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<article id="index">
<articleinfo>
<title>D-Bus Specification</title>
<releaseinfo>Version 0.12</releaseinfo>
<date>7 November 2006</date>
<authorgroup>
<author>
<firstname>Havoc</firstname>
<surname>Pennington</surname>
<affiliation>
<orgname>Red Hat, Inc.</orgname>
<address>
<email>hp@pobox.com</email>
</address>
</affiliation>
</author>
<author>
<firstname>Anders</firstname>
<surname>Carlsson</surname>
<affiliation>
<orgname>CodeFactory AB</orgname>
<address>
<email>andersca@codefactory.se</email>
</address>
</affiliation>
</author>
<author>
<firstname>Alexander</firstname>
<surname>Larsson</surname>
<affiliation>
<orgname>Red Hat, Inc.</orgname>
<address>
<email>alexl@redhat.com</email>
</address>
</affiliation>
</author>
</authorgroup>
</articleinfo>
<sect1 id="introduction">
<title>Introduction</title>
<para>
D-Bus is a system for low-latency, low-overhead, easy to use
interprocess communication (IPC). In more detail:
<itemizedlist>
<listitem>
<para>
D-Bus is <emphasis>low-latency</emphasis> because it is designed
to avoid round trips and allow asynchronous operation, much like
the X protocol.
</para>
</listitem>
<listitem>
<para>
D-Bus is <emphasis>low-overhead</emphasis> because it uses a
binary protocol, and does not have to convert to and from a text
format such as XML. Because D-Bus is intended for potentially
high-resolution same-machine IPC, not primarily for Internet IPC,
this is an interesting optimization.
</para>
</listitem>
<listitem>
<para>
D-Bus is <emphasis>easy to use</emphasis> because it works in terms
of <firstterm>messages</firstterm> rather than byte streams, and
automatically handles a lot of the hard IPC issues. Also, the D-Bus
library is designed to be wrapped in a way that lets developers use
their framework's existing object/type system, rather than learning
a new one specifically for IPC.
</para>
</listitem>
</itemizedlist>
</para>
<para>
The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
protocol, specified in <xref linkend="message-protocol"/>. That is, it is
a system for one application to talk to a single other
application. However, the primary intended application of the protocol is the
D-Bus <firstterm>message bus</firstterm>, specified in <xref
linkend="message-bus"/>. The message bus is a special application that
accepts connections from multiple other applications, and forwards
messages among them.
</para>
<para>
Uses of D-Bus include notification of system changes (notification of when
a camera is plugged in to a computer, or a new version of some software
has been installed), or desktop interoperability, for example a file
monitoring service or a configuration service.
</para>
<para>
D-Bus is designed for two specific use cases:
<itemizedlist>
<listitem>
<para>
A "system bus" for notifications from the system to user sessions,
and to allow the system to request input from user sessions.
</para>
</listitem>
<listitem>
<para>
A "session bus" used to implement desktop environments such as
GNOME and KDE.
</para>
</listitem>
</itemizedlist>
D-Bus is not intended to be a generic IPC system for any possible
application, and intentionally omits many features found in other
IPC systems for this reason.
</para>
<para>
At the same time, the bus daemons offer a number of features not found in
other IPC systems, such as single-owner "bus names" (similar to X
selections), on-demand startup of services, and security policies.
In many ways, these features are the primary motivation for developing
D-Bus; other systems would have sufficed if IPC were the only goal.
</para>
<para>
D-Bus may turn out to be useful in unanticipated applications, but future
versions of this spec and the reference implementation probably will not
incorporate features that interfere with the core use cases.
</para>
<para>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119. However, the
document could use a serious audit to be sure it makes sense to do
so. Also, they are not capitalized.
</para>
<sect2 id="stability">
<title>Protocol and Specification Stability</title>
<para>
The D-Bus protocol is frozen (only compatible extensions are allowed) as
of November 8, 2006. However, this specification could still use a fair
bit of work to make interoperable reimplementation possible without
reference to the D-Bus reference implementation. Thus, this
specification is not marked 1.0. To mark it 1.0, we'd like to see
someone invest significant effort in clarifying the specification
language, and growing the specification to cover more aspects of the
reference implementation's behavior.
</para>
<para>
Until this work is complete, any attempt to reimplement D-Bus will
probably require looking at the reference implementation and/or asking
questions on the D-Bus mailing list about intended behavior.
Questions on the list are very welcome.
</para>
<para>
Nonetheless, this document should be a useful starting point and is
to our knowledge accurate, though incomplete.
</para>
</sect2>
</sect1>
<sect1 id="message-protocol">
<title>Message Protocol</title>
<para>
A <firstterm>message</firstterm> consists of a
<firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
think of a message as a package, the header is the address, and the body
contains the package contents. The message delivery system uses the header
information to figure out where to send the message and how to interpret
it; the recipient interprets the body of the message.
</para>
<para>
The body of the message is made up of zero or more
<firstterm>arguments</firstterm>, which are typed values, such as an
integer or a byte array.
</para>
<para>
Both header and body use the same type system and format for
serializing data. Each type of value has a wire format.
Converting a value from some other representation into the wire
format is called <firstterm>marshaling</firstterm> and converting
it back from the wire format is <firstterm>unmarshaling</firstterm>.
</para>
<sect2 id="message-protocol-signatures">
<title>Type Signatures</title>
<para>
The D-Bus protocol does not include type tags in the marshaled data; a
block of marshaled values must have a known <firstterm>type
signature</firstterm>. The type signature is made up of <firstterm>type
codes</firstterm>. A type code is an ASCII character representing the
type of a value. Because ASCII characters are used, the type signature
will always form a valid ASCII string. A simple string compare
determines whether two type signatures are equivalent.
</para>
<para>
As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
the ASCII character 'i'. So the signature for a block of values
containing a single <literal>INT32</literal> would be:
<programlisting>
"i"
</programlisting>
A block of values containing two <literal>INT32</literal> would have this signature:
<programlisting>
"ii"
</programlisting>
</para>
<para>
All <firstterm>basic</firstterm> types work like
<literal>INT32</literal> in this example. To marshal and unmarshal
basic types, you simply read one value from the data
block corresponding to each type code in the signature.
In addition to basic types, there are four <firstterm>container</firstterm>
types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
and <literal>DICT_ENTRY</literal>.
</para>
<para>
<literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
code does not appear in signatures. Instead, ASCII characters
'(' and ')' are used to mark the beginning and end of the struct.
So for example, a struct containing two integers would have this
signature:
<programlisting>
"(ii)"
</programlisting>
Structs can be nested, so for example a struct containing
an integer and another struct:
<programlisting>
"(i(ii))"
</programlisting>
The value block storing that struct would contain three integers; the
type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
"(iii)" or "iii".
</para>
<para>
The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
but is useful in code that implements the protocol. This type code
is specified to allow such code to interoperate in non-protocol contexts.
</para>
<para>
<literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
followed by a <firstterm>single complete type</firstterm>. The single
complete type following the array is the type of each array element. So
the simple example is:
<programlisting>
"ai"
</programlisting>
which is an array of 32-bit integers. But an array can be of any type,
such as this array-of-struct-with-two-int32-fields:
<programlisting>
"a(ii)"
</programlisting>
Or this array of array of integer:
<programlisting>
"aai"
</programlisting>
</para>
<para>
The phrase <firstterm>single complete type</firstterm> deserves some
definition. A single complete type is a basic type code, a variant type code,
an array with its element type, or a struct with its fields.
So the following signatures are not single complete types:
<programlisting>
"aa"
</programlisting>
<programlisting>
"(ii"
</programlisting>
<programlisting>
"ii)"
</programlisting>
And the following signatures contain multiple complete types:
<programlisting>
"ii"
</programlisting>
<programlisting>
"aiai"
</programlisting>
<programlisting>
"(ii)(ii)"
</programlisting>
Note however that a single complete type may <emphasis>contain</emphasis>
multiple other single complete types.
</para>
<para>
<literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
type <literal>VARIANT</literal> will have the signature of a single complete type as part
of the <emphasis>value</emphasis>. This signature will be followed by a
marshaled value of that type.
</para>
<para>
A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
than parentheses it uses curly braces, and it has more restrictions.
The restrictions are: it occurs only as an array element type; it has
exactly two single complete types inside the curly braces; the first
single complete type (the "key") must be a basic type rather than a
container type. Implementations must not accept dict entries outside of
arrays, must not accept dict entries with zero, one, or more than two
fields, and must not accept dict entries with non-basic-typed keys. A
dict entry is always a key-value pair.
</para>
<para>
The first field in the <literal>DICT_ENTRY</literal> is always the key.
A message is considered corrupt if the same key occurs twice in the same
array of <literal>DICT_ENTRY</literal>. However, for performance reasons
implementations are not required to reject dicts with duplicate keys.
</para>
<para>
In most languages, an array of dict entry would be represented as a
map, hash table, or dict object.
</para>
<para>
The following table summarizes the D-Bus types.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Code</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>INVALID</literal></entry>
<entry>0 (ASCII NUL)</entry>
<entry>Not a valid type code, used to terminate signatures</entry>
</row><row>
<entry><literal>BYTE</literal></entry>
<entry>121 (ASCII 'y')</entry>
<entry>8-bit unsigned integer</entry>
</row><row>
<entry><literal>BOOLEAN</literal></entry>
<entry>98 (ASCII 'b')</entry>
<entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
</row><row>
<entry><literal>INT16</literal></entry>
<entry>110 (ASCII 'n')</entry>
<entry>16-bit signed integer</entry>
</row><row>
<entry><literal>UINT16</literal></entry>
<entry>113 (ASCII 'q')</entry>
<entry>16-bit unsigned integer</entry>
</row><row>
<entry><literal>INT32</literal></entry>
<entry>105 (ASCII 'i')</entry>
<entry>32-bit signed integer</entry>
</row><row>
<entry><literal>UINT32</literal></entry>
<entry>117 (ASCII 'u')</entry>
<entry>32-bit unsigned integer</entry>
</row><row>
<entry><literal>INT64</literal></entry>
<entry>120 (ASCII 'x')</entry>
<entry>64-bit signed integer</entry>
</row><row>
<entry><literal>UINT64</literal></entry>
<entry>116 (ASCII 't')</entry>
<entry>64-bit unsigned integer</entry>
</row><row>
<entry><literal>DOUBLE</literal></entry>
<entry>100 (ASCII 'd')</entry>
<entry>IEEE 754 double</entry>
</row><row>
<entry><literal>STRING</literal></entry>
<entry>115 (ASCII 's')</entry>
<entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated.</entry>
</row><row>
<entry><literal>OBJECT_PATH</literal></entry>
<entry>111 (ASCII 'o')</entry>
<entry>Name of an object instance</entry>
</row><row>
<entry><literal>SIGNATURE</literal></entry>
<entry>103 (ASCII 'g')</entry>
<entry>A type signature</entry>
</row><row>
<entry><literal>ARRAY</literal></entry>
<entry>97 (ASCII 'a')</entry>
<entry>Array</entry>
</row><row>
<entry><literal>STRUCT</literal></entry>
<entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
<entry>Struct</entry>
</row><row>
<entry><literal>VARIANT</literal></entry>
<entry>118 (ASCII 'v') </entry>
<entry>Variant type (the type of the value is part of the value itself)</entry>
</row><row>
<entry><literal>DICT_ENTRY</literal></entry>
<entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
<entry>Entry in a dict or map (array of key-value pairs)</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect2>
<sect2 id="message-protocol-marshaling">
<title>Marshaling (Wire Format)</title>
<para>
Given a type signature, a block of bytes can be converted into typed
values. This section describes the format of the block of bytes. Byte
order and alignment issues are handled uniformly for all D-Bus types.
