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> (0) to
 107 <structfield>maximum</structfield>, inclusive.</para>
 108
 109     <para>See also the examples in <xref linkend="control" />.</para>
 110
 111     <table pgwide="1" frame="none" id="v4l2-queryctrl">
 112       <title>struct <structname>v4l2_queryctrl</structname></title>
 113       <tgroup cols="3">
 114         &cs-str;
 115         <tbody valign="top">
 116           <row>
 117             <entry>__u32</entry>
 118             <entry><structfield>id</structfield></entry>
 119             <entry>Identifies the control, set by the application. See
 120 <xref linkend="control-id" /> for predefined IDs. When the ID is ORed
 121 with V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returns
 122 the first control with a higher ID. Drivers which do not support this
 123 flag yet always return an &EINVAL;.</entry>
 124           </row>
 125           <row>
 126             <entry>&v4l2-ctrl-type;</entry>
 127             <entry><structfield>type</structfield></entry>
 128             <entry>Type of control, see <xref
 129                 linkend="v4l2-ctrl-type" />.</entry>
 130           </row>
 131           <row>
 132             <entry>__u8</entry>
 133             <entry><structfield>name</structfield>[32]</entry>
 134             <entry>Name of the control, a NUL-terminated ASCII
 135 string. This information is intended for the user.</entry>
 136           </row>
 137           <row>
 138             <entry>__s32</entry>
 139             <entry><structfield>minimum</structfield></entry>
 140             <entry>Minimum value, inclusive. This field gives a lower
 141 bound for <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls and the
 142 lowest valid index (always 0) for <constant>V4L2_CTRL_TYPE_MENU</constant> controls.
 143 For <constant>V4L2_CTRL_TYPE_STRING</constant> controls the minimum value
 144 gives the minimum length of the string. This length <emphasis>does not include the terminating
 145 zero</emphasis>. It may not be valid for any other type of control, including
 146 <constant>V4L2_CTRL_TYPE_INTEGER64</constant> controls. Note that this is a
 147 signed value.</entry>
 148           </row>
 149           <row>
 150             <entry>__s32</entry>
 151             <entry><structfield>maximum</structfield></entry>
 152             <entry>Maximum value, inclusive. This field gives an upper
 153 bound for <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls and the
 154 highest valid index for <constant>V4L2_CTRL_TYPE_MENU</constant>
 155 controls.
 156 For <constant>V4L2_CTRL_TYPE_STRING</constant> controls the maximum value
 157 gives the maximum length of the string. This length <emphasis>does not include the terminating
 158 zero</emphasis>. It may not be valid for any other type of control, including
 159 <constant>V4L2_CTRL_TYPE_INTEGER64</constant> controls. Note that this is a
 160 signed value.</entry>
 161           </row>
 162           <row>
 163             <entry>__s32</entry>
 164             <entry><structfield>step</structfield></entry>
 165             <entry><para>This field gives a step size for
 166 <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls. For
 167 <constant>V4L2_CTRL_TYPE_STRING</constant> controls this field refers to
 168 the string length that has to be a multiple of this step size.
 169 It may not be valid for any other type of control, including
 170 <constant>V4L2_CTRL_TYPE_INTEGER64</constant>
 171 controls.</para><para>Generally drivers should not scale hardware
 172 control values. It may be necessary for example when the
 173 <structfield>name</structfield> or <structfield>id</structfield> imply
 174 a particular unit and the hardware actually accepts only multiples of
 175 said unit. If so, drivers must take care values are properly rounded
 176 when scaling, such that errors will not accumulate on repeated
 177 read-write cycles.</para><para>This field gives the smallest change of
 178 an integer control actually affecting hardware. Often the information
 179 is needed when the user can change controls by keyboard or GUI
 180 buttons, rather than a slider. When for example a hardware register
 181 accepts values 0-511 and the driver reports 0-65535, step should be
 182 128.</para><para>Note that although signed, the step value is supposed to
 183 be always positive.</para></entry>
 184           </row>
 185           <row>
 186             <entry>__s32</entry>
 187             <entry><structfield>default_value</structfield></entry>
 188             <entry>The default value of a
 189 <constant>V4L2_CTRL_TYPE_INTEGER</constant>,
 190 <constant>_BOOLEAN</constant> or <constant>_MENU</constant> control.
