Large groups of cooperating processes demand a dense mesh of individual communication channels (using one-to-one IPC methods) between them. D-Bus simplifies the IPC requirements with one single shared channel.
D-Bus provides a software-busabstraction that gathers all the communications among a group of processes over a single shared virtual channel.[4]Template:R/superscript Processes connected to a bus do not know how it is internally implemented, but the D-Bus specification guarantees that all processes connected to the bus can communicate with each other through it. D-Bus incurs at least a 2.5x performance loss over one-to-one IPC.[13]
a single system bus, available to all users and processes of the system, that provides access to system services (i.e. services provided by the operating system and also by any system daemons); and
a session bus for each user login session, that provides desktop services to user applications in the same desktop session, and allows the integration of the desktop session as a whole.
A process can connect to any number of buses, provided that it has been granted access to them. In practice, this means that any user process can connect to the system bus and to its current session bus, but not to another user's session buses, or even to a different session bus owned by the same user. The latter restriction may change in the future if all sessions of a user are combined into a single user bus.[16]Template:R/superscript
D-Bus provides additional or simplifies existing functionality to the applications, including information-sharing, modularity and privilege separation. For example, information on an incoming voice call received through Bluetooth or Skype can be propagated and interpreted by any currently running music player, which can react by muting the volume or by pausing playback until the call is finished.[17]Template:R/superscript
Every connection to a bus is identified in the context of D-Bus by what is called a bus name.[3]Template:R/superscript There are two types of bus names: unique and well-known. A well-known bus name consists of two or more dot-separated strings of letters, digits, dashes, and underscores—a reverse domain name. An example of a valid bus name is org.freedesktop.NetworkManager.[4]Template:R/superscript
When a process sets up a connection to a bus, the bus assigns to the connection a special bus name called unique connection name.[15]Template:R/superscript[4]Template:R/superscript Bus names of this type are immutable—it is guaranteed they will not change as long as the connection exists—and, more importantly, they cannot be reused during the bus lifetime.[3]Template:R/superscript[15]Template:R/superscript[4]Template:R/superscript This means that no other connection to that bus will ever have assigned such unique connection name, even if the same process closes down the connection to the bus and creates a new one. Unique connection names are easily recognizable because they start with the otherwise forbidden colon character.[15]Template:R/superscript[4]Template:R/superscript An example of a unique connection name is :1.1553 (the characters after the colon have no particular meaning[15]Template:R/superscript).
A process can ask for additional bus names for its connection,[15]Template:R/superscript provided that any requested name is not already being used by another connection to the bus. In D-Bus parlance, when a bus name is assigned to a connection, it is said the connection owns the bus name.[3]Template:R/superscript[15]Template:R/superscript In that sense, a bus name cannot be owned by two connections at the same time, but, unlike unique connection names, these names can be reused if they are available: a process may reclaim a bus name released—purposely or not—by another process.[3]Template:R/superscript[4]Template:R/superscript
The idea behind these additional bus names, commonly called well-known names, is to provide a way to refer to a service using a prearranged bus name.[15]Template:R/superscript[4]Template:R/superscript For instance, the service that reports the current time and date in the system bus lies in the process whose connection owns the Page Template:Mono/styles.css has no content.org.freedesktop.timedate1 bus name, regardless of which process it is.
Bus names can be used as a simple way to implement single-instance applications (second instances detect that the bus name is already taken).[15]Template:R/superscript It can also be used to track a service process lifecycle, since the bus sends a notification when a bus name is released due to a process termination.[15]Template:R/superscript
Object model
Because of its original conception as a replacement for several component oriented communications systems, D-Bus shares with its predecessors an object model in which to express the semantics of the communications between clients and services. The terms used in the D-Bus object model mimic those used by some object orientedprogramming languages. That does not mean that D-Bus is somehow limited to OOP languages—in fact, the most used implementation (Page Template:Mono/styles.css has no content.libdbus) is written in C, a procedural programming language.