</para>
<para>
A block of bytes has an associated byte order. The byte order
has to be discovered in some way; for D-Bus messages, the
byte order is part of the message header as described in
<xref linkend="message-protocol-messages"/>. For now, assume
that the byte order is known to be either little endian or big
endian.
</para>
<para>
Each value in a block of bytes is aligned "naturally," for example
4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
8-byte boundary. To properly align a value, <firstterm>alignment
padding</firstterm> may be necessary. The alignment padding must always
be the minimum required padding to properly align the following value;
and it must always be made up of nul bytes. The alignment padding must
not be left uninitialized (it can't contain garbage), and more padding
than required must not be used.
</para>
<para>
Given all this, the types are marshaled on the wire as follows:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Encoding</entry>
<entry>Alignment</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>INVALID</literal></entry>
<entry>Not applicable; cannot be marshaled.</entry>
<entry>N/A</entry>
</row><row>
<entry><literal>BYTE</literal></entry>
<entry>A single 8-bit byte.</entry>
<entry>1</entry>
</row><row>
<entry><literal>BOOLEAN</literal></entry>
<entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
<entry>4</entry>
</row><row>
<entry><literal>INT16</literal></entry>
<entry>16-bit signed integer in the message's byte order.</entry>
<entry>2</entry>
</row><row>
<entry><literal>UINT16</literal></entry>
<entry>16-bit unsigned integer in the message's byte order.</entry>
<entry>2</entry>
</row><row>
<entry><literal>INT32</literal></entry>
<entry>32-bit signed integer in the message's byte order.</entry>
<entry>4</entry>
</row><row>
<entry><literal>UINT32</literal></entry>
<entry>32-bit unsigned integer in the message's byte order.</entry>
<entry>4</entry>
</row><row>
<entry><literal>INT64</literal></entry>
<entry>64-bit signed integer in the message's byte order.</entry>
<entry>8</entry>
</row><row>
<entry><literal>UINT64</literal></entry>
<entry>64-bit unsigned integer in the message's byte order.</entry>
<entry>8</entry>
</row><row>
<entry><literal>DOUBLE</literal></entry>
<entry>64-bit IEEE 754 double in the message's byte order.</entry>
<entry>8</entry>
</row><row>
<entry><literal>STRING</literal></entry>
<entry>A <literal>UINT32</literal> indicating the string's
length in bytes excluding its terminating nul, followed by
string data of the given length, followed by a terminating nul
byte.
</entry>
<entry>
4 (for the length)
</entry>
</row><row>
<entry><literal>OBJECT_PATH</literal></entry>
<entry>Exactly the same as <literal>STRING</literal> except the
content must be a valid object path (see below).
</entry>
<entry>
4 (for the length)
</entry>
</row><row>
<entry><literal>SIGNATURE</literal></entry>
<entry>The same as <literal>STRING</literal> except the length is a single
byte (thus signatures have a maximum length of 255)
and the content must be a valid signature (see below).
</entry>
<entry>
1
</entry>
</row><row>
<entry><literal>ARRAY</literal></entry>
<entry>
A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
alignment padding to the alignment boundary of the array element type,
followed by each array element. The array length is from the
end of the alignment padding to the end of the last element,
i.e. it does not include the padding after the length,
or any padding after the last element.
Arrays have a maximum length defined to be 2 to the 26th power or
67108864. Implementations must not send or accept arrays exceeding this
length.
</entry>
<entry>
4 (for the length)
</entry>
</row><row>
<entry><literal>STRUCT</literal></entry>
<entry>
A struct must start on an 8-byte boundary regardless of the
type of the struct fields. The struct value consists of each
field marshaled in sequence starting from that 8-byte
alignment boundary.
</entry>
<entry>
8
</entry>
</row><row>
<entry><literal>VARIANT</literal></entry>
<entry>
A variant type has a marshaled <literal>SIGNATURE</literal>
followed by a marshaled value with the type
given in the signature.
Unlike a message signature, the variant signature
can contain only a single complete type.
So "i" is OK, "ii" is not.
</entry>
<entry>
1 (alignment of the signature)
</entry>
</row><row>
<entry><literal>DICT_ENTRY</literal></entry>
<entry>
Identical to STRUCT.
</entry>
<entry>
8
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<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>
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 ends with a nul byte.
</para>
</listitem>
<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 and open and close parentheses are
allowed in the signature. The <literal>STRUCT</literal> type code
is not allowed in signatures, because parentheses
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>
<listitem>
<para>
Signatures must be nul-terminated.
</para>
</listitem>
</itemizedlist>
</para>
</sect3>
</sect2>
<sect2 id="message-protocol-messages">
<title>Message Format</title>
<para>
A message consists of a header and a body. The header is a block of
values with a fixed signature and meaning. The body is a separate block
of values, with a signature specified in the header.
</para>
<para>
The length of the header must be a multiple of 8, allowing the body to
begin on an 8-byte boundary when storing the entire message in a single
buffer. If the header does not naturally end on an 8-byte boundary
up to 7 bytes of nul-initialized alignment padding must be added.
</para>
<para>
The message body need not end on an 8-byte boundary.
</para>
<para>
The maximum length of a message, including header, header alignment padding,
and body is 2 to the 27th power or 134217728. Implementations must not
send or accept messages exceeding this size.
</para>
<para>
The signature of the header is:
<programlisting>
"yyyyuua(yv)"
</programlisting>
Written out more readably, this is:
<programlisting>
BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
</programlisting>
</para>
<para>
These values have the following meanings:
<informaltable>
<tgroup cols="2">
<thead>
<row>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>1st <literal>BYTE</literal></entry>
<entry>Endianness flag; ASCII 'l' for little-endian
or ASCII 'B' for big-endian. Both header and body are
in this endianness.</entry>
</row>
<row>
<entry>2nd <literal>BYTE</literal></entry>
<entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
Currently-defined types are described below.
</entry>
</row>
<row>
<entry>3rd <literal>BYTE</literal></entry>
<entry>Bitwise OR of flags. Unknown flags
must be ignored. Currently-defined flags are described below.
</entry>
</row>
<row>
<entry>4th <literal>BYTE</literal></entry>
<entry>Major protocol version of the sending application. If
the major protocol version of the receiving application does not
match, the applications will not be able to communicate and the
D-Bus connection must be disconnected. The major protocol
version for this version of the specification is 0.
FIXME this field is stupid and pointless to put in
every message.
</entry>
</row>
<row>
<entry>1st <literal>UINT32</literal></entry>
<entry>Length in bytes of the message body, starting
from the end of the header. The header ends after
its alignment padding to an 8-boundary.
</entry>
</row>
<row>
<entry>2nd <literal>UINT32</literal></entry>
<entry>The serial of this message, used as a cookie
by the sender to identify the reply corresponding
to this request.
</entry>
</row>
<row>
<entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
<entry>An array of zero or more <firstterm>header
fields</firstterm> where the byte is the field code, and the
variant is the field value. The message type determines
which fields are required.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
<firstterm>Message types</firstterm> that can appear in the second byte
of the header are:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional name</entry>
<entry>Decimal value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>INVALID</literal></entry>
<entry>0</entry>
<entry>This is an invalid type.</entry>
</row>
<row>
<entry><literal>METHOD_CALL</literal></entry>
<entry>1</entry>
<entry>Method call.</entry>
</row>
<row>
<entry><literal>METHOD_RETURN</literal></entry>
<entry>2</entry>
<entry>Method reply with returned data.</entry>
</row>
<row>
<entry><literal>ERROR</literal></entry>
<entry>3</entry>
<entry>Error reply. If the first argument exists and is a
string, it is an error message.</entry>
</row>
<row>
<entry><literal>SIGNAL</literal></entry>
<entry>4</entry>
<entry>Signal emission.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Flags that can appear in the third byte of the header:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional name</entry>
<entry>Hex value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>NO_REPLY_EXPECTED</literal></entry>
<entry>0x1</entry>
<entry>This message does not expect method return replies or
error replies; the reply can be omitted as an
optimization. However, it is compliant with this specification
to return the reply despite this flag and the only harm
from doing so is extra network traffic.
</entry>
</row>
<row>
<entry><literal>NO_AUTO_START</literal></entry>
<entry>0x2</entry>
<entry>The bus must not launch an owner
for the destination name in response to this message.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<sect3 id="message-protocol-header-fields">
<title>Header Fields</title>
<para>
The array at the end of the header contains <firstterm>header
fields</firstterm>, where each field is a 1-byte field code followed
by a field value. A header must contain the required header fields for
its message type, and zero or more of any optional header
fields. Future versions of this protocol specification may add new
fields. Implementations must ignore fields they do not
understand. Implementations must not invent their own header fields;
only changes to this specification may introduce new header fields.
</para>
<para>
Again, if an implementation sees a header field code that it does not
expect, it must ignore that field, as it will be part of a new
(but compatible) version of this specification. This also applies
to known header fields appearing in unexpected messages, for
example: if a signal has a reply serial it must be ignored
even though it has no meaning as of this version of the spec.
</para>
<para>
However, implementations must not send or accept known header fields
with the wrong type stored in the field value. So for example a
message with an <literal>INTERFACE</literal> field of type
<literal>UINT32</literal> would be considered corrupt.
</para>
<para>
Here are the currently-defined header fields:
<informaltable>
<tgroup cols="5">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Decimal Code</entry>
<entry>Type</entry>
<entry>Required In</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>INVALID</literal></entry>
<entry>0</entry>
<entry>N/A</entry>
<entry>not allowed</entry>
<entry>Not a valid field name (error if it appears in a message)</entry>
</row>
<row>
<entry><literal>PATH</literal></entry>
<entry>1</entry>
<entry><literal>OBJECT_PATH</literal></entry>
<entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
<entry>The object to send a call to,
or the object a signal is emitted from.
</entry>
</row>
<row>
<entry><literal>INTERFACE</literal></entry>
<entry>2</entry>
<entry><literal>STRING</literal></entry>
<entry><literal>SIGNAL</literal></entry>
<entry>
The interface to invoke a method call on, or
that a signal is emitted from. Optional for
method calls, required for signals.
