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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>GLib Reference Manual: GVariant Text Format</title>
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<div class="refentry">
<a name="gvariant-text"></a><div class="titlepage"></div>
<div class="refnamediv"><table width="100%"><tr>
<td valign="top">
<h2><span class="refentrytitle">GVariant Text Format</span></h2>
<p>GVariant Text Format — textual representation of GVariants</p>
</td>
<td class="gallery_image" valign="top" align="right"></td>
</tr></table></div>
<div class="refsect1">
<a name="id-1.6.21.3"></a><h2>GVariant Text Format</h2>
<p>
This page attempts to document the GVariant text format as produced by
<a class="link" href="glib-GVariant.html#g-variant-print" title="g_variant_print ()"><code class="function">g_variant_print()</code></a> and parsed by the
<a class="link" href="glib-GVariant.html#g-variant-parse" title="g_variant_parse ()"><code class="function">g_variant_parse()</code></a> family of functions. In most
cases the style closely resembles the formatting of literals in Python but there are some additions and
exceptions.
</p>
<p>
The functions that deal with GVariant text format absolutely always deal in utf-8. Conceptually, GVariant
text format is a string of Unicode characters -- not bytes. Non-ASCII but otherwise printable Unicode
characters are not treated any differently from normal ASCII characters.
</p>
<p>
The parser makes two passes. The purpose of the first pass is to determine the type of the value being
parsed. The second pass does the actual parsing. Based on the fact that all elements in an array have to
have the same type, GVariant is able to make some deductions that would not otherwise be possible. As an
example:
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="gtkdoc opt">[[</span><span class="number">1</span><span class="gtkdoc opt">,</span> <span class="number">2</span><span class="gtkdoc opt">,</span> <span class="number">3</span><span class="gtkdoc opt">], [</span><span class="number">4</span><span class="gtkdoc opt">,</span> <span class="number">5</span><span class="gtkdoc opt">,</span> <span class="number">6</span><span class="gtkdoc opt">]]</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
is parsed as an array of arrays of integers (type '<code class="literal">aai</code>'), but
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="gtkdoc opt">[[</span><span class="number">1</span><span class="gtkdoc opt">,</span> <span class="number">2</span><span class="gtkdoc opt">,</span> <span class="number">3</span><span class="gtkdoc opt">], [</span><span class="number">4</span><span class="gtkdoc opt">,</span> <span class="number">5</span><span class="gtkdoc opt">,</span> <span class="number">6.0</span><span class="gtkdoc opt">]]</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
is parsed as a array of arrays of doubles (type '<code class="literal">aad</code>').
</p>
<p>
As another example, GVariant is able to determine that
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="gtkdoc opt">[</span><span class="string">"hello"</span><span class="gtkdoc opt">,</span> nothing<span class="gtkdoc opt">]</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
is an array of maybe strings (type '<code class="literal">ams</code>').
</p>
<p>
What the parser accepts as valid input is dependent on context. The API permits for out-of-band type
information to be supplied to the parser (which will change its behaviour). This can be seen in the
GSettings and GDBus command line utilities where the type information is available from the schema or the
remote introspection information. The additional information can cause parses to succeed when they would not
otherwise have been able to (by resolving ambiguous type information) or can cause them to fail (due to
conflicting type information). Unless stated otherwise, the examples given in this section assume that no
out-of-band type data has been given to the parser.
</p>
</div>
<div class="refsect1">
<a name="id-1.6.21.4"></a><h2>Syntax Summary</h2>
<p>
The following table describes the rough meaning of symbols that may appear inside GVariant text format.
Each symbol is described in detail in its own section, including usage examples.
</p>
<div class="informaltable"><table border="1">
<colgroup>
<col class="col_0">
<col class="col_1">
</colgroup>
<tbody>
<tr>
<td>
<p>
<span class="strong"><strong>Symbol</strong></span>
</p>
</td>
<td>
<p>
<span class="strong"><strong>Meaning</strong></span>
</p>
</td>
</tr>
<tr>
<td>
<p>
<span class="strong"><strong><code class="literal">true</code></strong></span>,
<span class="strong"><strong><code class="literal">false</code></strong></span>
</p>
</td>
<td>
<p>
<a href="gvariant-text.html#gvariant-text-booleans">Booleans</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<span class="strong"><strong><code class="literal">""</code></strong></span>,
<span class="strong"><strong><code class="literal">''</code></strong></span>
</p>
</td>
<td>
<p>
String literal. See <a href="gvariant-text.html#gvariant-text-strings">Strings</a> below.
