glib/docs/reference/glib/gvariant-varargs.xml

<?xml version='1.0' encoding='utf-8'?>

<refentry id='gvariant-format-strings'>
 <refmeta>
  <refentrytitle>GVariant Format Strings</refentrytitle>
 </refmeta>
 <refnamediv>
  <refname>GVariant Format Strings</refname>
  <refpurpose>varargs conversion of GVariants</refpurpose>
 </refnamediv>

 <refsect1>
  <title>Variable Argument Conversions</title>

  <para>
   This page attempts to document how to perform variable argument
   conversions with GVariant.
  </para>
  <para>
   Conversions occur according to format strings.  A format string is a two-way mapping between a single
   <link linkend='GVariant'>GVariant</link> value and one or more C values.
  </para>
  <para>
   A conversion from C values into a <link linkend='GVariant'>GVariant</link> value is made using the
   <link linkend='g-variant-new'><function>g_variant_new()</function></link> function.  A conversion from a
   <link linkend='GVariant'>GVariant</link> into C values is made using the
   <link linkend='g-variant-get'><function>g_variant_get()</function></link> function.
  </para>
 </refsect1>

 <refsect1>
  <title>Syntax</title>

  <para>
   This section exhaustively describes all possibilities for GVariant format strings.  There are no valid forms of
   format strings other than those described here.  Please note that the format string syntax is likely to expand in the
   future.
  </para>
  <para>
   Valid format strings have one of the following forms:
  </para>
  <itemizedlist>
   <listitem>
    <para>any type string</para>
   </listitem>
   <listitem>
    <para>
     a type string prefixed with a '<literal>@</literal>'
    </para>
   </listitem>
   <listitem>
    <para>
     '<literal>&amp;s</literal>' '<literal>&amp;o</literal>', '<literal>&amp;g</literal>', '<literal>^as</literal>',
     '<literal>^a&amp;s</literal>', '<literal>^ao</literal>', '<literal>^a&amp;o</literal>','<literal>^ay</literal>',
     '<literal>^&amp;ay</literal>', '<literal>^aay</literal>' or '<literal>^a&amp;ay</literal>'.
    </para>
   </listitem>
   <listitem>
    <para>
     any format string, prefixed with an '<literal>m</literal>'
    </para>
   </listitem>
   <listitem>
    <para>
     a sequence of zero or more format strings, concatenated and enclosed in parentheses
    </para>
   </listitem>
   <listitem>
    <para>
     an opening brace, followed by two format strings, followed by a closing brace (subject to the constraint that the
     first format string correspond to a type valid for use as the key type of a dictionary)
    </para>
   </listitem>
  </itemizedlist>
 </refsect1>
 <refsect1>
  <title>Symbols</title>

   <para>
    The following table describes the rough meaning of symbols that may appear inside a GVariant format string.  Each
    symbol is described in detail in its own section, including usage examples.
  </para>