</entry>
</row>
<row>
<entry><literal>MEMBER</literal></entry>
<entry>3</entry>
<entry><literal>STRING</literal></entry>
<entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
<entry>The member, either the method name or signal name.</entry>
</row>
<row>
<entry><literal>ERROR_NAME</literal></entry>
<entry>4</entry>
<entry><literal>STRING</literal></entry>
<entry><literal>ERROR</literal></entry>
<entry>The name of the error that occurred, for errors</entry>
</row>
<row>
<entry><literal>REPLY_SERIAL</literal></entry>
<entry>5</entry>
<entry><literal>UINT32</literal></entry>
<entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
<entry>The serial number of the message this message is a reply
to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
</row>
<row>
<entry><literal>DESTINATION</literal></entry>
<entry>6</entry>
<entry><literal>STRING</literal></entry>
<entry>optional</entry>
<entry>The name of the connection this message is intended for.
Only used in combination with the message bus, see
<xref linkend="message-bus"/>.</entry>
</row>
<row>
<entry><literal>SENDER</literal></entry>
<entry>7</entry>
<entry><literal>STRING</literal></entry>
<entry>optional</entry>
<entry>Unique name of the sending connection.
The message bus fills in this field so it is reliable; the field is
only meaningful in combination with the message bus.</entry>
</row>
<row>
<entry><literal>SIGNATURE</literal></entry>
<entry>8</entry>
<entry><literal>SIGNATURE</literal></entry>
<entry>optional</entry>
<entry>The signature of the message body.
If omitted, it is assumed to be the
empty signature "" (i.e. the body must be 0-length).</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
</sect2>
<sect2 id="message-protocol-names">
<title>Valid Names</title>
<para>
The various names in D-Bus messages have some restrictions.
</para>
<para>
There is a <firstterm>maximum name length</firstterm>
of 255 which applies to bus names, interfaces, and members.
</para>
<sect3 id="message-protocol-names-interface">
<title>Interface names</title>
<para>
Interfaces have names with type <literal>STRING</literal>, meaning that
they must be valid UTF-8. However, there are also some
additional restrictions that apply to interface names
specifically:
<itemizedlist>
<listitem><para>Interface names are composed of 1 or more elements separated by
a period ('.') character. All elements must contain at least
one character.
</para>
</listitem>
<listitem><para>Each element must only contain the ASCII characters
"[A-Z][a-z][0-9]_" and must not begin with a digit.
</para>
</listitem>
<listitem><para>Interface names must contain at least one '.' (period)
character (and thus at least two elements).
</para></listitem>
<listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
<listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
</itemizedlist>
</para>
</sect3>
<sect3 id="message-protocol-names-bus">
<title>Bus names</title>
<para>
Connections have one or more bus names associated with them.
A connection has exactly one bus name that is a unique connection
name. The unique connection name remains with the connection for
its entire lifetime.
A bus name is of type <literal>STRING</literal>,
meaning that it must be valid UTF-8. However, there are also
some additional restrictions that apply to bus names
specifically:
<itemizedlist>
<listitem><para>Bus names that start with a colon (':')
character are unique connection names.
</para>
</listitem>
<listitem><para>Bus names are composed of 1 or more elements separated by
a period ('.') character. All elements must contain at least
one character.
</para>
</listitem>
<listitem><para>Each element must only contain the ASCII characters
"[A-Z][a-z][0-9]_-". Only elements that are part of a unique
connection name may begin with a digit, elements in
other bus names must not begin with a digit.
</para>
</listitem>
<listitem><para>Bus names must contain at least one '.' (period)
character (and thus at least two elements).
</para></listitem>
<listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
<listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
</itemizedlist>
</para>
<para>
Note that the hyphen ('-') character is allowed in bus names but
not in interface names.
</para>
</sect3>
<sect3 id="message-protocol-names-member">
<title>Member names</title>
<para>
Member (i.e. method or signal) names:
<itemizedlist>
<listitem><para>Must only contain the ASCII characters
"[A-Z][a-z][0-9]_" and may not begin with a
digit.</para></listitem>
<listitem><para>Must not contain the '.' (period) character.</para></listitem>
<listitem><para>Must not exceed the maximum name length.</para></listitem>
<listitem><para>Must be at least 1 byte in length.</para></listitem>
</itemizedlist>
</para>
</sect3>
<sect3 id="message-protocol-names-error">
<title>Error names</title>
<para>
Error names have the same restrictions as interface names.
</para>
</sect3>
</sect2>
<sect2 id="message-protocol-types">
<title>Message Types</title>
<para>
Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
<literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
This section describes these conventions.
</para>
<sect3 id="message-protocol-types-method">
<title>Method Calls</title>
<para>
Some messages invoke an operation on a remote object. These are
called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
messages map naturally to methods on objects in a typical program.
</para>
<para>
A method call message is required to have a <literal>MEMBER</literal> header field
indicating the name of the method. Optionally, the message has an
<literal>INTERFACE</literal> field giving the interface the method is a part of. In the
absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
a method with the same name, it is undefined which of the two methods
will be invoked. Implementations may also choose to return an error in
this ambiguous case. However, if a method name is unique
implementations must not require an interface field.
</para>
<para>
Method call messages also include a <literal>PATH</literal> field
indicating the object to invoke the method on. If the call is passing
through a message bus, the message will also have a
<literal>DESTINATION</literal> field giving the name of the connection
to receive the message.
</para>
<para>
When an application handles a method call message, it is required to
return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
</para>
<para>
If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
are the return value(s) or "out parameters" of the method call.
If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
and the call fails; no return value will be provided. It makes
no sense to send multiple replies to the same method call.
</para>
<para>
Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
reply is required, so the caller will know the method
was successfully processed.
</para>
<para>
The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
header field.
</para>
<para>
If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
then as an optimization the application receiving the method
call may choose to omit the reply message (regardless of
whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
flag and reply anyway.
</para>
<para>
Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
destination name does not exist then a program to own the destination
name will be started before the message is delivered. The message
will be held until the new program is successfully started or has
failed to start; in case of failure, an error will be returned. This
flag is only relevant in the context of a message bus, it is ignored
during one-to-one communication with no intermediate bus.
</para>
<sect4 id="message-protocol-types-method-apis">
<title>Mapping method calls to native APIs</title>
<para>
APIs for D-Bus may map method calls to a method call in a specific
programming language, such as C++, or may map a method call written
in an IDL to a D-Bus message.
</para>
<para>
In APIs of this nature, arguments to a method are often termed "in"
(which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
"inout" arguments, which are both sent and received, i.e. the caller
passes in a value which is modified. Mapped to D-Bus, an "inout"
argument is equivalent to an "in" argument, followed by an "out"
argument. You can't pass things "by reference" over the wire, so
"inout" is purely an illusion of the in-process API.
</para>
<para>
Given a method with zero or one return values, followed by zero or more
arguments, where each argument may be "in", "out", or "inout", the
caller constructs a message by appending each "in" or "inout" argument,
in order. "out" arguments are not represented in the caller's message.
</para>
<para>
The recipient constructs a reply by appending first the return value
if any, then each "out" or "inout" argument, in order.
"in" arguments are not represented in the reply message.
</para>
<para>
Error replies are normally mapped to exceptions in languages that have
exceptions.
</para>
<para>
In converting from native APIs to D-Bus, it is perhaps nice to
map D-Bus naming conventions ("FooBar") to native conventions
such as "fooBar" or "foo_bar" automatically. This is OK
as long as you can say that the native API is one that
was specifically written for D-Bus. It makes the most sense
when writing object implementations that will be exported
over the bus. Object proxies used to invoke remote D-Bus
objects probably need the ability to call any D-Bus method,
and thus a magic name mapping like this could be a problem.
</para>
<para>
This specification doesn't require anything of native API bindings;
the preceding is only a suggested convention for consistency
among bindings.
</para>
</sect4>
</sect3>
<sect3 id="message-protocol-types-signal">
<title>Signal Emission</title>
<para>
Unlike method calls, signal emissions have no replies.
A signal emission is simply a single message of type <literal>SIGNAL</literal>.
It must have three header fields: <literal>PATH</literal> giving the object
the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
for signals, though it is optional for method calls.
</para>
</sect3>
<sect3 id="message-protocol-types-errors">
<title>Errors</title>
<para>
Messages of type <literal>ERROR</literal> are most commonly replies
to a <literal>METHOD_CALL</literal>, but may be returned in reply
to any kind of message. The message bus for example
will return an <literal>ERROR</literal> in reply to a signal emission if
the bus does not have enough memory to send the signal.
</para>
<para>
An <literal>ERROR</literal> may have any arguments, but if the first
argument is a <literal>STRING</literal>, it must be an error message.
The error message may be logged or shown to the user
in some way.
</para>
</sect3>
<sect3 id="message-protocol-types-notation">
<title>Notation in this document</title>
<para>
This document uses a simple pseudo-IDL to describe particular method
calls and signals. Here is an example of a method call:
<programlisting>
org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
out UINT32 resultcode)
</programlisting>
This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
<literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
characters so it's known that the last part of the name in
the "IDL" is the member name.
</para>
<para>
In C++ that might end up looking like this:
<programlisting>
unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
unsigned int flags);
</programlisting>
or equally valid, the return value could be done as an argument:
<programlisting>
void org::freedesktop::DBus::StartServiceByName (const char *name,
unsigned int flags,
unsigned int *resultcode);
</programlisting>
It's really up to the API designer how they want to make
this look. You could design an API where the namespace wasn't used
in C++, using STL or Qt, using varargs, or whatever you wanted.
</para>
<para>
Signals are written as follows:
<programlisting>
org.freedesktop.DBus.NameLost (STRING name)
</programlisting>
Signals don't specify "in" vs. "out" because only
a single direction is possible.
</para>
<para>
It isn't especially encouraged to use this lame pseudo-IDL in actual
API implementations; you might use the native notation for the
language you're using, or you might use COM or CORBA IDL, for example.
</para>
</sect3>
</sect2>
<sect2 id="message-protocol-handling-invalid">
<title>Invalid Protocol and Spec Extensions</title>
<para>
For security reasons, the D-Bus protocol should be strictly parsed and
validated, with the exception of defined extension points. Any invalid
protocol or spec violations should result in immediately dropping the
connection without notice to the other end. Exceptions should be
carefully considered, e.g. an exception may be warranted for a
well-understood idiosyncrasy of a widely-deployed implementation. In
cases where the other end of a connection is 100% trusted and known to
be friendly, skipping validation for performance reasons could also make
sense in certain cases.
</para>
<para>
Generally speaking violations of the "must" requirements in this spec
should be considered possible attempts to exploit security, and violations
of the "should" suggestions should be considered legitimate (though perhaps
they should generate an error in some cases).
