-D-BUS is a simple IPC library based on messages.
+Sections in this file describe:
+ - introduction and overview
+ - low-level vs. high-level API
+ - version numbers
+ - options to the configure script
+ - ABI stability policy
+
+Introduction
+===
+
+D-Bus is a simple system for interprocess communication and coordination.
-See also the file HACKING for notes of interest to developers working on D-BUS.
+The "and coordination" part is important; D-Bus provides a bus daemon that does things like:
+ - notify applications when other apps exit
+ - start services on demand
+ - support single-instance applications
See http://www.freedesktop.org/software/dbus/ for lots of documentation,
mailing lists, etc.
-Note
+See also the file HACKING for notes of interest to developers working on D-Bus.
+
+If you're considering D-Bus for use in a project, you should be aware
+that D-Bus was designed for a couple of specific use cases, a "system
+bus" and a "desktop session bus." These are documented in more detail
+in the D-Bus specification and FAQ available on the web site.
+
+If your use-case isn't one of these, D-Bus may still be useful, but
+only by accident; so you should evaluate carefully whether D-Bus makes
+sense for your project.
+
+Note: low-level API vs. high-level binding APIs
===
-A core concept of the D-BUS implementation is that "libdbus" is
-intended to be a low-level API, similar to Xlib. Most programmers are
-intended to use the bindings to GLib, Qt, Python, Mono, Java, or
-whatever. These bindings have varying levels of completeness.
+A core concept of the D-Bus implementation is that "libdbus" is
+intended to be a low-level API. Most programmers are intended to use
+the bindings to GLib, Qt, Python, Mono, Java, or whatever. These
+bindings have varying levels of completeness and are maintained as
+separate projects from the main D-Bus package. The main D-Bus package
+contains the low-level libdbus, the bus daemon, and a few command-line
+tools such as dbus-launch.
+
+If you use the low-level API directly, you're signing up for some
+pain. Think of the low-level API as analogous to Xlib or GDI, and the
+high-level API as analogous to Qt/GTK+/HTML.
+
+Version numbers
+===
+
+D-Bus uses the common "Linux kernel" versioning system, where
+even-numbered minor versions are stable and odd-numbered minor
+versions are development snapshots.
+
+So for example, development snapshots: 1.1.1, 1.1.2, 1.1.3, 1.3.4
+Stable versions: 1.0, 1.0.1, 1.0.2, 1.2.1, 1.2.3
+
+All pre-1.0 versions were development snapshots.
+
+Development snapshots make no ABI stability guarantees for new ABI
+introduced since the last stable release. Development snapshots are
+likely to have more bugs than stable releases, obviously.
Configuration flags
===
API/ABI Policy
===
-D-BUS API/ABI and protocol necessarily remain in flux until we are
-sure it will meet the various needs it's intended to meet. This means
-we need to see some significant sample usage in the contexts of GNOME,
-KDE, desktop applications, and systemwide uses such as print queue
-monitoring, hotplug events, or whatever. We need the flexibility to
-incorporate feedback from this sample usage.
-
-Once we feel confident in the protocol and the API, we will release a
-version 1.0. At that point, the intent is:
+Now that D-Bus has reached version 1.0, the objective is that all
+applications dynamically linked to libdbus will continue working
+indefinitely with the most recent system and session bus daemons.
- The protocol will never be broken again; any message bus should
work with any client forever. However, extensions are possible
it will always be possible to compile against and use the older
API, and apps will always get the API they expect.
-Until 1.0 is released, feedback that requires API changes may be
-incorporated into D-BUS. This may break the API, the ABI, the
-protocol, or all three.
+Interfaces can and probably will be _added_. This means both new
+functions and types in libdbus, and new methods exported to
+applications by the bus daemon.
+
+The above policy is intended to make D-Bus as API-stable as other
+widely-used libraries (such as GTK+, Qt, Xlib, or your favorite
+example). If you have questions or concerns they are very welcome on
+the D-Bus mailing list.
+
+NOTE ABOUT DEVELOPMENT SNAPSHOTS AND VERSIONING
+
+Odd-numbered minor releases (1.1.x, 1.3.x, 2.1.x, etc. -
+major.minor.micro) are devel snapshots for testing, and any new ABI
+they introduce relative to the last stable version is subject to
+change during the development cycle.
