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@@ -0,0 +1,153 @@ +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. + +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. + +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. 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 +=== + +These are the dbus-specific configuration flags that can be given to +the ./configure program. + + --enable-tests enable unit test code + --enable-verbose-mode support verbose debug mode + --enable-asserts include assertion checks + --enable-checks include sanity checks on public API + --enable-xml-docs build XML documentation (requires xmlto) + --enable-doxygen-docs build DOXYGEN documentation (requires Doxygen) + --enable-gcov compile with coverage profiling instrumentation (gcc only) + --enable-abstract-sockets + use abstract socket namespace (linux only) + --enable-selinux build with SELinux support + --enable-dnotify build with dnotify support (linux only) + --enable-kqueue build with kqueue support (*BSD only) + --with-xml=libxml/expat XML library to use + --with-init-scripts=redhat Style of init scripts to install + --with-session-socket-dir=dirname Where to put sockets for the per-login-session message bus + --with-test-socket-dir=dirname Where to put sockets for make check + --with-system-pid-file=pidfile PID file for systemwide daemon + --with-system-socket=filename UNIX domain socket for systemwide daemon + --with-console-auth-dir=dirname directory to check for console ownerhip + --with-dbus-user=<user> User for running the DBUS daemon (messagebus) + --with-gnu-ld assume the C compiler uses GNU ld [default=no] + --with-tags[=TAGS] include additional configurations [automatic] + --with-x use the X Window System + + +API/ABI Policy +=== + +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 + where the protocol is extensible. + + - If the library API is modified incompatibly, we will rename it + as in http://ometer.com/parallel.html - in other words, + it will always be possible to compile against and use the older + API, and apps will always get the API they expect. + +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. + +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. + +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. + +NOTE ABOUT HIGH-LEVEL BINDINGS + +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. |