WebSVN – nexmon – Blame – Rev 1 – /utilities/glib/docs/reference/glib/gvariant-text.xml

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office

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

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<refentry id='gvariant-text'>

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<refmeta>

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<refentrytitle>GVariant Text Format</refentrytitle>

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</refmeta>

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<refnamediv>

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<refname>GVariant Text Format</refname>

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<refpurpose>textual representation of GVariants</refpurpose>

</refnamediv>

<refsect1>

<title>GVariant Text Format</title>

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<para>

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This page attempts to document the GVariant text format as produced by

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<link linkend='g-variant-print'><function>g_variant_print()</function></link> and parsed by the

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<link linkend='g-variant-parse'><function>g_variant_parse()</function></link> family of functions. In most

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cases the style closely resembles the formatting of literals in Python but there are some additions and

exceptions.

</para>

<para>

The functions that deal with GVariant text format absolutely always deal in utf-8. Conceptually, GVariant

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text format is a string of Unicode characters -- not bytes. Non-ASCII but otherwise printable Unicode

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characters are not treated any differently from normal ASCII characters.

</para>

<para>

The parser makes two passes. The purpose of the first pass is to determine the type of the value being

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parsed. The second pass does the actual parsing. Based on the fact that all elements in an array have to

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have the same type, GVariant is able to make some deductions that would not otherwise be possible. As an

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example:

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<informalexample><programlisting>[[1, 2, 3], [4, 5, 6]]</programlisting></informalexample>

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is parsed as an array of arrays of integers (type '<literal>aai</literal>'), but

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<informalexample><programlisting>[[1, 2, 3], [4, 5, 6.0]]</programlisting></informalexample>

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is parsed as a array of arrays of doubles (type '<literal>aad</literal>').

</para>

<para>

As another example, GVariant is able to determine that

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<informalexample><programlisting>["hello", nothing]</programlisting></informalexample>

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is an array of maybe strings (type '<literal>ams</literal>').

</para>

<para>

What the parser accepts as valid input is dependent on context. The API permits for out-of-band type

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information to be supplied to the parser (which will change its behaviour). This can be seen in the

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GSettings and GDBus command line utilities where the type information is available from the schema or the

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remote introspection information. The additional information can cause parses to succeed when they would not

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otherwise have been able to (by resolving ambiguous type information) or can cause them to fail (due to

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conflicting type information). Unless stated otherwise, the examples given in this section assume that no

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out-of-band type data has been given to the parser.

</para>

</refsect1>

<refsect1>

<title>Syntax Summary</title>

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<para>

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The following table describes the rough meaning of symbols that may appear inside GVariant text format.

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Each symbol is described in detail in its own section, including usage examples.

</para>

<informaltable>

<tgroup cols='2'>

<colspec colname='col_0'/>

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<colspec colname='col_1'/>

<tbody>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'>Symbol</emphasis>

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</para>

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</entry>

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<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'>Meaning</emphasis>

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'><literal>true</literal></emphasis>,

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<emphasis role='strong'><literal>false</literal></emphasis>

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</para>

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</entry>

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<entry colsep='1' rowsep='1'>

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<para>

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<link linkend='gvariant-text-booleans'>Booleans</link>.

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'><literal>""</literal></emphasis>,

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<emphasis role='strong'><literal>''</literal></emphasis>

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</para>

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</entry>

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<entry colsep='1' rowsep='1'>

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<para>

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String literal. See <link linkend='gvariant-text-strings'>Strings</link> below.

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

<para>

numbers

</para>

</entry>

<entry colsep='1' rowsep='1'>

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<para>

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See <link linkend='gvariant-text-numbers'>Numbers</link> below.

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'><literal>()</literal></emphasis>

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</para>

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</entry>

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<entry colsep='1' rowsep='1'>

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<para>

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<link linkend='gvariant-text-tuples'>Tuples</link>.

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'><literal>[]</literal></emphasis>

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</para>

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</entry>

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<entry colsep='1' rowsep='1'>

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<para>

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<link linkend='gvariant-text-arrays'>Arrays</link>.