</para>
<para>
The following extension points are built in to D-Bus on purpose and must
not be treated as invalid protocol. The extension points are intended
for use by future versions of this spec, they are not intended for third
parties. At the moment, the only way a third party could extend D-Bus
without breaking interoperability would be to introduce a way to negotiate new
feature support as part of the auth protocol, using EXTENSION_-prefixed
commands. There is not yet a standard way to negotiate features.
<itemizedlist>
<listitem>
<para>
In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
commands result in an ERROR rather than a disconnect. This enables
future extensions to the protocol. Commands starting with EXTENSION_ are
reserved for third parties.
</para>
</listitem>
<listitem>
<para>
The authentication protocol supports pluggable auth mechanisms.
</para>
</listitem>
<listitem>
<para>
The address format (see <xref linkend="addresses"/>) supports new
kinds of transport.
</para>
</listitem>
<listitem>
<para>
Messages with an unknown type (something other than
<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
<literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
Unknown-type messages must still be well-formed in the same way
as the known messages, however. They still have the normal
header and body.
</para>
</listitem>
<listitem>
<para>
Header fields with an unknown or unexpected field code must be ignored,
though again they must still be well-formed.
</para>
</listitem>
<listitem>
<para>
New standard interfaces (with new methods and signals) can of course be added.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
</sect1>
<sect1 id="auth-protocol">
<title>Authentication Protocol</title>
<para>
Before the flow of messages begins, two applications must
authenticate. A simple plain-text protocol is used for
authentication; this protocol is a SASL profile, and maps fairly
directly from the SASL specification. The message encoding is
NOT used here, only plain text messages.
</para>
<para>
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
</para>
<sect2 id="auth-protocol-overview">
<title>Protocol Overview</title>
<para>
The protocol is a line-based protocol, where each line ends with
\r\n. Each line begins with an all-caps ASCII command name containing
only the character range [A-Z_], a space, then any arguments for the
command, then the \r\n ending the line. The protocol is
case-sensitive. All bytes must be in the ASCII character set.
Commands from the client to the server are as follows:
<itemizedlist>
<listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
<listitem><para>CANCEL</para></listitem>
<listitem><para>BEGIN</para></listitem>
<listitem><para>DATA <data in hex encoding></para></listitem>
<listitem><para>ERROR [human-readable error explanation]</para></listitem>
</itemizedlist>
From server to client are as follows:
<itemizedlist>
<listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
<listitem><para>OK <GUID in hex></para></listitem>
<listitem><para>DATA <data in hex encoding></para></listitem>
<listitem><para>ERROR</para></listitem>
</itemizedlist>
</para>
<para>
Unofficial extensions to the command set must begin with the letters
"EXTENSION_", to avoid conflicts with future official commands.
For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
</para>
</sect2>
<sect2 id="auth-nul-byte">
<title>Special credentials-passing nul byte</title>
<para>
Immediately after connecting to the server, the client must send a
single nul byte. This byte may be accompanied by credentials
information on some operating systems that use sendmsg() with
SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
sockets. However, the nul byte must be sent even on other kinds of
socket, and even on operating systems that do not require a byte to be
sent in order to transmit credentials. The text protocol described in
this document begins after the single nul byte. If the first byte
received from the client is not a nul byte, the server may disconnect
that client.
</para>
<para>
A nul byte in any context other than the initial byte is an error;
the protocol is ASCII-only.
</para>
<para>
The credentials sent along with the nul byte may be used with the
SASL mechanism EXTERNAL.
</para>
</sect2>
<sect2 id="auth-command-auth">
<title>AUTH command</title>
<para>
If an AUTH command has no arguments, it is a request to list
available mechanisms. The server must respond with a REJECTED
command listing the mechanisms it understands, or with an error.
</para>
<para>
If an AUTH command specifies a mechanism, and the server supports
said mechanism, the server should begin exchanging SASL
challenge-response data with the client using DATA commands.
</para>
<para>
If the server does not support the mechanism given in the AUTH
command, it must send either a REJECTED command listing the mechanisms
it does support, or an error.
</para>
<para>
If the [initial-response] argument is provided, it is intended for use
with mechanisms that have no initial challenge (or an empty initial
challenge), as if it were the argument to an initial DATA command. If
the selected mechanism has an initial challenge and [initial-response]
was provided, the server should reject authentication by sending
REJECTED.
</para>
<para>
If authentication succeeds after exchanging DATA commands,
an OK command must be sent to the client.
</para>
<para>
The first octet received by the client after the \r\n of the OK
command must be the first octet of the authenticated/encrypted
stream of D-Bus messages.
</para>
<para>
The first octet received by the server after the \r\n of the BEGIN
command from the client must be the first octet of the
authenticated/encrypted stream of D-Bus messages.
</para>
</sect2>
<sect2 id="auth-command-cancel">
<title>CANCEL Command</title>
<para>
At any time up to sending the BEGIN command, the client may send a
CANCEL command. On receiving the CANCEL command, the server must
send a REJECTED command and abort the current authentication
exchange.
</para>
</sect2>
<sect2 id="auth-command-data">
<title>DATA Command</title>
<para>
The DATA command may come from either client or server, and simply
contains a hex-encoded block of data to be interpreted
according to the SASL mechanism in use.
</para>
<para>
Some SASL mechanisms support sending an "empty string";
FIXME we need some way to do this.
</para>
</sect2>
<sect2 id="auth-command-begin">
<title>BEGIN Command</title>
<para>
The BEGIN command acknowledges that the client has received an
OK command from the server, and that the stream of messages
is about to begin.
</para>
<para>
The first octet received by the server after the \r\n of the BEGIN
command from the client must be the first octet of the
authenticated/encrypted stream of D-Bus messages.
</para>
</sect2>
<sect2 id="auth-command-rejected">
<title>REJECTED Command</title>
<para>
The REJECTED command indicates that the current authentication
exchange has failed, and further exchange of DATA is inappropriate.
The client would normally try another mechanism, or try providing
different responses to challenges.
</para><para>
Optionally, the REJECTED command has a space-separated list of
available auth mechanisms as arguments. If a server ever provides
a list of supported mechanisms, it must provide the same list
each time it sends a REJECTED message. Clients are free to
ignore all lists received after the first.
</para>
</sect2>
<sect2 id="auth-command-ok">
<title>OK Command</title>
<para>
The OK command indicates that the client has been authenticated,
and that further communication will be a stream of D-Bus messages
(optionally encrypted, as negotiated) rather than this protocol.
</para>
<para>
The first octet received by the client after the \r\n of the OK
command must be the first octet of the authenticated/encrypted
stream of D-Bus messages.
</para>
<para>
The client must respond to the OK command by sending a BEGIN
command, followed by its stream of messages, or by disconnecting.
The server must not accept additional commands using this protocol
after the OK command has been sent.
</para>
<para>
The OK command has one argument, which is the GUID of the server.
See <xref linkend="addresses"/> for more on server GUIDs.
</para>
</sect2>
<sect2 id="auth-command-error">
<title>ERROR Command</title>
<para>
The ERROR command indicates that either server or client did not
know a command, does not accept the given command in the current
context, or did not understand the arguments to the command. This
allows the protocol to be extended; a client or server can send a
command present or permitted only in new protocol versions, and if
an ERROR is received instead of an appropriate response, fall back
to using some other technique.
</para>
<para>
If an ERROR is sent, the server or client that sent the
error must continue as if the command causing the ERROR had never been
received. However, the the server or client receiving the error
should try something other than whatever caused the error;
if only canceling/rejecting the authentication.
</para>
<para>
If the D-Bus protocol changes incompatibly at some future time,
applications implementing the new protocol would probably be able to
check for support of the new protocol by sending a new command and
receiving an ERROR from applications that don't understand it. Thus the
ERROR feature of the auth protocol is an escape hatch that lets us
negotiate extensions or changes to the D-Bus protocol in the future.
</para>
</sect2>
<sect2 id="auth-examples">
<title>Authentication examples</title>
<para>
<figure>
<title>Example of successful magic cookie authentication</title>
<programlisting>
(MAGIC_COOKIE is a made up mechanism)
C: AUTH MAGIC_COOKIE 3138363935333137393635383634
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of finding out mechanisms then picking one</title>
<programlisting>
C: AUTH
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of client sends unknown command then falls back to regular auth</title>
<programlisting>
C: FOOBAR
S: ERROR
C: AUTH MAGIC_COOKIE 3736343435313230333039
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of server doesn't support initial auth mechanism</title>
<programlisting>
C: AUTH MAGIC_COOKIE 3736343435313230333039
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of wrong password or the like followed by successful retry</title>
<programlisting>
C: AUTH MAGIC_COOKIE 3736343435313230333039
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: REJECTED
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of skey cancelled and restarted</title>
<programlisting>
C: AUTH MAGIC_COOKIE 3736343435313230333039
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: CANCEL
S: REJECTED
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
</para>
</sect2>
<sect2 id="auth-states">
<title>Authentication state diagrams</title>
<para>
This section documents the auth protocol in terms of
a state machine for the client and the server. This is
probably the most robust way to implement the protocol.
</para>
<sect3 id="auth-states-client">
<title>Client states</title>
<para>
To more precisely describe the interaction between the
protocol state machine and the authentication mechanisms the
following notation is used: MECH(CHALL) means that the
server challenge CHALL was fed to the mechanism MECH, which
returns one of
<itemizedlist>
<listitem>
<para>
CONTINUE(RESP) means continue the auth conversation
and send RESP as the response to the server;
</para>
</listitem>
<listitem>
<para>
OK(RESP) means that after sending RESP to the server
the client side of the auth conversation is finished
and the server should return "OK";
</para>
</listitem>
<listitem>
<para>
ERROR means that CHALL was invalid and could not be
processed.
</para>
</listitem>
</itemizedlist>
Both RESP and CHALL may be empty.
</para>
<para>
The Client starts by getting an initial response from the
default mechanism and sends AUTH MECH RESP, or AUTH MECH if
the mechanism did not provide an initial response. If the
mechanism returns CONTINUE, the client starts in state
<emphasis>WaitingForData</emphasis>, if the mechanism
returns OK the client starts in state
<emphasis>WaitingForOK</emphasis>.
</para>
<para>
The client should keep track of available mechanisms and
which it mechanisms it has already attempted. This list is
used to decide which AUTH command to send. When the list is
exhausted, the client should give up and close the
connection.