  <informaltable>
   <tgroup cols='2'>
    <colspec colname='col_0'/>
    <colspec colname='col_1'/>
    <tbody>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'>Symbol</emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
         <emphasis role='strong'>Meaning</emphasis>
       </para>
      </entry>
     </row>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'>
         <literal>b</literal>, <literal>y</literal>, <literal>n</literal>, <literal>q</literal>, <literal>i</literal>,
         <literal>u</literal>, <literal>x</literal>, <literal>t</literal>, <literal>h</literal>, <literal>d</literal>
        </emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
        Used for building or deconstructing boolean, byte and numeric types.  See
        <link linkend='gvariant-format-strings-numeric-types'>Numeric Types</link> below.
       </para>
      </entry>
     </row>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'>
         <literal>s</literal>, <literal>o</literal>, <literal>g</literal>
        </emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
        Used for building or deconstructing string types.  See
        <link linkend='gvariant-format-strings-strings'>Strings</link> below.
       </para>
      </entry>
     </row>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'><literal>v</literal></emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
        Used for building or deconstructing variant types.  See
        <link linkend='gvariant-format-strings-variants'>Variants</link> below.
       </para>
      </entry>
     </row>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'>
         <literal>a</literal>
        </emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
        Used for building or deconstructing arrays.  See
        <link linkend='gvariant-format-strings-arrays'>Arrays</link> below.
       </para>
      </entry>
     </row>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'>
         <literal>m</literal>
        </emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
         Used for building or deconstructing maybe types.  See
         <link linkend='gvariant-format-strings-maybe-types'>Maybe Types</link> below.
       </para>
      </entry>
     </row>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'>
         <literal>()</literal>
        </emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
         Used for building or deconstructing tuples.  See
         <link linkend='gvariant-format-strings-tuples'>Tuples</link> below.
       </para>
      </entry>
     </row>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'>
         <literal>{}</literal>
        </emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
        Used for building or deconstructing dictionary entries. See
        <link linkend='gvariant-format-strings-dictionaries'>Dictionaries</link> below.
       </para>
      </entry>
     </row>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'>
         <literal>@</literal>
        </emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
        Used as a prefix for a GVariant type string (not a prefix for a format string, so <literal>@as</literal> is
        a valid format string but <literal>@^as</literal> is not).  Denotes that a pointer to a
        <link linkend='GVariant'>GVariant</link> should be used in place of the normal C type or types.  For
        <link linkend='g-variant-new'><function>g_variant_new()</function></link> this means that you must pass a
        non-<link linkend='NULL:CAPS'><literal>NULL</literal></link> <code>(<link linkend='GVariant'>GVariant</link>
        *)</code>; if it is a floating reference, ownership will be taken, as
        if by using <link linkend="g-variant-ref-sink"><function>g_variant_ref_sink()</function></link>.
        For <link linkend='g-variant-get'><function>g_variant_get()</function></link> this means that you
        must pass a pointer to a <code>(<link linkend='GVariant'>GVariant</link> *)</code> for the value to be returned
        by reference or <link linkend='NULL:CAPS'><literal>NULL</literal></link> to ignore the value.  See
        <link linkend='gvariant-format-strings-gvariant'><code>GVariant *</code></link> below.
       </para>
      </entry>
     </row>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'>
         <literal>*</literal>, <literal>?</literal>, <literal>r</literal>
        </emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
        Exactly equivalent to <literal>@*</literal>, <literal>@?</literal> and <literal>@r</literal>.  Provided only for
        completeness so that all GVariant type strings can be used also as format strings.  See <link
        linkend='gvariant-format-strings-gvariant'><code>GVariant *</code></link> below.
       </para>
      </entry>
     </row>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'><literal>&amp;</literal></emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
        Used as a prefix for a GVariant type string (not a prefix for a format string, so <literal>&amp;s</literal> is
        a valid format string but <literal>&amp;@s</literal> is not).
        Denotes that a C pointer to serialised data
        should be used in place of the normal C type.  See
        <link linkend='gvariant-format-strings-pointers'>Pointers</link> below.
       </para>
      </entry>
     </row>

     <row rowsep='1'>
      <entry colsep='1' rowsep='1'>
       <para>
        <emphasis role='strong'><literal>^</literal></emphasis>
       </para>
      </entry>
      <entry colsep='1' rowsep='1'>
       <para>
        Used as a prefix on some specific types of format strings.  See
        <link linkend='gvariant-format-strings-convenience'>Convenience Conversions</link> below.
       </para>
      </entry>
     </row>
    </tbody>
   </tgroup>
  </informaltable>


  <refsect2 id='gvariant-format-strings-numeric-types'>
   <title>Numeric Types</title>
   <para>
    <emphasis role='strong'>
     Characters: <literal>b</literal>, <literal>y</literal>, <literal>n</literal>, <literal>q</literal>,
     <literal>i</literal>, <literal>u</literal>, <literal>x</literal>, <literal>t</literal>, <literal>h</literal>,
     <literal>d</literal>
    </emphasis>
   </para>

   <para>
    Variable argument conversions from numeric types work in the most obvious way possible.  Upon encountering one of
    these characters, <link linkend='g-variant-new'><function>g_variant_new()</function></link> takes the equivalent C
    type as an argument.  <link linkend='g-variant-get'><function>g_variant_get()</function></link> takes a pointer to
    the equivalent C type (or <link linkend='NULL:CAPS'><literal>NULL</literal></link> to ignore the value).
   </para>