</p>
</td>
</tr>
<tr>
<td>
<p>
numbers
</p>
</td>
<td>
<p>
See <a href="gvariant-text.html#gvariant-text-numbers">Numbers</a> below.
</p>
</td>
</tr>
<tr>
<td>
<p>
<span class="strong"><strong><code class="literal">()</code></strong></span>
</p>
</td>
<td>
<p>
<a href="gvariant-text.html#gvariant-text-tuples">Tuples</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<span class="strong"><strong><code class="literal">[]</code></strong></span>
</p>
</td>
<td>
<p>
<a href="gvariant-text.html#gvariant-text-arrays">Arrays</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<span class="strong"><strong><code class="literal">{}</code></strong></span>
</p>
</td>
<td>
<p>
<a href="gvariant-text.html#gvariant-text-dictionaries">Dictionaries and Dictionary Entries</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<span class="strong"><strong><code class="literal"><></code></strong></span>
</p>
</td>
<td>
<p>
<a href="gvariant-text.html#gvariant-text-variants">Variants</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<span class="strong"><strong><code class="literal">just</code></strong></span>,
<span class="strong"><strong><code class="literal">nothing</code></strong></span>
</p>
</td>
<td>
<p>
<a href="gvariant-text.html#gvariant-text-maybe-types">Maybe Types</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<span class="strong"><strong><code class="literal">@</code></strong></span>
</p>
</td>
<td>
<p>
<a href="gvariant-text.html#gvariant-text-type-annotations">Type Annotations</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
type keywords
</p>
</td>
<td>
<p>
<code class="literal">boolean</code>,
<code class="literal">byte</code>,
<code class="literal">int16</code>,
<code class="literal">uint16</code>,
<code class="literal">int32</code>,
<code class="literal">uint32</code>,
<code class="literal">handle</code>,
<code class="literal">int64</code>,
<code class="literal">uint64</code>,
<code class="literal">double</code>,
<code class="literal">string</code>,
<code class="literal">objectpath</code>,
<code class="literal">signature</code>
</p>
<p>
See <a href="gvariant-text.html#gvariant-text-type-annotations">Type Annotations</a> below.
</p>
</td>
</tr>
<tr>
<td>
<p>
<span class="strong"><strong><code class="literal">b""</code></strong></span>,
<span class="strong"><strong><code class="literal">b''</code></strong></span>
</p>
</td>
<td>
<p>
<a href="gvariant-text.html#gvariant-text-bytestrings">Bytestrings</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<span class="strong"><strong><code class="literal">%</code></strong></span>
</p>
</td>
<td>
<p>
<a href="gvariant-text.html#gvariant-text-positional">Positional Parameters</a>.
</p>
</td>
</tr>
</tbody>
</table></div>
<div class="refsect2">
<a name="gvariant-text-booleans"></a><h3>Booleans</h3>
<p>
The strings <code class="literal">true</code> and <code class="literal">false</code> are parsed as booleans. This is the only
way to specify a boolean value.
</p>
</div>
<hr>
<div class="refsect2">
<a name="gvariant-text-strings"></a><h3>Strings</h3>
<p>
Strings literals must be quoted using <code class="literal">""</code> or <code class="literal">''</code>. The two are
completely equivalent (except for the fact that each one is unable to contain itself unescaped).
</p>
<p>
Strings are Unicode strings with no particular encoding. For example, to specify the character
<code class="literal">é</code>, you just write <code class="literal">'é'</code>. You could also give the Unicode codepoint of
that character (U+E9) as the escape sequence <code class="literal">'\u00e9'</code>. Since the strings are pure
Unicode, you should not attempt to encode the utf-8 byte sequence corresponding to the string using escapes;
it won't work and you'll end up with the individual characters corresponding to each byte.
</p>
<p>
Unicode escapes of the form <code class="literal">\uxxxx</code> and <code class="literal">\Uxxxxxxxx</code> are supported, in
hexidecimal. The usual control sequence escapes <code class="literal">\a</code>, <code class="literal">\b</code>,
<code class="literal">\f</code>, <code class="literal">\n</code>, <code class="literal">\r</code>, <code class="literal">\t</code> and
<code class="literal">\v</code> are supported. Additionally, a <code class="literal">\</code> before a newline character causes
the newline to be ignored. Finally, any other character following <code class="literal">\</code> is copied literally
(for example, <code class="literal">\"</code> or <code class="literal">\\</code>) but for forwards compatibility with future
additions you should only use this feature when necessary for escaping backslashes or quotes.