</para>
<formalpara>
<title><emphasis>WaitingForData</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive DATA CHALL
<simplelist>
<member>
MECH(CHALL) returns CONTINUE(RESP) → send
DATA RESP, goto
<emphasis>WaitingForData</emphasis>
</member>
<member>
MECH(CHALL) returns OK(RESP) → send DATA
RESP, goto <emphasis>WaitingForOK</emphasis>
</member>
<member>
MECH(CHALL) returns ERROR → send ERROR
[msg], goto <emphasis>WaitingForData</emphasis>
</member>
</simplelist>
</para>
</listitem>
<listitem>
<para>
Receive REJECTED [mechs] →
send AUTH [next mech], goto
WaitingForData or <emphasis>WaitingForOK</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR → send
CANCEL, goto
<emphasis>WaitingForReject</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive OK → send
BEGIN, terminate auth
conversation, authenticated
</para>
</listitem>
<listitem>
<para>
Receive anything else → send
ERROR, goto
<emphasis>WaitingForData</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForOK</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive OK → send BEGIN, terminate auth
conversation, <emphasis>authenticated</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive REJECT [mechs] → send AUTH [next mech],
goto <emphasis>WaitingForData</emphasis> or
<emphasis>WaitingForOK</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive DATA → send CANCEL, goto
<emphasis>WaitingForReject</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR → send CANCEL, goto
<emphasis>WaitingForReject</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else → send ERROR, goto
<emphasis>WaitingForOK</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForReject</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive REJECT [mechs] → send AUTH [next mech],
goto <emphasis>WaitingForData</emphasis> or
<emphasis>WaitingForOK</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else → terminate auth
conversation, disconnect
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
</sect3>
<sect3 id="auth-states-server">
<title>Server states</title>
<para>
For the server MECH(RESP) means that the client response
RESP was fed to the the mechanism MECH, which returns one of
<itemizedlist>
<listitem>
<para>
CONTINUE(CHALL) means continue the auth conversation and
send CHALL as the challenge to the client;
</para>
</listitem>
<listitem>
<para>
OK means that the client has been successfully
authenticated;
</para>
</listitem>
<listitem>
<para>
REJECT means that the client failed to authenticate or
there was an error in RESP.
</para>
</listitem>
</itemizedlist>
The server starts out in state
<emphasis>WaitingForAuth</emphasis>. If the client is
rejected too many times the server must disconnect the
client.
</para>
<formalpara>
<title><emphasis>WaitingForAuth</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive AUTH → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive AUTH MECH RESP
<simplelist>
<member>
MECH not valid mechanism → send REJECTED
[mechs], goto
<emphasis>WaitingForAuth</emphasis>
</member>
<member>
MECH(RESP) returns CONTINUE(CHALL) → send
DATA CHALL, goto
<emphasis>WaitingForData</emphasis>
</member>
<member>
MECH(RESP) returns OK → send OK, goto
<emphasis>WaitingForBegin</emphasis>
</member>
<member>
MECH(RESP) returns REJECT → send REJECTED
[mechs], goto
<emphasis>WaitingForAuth</emphasis>
</member>
</simplelist>
</para>
</listitem>
<listitem>
<para>
Receive BEGIN → terminate
auth conversation, disconnect
</para>
</listitem>
<listitem>
<para>
Receive ERROR → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else → send
ERROR, goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForData</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive DATA RESP
<simplelist>
<member>
MECH(RESP) returns CONTINUE(CHALL) → send
DATA CHALL, goto
<emphasis>WaitingForData</emphasis>
</member>
<member>
MECH(RESP) returns OK → send OK, goto
<emphasis>WaitingForBegin</emphasis>
</member>
<member>
MECH(RESP) returns REJECT → send REJECTED
[mechs], goto
<emphasis>WaitingForAuth</emphasis>
</member>
</simplelist>
</para>
</listitem>
<listitem>
<para>
Receive BEGIN → terminate auth conversation,
disconnect
</para>
</listitem>
<listitem>
<para>
Receive CANCEL → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else → send ERROR, goto
<emphasis>WaitingForData</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForBegin</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive BEGIN → terminate auth conversation,
client authenticated
</para>
</listitem>
<listitem>
<para>
Receive CANCEL → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else → send ERROR, goto
<emphasis>WaitingForBegin</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
</sect3>
</sect2>
<sect2 id="auth-mechanisms">
<title>Authentication mechanisms</title>
<para>
This section describes some new authentication mechanisms.
D-Bus also allows any standard SASL mechanism of course.
</para>
<sect3 id="auth-mechanisms-sha">
<title>DBUS_COOKIE_SHA1</title>
<para>
The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
has the ability to read a private file owned by the user being
authenticated. If the client can prove that it has access to a secret
cookie stored in this file, then the client is authenticated.
Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
directory.
</para>
<para>
Authentication proceeds as follows:
<itemizedlist>
<listitem>
<para>
The client sends the username it would like to authenticate
as.
</para>
</listitem>
<listitem>
<para>
The server sends the name of its "cookie context" (see below); a
space character; the integer ID of the secret cookie the client
must demonstrate knowledge of; a space character; then a
hex-encoded randomly-generated challenge string.
</para>
</listitem>
<listitem>
<para>
The client locates the cookie, and generates its own hex-encoded
randomly-generated challenge string. The client then
concatenates the server's hex-encoded challenge, a ":"
character, its own hex-encoded challenge, another ":" character,
and the hex-encoded cookie. It computes the SHA-1 hash of this
composite string. It sends back to the server the client's
hex-encoded challenge string, a space character, and the SHA-1
hash.
</para>
</listitem>
<listitem>
<para>
The server generates the same concatenated string used by the
client and computes its SHA-1 hash. It compares the hash with
the hash received from the client; if the two hashes match, the
client is authenticated.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Each server has a "cookie context," which is a name that identifies a
set of cookies that apply to that server. A sample context might be
"org_freedesktop_session_bus". Context names must be valid ASCII,
nonzero length, and may not contain the characters slash ("/"),
backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
tab ("\t"), or period ("."). There is a default context,
"org_freedesktop_general" that's used by servers that do not specify
otherwise.
</para>
<para>
Cookies are stored in a user's home directory, in the directory
<filename>~/.dbus-keyrings/</filename>. This directory must
not be readable or writable by other users. If it is,
clients and servers must ignore it. The directory
contains cookie files named after the cookie context.
</para>
<para>
A cookie file contains one cookie per line. Each line
has three space-separated fields:
<itemizedlist>
<listitem>
<para>
The cookie ID number, which must be a non-negative integer and
may not be used twice in the same file.
</para>
</listitem>
<listitem>
<para>
The cookie's creation time, in UNIX seconds-since-the-epoch
format.
</para>
</listitem>
<listitem>
<para>
The cookie itself, a hex-encoded random block of bytes. The cookie
may be of any length, though obviously security increases
as the length increases.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Only server processes modify the cookie file.
They must do so with this procedure:
<itemizedlist>
<listitem>
<para>
Create a lockfile name by appending ".lock" to the name of the
cookie file. The server should attempt to create this file
using <literal>O_CREAT | O_EXCL</literal>. If file creation
fails, the lock fails. Servers should retry for a reasonable
period of time, then they may choose to delete an existing lock
to keep users from having to manually delete a stale
lock. <footnote><para>Lockfiles are used instead of real file
locking <literal>fcntl()</literal> because real locking
implementations are still flaky on network
filesystems.</para></footnote>
</para>
</listitem>
<listitem>
<para>
Once the lockfile has been created, the server loads the cookie
file. It should then delete any cookies that are old (the
timeout can be fairly short), or more than a reasonable
time in the future (so that cookies never accidentally
become permanent, if the clock was set far into the future
at some point). If no recent keys remain, the
server may generate a new key.
</para>
</listitem>
<listitem>
<para>
The pruned and possibly added-to cookie file
must be resaved atomically (using a temporary
file which is rename()'d).
</para>
</listitem>
<listitem>
<para>
The lock must be dropped by deleting the lockfile.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Clients need not lock the file in order to load it,
because servers are required to save the file atomically.
</para>
</sect3>
</sect2>
</sect1>
<sect1 id="addresses">
<title>Server Addresses</title>
<para>
Server addresses consist of a transport name followed by a colon, and
then an optional, comma-separated list of keys and values in the form key=value.
Each value is escaped.
</para>
<para>
For example:
<programlisting>unix:path=/tmp/dbus-test</programlisting>
Which is the address to a unix socket with the path /tmp/dbus-test.
</para>
<para>
Value escaping is similar to URI escaping but simpler.
<itemizedlist>
<listitem>
<para>
The set of optionally-escaped bytes is:
<literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
<emphasis>byte</emphasis> (note, not character) which is not in the
set of optionally-escaped bytes must be replaced with an ASCII
percent (<literal>%</literal>) and the value of the byte in hex.
The hex value must always be two digits, even if the first digit is
zero. The optionally-escaped bytes may be escaped if desired.
</para>
</listitem>
<listitem>
<para>
To unescape, append each byte in the value; if a byte is an ASCII
percent (<literal>%</literal>) character then append the following
hex value instead. It is an error if a <literal>%</literal> byte
does not have two hex digits following. It is an error if a
non-optionally-escaped byte is seen unescaped.
</para>
</listitem>
</itemizedlist>
The set of optionally-escaped bytes is intended to preserve address
readability and convenience.
</para>
<para>
A server may specify a key-value pair with the key <literal>guid</literal>
and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
describes the format of the <literal>guid</literal> field. If present,
this UUID may be used to distinguish one server address from another. A
server should use a different UUID for each address it listens on. For
example, if a message bus daemon offers both UNIX domain socket and TCP
connections, but treats clients the same regardless of how they connect,
those two connections are equivalent post-connection but should have
distinct UUIDs to distinguish the kinds of connection.
</para>
<para>
The intent of the address UUID feature is to allow a client to avoid
opening multiple identical connections to the same server, by allowing the
client to check whether an address corresponds to an already-existing
connection. Comparing two addresses is insufficient, because addresses
can be recycled by distinct servers, and equivalent addresses may look
different if simply compared as strings (for example, the host in a TCP
address can be given as an IP address or as a hostname).
</para>
<para>
Note that the address key is <literal>guid</literal> even though the
rest of the API and documentation says "UUID," for historical reasons.
</para>
<para>
[FIXME clarify if attempting to connect to each is a requirement
or just a suggestion]
When connecting to a server, multiple server addresses can be
separated by a semi-colon. The library will then try to connect
to the first address and if that fails, it'll try to connect to
the next one specified, and so forth. For example
<programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
</para>
</sect1>
<sect1 id="transports">
<title>Transports</title>
<para>
[FIXME we need to specify in detail each transport and its possible arguments]
Current transports include: unix domain sockets (including
abstract namespace on linux), TCP/IP, and a debug/testing transport using
in-process pipes. Future possible transports include one that
tunnels over X11 protocol.
</para>
<sect2 id="transports-unix-domain-sockets">
<title>Unix Domain Sockets</title>
<para>
Unix domain sockets can be either paths in the file system or on Linux
kernels, they can be abstract which are similar to paths but
do not show up in the file system.
</para>
<para>
When a socket is opened by the D-Bus library it truncates the path
name right before the first trailing Nul byte. This is true for both
normal paths and abstract paths. Note that this is a departure from
previous versions of D-Bus that would create sockets with a fixed
length path name. Names which were shorter than the fixed length
would be padded by Nul bytes.
