 144             <entry>Minimum value, inclusive. This field gives a lower
 145 bound for <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls and the
 146 lowest valid index for <constant>V4L2_CTRL_TYPE_MENU</constant> controls.
 147 For <constant>V4L2_CTRL_TYPE_STRING</constant> controls the minimum value
 148 gives the minimum length of the string. This length <emphasis>does not include the terminating
 149 zero</emphasis>. It may not be valid for any other type of control, including
 150 <constant>V4L2_CTRL_TYPE_INTEGER64</constant> controls. Note that this is a
 151 signed value.</entry>
 152           </row>
 153           <row>
 154             <entry>__s32</entry>
 155             <entry><structfield>maximum</structfield></entry>
 156             <entry>Maximum value, inclusive. This field gives an upper
 157 bound for <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls and the
 158 highest valid index for <constant>V4L2_CTRL_TYPE_MENU</constant>
 159 controls.
 160 For <constant>V4L2_CTRL_TYPE_STRING</constant> controls the maximum value
 161 gives the maximum length of the string. This length <emphasis>does not include the terminating
 162 zero</emphasis>. It may not be valid for any other type of control, including
 163 <constant>V4L2_CTRL_TYPE_INTEGER64</constant> controls. Note that this is a
 164 signed value.</entry>
 165           </row>
 166           <row>
 167             <entry>__s32</entry>
 168             <entry><structfield>step</structfield></entry>
 169             <entry><para>This field gives a step size for
 170 <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls. For
 171 <constant>V4L2_CTRL_TYPE_STRING</constant> controls this field refers to
 172 the string length that has to be a multiple of this step size.
 173 It may not be valid for any other type of control, including
 174 <constant>V4L2_CTRL_TYPE_INTEGER64</constant>
 175 controls.</para><para>Generally drivers should not scale hardware
 176 control values. It may be necessary for example when the
 177 <structfield>name</structfield> or <structfield>id</structfield> imply
 178 a particular unit and the hardware actually accepts only multiples of
 179 said unit. If so, drivers must take care values are properly rounded
 180 when scaling, such that errors will not accumulate on repeated
 181 read-write cycles.</para><para>This field gives the smallest change of
 182 an integer control actually affecting hardware. Often the information
 183 is needed when the user can change controls by keyboard or GUI
 184 buttons, rather than a slider. When for example a hardware register
 185 accepts values 0-511 and the driver reports 0-65535, step should be
 186 128.</para><para>Note that although signed, the step value is supposed to
 187 be always positive.</para></entry>
 188           </row>
 189           <row>
 190             <entry>__s32</entry>
 191             <entry><structfield>default_value</structfield></entry>
 192             <entry>The default value of a
 193 <constant>V4L2_CTRL_TYPE_INTEGER</constant>,
 194 <constant>_BOOLEAN</constant> or <constant>_MENU</constant> control.
 195 Not valid for other types of controls. Drivers reset controls only
 196 when the driver is loaded, not later, in particular not when the
 197 func-open; is called.</entry>
 198           </row>
 199           <row>
 200             <entry>__u32</entry>
 201             <entry><structfield>flags</structfield></entry>
 202             <entry>Control flags, see <xref
 203                 linkend="control-flags" />.</entry>
 204           </row>
 205           <row>
 206             <entry>__u32</entry>
 207             <entry><structfield>reserved</structfield>[2]</entry>
 208             <entry>Reserved for future extensions. Drivers must set
 209 the array to zero.</entry>
 210           </row>
 211         </tbody>
 212       </tgroup>
 213     </table>
 214
 215     <table pgwide="1" frame="none" id="v4l2-querymenu">
 216       <title>struct <structname>v4l2_querymenu</structname></title>
 217       <tgroup cols="3">
 218         &cs-str;
 219         <tbody valign="top">
 220           <row>
 221             <entry>__u32</entry>
 222             <entry><structfield>id</structfield></entry>
 223             <entry>Identifies the control, set by the application
 224 from the respective &v4l2-queryctrl;
 225 <structfield>id</structfield>.</entry>
 226           </row>
 227           <row>
 228             <entry>__u32</entry>
 229             <entry><structfield>index</structfield></entry>
 230             <entry>Index of the menu item, starting at zero, set by
 231             the application.</entry>
 232           </row>
 233           <row>