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'><literal>{}</literal></emphasis>

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</para>

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</entry>

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<entry colsep='1' rowsep='1'>

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<para>

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<link linkend='gvariant-text-dictionaries'>Dictionaries and Dictionary Entries</link>.

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'><literal><></literal></emphasis>

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</para>

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</entry>

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<entry colsep='1' rowsep='1'>

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<para>

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<link linkend='gvariant-text-variants'>Variants</link>.

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'><literal>just</literal></emphasis>,

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<emphasis role='strong'><literal>nothing</literal></emphasis>

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</para>

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</entry>

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<entry colsep='1' rowsep='1'>

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<para>

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<link linkend='gvariant-text-maybe-types'>Maybe Types</link>.

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'><literal>@</literal></emphasis>

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</para>

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</entry>

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<entry colsep='1' rowsep='1'>

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<para>

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<link linkend='gvariant-text-type-annotations'>Type Annotations</link>.

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

<para>

type keywords

</para>

</entry>

<entry colsep='1' rowsep='1'>

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<para>

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<literal>boolean</literal>,

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<literal>byte</literal>,

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<literal>int16</literal>,

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<literal>uint16</literal>,

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<literal>int32</literal>,

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<literal>uint32</literal>,

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<literal>handle</literal>,

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<literal>int64</literal>,

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<literal>uint64</literal>,

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<literal>double</literal>,

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<literal>string</literal>,

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<literal>objectpath</literal>,

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<literal>signature</literal>

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</para>

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<para>

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See <link linkend='gvariant-text-type-annotations'>Type Annotations</link> below.

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'><literal>b""</literal></emphasis>,

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<emphasis role='strong'><literal>b''</literal></emphasis>

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</para>

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</entry>

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<entry colsep='1' rowsep='1'>

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<para>

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<link linkend='gvariant-text-bytestrings'>Bytestrings</link>.

</para>

</entry>

</row>

<row rowsep='1'>

<entry colsep='1' rowsep='1'>

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<para>

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<emphasis role='strong'><literal>%</literal></emphasis>

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</para>

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</entry>

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<entry colsep='1' rowsep='1'>

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<para>

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<link linkend='gvariant-text-positional'>Positional Parameters</link>.

</para>

</entry>

</row>

</tbody>

</tgroup>

</informaltable>

<refsect2 id='gvariant-text-booleans'>

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<title>Booleans</title>

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<para>

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The strings <literal>true</literal> and <literal>false</literal> are parsed as booleans. This is the only

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way to specify a boolean value.

</para>

</refsect2>

<refsect2 id='gvariant-text-strings'>

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<title>Strings</title>

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<para>

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Strings literals must be quoted using <literal>""</literal> or <literal>''</literal>. The two are

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completely equivalent (except for the fact that each one is unable to contain itself unescaped).

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</para>

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<para>

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Strings are Unicode strings with no particular encoding. For example, to specify the character

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<literal>é</literal>, you just write <literal>'é'</literal>. You could also give the Unicode codepoint of

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that character (U+E9) as the escape sequence <literal>'\u00e9'</literal>. Since the strings are pure

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Unicode, you should not attempt to encode the utf-8 byte sequence corresponding to the string using escapes;

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it won't work and you'll end up with the individual characters corresponding to each byte.

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</para>

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<para>

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Unicode escapes of the form <literal>\uxxxx</literal> and <literal>\Uxxxxxxxx</literal> are supported, in

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hexidecimal. The usual control sequence escapes <literal>\a</literal>, <literal>\b</literal>,

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<literal>\f</literal>, <literal>\n</literal>, <literal>\r</literal>, <literal>\t</literal> and

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<literal>\v</literal> are supported. Additionally, a <literal>\</literal> before a newline character causes

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the newline to be ignored. Finally, any other character following <literal>\</literal> is copied literally

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(for example, <literal>\"</literal> or <literal>\\</literal>) but for forwards compatibility with future

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additions you should only use this feature when necessary for escaping backslashes or quotes.

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</para>

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<para>

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The usual octal and hexidecimal escapes <literal>\0nnn</literal> and <literal>\xnn</literal> are not

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supported here. Those escapes are used to encode byte values and GVariant strings are Unicode.