</para>
</sect2>
</sect1>
<sect1 id="naming-conventions">
<title>Naming Conventions</title>
<para>
D-Bus namespaces are all lowercase and correspond to reversed domain
names, as with Java. e.g. "org.freedesktop"
</para>
<para>
Interface, signal, method, and property names are "WindowsStyleCaps", note
that the first letter is capitalized, unlike Java.
</para>
<para>
Object paths are normally all lowercase with underscores used rather than
hyphens.
</para>
</sect1>
<sect1 id="uuids">
<title>UUIDs</title>
<para>
A working D-Bus implementation uses universally-unique IDs in two places.
First, each server address has a UUID identifying the address,
as described in <xref linkend="addresses"/>. Second, each operating
system kernel instance running a D-Bus client or server has a UUID
identifying that kernel, retrieved by invoking the method
org.freedesktop.DBus.Peer.GetMachineId() (see <xref
linkend="standard-interfaces-peer"/>).
</para>
<para>
The term "UUID" in this document is intended literally, i.e. an
identifier that is universally unique. It is not intended to refer to
RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
</para>
<para>
The UUID must contain 128 bits of data and be hex-encoded. The
hex-encoded string may not contain hyphens or other non-hex-digit
characters, and it must be exactly 32 characters long. To generate a
UUID, the current reference implementation concatenates 96 bits of random
data followed by the 32-bit time in seconds since the UNIX epoch (in big
endian byte order).
</para>
<para>
It would also be acceptable and probably better to simply generate 128
bits of random data, as long as the random number generator is of high
quality. The timestamp could conceivably help if the random bits are not
very random. With a quality random number generator, collisions are
extremely unlikely even with only 96 bits, so it's somewhat academic.
</para>
<para>
Implementations should, however, stick to random data for the first 96 bits
of the UUID.
</para>
</sect1>
<sect1 id="standard-interfaces">
<title>Standard Interfaces</title>
<para>
See <xref linkend="message-protocol-types-notation"/> for details on
the notation used in this section. There are some standard interfaces
that may be useful across various D-Bus applications.
</para>
<sect2 id="standard-interfaces-peer">
<title><literal>org.freedesktop.DBus.Peer</literal></title>
<para>
The <literal>org.freedesktop.DBus.Peer</literal> interface
has two methods:
<programlisting>
org.freedesktop.DBus.Peer.Ping ()
org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
</programlisting>
</para>
<para>
On receipt of the <literal>METHOD_CALL</literal> message
<literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
nothing other than reply with a <literal>METHOD_RETURN</literal> as
usual. It does not matter which object path a ping is sent to. The
reference implementation handles this method automatically.
</para>
<para>
On receipt of the <literal>METHOD_CALL</literal> message
<literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
UUID representing the identity of the machine the process is running on.
This UUID must be the same for all processes on a single system at least
until that system next reboots. It should be the same across reboots
if possible, but this is not always possible to implement and is not
guaranteed.
It does not matter which object path a GetMachineId is sent to. The
reference implementation handles this method automatically.
</para>
<para>
The UUID is intended to be per-instance-of-the-operating-system, so may represent
a virtual machine running on a hypervisor, rather than a physical machine.
Basically if two processes see the same UUID, they should also see the same
shared memory, UNIX domain sockets, process IDs, and other features that require
a running OS kernel in common between the processes.
</para>
<para>
The UUID is often used where other programs might use a hostname. Hostnames
can change without rebooting, however, or just be "localhost" - so the UUID
is more robust.
</para>
<para>
<xref linkend="uuids"/> explains the format of the UUID.
</para>
</sect2>
<sect2 id="standard-interfaces-introspectable">
<title><literal>org.freedesktop.DBus.Introspectable</literal></title>
<para>
This interface has one method:
<programlisting>
org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
</programlisting>
</para>
<para>
Objects instances may implement
<literal>Introspect</literal> which returns an XML description of
the object, including its interfaces (with signals and methods), objects
below it in the object path tree, and its properties.
</para>
<para>
<xref linkend="introspection-format"/> describes the format of this XML string.
</para>
</sect2>
<sect2 id="standard-interfaces-properties">
<title><literal>org.freedesktop.DBus.Properties</literal></title>
<para>
Many native APIs will have a concept of object <firstterm>properties</firstterm>
or <firstterm>attributes</firstterm>. These can be exposed via the
<literal>org.freedesktop.DBus.Properties</literal> interface.
</para>
<para>
<programlisting>
org.freedesktop.DBus.Properties.Get (in STRING interface_name,
in STRING property_name,
out VARIANT value);
org.freedesktop.DBus.Properties.Set (in STRING interface_name,
in STRING property_name,
in VARIANT value);
</programlisting>
</para>
<para>
The available properties and whether they are writable can be determined
by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
see <xref linkend="standard-interfaces-introspectable"/>.
</para>
<para>
An empty string may be provided for the interface name; in this case,
if there are multiple properties on an object with the same name,
the results are undefined (picking one by according to an arbitrary
deterministic rule, or returning an error, are the reasonable
possibilities).
</para>
</sect2>
</sect1>
<sect1 id="introspection-format">
<title>Introspection Data Format</title>
<para>
As described in <xref linkend="standard-interfaces-introspectable"/>,
objects may be introspected at runtime, returning an XML string
that describes the object. The same XML format may be used in
other contexts as well, for example as an "IDL" for generating
static language bindings.
</para>
<para>
Here is an example of introspection data:
<programlisting>
<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node name="/org/freedesktop/sample_object">
<interface name="org.freedesktop.SampleInterface">
<method name="Frobate">
<arg name="foo" type="i" direction="in"/>
<arg name="bar" type="s" direction="out"/>
<arg name="baz" type="a{us}" direction="out"/>
<annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
</method>
<method name="Bazify">
<arg name="bar" type="(iiu)" direction="in"/>
<arg name="bar" type="v" direction="out"/>
</method>
<method name="Mogrify">
<arg name="bar" type="(iiav)" direction="in"/>
</method>
<signal name="Changed">
<arg name="new_value" type="b"/>
</signal>
<property name="Bar" type="y" access="readwrite"/>
</interface>
<node name="child_of_sample_object"/>
<node name="another_child_of_sample_object"/>
</node>
</programlisting>
</para>
<para>
A more formal DTD and spec needs writing, but here are some quick notes.
<itemizedlist>
<listitem>
<para>
Only the root <node> element can omit the node name, as it's
known to be the object that was introspected. If the root
<node> does have a name attribute, it must be an absolute
object path. If child <node> have object paths, they must be
relative.
</para>
</listitem>
<listitem>
<para>
If a child <node> has any sub-elements, then they
must represent a complete introspection of the child.
If a child <node> is empty, then it may or may
not have sub-elements; the child must be introspected
in order to find out. The intent is that if an object
knows that its children are "fast" to introspect
it can go ahead and return their information, but
otherwise it can omit it.
</para>
</listitem>
<listitem>
<para>
The direction element on <arg> may be omitted,
in which case it defaults to "in" for method calls
and "out" for signals. Signals only allow "out"
so while direction may be specified, it's pointless.
</para>
</listitem>
<listitem>
<para>
The possible directions are "in" and "out",
unlike CORBA there is no "inout"
</para>
</listitem>
<listitem>
<para>
The possible property access flags are
"readwrite", "read", and "write"
</para>
</listitem>
<listitem>
<para>
Multiple interfaces can of course be listed for
one <node>.
</para>
</listitem>
<listitem>
<para>
The "name" attribute on arguments is optional.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Method, interface, property, and signal elements may have
"annotations", which are generic key/value pairs of metadata.
They are similar conceptually to Java's annotations and C# attributes.
Well-known annotations:
</para>
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Values (separated by ,)</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>org.freedesktop.DBus.Deprecated</entry>
<entry>true,false</entry>
<entry>Whether or not the entity is deprecated; defaults to false</entry>
</row>
<row>
<entry>org.freedesktop.DBus.GLib.CSymbol</entry>
<entry>(string)</entry>
<entry>The C symbol; may be used for methods and interfaces</entry>
</row>
<row>
<entry>org.freedesktop.DBus.Method.NoReply</entry>
<entry>true,false</entry>
<entry>If set, don't expect a reply to the method call; defaults to false.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</sect1>
<sect1 id="message-bus">
<title>Message Bus Specification</title>
<sect2 id="message-bus-overview">
<title>Message Bus Overview</title>
<para>
The message bus accepts connections from one or more applications.
Once connected, applications can exchange messages with other
applications that are also connected to the bus.
</para>
<para>
In order to route messages among connections, the message bus keeps a
mapping from names to connections. Each connection has one
unique-for-the-lifetime-of-the-bus name automatically assigned.
Applications may request additional names for a connection. Additional
names are usually "well-known names" such as
"org.freedesktop.TextEditor". When a name is bound to a connection,
that connection is said to <firstterm>own</firstterm> the name.
</para>
<para>
The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
This name routes messages to the bus, allowing applications to make
administrative requests. For example, applications can ask the bus
to assign a name to a connection.
</para>
<para>
Each name may have <firstterm>queued owners</firstterm>. When an
application requests a name for a connection and the name is already in
use, the bus will optionally add the connection to a queue waiting for
the name. If the current owner of the name disconnects or releases
the name, the next connection in the queue will become the new owner.
</para>
<para>
This feature causes the right thing to happen if you start two text
editors for example; the first one may request "org.freedesktop.TextEditor",
and the second will be queued as a possible owner of that name. When
the first exits, the second will take over.
</para>
<para>
Messages may have a <literal>DESTINATION</literal> field (see <xref
linkend="message-protocol-header-fields"/>). If the
<literal>DESTINATION</literal> field is present, it specifies a message
recipient by name. Method calls and replies normally specify this field.
</para>
<para>
Signals normally do not specify a destination; they are sent to all
applications with <firstterm>message matching rules</firstterm> that
match the message.
</para>
<para>
When the message bus receives a method call, if the
<literal>DESTINATION</literal> field is absent, the call is taken to be
a standard one-to-one message and interpreted by the message bus
itself. For example, sending an
<literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
<literal>DESTINATION</literal> will cause the message bus itself to
reply to the ping immediately; the message bus will not make this
message visible to other applications.
</para>
<para>
Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
the ping message were sent with a <literal>DESTINATION</literal> name of
<literal>com.yoyodyne.Screensaver</literal>, then the ping would be
forwarded, and the Yoyodyne Corporation screensaver application would be
expected to reply to the ping.
</para>
</sect2>
<sect2 id="message-bus-names">
<title>Message Bus Names</title>
<para>
Each connection has at least one name, assigned at connection time and
returned in response to the
<literal>org.freedesktop.DBus.Hello</literal> method call. This
automatically-assigned name is called the connection's <firstterm>unique
name</firstterm>. Unique names are never reused for two different
connections to the same bus.
</para>
<para>
Ownership of a unique name is a prerequisite for interaction with
the message bus. It logically follows that the unique name is always
the first name that an application comes to own, and the last
one that it loses ownership of.