 234             <entry>__u8</entry>
 235             <entry><structfield>name</structfield>[32]</entry>
 236             <entry>Name of the menu item, a NUL-terminated ASCII
 237 string. This information is intended for the user.</entry>
 238           </row>
 239           <row>
 240             <entry>__u32</entry>
 241             <entry><structfield>reserved</structfield></entry>
 242             <entry>Reserved for future extensions. Drivers must set
 243 the array to zero.</entry>
 244           </row>
 245         </tbody>
 246       </tgroup>
 247     </table>
 248
 249     <table pgwide="1" frame="none" id="v4l2-ctrl-type">
 250       <title>enum v4l2_ctrl_type</title>
 251       <tgroup cols="5" align="left">
 252         <colspec colwidth="30*" />
 253         <colspec colwidth="5*" align="center" />
 254         <colspec colwidth="5*" align="center" />
 255         <colspec colwidth="5*" align="center" />
 256         <colspec colwidth="55*" />
 257         <thead>
 258           <row>
 259             <entry>Type</entry>
 260             <entry><structfield>minimum</structfield></entry>
 261             <entry><structfield>step</structfield></entry>
 262             <entry><structfield>maximum</structfield></entry>
 263             <entry>Description</entry>
 264           </row>
 265         </thead>
 266         <tbody valign="top">
 267           <row>
 268             <entry><constant>V4L2_CTRL_TYPE_INTEGER</constant></entry>
 269             <entry>any</entry>
 270             <entry>any</entry>
 271             <entry>any</entry>
 272             <entry>An integer-valued control ranging from minimum to
 273 maximum inclusive. The step value indicates the increment between
 274 values which are actually different on the hardware.</entry>
 275           </row>
 276           <row>
 277             <entry><constant>V4L2_CTRL_TYPE_BOOLEAN</constant></entry>
 278             <entry>0</entry>
 279             <entry>1</entry>
 280             <entry>1</entry>
 281             <entry>A boolean-valued control. Zero corresponds to
 282 "disabled", and one means "enabled".</entry>
 283           </row>
 284           <row>
 285             <entry><constant>V4L2_CTRL_TYPE_MENU</constant></entry>
 286             <entry>&ge; 0</entry>
 287             <entry>1</entry>
 288             <entry>N-1</entry>
 289             <entry>The control has a menu of N choices. The names of
 290 the menu items can be enumerated with the
 291 <constant>VIDIOC_QUERYMENU</constant> ioctl.</entry>
 292           </row>
 293           <row>
 294             <entry><constant>V4L2_CTRL_TYPE_BUTTON</constant></entry>
 295             <entry>0</entry>
 296             <entry>0</entry>
 297             <entry>0</entry>
 298             <entry>A control which performs an action when set.
 299 Drivers must ignore the value passed with
 300 <constant>VIDIOC_S_CTRL</constant> and return an &EINVAL; on a
 301 <constant>VIDIOC_G_CTRL</constant> attempt.</entry>
 302           </row>
 303           <row>
 304             <entry><constant>V4L2_CTRL_TYPE_INTEGER64</constant></entry>
 305             <entry>n/a</entry>
 306             <entry>n/a</entry>
 307             <entry>n/a</entry>
 308             <entry>A 64-bit integer valued control. Minimum, maximum
 309 and step size cannot be queried.</entry>
 310           </row>
 311           <row>
 312             <entry><constant>V4L2_CTRL_TYPE_STRING</constant></entry>
 313             <entry>&ge; 0</entry>
 314             <entry>&ge; 1</entry>
 315             <entry>&ge; 0</entry>
 316             <entry>The minimum and maximum string lengths. The step size
 317 means that the string must be (minimum + N * step) characters long for
 318 N &ge; 0. These lengths do not include the terminating zero, so in order to
 319 pass a string of length 8 to &VIDIOC-S-EXT-CTRLS; you need to set the
 320 <structfield>size</structfield> field of &v4l2-ext-control; to 9. For &VIDIOC-G-EXT-CTRLS; you can
 321 set the <structfield>size</structfield> field to <structfield>maximum</structfield> + 1.
 322 Which character encoding is used will depend on the string control itself and
 323 should be part of the control documentation.</entry>
 324           </row>
 325           <row>
 326             <entry><constant>V4L2_CTRL_TYPE_CTRL_CLASS</constant></entry>
 327             <entry>n/a</entry>
 328             <entry>n/a</entry>
 329             <entry>n/a</entry>
 330             <entry>This is not a control. When
 331 <constant>VIDIOC_QUERYCTRL</constant> is called with a control ID
 332 equal to a control class code (see <xref linkend="ctrl-class" />) + 1, the
 333 ioctl returns the name of the control class and this control type.
