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DBus redirects here. For dBus Tibetan, see Standard Tibetan; for the province, see Ü-Tsang.
D-Bus
Developer(s) Red Hat and the community
Stable release 1.2.16 / 2009-07-14; 5 months ago
Preview release 1.3.0 / 2009-07-29; 5 months ago
Operating system Cross-platform
Type Inter-process communication
License GNU General Public License or Academic Free License 2.1[1]
Website www.freedesktop.org/wiki/Software/dbus

In computing, D-Bus (Desktop Bus) is a simple inter-process communication (IPC) system for software applications to communicate with one another. D-Bus was heavily influenced by KDE2–3's DCOP system and has replaced it in the KDE 4 release; it is supported on most POSIX operating systems, and a port for Windows exists. It is used by Qt 4 and GNOME. In GNOME it has gradually replaced most parts of the earlier Bonobo mechanism.

Red Hat operates as the primary developer of D-Bus - as part of the freedesktop.org project. Released under the terms of the GNU General Public License and the Academic Free License, D-Bus is free software.

Contents

Design

D-Bus allows programs to register with it in order to offer services. It also allows client programs to check for the availability of services. Programs can also register as waiting for events of the kernel, as in the case of hot-swapping hardware.

D-Bus functionality runs as a daemon: typically dbus-daemon. Users can run several instances of it, each called a channel. There will usually be a privileged system channel, and a private instance for each logged-in user. The private instances are required because the system channel has access restrictions.

The main mission of the system channel is to deliver the signals from the HAL (hardware abstraction layer) daemon to the processes interested in them. The mission of the private instances is to provide unrestricted communication among any applications of the user.

Architecture

D-Bus has three architectural layers:[2]

  • 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 applications, thereby implementing the publish/subscribe paradigm.
  • Wrapper libraries based on particular application frameworks.

The design of D-Bus addresses 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
  • communication between the desktop session and the operating system, where the operating system would typically include the kernel and any system daemons or processes

Mechanisms

Each application using D-Bus contains objects that usually map to GObject, QObject, C++ objects, or Python objects. An object is an instance rather than a type. Messages received over a D-Bus connection get routed to a specific object, not to the application as a whole. In this way, D-Bus resembles software componentry, as it appears to users as if they are interacting with an object across the IPC connection, whether or not there is an object on the other side.

To allow messages to specify their destination object, the system needs a way to refer to an object. In many programming languages, this is usually called a pointer or reference. However, these references are implemented as memory addresses relative to the address space of the 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 have the name /org/kde/kspread/sheets/3/cells/4/5. Human-readable paths are preferred, but developers are free to create an object named /com/mycompany/c5yo817y0c1y1c5b if it makes sense for their application.

The D-Bus objects' names are namespaced to keep different code modules separated. Namespaces are generally prefixed with the developer's domain name components (eg. /org/kde).

References

External links

  • D-Bus at the Freedesktop.org home page







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