   <para>
    The equivalent C types are as follows:
   </para>

   <informaltable>
    <tgroup cols='2'>
     <colspec colname='col_0'/><colspec colname='col_1'/>
     <tbody>
      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
          <emphasis role='strong'>Character</emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
          <emphasis role='strong'>Equivalent C type</emphasis>
        </para>
       </entry>
      </row>
      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>b</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         <link linkend='gboolean'><type>gboolean</type></link>
        </para>
       </entry>
      </row>
      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>y</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         <link linkend='guchar'><type>guchar</type></link>
        </para>
       </entry>
      </row>
      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>n</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         <link linkend='gint16'><type>gint16</type></link>
        </para>
       </entry>
      </row>
      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>q</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         <link linkend='guint16'><type>guint16</type></link>
        </para>
       </entry>
      </row>
      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>i</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         <link linkend='gint32'><type>gint32</type></link>
        </para>
       </entry>
      </row>
      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>u</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         <link linkend='guint32'><type>guint32</type></link>
        </para>
       </entry>
      </row>
      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>x</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         <link linkend='gint64'><type>gint64</type></link>
        </para>
       </entry>
      </row>
      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>t</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         <link linkend='guint64'><type>guint64</type></link>
        </para>
       </entry>
      </row>
      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>h</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         <link linkend='gint32'><type>gint32</type></link>
        </para>
       </entry>
      </row>
      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>d</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         <link linkend='gdouble'><type>gdouble</type></link>
        </para>
       </entry>
      </row>
     </tbody>
    </tgroup>
   </informaltable>

   <anchor id='gvariant-varargs'/>
   <para>
    Note that in C, small integer types in variable argument lists are promoted up to <link
    linkend='gint'><type>int</type></link> or <link linkend='guint'><type>unsigned int</type></link> as appropriate, and
    read back accordingly.  <link linkend='gint'><type>int</type></link> is 32 bits on every platform on which GLib is
    currently supported.  This means that you can use C expressions of type <link linkend='gint'><type>int</type></link>
    with <link linkend='g-variant-new'><function>g_variant_new()</function></link> and format characters
    '<literal>b</literal>', '<literal>y</literal>', '<literal>n</literal>', '<literal>q</literal>',
    '<literal>i</literal>', '<literal>u</literal>' and '<literal>h</literal>'.  Specifically, you can use integer
    literals with these characters.
   </para>

   <para>
    When using the '<literal>x</literal>' and '<literal>t</literal>' characters, you must ensure that the value that you
    provide is 64 bit.  This means that you should use a cast or make use of the
    <link linkend='G-GINT64-CONSTANT:CAPS'><literal>G_GINT64_CONSTANT</literal></link> or
    <link linkend='G-GUINT64-CONSTANT:CAPS'><literal>G_GUINT64_CONSTANT</literal></link> macros.
   </para>

   <para>
    No type promotion occurs when using <link linkend='g-variant-get'><function>g_variant_get()</function></link> since
    it operates with pointers.  The pointers must always point to a memory region of exactly the correct size.
   </para>

   <refsect3>
    <title>Examples</title>
    <informalexample><programlisting>
<![CDATA[GVariant *value1, *value2, *value3, *value4;

value1 = g_variant_new ("y", 200);
value2 = g_variant_new ("b", TRUE);
value3 = g_variant_new ("d", 37.5):
value4 = g_variant_new ("x", G_GINT64_CONSTANT (998877665544332211));

{
  gdouble floating;
  gboolean truth;
  gint64 bignum;


  g_variant_get (value1, "y", NULL);      /* ignore the value. */
  g_variant_get (value2, "b", &truth);
  g_variant_get (value3, "d", &floating);
  g_variant_get (value4, "x", &bignum);
}]]></programlisting></informalexample>
   </refsect3>
  </refsect2>

  <refsect2 id='gvariant-format-strings-strings'>
   <title>Strings</title>
   <para>
    <emphasis role='strong'>
     Characters: <literal>s</literal>, <literal>o</literal>, <literal>g</literal>
    </emphasis>
   </para>

