D-Bus specification and reference implementation, including libdbus and dbus-daemon https://www.freedesktop.org/wiki/Software/dbus/ (LADI project modifications)
• seat support is now also possible via elogind
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 https://www.freedesktop.org/wiki/Software/dbus/ for lots of documentation, mailing lists, etc. See also the file CONTRIBUTING.md 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. Security == If you find a security vulnerability that is not known to the public, please report it privately to firstname.lastname@example.org or by reporting a Gitlab issue at https://gitlab.freedesktop.org/dbus/dbus/issues/new and marking it as "confidential". On Unix systems, the system bus (dbus-daemon --system) is designed to be a security boundary between users with different privileges. On Unix systems, the session bus (dbus-daemon --session) is designed to be used by a single user, and only accessible by that user. We do not currently consider D-Bus on Windows to be security-supported, and we do not recommend allowing untrusted users to access Windows D-Bus via TCP. 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 === This branch of dbus can be built by using Autotools, CMake or Meson. The Meson build system is currently considered experimental, but is likely to become the recommended build system in future. Older versions of dbus required Autotools or CMake, with Autotools recommended for Unix systems and CMake recommended for Windows systems. When using Autotools, the configure step is initiated by running ./configure with or without additional configuration flags. dbus requires GNU Make (on BSD systems, this is typically called gmake) or a "make" implementation with compatible extensions. When using CMake, the configure step is initiated by running the cmake program with or without additional configuration flags. Meson only supports out-of-tree builds, and must be passed a directory to put built and generated sources into. We'll call that directory "build" here. It's recommended to create a separate build directory for each configuration you might want to use. Basic configuration is done with: ``` sh meson build/ ``` This will create the build directory. If any dependencies are missing, you can install them, or try to remove the dependency with a Meson configuration option (see below). Configuration flags === When using Autotools, run "./configure --help" to see the possible configuration options and environment variables. When using CMake, inspect README.cmake to see the possible configuration options and environment variables. When using Meson, to review the options which Meson chose, run: ``` sh meson configure build/ ``` With additional arguments meson configure can be used to change options for a previously configured build directory. All options passed to this command are in the form `-D "option"="value"`. For example: ``` sh meson configure build/ -Dprefix=/tmp/install ``` 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. Bootstrapping D-Bus on new platforms === A full build of dbus, with all regression tests enabled and run, depends on GLib. A full build of GLib, with all regression tests enabled and run, depends on dbus. To break this cycle, don't enable full test coverage (for at least one of those projects) during bootstrapping. You can rebuild with full test coverage after you have built both dbus and GLib at least once.