</para>
<para>
Unique connection names must begin with the character ':' (ASCII colon
character); bus names that are not unique names must not begin
with this character. (The bus must reject any attempt by an application
to manually request a name beginning with ':'.) This restriction
categorically prevents "spoofing"; messages sent to a unique name
will always go to the expected connection.
</para>
<para>
When a connection is closed, all the names that it owns are deleted (or
transferred to the next connection in the queue if any).
</para>
<para>
A connection can request additional names to be associated with it using
the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
linkend="message-protocol-names-bus"/> describes the format of a valid
name. These names can be released again using the
<literal>org.freedesktop.DBus.ReleaseName</literal> message.
</para>
<sect3 id="bus-messages-request-name">
<title><literal>org.freedesktop.DBus.RequestName</literal></title>
<para>
As a method:
<programlisting>
UINT32 RequestName (in STRING name, in UINT32 flags)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to request</entry>
</row>
<row>
<entry>1</entry>
<entry>UINT32</entry>
<entry>Flags</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>Return value</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This method call should be sent to
<literal>org.freedesktop.DBus</literal> and asks the message bus to
assign the given name to the method caller. Each name maintains a
queue of possible owners, where the head of the queue is the primary
or current owner of the name. Each potential owner in the queue
maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
call. When RequestName is invoked the following occurs:
<itemizedlist>
<listitem>
<para>
If the method caller is currently the primary owner of the name,
the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
values are updated with the values from the new RequestName call,
and nothing further happens.
</para>
</listitem>
<listitem>
<para>
If the current primary owner (head of the queue) has
DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
the caller of RequestName replaces the current primary owner at
the head of the queue and the current primary owner moves to the
second position in the queue. If the caller of RequestName was
in the queue previously its flags are updated with the values from
the new RequestName in addition to moving it to the head of the queue.
</para>
</listitem>
<listitem>
<para>
If replacement is not possible, and the method caller is
currently in the queue but not the primary owner, its flags are
updated with the values from the new RequestName call.
</para>
</listitem>
<listitem>
<para>
If replacement is not possible, and the method caller is
currently not in the queue, the method caller is appended to the
queue.
</para>
</listitem>
<listitem>
<para>
If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
set and is not the primary owner, it is removed from the
queue. This can apply to the previous primary owner (if it
was replaced) or the method caller (if it updated the
DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
queue, or if it was just added to the queue with that flag set).
</para>
</listitem>
</itemizedlist>
</para>
<para>
Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
queue," even if another application already in the queue had specified
DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
that does not allow replacement goes away, and the next primary owner
does allow replacement. In this case, queued items that specified
DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
automatically replace the new primary owner. In other words,
DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
time RequestName is called. This is deliberate to avoid an infinite loop
anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
and DBUS_NAME_FLAG_REPLACE_EXISTING.
</para>
<para>
The flags argument contains any of the following values logically ORed
together:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
<entry>0x1</entry>
<entry>
If an application A specifies this flag and succeeds in
becoming the owner of the name, and another application B
later calls RequestName with the
DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
will lose ownership and receive a
<literal>org.freedesktop.DBus.NameLost</literal> signal, and
application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
is not specified by application B, then application B will not replace
application A as the owner.
</entry>
</row>
<row>
<entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
<entry>0x2</entry>
<entry>
Try to replace the current owner if there is one. If this
flag is not set the application will only become the owner of
the name if there is no current owner. If this flag is set,
the application will replace the current owner if
the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
</entry>
</row>
<row>
<entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
<entry>0x4</entry>
<entry>
Without this flag, if an application requests a name that is
already owned, the application will be placed in a queue to
own the name when the current owner gives it up. If this
flag is given, the application will not be placed in the
queue, the request for the name will simply fail. This flag
also affects behavior when an application is replaced as
name owner; by default the application moves back into the
waiting queue, unless this flag was provided when the application
became the name owner.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
The return code can be one of the following values:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
<entry>1</entry> <entry>The caller is now the primary owner of
the name, replacing any previous owner. Either the name had no
owner before, or the caller specified
DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
</row>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
<entry>2</entry>
<entry>The name already had an owner,
DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
the current owner did not specify
DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
</entry>
</row>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
<entry>The name already has an owner,
DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
specified by the requesting application.</entry>
</row>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
<entry>4</entry>
<entry>The application trying to request ownership of a name is already the owner of it.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
<sect3 id="bus-messages-release-name">
<title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
<para>
As a method:
<programlisting>
UINT32 ReleaseName (in STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to release</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>Return value</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This method call should be sent to
<literal>org.freedesktop.DBus</literal> and asks the message bus to
release the method caller's claim to the given name. If the caller is
the primary owner, a new primary owner will be selected from the
queue if any other owners are waiting. If the caller is waiting in
the queue for the name, the caller will removed from the queue and
will not be made an owner of the name if it later becomes available.
If there are no other owners in the queue for the name, it will be
removed from the bus entirely.
The return code can be one of the following values:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
<entry>1</entry> <entry>The caller has released his claim on
the given name. Either the caller was the primary owner of
the name, and the name is now unused or taken by somebody
waiting in the queue for the name, or the caller was waiting
in the queue for the name and has now been removed from the
queue.</entry>
</row>
<row>
<entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
<entry>2</entry>
<entry>The given name does not exist on this bus.</entry>
</row>
<row>
<entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
<entry>3</entry>
<entry>The caller was not the primary owner of this name,
and was also not waiting in the queue to own this name.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
</sect2>
<sect2 id="message-bus-routing">
<title>Message Bus Message Routing</title>
<para>
FIXME
</para>
<sect3 id="message-bus-routing-match-rules">
<title>Match Rules</title>
<para>
An important part of the message bus routing protocol is match
rules. Match rules describe what messages can be sent to a client
based on the contents of the message. When a message is routed
through the bus it is compared to clients' match rules. If any
of the rules match, the message is dispatched to the client.
If none of the rules match the message never leaves the bus. This
is an effective way to control traffic over the bus and to make sure
only relevant message need to be processed by the client.
</para>
<para>
Match rules are added using the AddMatch bus method
(see xref linkend="bus-messages-add-match"/>). Rules are
specified as a string of comma separated key/value pairs.
Excluding a key from the rule indicates a wildcard match.
For instance excluding the the member from a match rule but
adding a sender would let all messages from that sender through.
An example of a complete rule would be
"type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
</para>
<para>
The following table describes the keys that can be used to create
a match rule:
The following table summarizes the D-Bus types.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Key</entry>
<entry>Possible Values</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>type</literal></entry>
<entry>'signal', 'method_call', 'method_return', 'error'</entry>
<entry>Match on the message type. An example of a type match is type='signal'</entry>
</row>
<row>
<entry><literal>sender</literal></entry>
<entry>A bus or unique name (see <xref linkend="term-bus-name"/>
and <xref linkend="term-unique-name"/> respectively)
</entry>
<entry>Match messages sent by a particular sender. An example of a sender match
is sender='org.freedesktop.Hal'</entry>
</row>
<row>
<entry><literal>interface</literal></entry>
<entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
<entry>Match messages sent over or to a particular interface. An example of an
interface match is interface='org.freedesktop.Hal.Manager'.
If a message omits the interface header, it must not match any rule
that specifies this key.</entry>
</row>
<row>
<entry><literal>member</literal></entry>
<entry>Any valid method or signal name</entry>
<entry>Matches messages which have the give method or signal name. An example of
a member match is member='NameOwnerChanged'</entry>
</row>
<row>
<entry><literal>path</literal></entry>
<entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
<entry>Matches messages which are sent from or to the given object. An example of a
path match is path='/org/freedesktop/Hal/Manager'</entry>
</row>
<row>
<entry><literal>destination</literal></entry>
<entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
<entry>Matches messages which are being sent to the given unique name. An
example of a destination match is destination=':1.0'</entry>
</row>
<row>
<entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
<entry>Any string</entry>
<entry>Arg matches are special and are used for further restricting the
match based on the arguments in the body of a message. As of this time
only string arguments can be matched. An example of an argument match
would be arg3='Foo'. Only argument indexes from 0 to 63 should be
accepted.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
</sect2>
<sect2 id="message-bus-starting-services">
<title>Message Bus Starting Services</title>
<para>
The message bus can start applications on behalf of other applications.
In CORBA terms, this would be called <firstterm>activation</firstterm>.
An application that can be started in this way is called a
<firstterm>service</firstterm>.
</para>
<para>
With D-Bus, starting a service is normally done by name. That is,
applications ask the message bus to start some program that will own a
well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
This implies a contract documented along with the name
<literal>org.freedesktop.TextEditor</literal> for which objects
the owner of that name will provide, and what interfaces those
objects will have.
</para>
<para>
To find an executable corresponding to a particular name, the bus daemon
looks for <firstterm>service description files</firstterm>. Service
description files define a mapping from names to executables. Different
kinds of message bus will look for these files in different places, see
<xref linkend="message-bus-types"/>.
</para>
<para>
[FIXME the file format should be much better specified than "similar to
.desktop entries" esp. since desktop entries are already
badly-specified. ;-)] Service description files have the ".service" file
extension. The message bus will only load service description files
ending with .service; all other files will be ignored. The file format
is similar to that of <ulink
url="http://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.html">desktop
entries</ulink>. All service description files must be in UTF-8
encoding. To ensure that there will be no name collisions, service files
must be namespaced using the same mechanism as messages and service
names.
<figure>
<title>Example service description file</title>
<programlisting>
# Sample service description file
[D-BUS Service]
Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
Exec=/usr/libexec/gconfd-2
</programlisting>
</figure>
</para>
<para>
When an application asks to start a service by name, the bus daemon tries to
find a service that will own that name. It then tries to spawn the
executable associated with it. If this fails, it will report an
error. [FIXME what happens if two .service files offer the same service;
what kind of error is reported, should we have a way for the client to
choose one?]
</para>
<para>
The executable launched will have the environment variable
<literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
message bus so it can connect and request the appropriate names.
</para>
<para>
The executable being launched may want to know whether the message bus
starting it is one of the well-known message buses (see <xref
linkend="message-bus-types"/>). To facilitate this, the bus must also set
the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
of the well-known buses. The currently-defined values for this variable
are <literal>system</literal> for the systemwide message bus,
and <literal>session</literal> for the per-login-session message
bus. The new executable must still connect to the address given
in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
resulting connection is to the well-known bus.
</para>
<para>
[FIXME there should be a timeout somewhere, either specified
in the .service file, by the client, or just a global value
and if the client being activated fails to connect within that
timeout, an error should be sent back.]
</para>
<sect3 id="message-bus-starting-services-scope">
<title>Message Bus Service Scope</title>
<para>
The "scope" of a service is its "per-", such as per-session,
per-machine, per-home-directory, or per-display. The reference
implementation doesn't yet support starting services in a different
scope from the message bus itself. So e.g. if you start a service
on the session bus its scope is per-session.