   <para>
    String conversions occur to and from standard nul-terminated C strings.  Upon encountering an
    '<literal>s</literal>', '<literal>o</literal>' or '<literal>g</literal>' in a format string,
    <link linkend='g-variant-new'><function>g_variant_new()</function></link> takes a <code>(const
    <link linkend='gchar'>gchar</link> *)</code> and makes a copy of it.
    <link linkend='NULL:CAPS'><literal>NULL</literal></link> is not a valid string; use
    <link linkend='gvariant-format-strings-maybe-types'>maybe types</link> to encode that.  If the '<literal>o</literal>' or
    '<literal>g</literal>' characters are used, care must be taken to ensure that the passed string is a valid DBus
    object path or DBus type signature, respectively.
   </para>
   <para>
    Upon encounting '<literal>s</literal>', '<literal>o</literal>' or '<literal>g</literal>', <link
    linkend='g-variant-get'><function>g_variant_get()</function></link> takes a pointer to a
    <code>(<link linkend='gchar'>gchar</link> *)</code> (ie: <code>(<link linkend='gchar'>gchar</link> **)</code>) and
    sets it to a newly-allocated copy of the string.  It is appropriate to free this copy using
    <link linkend='g-free'><function>g_free()</function></link>.
    <link linkend='NULL:CAPS'><literal>NULL</literal></link> may also be passed to indicate that the value of the
    string should be ignored (in which case no copy is made).
   </para>

   <refsect3>
    <title>Examples</title>
    <informalexample><programlisting>
<![CDATA[GVariant *value1, *value2, *value3;

value1 = g_variant_new ("s", "hello world!");
value2 = g_variant_new ("o", "/must/be/a/valid/path");
value3 = g_variant_new ("g", "iias");

#if 0
  g_variant_new ("s", NULL);      /* not valid: NULL is not a string. */
#endif

{
  gchar *result;

  g_variant_get (value1, "s", &result);
  g_print ("It was '%s'\n", result);
  g_free (result);
}]]></programlisting></informalexample>
   </refsect3>
  </refsect2>

  <refsect2 id='gvariant-format-strings-variants'>
   <title>Variants</title>
   <para>
    <emphasis role='strong'>
     Characters: <literal>v</literal>
    </emphasis>
   </para>

   <para>
    Upon encountering a '<literal>v</literal>',
    <link linkend='g-variant-new'><function>g_variant_new()</function></link> takes a <code>(<link
    linkend='GVariant'>GVariant</link> *)</code>.  The value of the
    <link linkend='GVariant'><type>GVariant</type></link> is used as the contents of the variant value.
   </para>
   <para>
    Upon encountering a '<literal>v</literal>', <link
    linkend='g-variant-get'><function>g_variant_get()</function></link> takes a pointer to a
    <code>(<link linkend='GVariant'>GVariant</link> *)</code> (ie: <code>(<link linkend='GVariant'>GVariant</link> **)
    </code>).  It is set to a new reference to a <link linkend='GVariant'><type>GVariant</type></link> instance
    containing the contents of the variant value.  It is appropriate to free this reference using
    <link linkend='g-variant-unref'><function>g_variant_unref()</function></link>.
    <link linkend='NULL:CAPS'><literal>NULL</literal></link> may also be passed to indicate that the value should be
    ignored (in which case no new reference is created).
   </para>

   <refsect3>
    <title>Examples</title>
    <informalexample><programlisting>
<![CDATA[GVariant *x, *y;

/* the following two lines are equivalent: */
x = g_variant_new ("v", y);
x = g_variant_new_variant (y);

/* as are these: */
g_variant_get (x, "v", &y);
y = g_variant_get_variant (x);]]></programlisting></informalexample>
   </refsect3>
  </refsect2>


  <refsect2 id='gvariant-format-strings-arrays'>
   <title>Arrays</title>
   <para>
    <emphasis role='strong'>
     Characters: <literal>a</literal>
    </emphasis>
   </para>

   <para>
    Upon encountering an '<literal>a</literal>' character followed by a type string,
    <link linkend='g-variant-new'><function>g_variant_new()</function></link> will take a
    <code>(<link linkend='GVariantBuilder'>GVariantBuilder</link> *)</code> that has been created as an array builder
    for an array of the type given in the type string.  The builder will have
    <link linkend='g-variant-builder-end'><function>g_variant_builder_end()</function></link> called on it and the
    result will be used as the value.  As a special exception, if the given type string is a definite type, then
    <link linkend='NULL:CAPS'><literal>NULL</literal></link> may be given to mean an empty array of that type.
   </para>