 191 Not valid for other types of controls. Drivers reset controls only
 192 when the driver is loaded, not later, in particular not when the
 193 func-open; is called.</entry>
 194           </row>
 195           <row>
 196             <entry>__u32</entry>
 197             <entry><structfield>flags</structfield></entry>
 198             <entry>Control flags, see <xref
 199                 linkend="control-flags" />.</entry>
 200           </row>
 201           <row>
 202             <entry>__u32</entry>
 203             <entry><structfield>reserved</structfield>[2]</entry>
 204             <entry>Reserved for future extensions. Drivers must set
 205 the array to zero.</entry>
 206           </row>
 207         </tbody>
 208       </tgroup>
 209     </table>
 210
 211     <table pgwide="1" frame="none" id="v4l2-querymenu">
 212       <title>struct <structname>v4l2_querymenu</structname></title>
 213       <tgroup cols="3">
 214         &cs-str;
 215         <tbody valign="top">
 216           <row>
 217             <entry>__u32</entry>
 218             <entry><structfield>id</structfield></entry>
 219             <entry>Identifies the control, set by the application
 220 from the respective &v4l2-queryctrl;
 221 <structfield>id</structfield>.</entry>
 222           </row>
 223           <row>
 224             <entry>__u32</entry>
 225             <entry><structfield>index</structfield></entry>
 226             <entry>Index of the menu item, starting at zero, set by
 227             the application.</entry>
 228           </row>
 229           <row>
 230             <entry>__u8</entry>
 231             <entry><structfield>name</structfield>[32]</entry>
 232             <entry>Name of the menu item, a NUL-terminated ASCII
 233 string. This information is intended for the user.</entry>
 234           </row>
 235           <row>
 236             <entry>__u32</entry>
 237             <entry><structfield>reserved</structfield></entry>
 238             <entry>Reserved for future extensions. Drivers must set
 239 the array to zero.</entry>
 240           </row>
 241         </tbody>
 242       </tgroup>
 243     </table>
 244
 245     <table pgwide="1" frame="none" id="v4l2-ctrl-type">
 246       <title>enum v4l2_ctrl_type</title>
 247       <tgroup cols="5" align="left">
 248         <colspec colwidth="30*" />
 249         <colspec colwidth="5*" align="center" />
 250         <colspec colwidth="5*" align="center" />
 251         <colspec colwidth="5*" align="center" />
 252         <colspec colwidth="55*" />
 253         <thead>
 254           <row>
 255             <entry>Type</entry>
 256             <entry><structfield>minimum</structfield></entry>
 257             <entry><structfield>step</structfield></entry>
 258             <entry><structfield>maximum</structfield></entry>
 259             <entry>Description</entry>
 260           </row>
 261         </thead>
 262         <tbody valign="top">
 263           <row>
 264             <entry><constant>V4L2_CTRL_TYPE_INTEGER</constant></entry>
 265             <entry>any</entry>
 266             <entry>any</entry>
 267             <entry>any</entry>
 268             <entry>An integer-valued control ranging from minimum to
 269 maximum inclusive. The step value indicates the increment between
 270 values which are actually different on the hardware.</entry>
 271           </row>
 272           <row>
 273             <entry><constant>V4L2_CTRL_TYPE_BOOLEAN</constant></entry>
 274             <entry>0</entry>
 275             <entry>1</entry>
 276             <entry>1</entry>
 277             <entry>A boolean-valued control. Zero corresponds to
 278 "disabled", and one means "enabled".</entry>
 279           </row>
 280           <row>
 281             <entry><constant>V4L2_CTRL_TYPE_MENU</constant></entry>
 282             <entry>0</entry>
 283             <entry>1</entry>
 284             <entry>N-1</entry>
 285             <entry>The control has a menu of N choices. The names of
 286 the menu items can be enumerated with the
 287 <constant>VIDIOC_QUERYMENU</constant> ioctl.</entry>
 288           </row>
 289           <row>
 290             <entry><constant>V4L2_CTRL_TYPE_BUTTON</constant></entry>
 291             <entry>0</entry>
 292             <entry>0</entry>
 293             <entry>0</entry>
 294             <entry>A control which performs an action when set.
 295 Drivers must ignore the value passed with
 296 <constant>VIDIOC_S_CTRL</constant> and return an &EINVAL; on a
 297 <constant>VIDIOC_G_CTRL</constant> attempt.</entry>
 298           </row>
 299           <row>
 300             <entry><constant>V4L2_CTRL_TYPE_INTEGER64</constant></entry>
 301             <entry>n/a</entry>
 302             <entry>n/a</entry>
 303             <entry>n/a</entry>
 304             <entry>A 64-bit integer valued control. Minimum, maximum
 305 and step size cannot be queried.</entry>
 306           </row>
 307           <row>
 308             <entry><constant>V4L2_CTRL_TYPE_STRING</constant></entry>
 309             <entry>&ge; 0</entry>
 310             <entry>&ge; 1</entry>
 311             <entry>&ge; 0</entry>
 312             <entry>The minimum and maximum string lengths. The step size
 313 means that the string must be (minimum + N * step) characters long for
 314 N &ge; 0. These lengths do not include the terminating zero, so in order to
 315 pass a string of length 8 to &VIDIOC-S-EXT-CTRLS; you need to set the
 316 <structfield>size</structfield> field of &v4l2-ext-control; to 9. For &VIDIOC-G-EXT-CTRLS; you can
 317 set the <structfield>size</structfield> field to <structfield>maximum</structfield> + 1.