</p>
<p>
The usual octal and hexidecimal escapes <code class="literal">\0nnn</code> and <code class="literal">\xnn</code> are not
supported here. Those escapes are used to encode byte values and GVariant strings are Unicode.
</p>
<p>
Single-character strings are not interpreted as bytes. Bytes must be specified by their numerical value.
</p>
</div>
<hr>
<div class="refsect2">
<a name="gvariant-text-numbers"></a><h3>Numbers</h3>
<p>
Numbers are given by default as decimal values. Octal and hex values can be given in the usual way (by
prefixing with <code class="literal">0</code> or <code class="literal">0x</code>). Note that GVariant considers bytes to be
unsigned integers and will print them as a two digit hexidecimal number by default.
</p>
<p>
Floating point numbers can also be given in the usual ways, including scientific and hexidecimal notations.
</p>
<p>
For lack of additional information, integers will be parsed as int32 values by default. If the number has a
point or an 'e' in it, then it will be parsed as a double precision floating point number by default. If
type information is available (either explicitly or inferred) then that type will be used instead.
</p>
<p>
Some examples:
</p>
<p>
<code class="literal">5</code> parses as the int32 value five.
</p>
<p>
<code class="literal">37.5</code> parses as a floating point value.
</p>
<p>
<code class="literal">3.75e1</code> parses the same as the value above.
</p>
<p>
<code class="literal">uint64 7</code> parses seven as a uint64.
See <a href="gvariant-text.html#gvariant-text-type-annotations">Type Annotations</a>.
</p>
</div>
<hr>
<div class="refsect2">
<a name="gvariant-text-tuples"></a><h3>Tuples</h3>
<p>
Tuples are formed using the same syntax as Python. Here are some examples:
</p>
<p>
<code class="literal">()</code> parses as the empty tuple.
</p>
<p>
<code class="literal">(5,)</code> is a tuple containing a single value.
</p>
<p>
<code class="literal">("hello", 42)</code> is a pair. Note that values of different types are permitted.
</p>
</div>
<hr>
<div class="refsect2">
<a name="gvariant-text-arrays"></a><h3>Arrays</h3>
<p>
Arrays are formed using the same syntax as Python uses for lists (which is arguably the term that GVariant
should have used). Note that, unlike Python lists, GVariant arrays are statically typed. This has two
implications.
</p>
<p>
First, all items in the array must have the same type. Second, the type of the array must be known, even in
the case that it is empty. This means that (unless there is some other way to infer it) type information
will need to be given explicitly for empty arrays.
</p>
<p>
The parser is able to infer some types based on the fact that all items in an array must have the same type.
See the examples below:
</p>
<p>
<code class="literal">[1]</code> parses (without additional type information) as a one-item array of signed integers.
</p>
<p>
<code class="literal">[1, 2, 3]</code> parses (similarly) as a three-item array.
</p>
<p>
<code class="literal">[1, 2, 3.0]</code> parses as an array of doubles. This is the most simple case of the type
inferencing in action.
</p>
<p>
<code class="literal">[(1, 2), (3, 4.0)]</code> causes the 2 to also be parsed as a double (but the 1 and 4 are still
integers).
</p>
<p>
<code class="literal">["", nothing]</code> parses as an array of maybe strings. The presence of
"<code class="literal">nothing</code>" clearly implies that the array elements are nullable.
</p>
<p>
<code class="literal">[[], [""]]</code> will parse properly because the type of the first (empty) array can be
inferred to be equal to the type of the second array (both are arrays of strings).
</p>
<p>
<code class="literal">[b'hello', []]</code> looks odd but will parse properly.
See <a href="gvariant-text.html#gvariant-text-bytestrings">Bytestrings</a>
</p>
<p>
And some examples of errors:
</p>
<p>
<code class="literal">["hello", 42]</code> fails to parse due to conflicting types.
</p>
<p>
<code class="literal">[]</code> will fail to parse without additional type information.
</p>
</div>
<hr>
<div class="refsect2">
<a name="gvariant-text-dictionaries"></a><h3>Dictionaries and Dictionary Entries</h3>
<p>
Dictionaries and dictionary entries are both specified using the <code class="literal">{}</code> characters.