   <para>
    Upon encountering an '<literal>a</literal>' character followed by a type string,
    <link linkend='g-variant-get'><function>g_variant_get()</function></link> will take a pointer to a
    <code>(<link linkend='GVariantIter'>GVariantIter</link> *)</code> (ie:
    <code>(<link linkend='GVariantIter'>GVariantIter</link> **)</code>).
    A new heap-allocated iterator is created and returned, initialised for iterating over the elements of the array.
    This iterator should be freed when you are done with it, using
    <link linkend='g-variant-iter-free'><function>g_variant_iter_free()</function></link>.
    <link linkend='NULL:CAPS'><literal>NULL</literal></link> may also be given to indicate that the value of the array
    should be ignored.
   </para>

   <refsect3>
    <title>Examples</title>
    <informalexample><programlisting>
<![CDATA[GVariantBuilder *builder;
GVariant *value;

builder = g_variant_builder_new (G_VARIANT_TYPE ("as"));
g_variant_builder_add (builder, "s", "when");
g_variant_builder_add (builder, "s", "in");
g_variant_builder_add (builder, "s", "the");
g_variant_builder_add (builder, "s", "course");
value = g_variant_new ("as", builder);
g_variant_builder_unref (builder);

{
  GVariantIter *iter;
  gchar *str;

  g_variant_get (value, "as", &iter);
  while (g_variant_iter_loop (iter, "s", &str))
    g_print ("%s\n", str);
  g_variant_iter_free (iter);
}

g_variant_unref (value);]]></programlisting></informalexample>
   </refsect3>
  </refsect2>

  <refsect2 id='gvariant-format-strings-maybe-types'>
   <title>Maybe Types</title>
   <para>
    <emphasis role='strong'>
     Characters: <literal>m</literal>
    </emphasis>
   </para>
   <para>
    Maybe types are handled in two separate ways depending on the format string that follows the
    '<literal>m</literal>'.  The method that is used currently depends entirely on the character immediately following the
    '<literal>m</literal>'.
   </para>

   <para>
    The first way is used with format strings starting with '<literal>a</literal>', '<literal>s</literal>',
    '<literal>o</literal>', '<literal>g</literal>', '<literal>v</literal>', '<literal>@</literal>',
    '<literal>*</literal>', '<literal>?</literal>', '<literal>r</literal>', '<literal>&amp;</literal>', or
    '<literal>^</literal>'.  In all of these cases, for non-maybe types,
    <link linkend='g-variant-new'><function>g_variant_new()</function></link> takes a pointer to a
    non-<link linkend='NULL:CAPS'><literal>NULL</literal></link> value and
    <link linkend='g-variant-get'><function>g_variant_get()</function></link> returns (by reference) a
    non-<link linkend='NULL:CAPS'><literal>NULL</literal></link> pointer.  When any of these format strings are
    prefixed with an '<literal>m</literal>', the type of arguments that are collected does not change in any way, but
    <link linkend='NULL:CAPS'><literal>NULL</literal></link> becomes a permissable value, to indicate the Nothing case.
   </para>
   <para>
    Note that the "special exception" introduced in the array section for constructing empty arrays is ignored
    here.  Using a <literal>NULL</literal> pointer with the format string '<literal>mas</literal>' constructs
    the Nothing value -- not an empty array.
   </para>
   <para>
    The second way is used with all other format strings.  For
    <link linkend='g-variant-new'><function>g_variant_new()</function></link> an additional
    <link linkend='gboolean'><type>gboolean</type></link> argument is collected and for
    <link linkend='g-variant-get'><function>g_variant_get()</function></link> an additional
    <code>(<link linkend='gboolean'>gboolean</link> *)</code>.  Following this argument, the arguments that are normally
    collected for the equivalent non-maybe type will be collected.
   </para>
   <para>
    If <link linkend='FALSE:CAPS'><literal>FALSE</literal></link> is given to
    <link linkend='g-variant-new'><function>g_variant_new()</function></link> then the Nothing value is constructed and
    the collected arguments are ignored.  Otherwise (if <link linkend='TRUE:CAPS'><literal>TRUE</literal></link> was
    given), the arguments are used in the normal way to create the Just value.
   </para>
   <para>
    If <link linkend='NULL:CAPS'><literal>NULL</literal></link> is given to
    <link linkend='g-variant-get'><function>g_variant_get()</function></link> then the value is ignored.  If a
    non-<link linkend='NULL:CAPS'><literal>NULL</literal></link> pointer is given then it is used to return by reference
    whether the value was Just.  In the case that the value was Just, the
    <link linkend='gboolean'><type>gboolean</type></link> will be set to
    <link linkend='TRUE:CAPS'><literal>TRUE</literal></link> and the value will be stored in the arguments in the usual
    way.  In the case that the value was Nothing, the <link linkend='gboolean'><type>gboolean</type></link> will be set to
    <link linkend='FALSE:CAPS'><literal>FALSE</literal></link> and the arguments will be collected in the normal way
    but have their values set to binary zero.
   </para>