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</para>

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<para>

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Single-character strings are not interpreted as bytes. Bytes must be specified by their numerical value.

</para>

</refsect2>

<refsect2 id='gvariant-text-numbers'>

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<title>Numbers</title>

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<para>

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Numbers are given by default as decimal values. Octal and hex values can be given in the usual way (by

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prefixing with <literal>0</literal> or <literal>0x</literal>). Note that GVariant considers bytes to be

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unsigned integers and will print them as a two digit hexidecimal number by default.

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</para>

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<para>

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Floating point numbers can also be given in the usual ways, including scientific and hexidecimal notations.

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</para>

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<para>

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For lack of additional information, integers will be parsed as int32 values by default. If the number has a

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point or an 'e' in it, then it will be parsed as a double precision floating point number by default. If

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type information is available (either explicitly or inferred) then that type will be used instead.

</para>

<para>

Some examples:

</para>

<para>

<literal>5</literal> parses as the int32 value five.

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</para>

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<para>

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<literal>37.5</literal> parses as a floating point value.

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</para>

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<para>

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<literal>3.75e1</literal> parses the same as the value above.

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</para>

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<para>

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<literal>uint64 7</literal> parses seven as a uint64.

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See <link linkend='gvariant-text-type-annotations'>Type Annotations</link>.

</para>

</refsect2>

<refsect2 id='gvariant-text-tuples'>

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<title>Tuples</title>

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<para>

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Tuples are formed using the same syntax as Python. Here are some examples:

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</para>

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<para>

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<literal>()</literal> parses as the empty tuple.

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</para>

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<para>

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<literal>(5,)</literal> is a tuple containing a single value.

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</para>

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<para>

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<literal>("hello", 42)</literal> is a pair. Note that values of different types are permitted.

</para>

</refsect2>

<refsect2 id='gvariant-text-arrays'>

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<title>Arrays</title>

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<para>

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Arrays are formed using the same syntax as Python uses for lists (which is arguably the term that GVariant

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should have used). Note that, unlike Python lists, GVariant arrays are statically typed. This has two

implications.

</para>

<para>

First, all items in the array must have the same type. Second, the type of the array must be known, even in

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the case that it is empty. This means that (unless there is some other way to infer it) type information

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will need to be given explicitly for empty arrays.

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</para>

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<para>

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The parser is able to infer some types based on the fact that all items in an array must have the same type.

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See the examples below:

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</para>

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<para>

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<literal>[1]</literal> parses (without additional type information) as a one-item array of signed integers.

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</para>

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<para>

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<literal>[1, 2, 3]</literal> parses (similarly) as a three-item array.

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</para>

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<para>

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<literal>[1, 2, 3.0]</literal> parses as an array of doubles. This is the most simple case of the type

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inferencing in action.

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</para>

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<para>

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<literal>[(1, 2), (3, 4.0)]</literal> causes the 2 to also be parsed as a double (but the 1 and 4 are still

integers).

</para>

<para>

<literal>["", nothing]</literal> parses as an array of maybe strings. The presence of

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"<literal>nothing</literal>" clearly implies that the array elements are nullable.

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</para>

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<para>

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<literal>[[], [""]]</literal> will parse properly because the type of the first (empty) array can be

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inferred to be equal to the type of the second array (both are arrays of strings).

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</para>

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<para>

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<literal>[b'hello', []]</literal> looks odd but will parse properly.

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See <link linkend='gvariant-text-bytestrings'>Bytestrings</link>

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</para>

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<para>

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And some examples of errors:

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</para>

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<para>

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<literal>["hello", 42]</literal> fails to parse due to conflicting types.

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</para>

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<para>

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<literal>[]</literal> will fail to parse without additional type information.

</para>

</refsect2>

<refsect2 id='gvariant-text-dictionaries'>

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<title>Dictionaries and Dictionary Entries</title>

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<para>

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Dictionaries and dictionary entries are both specified using the <literal>{}</literal> characters.