Browsing the existing bus names, objects, interfaces, methods and signals in a D-Bus bus using D-Feet
In D-Bus, a process offers its services by exposing objects. These objects have methods that can be invoked, and signals that the object can emit.[15]Template:R/superscript Methods and signals are collectively referred to as the members of the object.[3]Template:R/superscript Any client connected to the bus can interact with an object by using its methods, making requests or commanding the object to perform actions.[15]Template:R/superscript For instance, an object representing a time service can be queried by a client using a method that returns the current date and time. A client can also listen to signals that an object emits when its state changes due to certain events, usually related to the underlying service. An example would be when a service that manages hardware devices—such as USB or network drivers—signals a "new hardware device added" event. Clients should instruct the bus that they are interested in receiving certain signals from a particular object, since a D-Bus bus only passes signals to those processes with a registered interest in them.[4]Template:R/superscript
A process connected to a D-Bus bus can request it to export as many D-Bus objects as it wants. Each object is identified by an object path, a string of numbers, letters and underscores separated and prefixed by the slash character, called that because of their resemblance to Unix filesystem paths.[3]Template:R/superscript[15]Template:R/superscript The object path is selected by the requesting process, and must be unique in the context of that bus connection. An example of a valid object path is /org/kde/kspread/sheets/3/cells/4/5.[15]Template:R/superscript However, it is not enforced—but also not discouraged—to form hierarchies within object paths.[4]Template:R/superscript The particular naming convention for the objects of a service is entirely up to the developers of such service, but many developers choose to namespace them using the reserved domain name of the project as a prefix (e.g. Page Template:Mono/styles.css has no content./org/kde).[15]Template:R/superscript
Every object is inextricably associated to the particular bus connection where it was exported, and, from the D-Bus point of view, only lives in the context of such connection. Therefore, in order to be able to use a certain service, a client must indicate not only the object path providing the desired service, but also the bus name under which the service process is connected to the bus.[3]Template:R/superscript This in turn allows that several processes connected to the bus can export different objects with identical object paths unambiguously.
An interface specifies members—methods and signals—that can be used with an object.[15]Template:R/superscript It is a set of declarations of methods (including its passing and returning parameters) and signals (including its parameters) identified by a dot-separated name resembling the Java language interfaces notation.[15]Template:R/superscript[4]Template:R/superscript An example of a valid interface name is org.freedesktop.Introspectable.[4]Template:R/superscript Despite their similarity, interface names and bus names should not be mistaken. A D-Bus object can implement several interfaces, but at least must implement one, providing support for every method and signal defined by it. The combination of all interfaces implemented by an object is called the object type.[3]Template:R/superscript[15]Template:R/superscript
When using an object, it is a good practice for the client process to provide the member's interface name besides the member's name, but is only mandatory when there is an ambiguity caused by duplicated member names available from different interfaces implemented by the object[3]Template:R/superscript[15]Template:R/superscript—otherwise, the selected member is undefined or erroneous. An emitted signal, on the other hand, must always indicate to which interface it belongs.
The D-Bus specification also defines several standard interfaces that objects may want to implement in addition to its own interfaces.[14]Template:R/superscript Although technically optional, most D-Bus service developers choose to support them in their exported objects since they offer important additional features to D-Bus clients, such as introspection.[4]Template:R/superscript These standard interfaces are:[14]Template:R/superscript[4]Template:R/superscript
Page Template:Mono/styles.css has no content.org.freedesktop.DBus.ObjectManager: when a D-Bus service arranges its objects hierarchically, this interface provides a way to query an object about all sub-objects under its path, as well as their interfaces and properties, using a single method call.[14]Template:R/superscript
The D-Bus specification defines a number of administrative bus operations (called "bus services") to be performed using the Page Template:Mono/styles.css has no content./org/freedesktop/DBus object that resides in the Page Template:Mono/styles.css has no content.org.freedesktop.DBus bus name.[14]Template:R/superscript Each bus reserves this special bus name for itself, and manages any requests made specifically to this combination of bus name and object path. The administrative operations provided by the bus are those defined by the object's interface Page Template:Mono/styles.css has no content.org.freedesktop.DBus. These operations are used for example to provide information about the status of the bus,[3]Template:R/superscript or to manage the request and release of additional well-known bus names.[14]Template:R/superscript[4]Template:R/superscript
Communications model
D-Bus was conceived as a generic, high-level inter-process communication system. To accomplish such goals, D-Bus communications are based on the exchange of messages between processes instead of "raw bytes".[3]Template:R/superscript[15]Template:R/superscript D-Bus messages are high-level discrete items that a process can send through the bus to another connected process. Messages have a well-defined structure (even the types of the data carried in their payload are defined), allowing the bus to validate them and to reject any ill-formed message.
