D-BUS Tutorial
Version 0.3
18 January 2005
Havoc
Pennington
Red Hat, Inc.
hp@pobox.com
David
Wheeler
What is D-BUS?
D-BUS is a system for interprocess communication
(IPC). Architecturally, it has several layers:
A library, libdbus, that allows two
applications to connect to each other and exchange messages.
A message bus daemon executable, built on
libdbus, that multiple applications can connect to. The daemon can
route messages from one application to zero or more other
applications.
Wrapper libraries based on particular
application frameworks. For example, libdbus-glib and
libdbus-qt. There are also bindings to languages such as
Python. These wrapper libraries are the API most people should use,
as they simplify the details of D-BUS programming. libdbus is
intended to be a low-level backend for the higher level bindings.
Much of the libdbus API is only useful for binding implementation.
If you just want to use D-BUS and don't care how it works, jump directly
to .
Otherwise, read on.
libdbus only supports one-to-one connections, just like a raw network
socket. However, rather than sending byte streams over the connection, you
send messages. Messages have a header identifying
the kind of message, and a body containing a data payload. libdbus also
abstracts the exact transport used (sockets vs. whatever else), and
handles details such as authentication.
The message bus daemon forms the hub of a wheel. Each spoke of the wheel
is a one-to-one connection to an application using libdbus. An
application sends a message to the bus daemon over its spoke, and the bus
daemon forwards the message to other connected applications as
appropriate. Think of the daemon as a router.
The bus daemon has multiple instances on a typical computer. The
first instance is a machine-global singleton, that is, a system daemon
similar to sendmail or Apache. This instance has heavy security
restrictions on what messages it will accept, and is used for systemwide
communication. The other instances are created one per user login session.
These instances allow applications in the user's session to communicate
with one another.
The systemwide and per-user daemons are separate. Normal within-session
IPC does not involve the systemwide message bus process and vice versa.
D-BUS applications
There are many, many technologies in the world that have "Inter-process
communication" or "networking" in their stated purpose: CORBA, DCE, DCOM, DCOP, XML-RPC, SOAP, MBUS, Internet Communications Engine (ICE),
and probably hundreds more.
Each of these is tailored for particular kinds of application.
D-BUS is designed for two specific cases:
Communication between desktop applications in the same desktop
session; to allow integration of the desktop session as a whole,
and address issues of process lifecycle (when do desktop components
start and stop running).
Communication between the desktop session and the operating system,
where the operating system would typically include the kernel
and any system daemons or processes.
For the within-desktop-session use case, the GNOME and KDE desktops
have significant previous experience with different IPC solutions
such as CORBA and DCOP. D-BUS is built on that experience and
carefully tailored to meet the needs of these desktop projects
in particular. D-BUS may or may not be appropriate for other
applications; the FAQ has some comparisons to other IPC systems.
The problem solved by the systemwide or communication-with-the-OS case
is explained well by the following text from the Linux Hotplug project:
A gap in current Linux support is that policies with any sort of
dynamic "interact with user" component aren't currently
supported. For example, that's often needed the first time a network
adapter or printer is connected, and to determine appropriate places
to mount disk drives. It would seem that such actions could be
supported for any case where a responsible human can be identified:
single user workstations, or any system which is remotely
administered.
This is a classic "remote sysadmin" problem, where in this case
hotplugging needs to deliver an event from one security domain
(operating system kernel, in this case) to another (desktop for
logged-in user, or remote sysadmin). Any effective response must go
the other way: the remote domain taking some action that lets the
kernel expose the desired device capabilities. (The action can often
be taken asynchronously, for example letting new hardware be idle
until a meeting finishes.) At this writing, Linux doesn't have
widely adopted solutions to such problems. However, the new D-Bus
work may begin to solve that problem.
D-BUS may happen to be useful for purposes other than the one it was
designed for. Its general properties that distinguish it from
other forms of IPC are:
Binary protocol designed to be used asynchronously
(similar in spirit to the X Window System protocol).
Stateful, reliable connections held open over time.
The message bus is a daemon, not a "swarm" or
distributed architecture.
Many implementation and deployment issues are specified rather
than left ambiguous.
Semantics are similar to the existing DCOP system, allowing
KDE to adopt it more easily.
Security features to support the systemwide mode of the
message bus.
Concepts
Some basic concepts apply no matter what application framework you're
using to write a D-BUS application. The exact code you write will be
different for GLib vs. Qt vs. Python applications, however.