+
+Any ABI found in a stable release, however, is frozen.
+
+ABI will not be added in a stable series if we can help it. i.e. the
+ABI of 1.2.0 and 1.2.5 you can expect to be the same, while the ABI of
+1.4.x may add more stuff not found in 1.2.x.
+
+NOTE ABOUT STATIC LINKING
+
+We are not yet firmly freezing all runtime dependencies of the libdbus
+library. For example, the library may read certain files as part of
+its implementation, and these files may move around between versions.
-To avoid a huge soname, the plan is to increment the soname only
-between official stable releases, not with every development snapshot.
-Versions numbered 0.x are considered development snapshots.
+As a result, we don't yet recommend statically linking to
+libdbus. Also, reimplementations of the protocol from scratch might
+have to work to stay in sync with how libdbus behaves.
-Until 1.0 is released, you have to define -DDBUS_API_SUBJECT_TO_CHANGE
-just as a safety check to be sure everyone is aware of this API/ABI
-policy and has the right expectations.
+To lock things down and declare static linking and reimplementation to
+be safe, we'd like to see all the internal dependencies of libdbus
+(for example, files read) well-documented in the specification, and
+we'd like to have a high degree of confidence that these dependencies
+are supportable over the long term and extensible where required.
-We do need people to test the APIs, so please do use the development
-snapshots of D-BUS. They are intended to work and we do actively
-address bugs.
+NOTE ABOUT HIGH-LEVEL BINDINGS
-However, if you're shipping a commercial binary-only application that
-needs to keep running on M future versions of N operating systems, you
-might want to include your own copy of D-BUS rather than relying on
-the installed copy, for example.
+Note that the high-level bindings are _separate projects_ from the
+main D-Bus package, and have their own release cycles, levels of
+maturity, and ABI stability policies. Please consult the documentation
+for your binding.
<article id="index">
<articleinfo>
<title>D-Bus FAQ</title>
- <releaseinfo>Version 0.1</releaseinfo>
- <date>22 January 2005</date>
+ <releaseinfo>Version 0.2</releaseinfo>
+ <date>07 November 2006</date>
<authorgroup>
<author>
<firstname>Havoc</firstname>
</question>
<answer>
<para>
- This is probably best answered by reading the D-Bus <ulink url="dbus-tutorial.html">tutorial</ulink>. In
+ This is probably best answered by reading the D-Bus <ulink url="dbus-tutorial.html">tutorial</ulink> or
+ the introduction to the <ulink url="dbus-specification.html">specification</ulink>. In
short, it is a system consisting of 1) a wire protocol for exposing a
typical object-oriented language/framework to other applications; and
2) a bus daemon that allows applications to find and monitor one another.
+ Phrased differently, D-Bus is 1) an interprocess communication (IPC) system and 2) some higher-level
+ structure (lifecycle tracking, service activation, security policy) provided by two bus daemons,
+ one systemwide and one per-user-session.
</para>
</answer>
</qandaentry>
</question>
<answer>
<para>
- D-Bus has not yet reached 1.0. The <ulink url="README">README</ulink>
- file has a discussion of the API/ABI stability guarantees before and
- after 1.0. In short, there are no guarantees before 1.0, and stability
- of both protocol and reference library will be maintained after 1.0.
- As of January 2005 we don't expect major protocol or API changes prior
- to the 1.0 release, but anything is possible.
+ The low-level library "libdbus" and the protocol specification are considered
+ ABI stable. The <ulink url="README">README</ulink>
+ file has a discussion of the API/ABI stability guarantees.
+ Higher-level bindings (such as those for Qt, GLib, Python, Java, C#) each
+ have their own release schedules and degree of maturity, not linked to
+ the low-level library and bus daemon release. Check the project page for
+ the binding you're considering to understand that project's policies.
</para>
</answer>
</qandaentry>
are normally launched according to the bus name they will
have.
</para>
+ <para>
+ People often misuse the word "service" for any
+ bus name, but this tends to be ambiguous and confusing so is discouraged.
+ In the D-Bus docs we try to use "service" only when talking about
+ programs the bus knows how to launch, i.e. a service always has a
+ .service file.
+ </para>
</answer>
</qandaentry>
projects / platform / upstream / dbus.git / commitdiff