+ <sect3 id="message-protocol-marshaling-object-path">
+ <title>Valid Object Paths</title>
+
+ <para>
+ An object path is a name used to refer to an object instance.
+ Conceptually, each participant in a D-Bus message exchange may have
+ any number of object instances (think of C++ or Java objects) and each
+ such instance will have a path. Like a filesystem, the object
+ instances in an application form a hierarchical tree.
+ </para>
+
+ <para>
+ Object paths are often namespaced by starting with a reversed
+ domain name and containing an interface version number, in the
+ same way as
+ <link linkend="message-protocol-names-interface">interface
+ names</link> and
+ <link linkend="message-protocol-names-bus">well-known
+ bus names</link>.
+ This makes it possible to implement more than one service, or
+ more than one version of a service, in the same process,
+ even if the services share a connection but cannot otherwise
+ co-operate (for instance, if they are implemented by different
+ plugins).
+ </para>
+
+ <para>
+ For instance, if the owner of <literal>example.com</literal> is
+ developing a D-Bus API for a music player, they might use the
+ hierarchy of object paths that start with
+ <literal>/com/example/MusicPlayer1</literal> for its objects.
+ </para>
+
+ <para>
+ The following rules define a valid object path. Implementations must
+ not send or accept messages with invalid object paths.
+ <itemizedlist>
+ <listitem>
+ <para>
+ The path may be of any length.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ The path must begin with an ASCII '/' (integer 47) character,
+ and must consist of elements separated by slash characters.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Each element must only contain the ASCII characters
+ "[A-Z][a-z][0-9]_"
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ No element may be the empty string.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Multiple '/' characters cannot occur in sequence.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ A trailing '/' character is not allowed unless the
+ path is the root path (a single '/' character).
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ </sect3>
+
+ <sect3 id="message-protocol-marshaling-signature">
+ <title>Valid Signatures</title>
+ <para>
+ An implementation must not send or accept invalid signatures.
+ Valid signatures will conform to the following rules:
+ <itemizedlist>
+ <listitem>
+ <para>
+ The signature is a list of single complete types.
+ Arrays must have element types, and structs must
+ have both open and close parentheses.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Only type codes, open and close parentheses, and open and
+ close curly brackets are allowed in the signature. The
+ <literal>STRUCT</literal> type code
+ is not allowed in signatures, because parentheses
+ are used instead. Similarly, the
+ <literal>DICT_ENTRY</literal> type code is not allowed in
+ signatures, because curly brackets are used instead.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ The maximum depth of container type nesting is 32 array type
+ codes and 32 open parentheses. This implies that the maximum
+ total depth of recursion is 64, for an "array of array of array
+ of ... struct of struct of struct of ..." where there are 32
+ array and 32 struct.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ The maximum length of a signature is 255.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
+ When signatures appear in messages, the marshalling format
+ guarantees that they will be followed by a nul byte (which can
+ be interpreted as either C-style string termination or the INVALID
+ type-code), but this is not conceptually part of the signature.
+ </para>
+ </sect3>
+