path: root/Documentation/DocBook/v4l/vidioc-queryctrl.xml

<refentry id="vidioc-queryctrl">
  <refmeta>
    <refentrytitle>ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>VIDIOC_QUERYCTRL</refname>
    <refname>VIDIOC_QUERYMENU</refname>
    <refpurpose>Enumerate controls and menu control items</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcprototype>
	<funcdef>int <function>ioctl</function></funcdef>
	<paramdef>int <parameter>fd</parameter></paramdef>
	<paramdef>int <parameter>request</parameter></paramdef>
	<paramdef>struct v4l2_queryctrl *<parameter>argp</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
    <funcsynopsis>
      <funcprototype>
	<funcdef>int <function>ioctl</function></funcdef>
	<paramdef>int <parameter>fd</parameter></paramdef>
	<paramdef>int <parameter>request</parameter></paramdef>
	<paramdef>struct v4l2_querymenu *<parameter>argp</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Arguments</title>

    <variablelist>
      <varlistentry>
	<term><parameter>fd</parameter></term>
	<listitem>
	  <para>&fd;</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>request</parameter></term>
	<listitem>
	  <para>VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>argp</parameter></term>
	<listitem>
	  <para></para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

    <para>To query the attributes of a control applications set the
<structfield>id</structfield> field of a &v4l2-queryctrl; and call the
<constant>VIDIOC_QUERYCTRL</constant> ioctl with a pointer to this
structure. The driver fills the rest of the structure or returns an
&EINVAL; when the <structfield>id</structfield> is invalid.</para>

    <para>It is possible to enumerate controls by calling
<constant>VIDIOC_QUERYCTRL</constant> with successive
<structfield>id</structfield> values starting from
<constant>V4L2_CID_BASE</constant> up to and exclusive
<constant>V4L2_CID_BASE_LASTP1</constant>. Drivers may return
<errorcode>EINVAL</errorcode> if a control in this range is not
supported. Further applications can enumerate private controls, which
are not defined in this specification, by starting at
<constant>V4L2_CID_PRIVATE_BASE</constant> and incrementing
<structfield>id</structfield> until the driver returns
<errorcode>EINVAL</errorcode>.</para>

    <para>In both cases, when the driver sets the
<constant>V4L2_CTRL_FLAG_DISABLED</constant> flag in the
<structfield>flags</structfield> field this control is permanently
disabled and should be ignored by the application.<footnote>
	<para><constant>V4L2_CTRL_FLAG_DISABLED</constant> was
intended for two purposes: Drivers can skip predefined controls not
supported by the hardware (although returning EINVAL would do as
well), or disable predefined and private controls after hardware
detection without the trouble of reordering control arrays and indices
(EINVAL cannot be used to skip private controls because it would
prematurely end the enumeration).</para></footnote></para>

    <para>When the application ORs <structfield>id</structfield> with
<constant>V4L2_CTRL_FLAG_NEXT_CTRL</constant> the driver returns the
next supported control, or <errorcode>EINVAL</errorcode> if there is
none. Drivers which do not support this flag yet always return
<errorcode>EINVAL</errorcode>.</para>

    <para>Additional information is required for menu controls: the
names of the menu items. To query them applications set the
<structfield>id</structfield> and <structfield>index</structfield>
fields of &v4l2-querymenu; and call the
<constant>VIDIOC_QUERYMENU</constant> ioctl with a pointer to this
structure. The driver fills the rest of the structure or returns an
&EINVAL; when the <structfield>id</structfield> or
<structfield>index</structfield> is invalid. Menu items are enumerated
by calling <constant>VIDIOC_QUERYMENU</constant> with successive
<structfield>index</structfield> values from &v4l2-queryctrl;
<structfield>minimum</structfield> (0) to
<structfield>maximum</structfield>, inclusive.</para>