</p>
<p>
The dictionary syntax is more commonly used. This is what the printer elects to use in the normal case of
dictionary entries appearing in an array (aka "a dictionary"). The separate syntax for dictionary entries
is typically only used for when the entries appear on their own, outside of an array (which is valid but
unusual). Of course, you are free to use the dictionary entry syntax within arrays but there is no good
reason to do so (and the printer itself will never do so). Note that, as with arrays, the type of empty
dictionaries must be established (either explicitly or through inference).
</p>
<p>
The dictionary syntax is the same as Python's syntax for dictionaries. Some examples:
</p>
<p>
<code class="literal">@a{sv} {}</code> parses as the empty dictionary of everyone's favourite type.
</p>
<p>
<code class="literal">@a{sv} []</code> is the same as above (owing to the fact that dictionaries are really arrays).
</p>
<p>
<code class="literal">{1: "one", 2: "two", 3: "three"}</code> parses as a dictionary mapping integers to strings.
</p>
<p>
The dictionary entry syntax looks just like a pair (2-tuple) that uses braces instead of parens. The
presence of a comma immediately following the key differentiates it from the dictionary syntax (which
features a colon after the first key). Some examples:
</p>
<p>
<code class="literal">{1, "one"}</code> is a free-standing dictionary entry that can be parsed on its own or as part
of another container value.
</p>
<p>
<code class="literal">[{1, "one"}, {2, "two"}, {3, "three"}]</code> is exactly equivalent to the dictionary example
given above.
</p>
</div>
<hr>
<div class="refsect2">
<a name="gvariant-text-variants"></a><h3>Variants</h3>
<p>
Variants are denoted using angle brackets (aka "XML brackets"), <code class="literal"><></code>. They may not
be omitted.
</p>
<p>
Using <code class="literal"><></code> effectively disrupts the type inferencing that occurs between array
elements. This can have positive and negative effects.
</p>
<p>
<code class="literal">[<"hello">, <42>]</code> will parse whereas <code class="literal">["hello", 42]</code> would
not.
</p>
<p>
<code class="literal">[<['']>, <[]>]</code> will fail to parse even though <code class="literal">[[''], []]</code>
parses successfully. You would need to specify <code class="literal">[<['']>, <@as []>]</code>.
</p>
<p>
<code class="literal">{"title": <"frobit">, "enabled": <true>, width: <800>}</code> is an example of
perhaps the most pervasive use of both dictionaries and variants.
</p>
</div>
<hr>
<div class="refsect2">
<a name="gvariant-text-maybe-types"></a><h3>Maybe Types</h3>
<p>
The syntax for specifying maybe types is inspired by Haskell.
</p>
<p>
The null case is specified using the keyword <code class="literal">nothing</code> and the non-null case is explicitly
specified using the keyword <code class="literal">just</code>. GVariant allows <code class="literal">just</code> to be omitted
in every case that it is able to unambiguously determine the intention of the writer. There are two cases
where it must be specified:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>when using nested maybes, in order to specify the <code class="literal">just nothing</code> case</p></li>
<li class="listitem"><p>
to establish the nullability of the type of a value without explicitly specifying its full type
</p></li>
</ul></div>
<p>
Some examples:
</p>
<p>
<code class="literal">just 'hello'</code> parses as a non-null nullable string.
</p>
<p>
<code class="literal">@ms 'hello'</code> is the same (demonstrating how <code class="literal">just</code> can be dropped if the type is already
known).
</p>
<p>
<code class="literal">nothing</code> will not parse wtihout extra type information.
</p>
<p>
<code class="literal">@ms nothing</code> parses as a null nullable string.
</p>
<p>
<code class="literal">[just 3, nothing]</code> is an array of nullable integers
</p>
<p>
<code class="literal">[3, nothing]</code> is the same as the above (demonstrating another place were
<code class="literal">just</code> can be dropped).
</p>
<p>
<code class="literal">[3, just nothing]</code> parses as an array of maybe maybe integers (type
<code class="literal">'ammi'</code>).
</p>
</div>
<hr>
<div class="refsect2">
<a name="gvariant-text-type-annotations"></a><h3>Type Annotations</h3>
<p>
Type annotations allow additional type information to be given to the parser. Depending on the context,
this type information can change the output of the parser, cause an error when parsing would otherwise have
succeeded or resolve an error when parsing would have otherwise failed.
</p>
<p>
Type annotations come in two forms: type codes and type keywords.
</p>
<p>
Type keywords can be seen as more verbose (and more legible) versions of a common subset of the type codes.