   <refsect3>
    <title>Examples</title>
    <informalexample><programlisting>
<![CDATA[GVariant *value1, *value2, *value3, *value4, *value5, *value6;
value1 = g_variant_new ("ms", "Hello world");
value2 = g_variant_new ("ms", NULL);
value3 = g_variant_new ("(m(ii)s)", TRUE, 123, 456, "Done");
value4 = g_variant_new ("(m(ii)s)", FALSE, -1, -1, "Done");          /* both '-1' are ignored. */
value5 = g_variant_new ("(m@(ii)s)", NULL, "Done");

{
  GVariant *contents;
  const gchar *cstr;
  gboolean just;
  gint32 x, y;
  gchar *str;

  g_variant_get (value1, "ms", &str);
  if (str != NULL)
    g_print ("str: %s\n", str);
  else
    g_print ("it was null\n");
  g_free (str);


  g_variant_get (value2, "m&s", &cstr);
  if (cstr != NULL)
    g_print ("str: %s\n", cstr);
  else
    g_print ("it was null\n");
  /* don't free 'cstr' */


  /* NULL passed for the gboolean *, but two 'gint32 *' still collected */
  g_variant_get (value3, "(m(ii)s)", NULL, NULL, NULL, &str);
  g_print ("string is %s\n", str);
  g_free (str);

  /* note: &s used, so g_free() not needed */
  g_variant_get (value4, "(m(ii)&s)", &just, &x, &y, &cstr);
  if (just)
    g_print ("it was (%d, %d)\n", x, y);
  else
    g_print ("it was null\n");
  g_print ("string is %s\n", cstr);
  /* don't free 'cstr' */


  g_variant_get (value5, "(m*s)", &contents, NULL); /* ignore the string. */
  if (contents != NULL)
    {
      g_variant_get (contents, "(ii)", &x, &y);
      g_print ("it was (%d, %d)\n", x, y);
      g_variant_unref (contents);
    }
  else
    g_print ("it was null\n");
}]]></programlisting></informalexample>
   </refsect3>
  </refsect2>

  <refsect2 id='gvariant-format-strings-tuples'>
   <title>Tuples</title>
   <para>
    <emphasis role='strong'>
     Characters: <code>()</code>
    </emphasis>
   </para>

   <para>
    Tuples are handled by handling each item in the tuple, in sequence.  Each item is handled in the usual way.
   </para>

   <refsect3>
    <title>Examples</title>
    <informalexample><programlisting>
<![CDATA[GVariant *value1, *value2;

value1 = g_variant_new ("(s(ii))", "Hello", 55, 77);
value2 = g_variant_new ("()");

{
  gchar *string;
  gint x, y;

  g_variant_get (value1, "(s(ii))", &string, &x, &y);
  g_print ("%s, %d, %d\n", string, x, y);
  g_free (string);

  g_variant_get (value2, "()");   /* do nothing... */
}]]></programlisting></informalexample>
   </refsect3>
  </refsect2>

  <refsect2 id='gvariant-format-strings-dictionaries'>
   <title>Dictionaries</title>
   <para>
    <emphasis role='strong'>
     Characters: <code>{}</code>
    </emphasis>
   </para>

   <para>
    Dictionary entries are handled by handling first the key, then the value.  Each is handled in the usual way.
   </para>

   <refsect3>
    <title>Examples</title>
    <informalexample><programlisting>
<![CDATA[GVariantBuilder *b;
GVariant *dict;

b = g_variant_builder_new (G_VARIANT_TYPE ("a{sv}"));
g_variant_builder_add (b, "{sv}", "name", g_variant_new_string ("foo"));
g_variant_builder_add (b, "{sv}", "timeout", g_variant_new_int32 (10));
dict = g_variant_builder_end (b);]]></programlisting></informalexample>
   </refsect3>
  </refsect2>