    <para>See also the examples in <xref linkend="control" />.</para>

    <table pgwide="1" frame="none" id="v4l2-queryctrl">
      <title>struct <structname>v4l2_queryctrl</structname></title>
      <tgroup cols="3">
	&cs-str;
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>id</structfield></entry>
	    <entry>Identifies the control, set by the application. See
<xref linkend="control-id" /> for predefined IDs. When the ID is ORed
with V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returns
the first control with a higher ID. Drivers which do not support this
flag yet always return an &EINVAL;.</entry>
	  </row>
	  <row>
	    <entry>&v4l2-ctrl-type;</entry>
	    <entry><structfield>type</structfield></entry>
	    <entry>Type of control, see <xref
		linkend="v4l2-ctrl-type" />.</entry>
	  </row>
	  <row>
	    <entry>__u8</entry>
	    <entry><structfield>name</structfield>[32]</entry>
	    <entry>Name of the control, a NUL-terminated ASCII
string. This information is intended for the user.</entry>
	  </row>
	  <row>
	    <entry>__s32</entry>
	    <entry><structfield>minimum</structfield></entry>
	    <entry>Minimum value, inclusive. This field gives a lower
bound for <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls and the
lowest valid index (always 0) for <constant>V4L2_CTRL_TYPE_MENU</constant> controls.
For <constant>V4L2_CTRL_TYPE_STRING</constant> controls the minimum value
gives the minimum length of the string. This length <emphasis>does not include the terminating
zero</emphasis>. It may not be valid for any other type of control, including
<constant>V4L2_CTRL_TYPE_INTEGER64</constant> controls. Note that this is a
signed value.</entry>
	  </row>
	  <row>
	    <entry>__s32</entry>
	    <entry><structfield>maximum</structfield></entry>
	    <entry>Maximum value, inclusive. This field gives an upper
bound for <constant>V4L2_CTRL_TYPE_INTEGER</constant> controls and the
highest valid index for <constant>V4L2_CTRL_TYPE_MENU</constant>
controls.
For <constant>V4L2_CTRL_TYPE_STRING</constant> controls the maximum value
gives the maximum length of the string. This length <emphasis>does not include the terminating
zero</emphasis>. It may not be valid for any other type of control, including
<constant>V4L2_CTRL_TYPE_INTEGER64</constant> controls. Note that this is a
signed value.</entry>
	  </row>
	  <row>
	    <entry>__s32</entry>
	    <entry><structfield>step</structfield></entry>
	    <entry><para>This field gives a step size for
<constant>V4L2_CTRL_TYPE_INTEGER</constant> controls. For
<constant>V4L2_CTRL_TYPE_STRING</constant> controls this field refers to
the string length that has to be a multiple of this step size.
It may not be valid for any other type of control, including
<constant>V4L2_CTRL_TYPE_INTEGER64</constant>
controls.</para><para>Generally drivers should not scale hardware
control values. It may be necessary for example when the
<structfield>name</structfield> or <structfield>id</structfield> imply
a particular unit and the hardware actually accepts only multiples of
said unit. If so, drivers must take care values are properly rounded
when scaling, such that errors will not accumulate on repeated
read-write cycles.</para><para>This field gives the smallest change of
an integer control actually affecting hardware. Often the information
is needed when the user can change controls by keyboard or GUI
buttons, rather than a slider. When for example a hardware register
accepts values 0-511 and the driver reports 0-65535, step should be
128.</para><para>Note that although signed, the step value is supposed to
be always positive.</para></entry>
	  </row>
	  <row>
	    <entry>__s32</entry>
	    <entry><structfield>default_value</structfield></entry>
	    <entry>The default value of a
<constant>V4L2_CTRL_TYPE_INTEGER</constant>,
<constant>_BOOLEAN</constant> or <constant>_MENU</constant> control.
Not valid for other types of controls. Drivers reset controls only
when the driver is loaded, not later, in particular not when the
func-open; is called.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>flags</structfield></entry>
	    <entry>Control flags, see <xref
		linkend="control-flags" />.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>reserved</structfield>[2]</entry>
	    <entry>Reserved for future extensions. Drivers must set
the array to zero.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <table pgwide="1" frame="none" id="v4l2-querymenu">
      <title>struct <structname>v4l2_querymenu</structname></title>
      <tgroup cols="3">
	&cs-str;
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>id</structfield></entry>
	    <entry>Identifies the control, set by the application
from the respective &v4l2-queryctrl;
<structfield>id</structfield>.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>index</structfield></entry>
	    <entry>Index of the menu item, starting at zero, set by
	    the application.</entry>
	  </row>
	  <row>
	    <entry>__u8</entry>
	    <entry><structfield>name</structfield>[32]</entry>
	    <entry>Name of the menu item, a NUL-terminated ASCII
string. This information is intended for the user.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>reserved</structfield></entry>
	    <entry>Reserved for future extensions. Drivers must set
the array to zero.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <table pgwide="1" frame="none" id="v4l2-ctrl-type">
      <title>enum v4l2_ctrl_type</title>
      <tgroup cols="5" align="left">
	<colspec colwidth="30*" />
	<colspec colwidth="5*" align="center" />
	<colspec colwidth="5*" align="center" />
	<colspec colwidth="5*" align="center" />
	<colspec colwidth="55*" />
	<thead>
	  <row>