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</para>

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<para>

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The dictionary syntax is more commonly used. This is what the printer elects to use in the normal case of

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dictionary entries appearing in an array (aka "a dictionary"). The separate syntax for dictionary entries

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is typically only used for when the entries appear on their own, outside of an array (which is valid but

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unusual). Of course, you are free to use the dictionary entry syntax within arrays but there is no good

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reason to do so (and the printer itself will never do so). Note that, as with arrays, the type of empty

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dictionaries must be established (either explicitly or through inference).

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</para>

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<para>

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The dictionary syntax is the same as Python's syntax for dictionaries. Some examples:

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</para>

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<para>

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<literal>@a{sv} {}</literal> parses as the empty dictionary of everyone's favourite type.

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</para>

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<para>

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<literal>@a{sv} []</literal> is the same as above (owing to the fact that dictionaries are really arrays).

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</para>

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<para>

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<literal>{1: "one", 2: "two", 3: "three"}</literal> parses as a dictionary mapping integers to strings.

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</para>

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<para>

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The dictionary entry syntax looks just like a pair (2-tuple) that uses braces instead of parens. The

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presence of a comma immediately following the key differentiates it from the dictionary syntax (which

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features a colon after the first key). Some examples:

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</para>

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<para>

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<literal>{1, "one"}</literal> is a free-standing dictionary entry that can be parsed on its own or as part

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of another container value.

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</para>

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<para>

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<literal>[{1, "one"}, {2, "two"}, {3, "three"}]</literal> is exactly equivalent to the dictionary example

given above.

</para>

</refsect2>

<refsect2 id='gvariant-text-variants'>

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<title>Variants</title>

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<para>

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Variants are denoted using angle brackets (aka "XML brackets"), <literal><></literal>. They may not

be omitted.

</para>

<para>

Using <literal><></literal> effectively disrupts the type inferencing that occurs between array

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elements. This can have positive and negative effects.

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</para>

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<para>

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<literal>[<"hello">, <42>]</literal> will parse whereas <literal>["hello", 42]</literal> would

not.

</para>

<para>

<literal>[<['']>, <[]>]</literal> will fail to parse even though <literal>[[''], []]</literal>

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parses successfully. You would need to specify <literal>[<['']>, <@as []>]</literal>.

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</para>

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<para>

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<literal>{"title": <"frobit">, "enabled": <true>, width: <800>}</literal> is an example of

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perhaps the most pervasive use of both dictionaries and variants.

</para>

</refsect2>

<refsect2 id='gvariant-text-maybe-types'>

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<title>Maybe Types</title>

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<para>

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The syntax for specifying maybe types is inspired by Haskell.

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</para>

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<para>

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The null case is specified using the keyword <literal>nothing</literal> and the non-null case is explicitly

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specified using the keyword <literal>just</literal>. GVariant allows <literal>just</literal> to be omitted

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in every case that it is able to unambiguously determine the intention of the writer. There are two cases

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where it must be specified:

</para>

<itemizedlist>

<listitem>

<para>when using nested maybes, in order to specify the <literal>just nothing</literal> case</para>

</listitem>

<listitem>

<para>

to establish the nullability of the type of a value without explicitly specifying its full type

</para>

</listitem>

</itemizedlist>

<para>

Some examples:

</para>

<para>

<literal>just 'hello'</literal> parses as a non-null nullable string.

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</para>

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<para>

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    <literal>@ms 'hello'</literal> is the same (demonstrating how <literal>just</literal> can be dropped if the type is already

known).

</para>

<para>

<literal>nothing</literal> will not parse wtihout extra type information.

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</para>

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<para>

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<literal>@ms nothing</literal> parses as a null nullable string.

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</para>

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<para>

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<literal>[just 3, nothing]</literal> is an array of nullable integers

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</para>

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<para>

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<literal>[3, nothing]</literal> is the same as the above (demonstrating another place were

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<literal>just</literal> can be dropped).

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</para>

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<para>

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<literal>[3, just nothing]</literal> parses as an array of maybe maybe integers (type

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<literal>'ammi'</literal>).

</para>

</refsect2>

<refsect2 id='gvariant-text-type-annotations'>

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<title>Type Annotations</title>

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<para>

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Type annotations allow additional type information to be given to the parser. Depending on the context,

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this type information can change the output of the parser, cause an error when parsing would otherwise have

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succeeded or resolve an error when parsing would have otherwise failed.