In this regard, D-Bus is closer to an RPC mechanism than to a classic IPC mechanism, with its own type definition system and its own marshaling.[3]Template:R/superscript
Example of one-to-one request-response message exchange to invoke a method over D-Bus. Here the client process invokes the Page Template:Mono/styles.css has no content.SetFoo() method of the Page Template:Mono/styles.css has no content./org/example/object1 object from the service process named Page Template:Mono/styles.css has no content.org.example.foo (or :1.14) in the bus.
The bus supports two modes of interchanging messages between a client and a service process[3]Template:R/superscript:
One-to-one request-response: This is the way for a client to invoke an object's method. The client sends a message to the service process exporting the object, and the service in turn replies with a message back to the client process.[15]Template:R/superscript The message sent by the client must contain the object path, the name of the invoked method (and optionally the name of its interface), and the values of the input parameters (if any) as defined by the object's selected interface. The reply message carries the result of the request, including the values of the output parameters returned by the object's method invocation, or exception information if there was an error.[3]Template:R/superscript[15]Template:R/superscript
Publish/subscribe: This is the way for an object to announce the occurrence of a signal to the interested parties. The object's service process broadcasts a message that the bus passes only to the connected clients subscribed to the object's signal.[15]Template:R/superscript The message carries the object path, the name of the signal, the interface to which the signal belongs, and also the values of the signal's parameters (if any). The communication is one-way: there are no response messages to the original message from any client process, since the sender knows neither the identities nor the number of the recipients.[3]Template:R/superscript[15]Template:R/superscript
Every D-Bus message consists of a header and a body.[15]Template:R/superscript The header is formed by several fields that identify the type of message, the sender, as well as information required to deliver the message to its recipient (destination bus name, object path, method or signal name, interface name, etc.).[15]Template:R/superscript[14]Template:R/superscript The body contains the data payload that the receiver process interprets—for instance the input or output arguments. All the data is encoded in a well known binary format called the wire format which supports the serialization of various types, such as integers and floating-point numbers, strings, compound types, and so on,[14]Template:R/superscript also referred to as marshaling.
The D-Bus specification defines the wire protocol: how to build the D-Bus messages to be exchanged between processes within a D-Bus connection. However, it does not define the underlying transport method for delivering these messages.
Internals
Most existing D-Bus implementations follow the architecture of the reference implementation. This architecture consists of two main components:[3]Template:R/superscript
a point-to-point communications library that implements the D-Bus wire protocol in order to exchange messages between two processes. In the reference implementation this library is Page Template:Mono/styles.css has no content.libdbus. In other implementations Page Template:Mono/styles.css has no content.libdbus may be wrapped by another higher-level library, language binding, or entirely replaced by a different standalone implementation that serves the same purpose.[18]Template:R/superscript This library only supports one-to-one communications between two processes.[15]Template:R/superscript
A Page Template:Mono/styles.css has no content.dbus-daemon process acting as a D-Bus message bus daemon. Every process connected to the bus keeps one D-Bus connection with it.a special daemon process that plays the bus role and to which the rest of the processes connect using any D-Bus point-to-point communications library. This process is also known as the message bus daemon,[17]Template:R/superscript since it is responsible for routing messages from any process connected to the bus to another. In the reference implementation this role is performed by Page Template:Mono/styles.css has no content.dbus-daemon, which itself is built on top of Page Template:Mono/styles.css has no content.libdbus. Another implementation of the message bus daemon is Page Template:Mono/styles.css has no content.dbus-broker, which is built on top of Page Template:Mono/styles.css has no content.sd-bus.
Process A and B both connected to a Page Template:Mono/styles.css has no content.dbus-daemon using Page Template:Mono/styles.css has no content.libdbus over a Unix domain socket. They can exchange messages sending them to the message bus process, which in turn will deliver the messages to the appropriate process. In this scenario bus names are mandatory to identify the destination process.