  <refsect2 id='gvariant-format-strings-gvariant'>
   <title>GVariant *</title>
   <para>
    <emphasis role='strong'>
     Characters: <literal>@</literal>, <literal>*</literal>, <literal>?</literal>, <literal>r</literal>
    </emphasis>

   </para>
   <para>
    Upon encountering a '<literal>@</literal>' in front of a type string,
    <link linkend='g-variant-new'><function>g_variant_new()</function></link> takes a
    non-<link linkend='NULL:CAPS'><literal>NULL</literal></link> pointer to a
    <link linkend='GVariant'><type>GVariant</type></link> and uses its value directly instead of collecting arguments to
    create the value.  The provided <link linkend='GVariant'><type>GVariant</type></link> must have a type that matches the
    type string following the '<literal>@</literal>'.  '<literal>*</literal>' is
    the same as '<literal>@*</literal>' (ie: take a <link linkend='GVariant'><type>GVariant</type></link> of any type).
    '<literal>?</literal>' is the same as '<literal>@?</literal>' (ie: take a
    <link linkend='GVariant'><type>GVariant</type></link> of any basic type).  '<literal>r</literal>' is the same as
    '<literal>@r</literal>' (ie: take a <link linkend='GVariant'><type>GVariant</type></link> of any tuple type).
   </para>
   <para>
    Upon encountering a '<literal>@</literal>' in front of a type string,
    <link linkend='g-variant-get'><function>g_variant_get()</function></link>
    takes a pointer to a <code>(<link linkend='GVariant'>GVariant</link> *)</code> (ie: a
    <code>(<link linkend='GVariant'>GVariant</link> **)</code>) and sets it to a new reference to a
    <link linkend='GVariant'><type>GVariant</type></link> containing the value (instead of deconstructing the value into
    C types in the usual way).  <link linkend='NULL:CAPS'><literal>NULL</literal></link> can be given to ignore the
    value.  '<literal>*</literal>', '<literal>?</literal>' and '<literal>r</literal>' are handled in a way analogous to
    what is stated above.
   </para>
   <para>
    You can always use '<literal>*</literal>' as an alternative to '<literal>?</literal>', '<literal>r</literal>' or any
    use of '<literal>@</literal>'.  Using the other characters where possible is recommended, however, due to the
    improvements in type safety and code self-documentation.
   </para>

   <refsect3>
    <title>Examples</title>
    <informalexample><programlisting>
<![CDATA[GVariant *value1, *value2;

value1 = g_variant_new ("(i@ii)", 44, g_variant_new_int32 (55), 66);

/* note: consumes floating reference count on 'value1' */
value2 = g_variant_new ("(@(iii)*)", value1, g_variant_new_string ("foo"));

{
  const gchar *string;
  GVariant *tmp;
  gsize length;
  gint x, y, z;

  g_variant_get (value2, "((iii)*)", &x, &y, &z, &tmp);
  string = g_variant_get_string (tmp, &length);
  g_print ("it is %d %d %d %s (length=%d)\n", x, y, z, string, (int) length);
  g_variant_unref (tmp);

  /* quick way to skip all the values in a tuple */
  g_variant_get (value2, "(rs)", NULL, &string); /* or "(@(iii)s)" */
  g_print ("i only got the string: %s\n", string);
  g_free (string);
}]]></programlisting></informalexample>
   </refsect3>
  </refsect2>

  <refsect2 id='gvariant-format-strings-pointers'>
   <title>Pointers</title>
   <para>
    <emphasis role='strong'>
     Characters: <code>&amp;</code>
    </emphasis>
   </para>

   <para>
    The '<code>&amp;</code>' character is used to indicate that serialised data should be directly exchanged via a
    pointer.
   </para>
   <para>
    Currently, the only use for this character is when it is applied to a string (ie: '<literal>&amp;s</literal>',
    '<literal>&amp;o</literal>' or '<code>&amp;g</code>').  For
    <link linkend='g-variant-new'><function>g_variant_new()</function></link> this has absolutely no effect.  The string
    is collected and duplicated normally.  For <link linkend='g-variant-get'><function>g_variant_get()</function></link>
    it means that instead of creating a newly allocated copy of the string, a pointer to the serialised data is
    returned.  This pointer should not be freed.  Validity checks are performed to ensure that the string data will
    always be properly nul-terminated.
   </para>