 318 Which character encoding is used will depend on the string control itself and
 319 should be part of the control documentation.</entry>
 320           </row>
 321           <row>
 322             <entry><constant>V4L2_CTRL_TYPE_CTRL_CLASS</constant></entry>
 323             <entry>n/a</entry>
 324             <entry>n/a</entry>
 325             <entry>n/a</entry>
 326             <entry>This is not a control. When
 327 <constant>VIDIOC_QUERYCTRL</constant> is called with a control ID
 328 equal to a control class code (see <xref linkend="ctrl-class" />), the
 329 ioctl returns the name of the control class and this control type.
 330 Older drivers which do not support this feature return an
 331 &EINVAL;.</entry>
 332           </row>
 333         </tbody>
 334       </tgroup>
 335     </table>
 336
 337     <table pgwide="1" frame="none" id="control-flags">
 338       <title>Control Flags</title>
 339       <tgroup cols="3">
 340         &cs-def;
 341         <tbody valign="top">
 342           <row>
 343             <entry><constant>V4L2_CTRL_FLAG_DISABLED</constant></entry>
 344             <entry>0x0001</entry>
 345             <entry>This control is permanently disabled and should be
 346 ignored by the application. Any attempt to change the control will
 347 result in an &EINVAL;.</entry>
 348           </row>
 349           <row>
 350             <entry><constant>V4L2_CTRL_FLAG_GRABBED</constant></entry>
 351             <entry>0x0002</entry>
 352             <entry>This control is temporarily unchangeable, for
 353 example because another application took over control of the
 354 respective resource. Such controls may be displayed specially in a
 355 user interface. Attempts to change the control may result in an
 356 &EBUSY;.</entry>
 357           </row>
 358           <row>
 359             <entry><constant>V4L2_CTRL_FLAG_READ_ONLY</constant></entry>
 360             <entry>0x0004</entry>
 361             <entry>This control is permanently readable only. Any
 362 attempt to change the control will result in an &EINVAL;.</entry>
 363           </row>
 364           <row>
 365             <entry><constant>V4L2_CTRL_FLAG_UPDATE</constant></entry>
 366             <entry>0x0008</entry>
 367             <entry>A hint that changing this control may affect the
 368 value of other controls within the same control class. Applications
 369 should update their user interface accordingly.</entry>
 370           </row>
 371           <row>
 372             <entry><constant>V4L2_CTRL_FLAG_INACTIVE</constant></entry>
 373             <entry>0x0010</entry>
 374             <entry>This control is not applicable to the current
 375 configuration and should be displayed accordingly in a user interface.
 376 For example the flag may be set on a MPEG audio level 2 bitrate
 377 control when MPEG audio encoding level 1 was selected with another
 378 control.</entry>
 379           </row>
 380           <row>
 381             <entry><constant>V4L2_CTRL_FLAG_SLIDER</constant></entry>
 382             <entry>0x0020</entry>
 383             <entry>A hint that this control is best represented as a
 384 slider-like element in a user interface.</entry>
 385           </row>
 386           <row>
 387             <entry><constant>V4L2_CTRL_FLAG_WRITE_ONLY</constant></entry>
 388             <entry>0x0040</entry>
 389             <entry>This control is permanently writable only. Any
 390 attempt to read the control will result in an &EACCES; error code. This
 391 flag is typically present for relative controls or action controls where
 392 writing a value will cause the device to carry out a given action
 393 (&eg; motor control) but no meaningful value can be returned.</entry>
 394           </row>
 395         </tbody>
 396       </tgroup>
 397     </table>
 398   </refsect1>
 399
 400   <refsect1>
 401     &return-value;
 402
 403     <variablelist>
 404       <varlistentry>
 405         <term><errorcode>EINVAL</errorcode></term>
 406         <listitem>
 407           <para>The &v4l2-queryctrl; <structfield>id</structfield>
 408 is invalid. The &v4l2-querymenu; <structfield>id</structfield> or
 409 <structfield>index</structfield> is invalid.</para>
 410         </listitem>
 411       </varlistentry>
 412       <varlistentry>
 413         <term><errorcode>EACCES</errorcode></term>
 414         <listitem>
 415           <para>An attempt was made to read a write-only control.</para>
 416         </listitem>
 417       </varlistentry>
 418     </variablelist>
 419   </refsect1>
 420 </refentry>
 421
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