The Page Template:Mono/styles.css has no content.libdbus library (or its equivalent) internally uses a native lower-level IPC mechanism to transport the required D-Bus messages between the two processes in both ends of the D-Bus connection. D-Bus specification does not mandate which particular IPC transport mechanisms should be available to use, as it is the communications library that decides what transport methods it supports. For instance, in Unix-like operating systems such as Linux Page Template:Mono/styles.css has no content.libdbus typically uses Unix domain sockets as the underlying transport method, but it also supports TCP sockets.[3]Template:R/superscript[15]Template:R/superscript
The communications libraries of both processes must agree on the selected transport method and also on the particular channel used for their communication. This information is defined by what D-Bus calls an address.[4]Template:R/superscript[15]Template:R/superscript Unix-domain sockets are filesystem objects, and therefore they can be identified by a filename, so a valid address would be unix:path=/tmp/.hiddensocket.[3]Template:R/superscript[14]Template:R/superscript Both processes must pass the same address to their respective communications libraries to establish the D-Bus connection between them. An address can also provide additional data to the communications library in the form of comma-separated key=value pairs.[4]Template:R/superscript[14]Template:R/superscript This way, for example, it can provide authentication information to a specific type of connection that supports it.
When a message bus daemon like Page Template:Mono/styles.css has no content.dbus-daemon is used to implement a D-Bus bus, all processes that want to connect to the bus must know the bus address, the address by which a process can establish a D-Bus connection to the central message bus process.[3]Template:R/superscript[15]Template:R/superscript In this scenario, the message bus daemon selects the bus address and the remainder processes must pass that value to their corresponding Page Template:Mono/styles.css has no content.libdbus or equivalent libraries. Page Template:Mono/styles.css has no content.dbus-daemon defines a different bus address for every bus instance it provides. These addresses are defined in the daemon's configuration files.
Two processes can use a D-Bus connection to exchange messages directly between them,[19]Template:R/superscript but this is not the way in which D-Bus is normally intended to be used. The usual way is to always use a message bus daemon (i.e. Page Template:Mono/styles.css has no content.dbus-daemon) as a communications central point to which each process should establish its point-to-point D-Bus connection. When a process—client or service—sends a D-Bus message, the message bus process receives it in the first instance and delivers it to the appropriate recipient. The message bus daemon may be seen as a hub or router in charge of getting each message to its destination by repeating it through the D-Bus connection to the recipient process.[15]Template:R/superscript The recipient process is determined by the destination bus name in the message's header field,[14]Template:R/superscript or by the subscription information to signals maintained by the message bus daemon in the case of signal propagation messages.[4]Template:R/superscript The message bus daemon can also produce its own messages as a response to certain conditions, such as an error message to a process that sent a message to a nonexistent bus name.[15]Template:R/superscript
Page Template:Mono/styles.css has no content.dbus-daemon improves the feature set already provided by D-Bus itself with additional functionality. For example, service activation allows automatic starting of services when needed—when the first request to any bus name of such service arrives at the message bus daemon.[3]Template:R/superscript This way, service processes neither need to be launched during the system initialization or user initialization stage nor need they consume memory or other resources when not being used. This feature was originally implemented using setuid helpers,[20]Template:R/superscript but nowadays it can also be provided by systemd's service activation framework.[citation needed] Service activation is an important feature that facilitates the management of the process lifecycle of services (for example when a desktop component should start or stop).[15]Template:R/superscript
Heavily influenced by the DCOP system used by versions 2 and 3 of KDE, D-Bus has replaced DCOP in the KDE 4 release.[23][22]Template:R/superscript[24]Template:R/superscript An implementation of D-Bus supports most POSIX operating systems, and a port for Windows exists. It is used by Qt 4 and later by GNOME. In GNOME it has gradually replaced most parts of the earlier Bonobo mechanism. It is also used by Xfce.