Here is a diagram (png svg) that may help you visualize the concepts
that follow.
Objects and Object Paths
Each application using D-BUS contains objects,
which generally map to GObject, QObject, C++ objects, or Python objects
(but need not). An object is an instance rather
than a type. When messages are received over a D-BUS connection, they
are sent to a specific object, not to the application as a whole.
To allow messages to specify their destination object, there has to be a
way to refer to an object. In your favorite programming language, this
is normally called a pointer or
reference. However, these references are
implemented as memory addresses relative to the address space of your
application, and thus can't be passed from one application to another.
To solve this, D-BUS introduces a name for each object. The name
looks like a filesystem path, for example an object could be
named /org/kde/kspread/sheets/3/cells/4/5.
Human-readable paths are nice, but you are free to create an
object named /com/mycompany/c5yo817y0c1y1c5b
if it makes sense for your application.
Namespacing object paths is smart, by starting them with the components
of a domain name you own (e.g. /org/kde). This
keeps different code modules in the same process from stepping
on one another's toes.
Interfaces
Each object supports one or more interfaces.
Think of an interface as a named group of methods and signals,
just as it is in GLib or Qt or Java. Interfaces define the
type of an object instance.
Message Types
Messages are not all the same; in particular, D-BUS has
4 built-in message types:
Method call messages ask to invoke a method
on an object.
Method return messages return the results
of invoking a method.
Error messages return an exception caused by
invoking a method.
Signal messages are notifications that a given signal
has been emitted (that an event has occurred).
You could also think of these as "event" messages.
A method call maps very simply to messages, then: you send a method call
message, and receive either a method return message or an error message
in reply.
Bus Names
Object paths, interfaces, and messages exist on the level of
libdbus and the D-BUS protocol; they are used even in the
1-to-1 case with no message bus involved.
Bus names, on the other hand, are a property of the message bus daemon.
The bus maintains a mapping from names to message bus connections.
These names are used to specify the origin and destination
of messages passing through the message bus. When a name is mapped
to a particular application's connection, that application is said to
own that name.
On connecting to the bus daemon, each application immediately owns a
special name called the unique connection name.
A unique name begins with a ':' (colon) character; no other names are
allowed to begin with that character. Unique names are special because
they are created dynamically, and are never re-used during the lifetime
of the same bus daemon. You know that a given unique name will have the
same owner at all times. An example of a unique name might be
:34-907. The numbers after the colon have
no meaning other than their uniqueness.
Applications may ask to own additional well-known
names. For example, you could write a specification to
define a name called com.mycompany.TextEditor.
Your definition could specify that to own this name, an application
should have an object at the path
/com/mycompany/TextFileManager supporting the
interface org.freedesktop.FileHandler.
Applications could then send messages to this bus name,
object, and interface to execute method calls.
You could think of the unique names as IP addresses, and the
well-known names as domain names. So
com.mycompany.TextEditor might map to something like
:34-907 just as mycompany.com maps
to something like 192.168.0.5.
Names have a second important use, other than routing messages. They
are used to track lifecycle. When an application exits (or crashes), its
connection to the message bus will be closed by the operating system
kernel. The message bus then sends out notification messages telling
remaining applications that the application's names have lost their
owner. By tracking these notifications, your application can reliably
monitor the lifetime of other applications.
Addresses
Applications using D-BUS are either servers or clients. A server
listens for incoming connections; a client connects to a server. Once
the connection is established, it is a symmetric flow of messages; the
client-server distinction only matters when setting up the
connection.
A D-BUS address specifies where a server will
listen, and where a client will connect. For example, the address
unix:path=/tmp/abcdef specifies that the server will
listen on a UNIX domain socket at the path
/tmp/abcdef and the client will connect to that
socket. An address can also specify TCP/IP sockets, or any other
transport defined in future iterations of the D-BUS specification.
When using D-BUS with a message bus, the bus daemon is a server
and all other applications are clients of the bus daemon.
libdbus automatically discovers the address of the per-session bus
daemon by reading an environment variable. It discovers the
systemwide bus daemon by checking a well-known UNIX domain socket path
(though you can override this address with an environment variable).
If you're using D-BUS without a bus daemon, it's up to you to
define which application will be the server and which will be
the client, and specify a mechanism for them to agree on
the server's address.