The type keywords <code class="literal">boolean</code>, <code class="literal">byte</code>, <code class="literal">int16</code>,
<code class="literal">uint16</code>, <code class="literal">int32</code>, <code class="literal">uint32</code>, <code class="literal">handle</code>,
<code class="literal">int64</code>, <code class="literal">uint64</code>, <code class="literal">double</code>, <code class="literal">string</code>,
<code class="literal">objectpath</code> and literal <code class="literal">signature</code> are each exactly equivalent to their
corresponding type code.
</p>
<p>
Type codes are an <code class="literal">@</code> ("at" sign) followed by a definite GVariant type string. Some
examples:
</p>
<p>
<code class="literal">uint32 5</code> causes the number to be parsed unsigned instead of signed (the default).
</p>
<p>
<code class="literal">@u 5</code> is the same
</p>
<p>
<code class="literal">objectpath "/org/gnome/xyz"</code> creates an object path instead of a normal string
</p>
<p>
<code class="literal">@au []</code> specifies the type of the empty array (which would not parse otherwise)
</p>
<p>
<code class="literal">@ms ""</code> indicates that a string value is meant to have a maybe type
</p>
</div>
<hr>
<div class="refsect2">
<a name="gvariant-text-bytestrings"></a><h3>Bytestrings</h3>
<p>
The bytestring syntax is a piece of syntactic sugar meant to complement the bytestring APIs in GVariant. It
constructs arrays of non-nul bytes (type '<code class="literal">ay</code>') with a nul terminator at the end.
</p>
<p>
Bytestrings are specified with either <code class="literal">b""</code> or <code class="literal">b''</code>. As with strings,
there is no fundamental difference between the two different types of quotes.
</p>
<p>
Bytestrings support the full range of escapes that you would expect (ie: those supported by
<a class="link" href="glib-String-Utility-Functions.html#g-strcompress" title="g_strcompress ()"><code class="function">g_strcompress()</code></a>. This includes the normal control
sequence escapes (as mentioned in the section on strings) as well as octal and hexidecimal escapes of the
forms <code class="literal">\0nnn</code> and <code class="literal">\xnn</code>.
</p>
<p>
<code class="literal">b'abc'</code> is equivalent to <code class="literal">[byte 0x97, 0x98, 0x99, 0]</code>.
</p>
<p>
When formatting arrays of bytes, the printer will choose to display the array as a bytestring if it contains
a nul character at the end and no other nul bytes within. Otherwise, it is formatted as a normal array.
</p>
</div>
<hr>
<div class="refsect2">
<a name="gvariant-text-positional"></a><h3>Positional Parameters</h3>
<p>
Positional parameters are not a part of the normal GVariant text format, but they are mentioned here because
they can be used with <a class="link" href="glib-GVariant.html#g-variant-new-parsed" title="g_variant_new_parsed ()"><code class="function">g_variant_new_parsed()</code></a>.
</p>
<p>
A positional parameter is indicated with a <code class="literal">%</code> followed by any valid
<a href="gvariant-format-strings.html">GVariant Format String</a>. Variable arguments are collected as
specified by the format string and the resulting value is inserted at the current position.
</p>
<p>
This feature is best explained by example:
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="gtkdoc kwb">char</span> <span class="gtkdoc opt">*</span>t <span class="gtkdoc opt">=</span> <span class="string">"xyz"</span><span class="gtkdoc opt">;</span>
gboolean en <span class="gtkdoc opt">=</span> <span class="keyword">false</span><span class="gtkdoc opt">;</span>
GVariant <span class="gtkdoc opt">*</span>value<span class="gtkdoc opt">;</span>
value <span class="gtkdoc opt">=</span> <span class="function"><a href="glib-GVariant.html#g-variant-new-parsed">g_variant_new_parsed</a></span> <span class="gtkdoc opt">(</span><span class="string">"{'title': <%s>, 'enabled': <%b>}"</span><span class="gtkdoc opt">,</span> t<span class="gtkdoc opt">,</span> en<span class="gtkdoc opt">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
This constructs a dictionary mapping strings to variants (type '<code class="literal">a{sv}</code>') with two items in
it. The key names are parsed from the string and the values for those keys are taken as variable arguments
parameters.
</p>
<p>
The arguments are always collected in the order that they appear in the string to be parsed. Format strings
that collect multiple arguments are permitted, so you may require more varargs parameters than the number of
<code class="literal">%</code> signs that appear. You can also give format strings that collect no arguments, but
there's no good reason to do so.
</p>
</div>
</div>
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