 334 Older drivers which do not support this feature return an
 335 &EINVAL;.</entry>
 336           </row>
 337         </tbody>
 338       </tgroup>
 339     </table>
 340
 341     <table pgwide="1" frame="none" id="control-flags">
 342       <title>Control Flags</title>
 343       <tgroup cols="3">
 344         &cs-def;
 345         <tbody valign="top">
 346           <row>
 347             <entry><constant>V4L2_CTRL_FLAG_DISABLED</constant></entry>
 348             <entry>0x0001</entry>
 349             <entry>This control is permanently disabled and should be
 350 ignored by the application. Any attempt to change the control will
 351 result in an &EINVAL;.</entry>
 352           </row>
 353           <row>
 354             <entry><constant>V4L2_CTRL_FLAG_GRABBED</constant></entry>
 355             <entry>0x0002</entry>
 356             <entry>This control is temporarily unchangeable, for
 357 example because another application took over control of the
 358 respective resource. Such controls may be displayed specially in a
 359 user interface. Attempts to change the control may result in an
 360 &EBUSY;.</entry>
 361           </row>
 362           <row>
 363             <entry><constant>V4L2_CTRL_FLAG_READ_ONLY</constant></entry>
 364             <entry>0x0004</entry>
 365             <entry>This control is permanently readable only. Any
 366 attempt to change the control will result in an &EINVAL;.</entry>
 367           </row>
 368           <row>
 369             <entry><constant>V4L2_CTRL_FLAG_UPDATE</constant></entry>
 370             <entry>0x0008</entry>
 371             <entry>A hint that changing this control may affect the
 372 value of other controls within the same control class. Applications
 373 should update their user interface accordingly.</entry>
 374           </row>
 375           <row>
 376             <entry><constant>V4L2_CTRL_FLAG_INACTIVE</constant></entry>
 377             <entry>0x0010</entry>
 378             <entry>This control is not applicable to the current
 379 configuration and should be displayed accordingly in a user interface.
 380 For example the flag may be set on a MPEG audio level 2 bitrate
 381 control when MPEG audio encoding level 1 was selected with another
 382 control.</entry>
 383           </row>
 384           <row>
 385             <entry><constant>V4L2_CTRL_FLAG_SLIDER</constant></entry>
 386             <entry>0x0020</entry>
 387             <entry>A hint that this control is best represented as a
 388 slider-like element in a user interface.</entry>
 389           </row>
 390           <row>
 391             <entry><constant>V4L2_CTRL_FLAG_WRITE_ONLY</constant></entry>
 392             <entry>0x0040</entry>
 393             <entry>This control is permanently writable only. Any
 394 attempt to read the control will result in an &EACCES; error code. This
 395 flag is typically present for relative controls or action controls where
 396 writing a value will cause the device to carry out a given action
 397 (&eg; motor control) but no meaningful value can be returned.</entry>
 398           </row>
 399         </tbody>
 400       </tgroup>
 401     </table>
 402   </refsect1>
 403
 404   <refsect1>
 405     &return-value;
 406
 407     <variablelist>
 408       <varlistentry>
 409         <term><errorcode>EINVAL</errorcode></term>
 410         <listitem>
 411           <para>The &v4l2-queryctrl; <structfield>id</structfield>
 412 is invalid. The &v4l2-querymenu; <structfield>id</structfield> is
 413 invalid or <structfield>index</structfield> is out of range (less than
 414 <structfield>minimum</structfield> or greater than <structfield>maximum</structfield>)
 415 or this particular menu item is not supported by the driver.</para>
 416         </listitem>
 417       </varlistentry>
 418       <varlistentry>
 419         <term><errorcode>EACCES</errorcode></term>
 420         <listitem>
 421           <para>An attempt was made to read a write-only control.</para>
 422         </listitem>
 423       </varlistentry>
 424     </variablelist>
 425   </refsect1>
 426 </refentry>
 427
 428 <!--
 429 Local Variables:
 430 mode: sgml
 431 sgml-parent-document: "v4l2.sgml"
 432 indent-tabs-mode: nil
 433 End:
 434 -->