Big Conceptual Picture
Pulling all these concepts together, to specify a particular
method call on a particular object instance, a number of
nested components have to be named:
Address -> [Bus Name] -> Path -> Interface -> Method
The bus name is in brackets to indicate that it's optional -- you only
provide a name to route the method call to the right application
when using the bus daemon. If you have a direct connection to another
application, bus names aren't used; there's no bus daemon.
The interface is also optional, primarily for historical
reasons; DCOP does not require specifying the interface,
instead simply forbidding duplicate method names
on the same object instance. D-BUS will thus let you
omit the interface, but if your method name is ambiguous
it is undefined which method will be invoked.
GLib API: Using Remote Objects
The GLib binding is defined in the header file
<dbus/dbus-glib.h>. The API is very small, in sharp contrast to the
low-level <dbus/dbus.h>.
The GLib bindings are incomplete, see the TODO file and comments in the
source code.
Here is a D-BUS program using the GLib bindings.
int
main (int argc, char **argv)
{
DBusGConnection *connection;
GError *error;
DBusGProxy *proxy;
DBusGPendingCall *call;
char **name_list;
int name_list_len;
int i;
g_type_init ();
error = NULL;
connection = dbus_g_bus_get (DBUS_BUS_SESSION,
&error);
if (connection == NULL)
{
g_printerr ("Failed to open connection to bus: %s\n",
error->message);
g_error_free (error);
exit (1);
}
/* Create a proxy object for the "bus driver" (name "org.freedesktop.DBus") */
proxy = dbus_g_proxy_new_for_name (connection,
DBUS_SERVICE_ORG_FREEDESKTOP_DBUS,
DBUS_PATH_ORG_FREEDESKTOP_DBUS,
DBUS_INTERFACE_ORG_FREEDESKTOP_DBUS);
/* Call ListNames method */
call = dbus_g_proxy_begin_call (proxy, "ListNames", DBUS_TYPE_INVALID);
error = NULL;
if (!dbus_g_proxy_end_call (proxy, call, &error,
DBUS_TYPE_ARRAY, DBUS_TYPE_STRING,
&name_list, &name_list_len,
DBUS_TYPE_INVALID))
{
g_printerr ("Failed to complete ListNames call: %s\n",
error->message);
g_error_free (error);
exit (1);
}
/* Print the results */
g_print ("Names on the message bus:\n");
i = 0;
while (i < name_list_len)
{
g_assert (name_list[i] != NULL);
g_print (" %s\n", name_list[i]);
++i;
}
g_assert (name_list[i] == NULL);
g_strfreev (name_list);
return 0;
}
DBusGProxy represents a remote object. dbus_g_proxy_begin_call() sends
a method call to the remote object, and dbus_g_proxy_end_call() retrieves
any return values or exceptions resulting from the method call.
There are also DBusGProxy functions to connect and disconnect signals,
not shown in the code example.
dbus_g_bus_get() assumes that the application will use GMainLoop. The
created connection will be associated with the main loop such that
messages will be sent and received when the main loop runs. However, in
the above code example the main loop never runs; D-BUS will not run the
loop implicitly. Instead, dbus_g_proxy_end_call() will block until the
method call has been sent and the reply received. A more complex GUI
application might run the main loop while waiting for the method call
reply. (DBusGPendingCall is currently missing the "notify me when the
call is complete" functionality found in DBusPendingCall, but it should be
added.)
Future plans (see doc/TODO) are to use G_TYPE_STRING in place of
DBUS_TYPE_STRING and so forth. In fact the above code is slightly
incorrect at the moment, since it uses g_strfreev() to free a string array
that was not allocated with g_malloc(). dbus_free_string_array() should
really be used. However, once the GLib bindings are complete the returned
data from dbus_g_proxy_end_call() will be allocated with g_malloc().
GLib API: Implementing Objects
The GLib binding is defined in the header file
<dbus/dbus-glib.h>. To implement an object, it's also necessary
to use the dbus-glib-tool command line tool.
The GLib bindings are incomplete. Implementing an object is not yet
possible, see the TODO file and comments in the source code for details
on what work needs doing.
Qt API: Using Remote Objects
The Qt bindings are not yet documented.
Qt API: Implementing Objects
The Qt bindings are not yet documented.
Python API: Using Remote Objects
The Python bindings are not yet documented, but the
bindings themselves are in good shape.
Python API: Implementing Objects
The Python bindings are not yet documented, but the
bindings themselves are in good shape.