One of the earlier adopters was the (nowadays deprecated) Hardware Abstraction Layer. HAL used D-Bus to export information about hardware that has been added to or removed from the computer.[6]Template:R/superscript
The usage of D-Bus is steadily expanding beyond the initial scope of desktop environments to cover an increasing amount of system services. For instance, the NetworkManager network daemon, BlueZ bluetooth stack and PulseAudio sound server use D-Bus to provide part or all of their services. systemd uses the D-Bus wire protocol for communication between Page Template:Mono/styles.css has no content.systemctl and systemd, and is also promoting traditional system daemons to D-Bus services, such as logind.[25]Template:R/superscript Another heavy user of D-Bus is Polkit, whose policy authority daemon is implemented as a service connected to the system bus.[26]Template:R/superscript
Implementations
libdbus
Although there are several implementations of D-Bus, the most widely used is the reference implementation libdbus, developed by the same freedesktop.org project that designed the specification. However, libdbus is a low-level implementation that was never meant to be used directly by application developers, but as a reference guide for other reimplementations of D-Bus (such as those included in standard libraries of desktop environments, or in programming language bindings). The freedesktop.org project itself recommends applications authors to "use one of the higher level bindings or implementations" instead.[27]Template:R/superscript
In 2013, the systemd project rewrote libdbus in an effort to simplify the code,[30]Template:R/superscript but it also resulted in a significant increase of the overall D-Bus performance. In preliminary benchmarks, BMW found that the systemd's D-Bus library increased performance by 360%.[31]Template:R/superscript By version 221 of systemd, released in 2015, the sd-bus API was declared stable.[32]Template:R/superscript
kdbus
kdbus is implemented as a character device driver.[33][34] All communication between processes take place over special character device nodes in /dev/kdbus (cf. devfs).
^Page Module:Citation/CS1/styles.css has no content.Ward, Brian (2004). "14: A brief survey of the Linux desktop". How Linux Works: What Every Superuser Should Know (2 ed.). San Francisco: No Starch Press (published 2014). p. 305. ISBN9781593275679. Retrieved 2016-11-07. One of the most important developments to come out of the Linux desktop is the Desktop Bus (D-Bus), a message-passing system. D-Bus is important because it serves as an interprocess communication mechanism that allows desktop applications to talk to each other [...].
^Page Module:Citation/CS1/styles.css has no content.Vermeulen, Jeroen (14 July 2013). "Introduction to D-Bus". FreeDesktop.org. Retrieved 3 October 2015. D-Bus [...] is designed for use as a unified middleware layer underneath the main free desktop environments.
^Page Module:Citation/CS1/styles.css has no content.Pennington, Havoc; Wheeler, David; Walters, Colin. "D-Bus Tutorial". Retrieved 21 October 2015. 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.
^Page Module:Citation/CS1/styles.css has no content.Vermeulen, Jeroen (14 July 2013). "Introduction to D-Bus". FreeDesktop.org. Retrieved 3 October 2015. D-Bus was first built to replace the CORBA-like component model underlying the GNOME desktop environment. Similar to DCOP (which is used by KDE), D-Bus is set to become a standard component of the major free desktop environments for GNU/Linux and other platforms.
^Page Module:Citation/CS1/styles.css has no content.Poettering, Lennart (19 June 2015). "The new sd-bus API of systemd". Retrieved 21 October 2015. we are working on moving things to a true user bus, of which there is only one per user on a system, regardless how many times that user happens to log in
^Page Module:Citation/CS1/styles.css has no content."What is D-Bus?". FreeDesktop.org. Retrieved 29 October 2015. There are also some reimplementations of the D-Bus protocol for languages such as C#, Java, and Ruby. These do not use the libdbus reference implementation
^ abPage Module:Citation/CS1/styles.css has no content."What is D-Bus?". FreeDesktop.org. Retrieved 29 October 2015. is built on top of a general one-to-one message passing framework, which can be used by any two apps to communicate directly (without going through the message bus daemon)
^Page Module:Citation/CS1/styles.css has no content."What is D-Bus?". FreeDesktop.org. Retrieved 5 January 2015. The low-level implementation is not primarily designed for application authors to use. Rather, it is a basis for binding authors and a reference for reimplementations. If you are able to do so it is recommended that you use one of the higher level bindings or implementations.
^Page Module:Citation/CS1/styles.css has no content."Migrating to GDBus". GNOME Developer. Retrieved 21 October 2015. dbus-glib uses the libdbus reference implementation, GDBus doesn't. Instead, it relies on GIO streams as transport layer, and has its own implementation for the D-Bus connection setup and authentication.
Lua error in package.lua at line 80: module 'Module:Navbox/configuration' not found.Lua error in package.lua at line 80: module 'Module:Navbox/configuration' not found.