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</para>

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<para>

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Type annotations come in two forms: type codes and type keywords.

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</para>

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<para>

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Type keywords can be seen as more verbose (and more legible) versions of a common subset of the type codes.

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The type keywords <literal>boolean</literal>, <literal>byte</literal>, <literal>int16</literal>,

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<literal>uint16</literal>, <literal>int32</literal>, <literal>uint32</literal>, <literal>handle</literal>,

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<literal>int64</literal>, <literal>uint64</literal>, <literal>double</literal>, <literal>string</literal>,

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<literal>objectpath</literal> and literal <literal>signature</literal> are each exactly equivalent to their

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corresponding type code.

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</para>

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<para>

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Type codes are an <literal>@</literal> ("at" sign) followed by a definite GVariant type string. Some

examples:

</para>

<para>

<literal>uint32 5</literal> causes the number to be parsed unsigned instead of signed (the default).

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</para>

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<para>

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<literal>@u 5</literal> is the same

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</para>

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<para>

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<literal>objectpath "/org/gnome/xyz"</literal> creates an object path instead of a normal string

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</para>

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<para>

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<literal>@au []</literal> specifies the type of the empty array (which would not parse otherwise)

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</para>

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<para>

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<literal>@ms ""</literal> indicates that a string value is meant to have a maybe type

</para>

</refsect2>

<refsect2 id='gvariant-text-bytestrings'>

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<title>Bytestrings</title>

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<para>

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The bytestring syntax is a piece of syntactic sugar meant to complement the bytestring APIs in GVariant. It

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constructs arrays of non-nul bytes (type '<literal>ay</literal>') with a nul terminator at the end.

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</para>

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<para>

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Bytestrings are specified with either <literal>b""</literal> or <literal>b''</literal>. As with strings,

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there is no fundamental difference between the two different types of quotes.

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</para>

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<para>

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Bytestrings support the full range of escapes that you would expect (ie: those supported by

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<link linkend='g-strcompress'><function>g_strcompress()</function></link>. This includes the normal control

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sequence escapes (as mentioned in the section on strings) as well as octal and hexidecimal escapes of the

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forms <literal>\0nnn</literal> and <literal>\xnn</literal>.

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</para>

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<para>

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<literal>b'abc'</literal> is equivalent to <literal>[byte 0x97, 0x98, 0x99, 0]</literal>.

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</para>

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<para>

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When formatting arrays of bytes, the printer will choose to display the array as a bytestring if it contains

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a nul character at the end and no other nul bytes within. Otherwise, it is formatted as a normal array.

</para>

</refsect2>

<refsect2 id='gvariant-text-positional'>

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<title>Positional Parameters</title>

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<para>

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Positional parameters are not a part of the normal GVariant text format, but they are mentioned here because

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they can be used with <link linkend='g-variant-new-parsed'><function>g_variant_new_parsed()</function></link>.

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</para>

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<para>

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A positional parameter is indicated with a <literal>%</literal> followed by any valid

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<link linkend='gvariant-format-strings'>GVariant Format String</link>. Variable arguments are collected as

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specified by the format string and the resulting value is inserted at the current position.

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</para>

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<para>

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This feature is best explained by example:

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</para>

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<informalexample><programlisting><![CDATA[char *t = "xyz";

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gboolean en = false;

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GVariant *value;

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value = g_variant_new_parsed ("{'title': <%s>, 'enabled': <%b>}", t, en);]]></programlisting></informalexample>

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<para>

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This constructs a dictionary mapping strings to variants (type '<literal>a{sv}</literal>') with two items in

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it. The key names are parsed from the string and the values for those keys are taken as variable arguments

parameters.

</para>

<para>

The arguments are always collected in the order that they appear in the string to be parsed. Format strings

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that collect multiple arguments are permitted, so you may require more varargs parameters than the number of

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<literal>%</literal> signs that appear. You can also give format strings that collect no arguments, but

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there's no good reason to do so.

</para>

</refsect2>

</refsect1>

</refentry>

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