   <refsect3>
    <title>Examples</title>
    <informalexample><programlisting>
<![CDATA[{
  const gchar *str;
  GVariant *value;

  value = g_variant_new ("&s", "hello world");
  g_variant_get (value, "&s", &str);
  g_print ("string is: %s\n", str);
  /* no need to free str */
}]]></programlisting></informalexample>
   </refsect3>
  </refsect2>

  <refsect2 id='gvariant-format-strings-convenience'>
   <title>Convenience Conversions</title>
   <para>
    <emphasis role='strong'>
     Characters: <literal>^</literal>
    </emphasis>
   </para>

   <para>
    The '<literal>^</literal>' character currently supports conversion to and from bytestrings or to and from arrays
    of strings or bytestrings.  It has a number of forms.
   </para>

   <para>
    In all forms, when used with <link linkend='g-variant-new'><function>g_variant_new()</function></link> one
    pointer value is collected from the variable arguments and passed to a function (as given in the table below).
    The result of that function is used as the value for this position.  When used with
    <link linkend='g-variant-get'><function>g_variant_get()</function></link> one pointer value is produced by using
    the function (given in the table) and returned by reference.
   </para>

   <informaltable>
    <tgroup cols='2'>
     <colspec colname='col_0'/>
     <colspec colname='col_1'/>
     <colspec colname='col_2'/>
     <tbody>

      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>Conversion</emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
          <emphasis role='strong'>
            Used with <link linkend='g-variant-new'><function>g_variant_new()</function></link>
          </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
          <emphasis role='strong'>
            Used with <link linkend='g-variant-get'><function>g_variant_get()</function></link>
          </emphasis>
        </para>
       </entry>
      </row>

      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>^as</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1' morerows='1'>
        <para>
         equivalent to <link linkend='g-variant-new-strv'><function>g_variant_new_strv()</function></link>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         equivalent to <link linkend='g-variant-dup-strv'><function>g_variant_dup_strv()</function></link>
        </para>
       </entry>
      </row>

      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>^a&amp;s</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         equivalent to <link linkend='g-variant-get-strv'><function>g_variant_get_strv()</function></link>
        </para>
       </entry>
      </row>

      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>^ao</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1' morerows='1'>
        <para>
         equivalent to <link linkend='g-variant-new-objv'><function>g_variant_new_objv()</function></link>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         equivalent to <link linkend='g-variant-dup-objv'><function>g_variant_dup_objv()</function></link>
        </para>
       </entry>
      </row>

      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>^a&amp;o</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         equivalent to <link linkend='g-variant-get-objv'><function>g_variant_get_objv()</function></link>
        </para>
       </entry>
      </row>

      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>^ay</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1' morerows='1'>
        <para>
         equivalent to <link linkend='g-variant-new-bytestring'><function>g_variant_new_bytestring()</function></link>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         equivalent to <link linkend='g-variant-dup-bytestring'><function>g_variant_dup_bytestring()</function></link>
        </para>
       </entry>
      </row>

      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>^&amp;ay</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         equivalent to <link linkend='g-variant-get-bytestring'><function>g_variant_get_bytestring()</function></link>
        </para>
       </entry>
      </row>

      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>^aay</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1' morerows='1'>
        <para>
         equivalent to <link linkend='g-variant-new-bytestring-array'><function>g_variant_new_bytestring_array()</function></link>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         equivalent to <link linkend='g-variant-dup-bytestring-array'><function>g_variant_dup_bytestring_array()</function></link>
        </para>
       </entry>
      </row>

      <row rowsep='1'>
       <entry colsep='1' rowsep='1'>
        <para>
         <emphasis role='strong'>
          <literal>^a&amp;ay</literal>
         </emphasis>
        </para>
       </entry>
       <entry colsep='1' rowsep='1'>
        <para>
         equivalent to <link linkend='g-variant-get-bytestring-array'><function>g_variant_get_bytestring_array()</function></link>
        </para>
       </entry>
      </row>

     </tbody>
    </tgroup>
   </informaltable>
  </refsect2>
 </refsect1>
</refentry>