</para>
<para>
We could add an optional scope to a bus name. For example, for
per-(display,session pair), we could have a unique ID for each display
generated automatically at login and set on screen 0 by executing a
special "set display ID" binary. The ID would be stored in a
<literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
random bytes. This ID would then be used to scope names.
Starting/locating a service could be done by ID-name pair rather than
only by name.
</para>
<para>
Contrast this with a per-display scope. To achieve that, we would
want a single bus spanning all sessions using a given display.
So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
property on screen 0 of the display, pointing to this bus.
</para>
</sect3>
</sect2>
<sect2 id="message-bus-types">
<title>Well-known Message Bus Instances</title>
<para>
Two standard message bus instances are defined here, along with how
to locate them and where their service files live.
</para>
<sect3 id="message-bus-types-login">
<title>Login session message bus</title>
<para>
Each time a user logs in, a <firstterm>login session message
bus</firstterm> may be started. All applications in the user's login
session may interact with one another using this message bus.
</para>
<para>
The address of the login session message bus is given
in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
variable. If that variable is not set, applications may
also try to read the address from the X Window System root
window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
The root window property must have type <literal>STRING</literal>.
The environment variable should have precedence over the
root window property.
</para>
<para>
[FIXME specify location of .service files, probably using
DESKTOP_DIRS etc. from basedir specification, though login session
bus is not really desktop-specific]
</para>
</sect3>
<sect3 id="message-bus-types-system">
<title>System message bus</title>
<para>
A computer may have a <firstterm>system message bus</firstterm>,
accessible to all applications on the system. This message bus may be
used to broadcast system events, such as adding new hardware devices,
changes in the printer queue, and so forth.
</para>
<para>
The address of the system message bus is given
in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
variable. If that variable is not set, applications should try
to connect to the well-known address
<literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
<footnote>
<para>
The D-Bus reference implementation actually honors the
<literal>$(localstatedir)</literal> configure option
for this address, on both client and server side.
</para>
</footnote>
</para>
<para>
[FIXME specify location of system bus .service files]
</para>
</sect3>
</sect2>
<sect2 id="message-bus-messages">
<title>Message Bus Messages</title>
<para>
The special message bus name <literal>org.freedesktop.DBus</literal>
responds to a number of additional messages.
</para>
<sect3 id="bus-messages-hello">
<title><literal>org.freedesktop.DBus.Hello</literal></title>
<para>
As a method:
<programlisting>
STRING Hello ()
</programlisting>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Unique name assigned to the connection</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Before an application is able to send messages to other applications
it must send the <literal>org.freedesktop.DBus.Hello</literal> message
to the message bus to obtain a unique name. If an application without
a unique name tries to send a message to another application, or a
message to the message bus itself that isn't the
<literal>org.freedesktop.DBus.Hello</literal> message, it will be
disconnected from the bus.
</para>
<para>
There is no corresponding "disconnect" request; if a client wishes to
disconnect from the bus, it simply closes the socket (or other
communication channel).
</para>
</sect3>
<sect3 id="bus-messages-list-names">
<title><literal>org.freedesktop.DBus.ListNames</literal></title>
<para>
As a method:
<programlisting>
ARRAY of STRING ListNames ()
</programlisting>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of STRING</entry>
<entry>Array of strings where each string is a bus name</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Returns a list of all currently-owned names on the bus.
</para>
</sect3>
<sect3 id="bus-messages-list-activatable-names">
<title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
<para>
As a method:
<programlisting>
ARRAY of STRING ListActivatableNames ()
</programlisting>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of STRING</entry>
<entry>Array of strings where each string is a bus name</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Returns a list of all names that can be activated on the bus.
</para>
</sect3>
<sect3 id="bus-messages-name-exists">
<title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
<para>
As a method:
<programlisting>
BOOLEAN NameHasOwner (in STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to check</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>BOOLEAN</entry>
<entry>Return value, true if the name exists</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Checks if the specified name exists (currently has an owner).
</para>
</sect3>
<sect3 id="bus-messages-name-owner-changed">
<title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
<para>
This is a signal:
<programlisting>
NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name with a new owner</entry>
</row>
<row>
<entry>1</entry>
<entry>STRING</entry>
<entry>Old owner or empty string if none</entry>
</row>
<row>
<entry>2</entry>
<entry>STRING</entry>
<entry>New owner or empty string if none</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This signal indicates that the owner of a name has changed.
It's also the signal to use to detect the appearance of
new names on the bus.
</para>
</sect3>
<sect3 id="bus-messages-name-lost">
<title><literal>org.freedesktop.DBus.NameLost</literal></title>
<para>
This is a signal:
<programlisting>
NameLost (STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name which was lost</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This signal is sent to a specific application when it loses
ownership of a name.
</para>
</sect3>
<sect3 id="bus-messages-name-acquired">
<title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
<para>
This is a signal:
<programlisting>
NameAcquired (STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name which was acquired</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This signal is sent to a specific application when it gains
ownership of a name.
</para>
</sect3>
<sect3 id="bus-messages-start-service-by-name">
<title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
<para>
As a method:
<programlisting>
UINT32 StartServiceByName (in STRING name, in UINT32 flags)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name of the service to start</entry>
</row>
<row>
<entry>1</entry>
<entry>UINT32</entry>
<entry>Flags (currently not used)</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>Return value</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
</para>
<para>
The return value can be one of the following values:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Identifier</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_START_REPLY_SUCCESS</entry>
<entry>1</entry>
<entry>The service was successfully started.</entry>
</row>
<row>
<entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
<entry>2</entry>
<entry>A connection already owns the given name.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
<sect3 id="bus-messages-get-name-owner">
<title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
<para>
As a method:
<programlisting>
STRING GetNameOwner (in STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to get the owner of</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Return value, a unique connection name</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Returns the unique connection name of the primary owner of the name
given. If the requested name doesn't have an owner, returns a
<literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
</para>
</sect3>
<sect3 id="bus-messages-get-connection-unix-user">
<title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
<para>
As a method:
<programlisting>
UINT32 GetConnectionUnixUser (in STRING connection_name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name of the connection to query</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>unix user id</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Returns the unix uid of the process connected to the server. If unable to
determine it, a <literal>org.freedesktop.DBus.Error.Failed</literal>
error is returned.
</para>
</sect3>
<sect3 id="bus-messages-add-match">
<title><literal>org.freedesktop.DBus.AddMatch</literal></title>
<para>
As a method:
<programlisting>
AddMatch (in STRING rule)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Match rule to add to the connection</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
error is returned.
</para>
</sect3>
<sect3 id="bus-messages-remove-match">
<title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
<para>
As a method:
<programlisting>
RemoveMatch (in STRING rule)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Match rule to remove from the connection</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
error is returned.
</para>
</sect3>
</sect2>
</sect1>
<!--
<appendix id="implementation-notes">
<title>Implementation notes</title>
<sect1 id="implementation-notes-subsection">
<title></title>
<para>
</para>
</sect1>
</appendix>
-->
<glossary><title>Glossary</title>
<para>
This glossary defines some of the terms used in this specification.
</para>
<glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
<glossdef>
<para>
The message bus maintains an association between names and
connections. (Normally, there's one connection per application.) A
bus name is simply an identifier used to locate connections. For
example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
name might be used to send a message to a screensaver from Yoyodyne
Corporation. An application is said to <firstterm>own</firstterm> a
name if the message bus has associated the application's connection
with the name. Names may also have <firstterm>queued
owners</firstterm> (see <xref linkend="term-queued-owner"/>).
The bus assigns a unique name to each connection,
see <xref linkend="term-unique-name"/>. Other names
can be thought of as "well-known names" and are
used to find applications that offer specific functionality.
</para>
</glossdef>
</glossentry>
<glossentry id="term-message"><glossterm>Message</glossterm>
<glossdef>
<para>
A message is the atomic unit of communication via the D-Bus
protocol. It consists of a <firstterm>header</firstterm> and a
<firstterm>body</firstterm>; the body is made up of
<firstterm>arguments</firstterm>.
</para>
</glossdef>
</glossentry>
<glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
<glossdef>
<para>
The message bus is a special application that forwards
or routes messages between a group of applications
connected to the message bus. It also manages
<firstterm>names</firstterm> used for routing
messages.
</para>
</glossdef>
</glossentry>
<glossentry id="term-name"><glossterm>Name</glossterm>
<glossdef>
<para>
See <xref linkend="term-bus-name"/>. "Name" may
also be used to refer to some of the other names
in D-Bus, such as interface names.
</para>
</glossdef>
</glossentry>
<glossentry id="namespace"><glossterm>Namespace</glossterm>
<glossdef>
<para>
Used to prevent collisions when defining new interfaces or bus
names. The convention used is the same one Java uses for defining
classes: a reversed domain name.
</para>
</glossdef>
</glossentry>
<glossentry id="term-object"><glossterm>Object</glossterm>
<glossdef>
<para>
Each application contains <firstterm>objects</firstterm>, which have
<firstterm>interfaces</firstterm> and
<firstterm>methods</firstterm>. Objects are referred to by a name,
called a <firstterm>path</firstterm>.
</para>
</glossdef>
</glossentry>
<glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
<glossdef>
<para>
An application talking directly to another application, without going
through a message bus. One-to-one connections may be "peer to peer" or
"client to server." The D-Bus protocol has no concept of client
vs. server after a connection has authenticated; the flow of messages
is symmetrical (full duplex).
</para>
</glossdef>
</glossentry>
<glossentry id="term-path"><glossterm>Path</glossterm>
<glossdef>
<para>
Object references (object names) in D-Bus are organized into a
filesystem-style hierarchy, so each object is named by a path. As in
LDAP, there's no difference between "files" and "directories"; a path
can refer to an object, while still having child objects below it.
</para>
</glossdef>
</glossentry>
<glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
<glossdef>
<para>
Each bus name has a primary owner; messages sent to the name go to the
primary owner. However, certain names also maintain a queue of
secondary owners "waiting in the wings." If the primary owner releases
the name, then the first secondary owner in the queue automatically
becomes the new owner of the name.
</para>
</glossdef>
</glossentry>
<glossentry id="term-service"><glossterm>Service</glossterm>
<glossdef>
<para>
A service is an executable that can be launched by the bus daemon.
Services normally guarantee some particular features, for example they
may guarantee that they will request a specific name such as
"org.freedesktop.Screensaver", have a singleton object
"/org/freedesktop/Application", and that object will implement the
interface "org.freedesktop.ScreensaverControl".
</para>
</glossdef>
</glossentry>
<glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
<glossdef>
<para>
".service files" tell the bus about service applications that can be
launched (see <xref linkend="term-service"/>). Most importantly they
provide a mapping from bus names to services that will request those
names when they start up.
</para>
</glossdef>
</glossentry>
<glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
<glossdef>
<para>
The special name automatically assigned to each connection by the
message bus. This name will never change owner, and will be unique
(never reused during the lifetime of the message bus).
It will begin with a ':' character.
</para>
</glossdef>
</glossentry>
</glossary>
</article>