1 = D-Bus API Design Guidelines
3 @credit[type="author copyright"]
5 @email philip.withnall@collabora.co.uk
7 @desc Guidelines for writing high quality D-Bus APIs
8 @revision[date=2015-02-05 status=draft]
14 The most common use for D-Bus is in implementing a service which will be
15 consumed by multiple client programs, and hence all interfaces exported on the
16 bus form a public API. Designing a D-Bus API is like designing any other API:
17 there is a lot of flexibility, but there are design patterns to follow and
18 anti-patterns to avoid.
20 This guide aims to explain the best practices for writing D-Bus APIs. These
21 have been refined over several years of use of D-Bus in many projects.
22 Pointers will be given for implementing APIs using common D-Bus
24 $link[>>https://developer.gnome.org/gio/stable/gdbus-convenience.html](GDBus),
25 but detailed implementation instructions are left to the libraries’
26 documentation. Note that you should $em(not) use dbus-glib to implement D-Bus
27 services as it is deprecated and unmaintained. Most services should also avoid
28 libdbus (dbus-1), which is a low-level library and is awkward to use
29 correctly: it is designed to be used via a language binding such as
30 $link[>>http://qt-project.org/doc/qt-4.8/qtdbus.html](QtDBus).
32 For documentation on D-Bus itself, see the
33 $link[>>http://dbus.freedesktop.org/doc/dbus-specification.html](D-Bus
41 A D-Bus API is a specification of one or more interfaces, which will be
42 implemented by objects exposed by a service on the bus. Typically an API is
43 designed as a set of $link[>#interface-files](interface files), and the
44 implementation of the service follows those files. Some projects, however,
45 choose to define the API in the code for the service, and to export XML
46 interface files from the running service
47 $link[>>http://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-introspectable](using
48 D-Bus introspection). Both are valid approaches.
50 For simplicity, this document uses the XML descriptions of D-Bus interfaces as
51 the canonical representation.
54 [id="interface-files"]
56 A D-Bus interface file is an XML file which describes one or more D-Bus
57 interfaces, and is the best way of describing a D-Bus API in a machine
58 readable way. The format is described in the
59 $link[>>http://dbus.freedesktop.org/doc/dbus-specification.html#introspection-format](D-Bus
60 specification), and is supported by tools such as $cmd(gdbus-codegen).
62 Interface files for public API should be installed to
63 $code($var($$(datadir))/dbus-1/interfaces) so that other services can load
64 them. Private APIs should not be installed. There should be one file installed
65 per D-Bus interface, named after the interface, containing a single top-level
66 $code(<node>) element with a single $code(<interface>) beneath it. For example,
67 interface $code(com.example.MyService1.Manager) would be described by file
68 $file($var($$(datadir))/dbus-1/interfaces/com.example.MyService1.Manager.xml):
72 Example D-Bus Interface XML
74 A brief example interface XML document.
75 [code mime="application/xml"]
77 "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
78 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd" >
79 <node xmlns:doc="http://www.freedesktop.org/dbus/1.0/doc.dtd">
80 <interface name="com.example.MyService1.InterestingInterface">
81 <method name="AddContact">
82 <arg name="name" direction="in" type="s">
83 <doc:doc><doc:summary>Name of new contact</doc:summary></doc:doc>
85 <arg name="email" direction="in" type="s">
86 <doc:doc><doc:summary>E-mail address of new contact</doc:summary></doc:doc>
88 <arg name="id" direction="out" type="u">
89 <doc:doc><doc:summary>ID of newly added contact</doc:summary></doc:doc>
94 Adds a new contact to the address book with their name and
103 If an interface defined by service A needs to be used by client B, client B
104 should declare a build time dependency on service A, and use the installed copy
105 of the interface file for any code generation it has to do. It should $em(not)
106 have a local copy of the interface, as that could then go out of sync with the
107 canonical copy in service A’s git repository.
110 [id="api-versioning"]
112 $link[>>http://ometer.com/parallel.html](Just like C APIs), D-Bus interfaces
113 should be designed to be usable in parallel with API-incompatible versions. This
114 is achieved by including a version number in each interface name, service name
115 and object path which is incremented on every backwards-incompatible change.
117 Version numbers should be included in all APIs from the first release, as that
118 means moving to a new version is as simple as incrementing the number, rather
119 than inserting a number everywhere, which takes more effort.
121 New API can be added to a D-Bus interface without incrementing the version
122 number, as such additions are still backwards-compatible. However, clients
123 should gracefully handle the $code(org.freedesktop.DBus.Error.UnknownMethod)
124 error reply from all D-Bus method calls if they want to run against older
125 versions of the service which don’t implement new methods. (This also prevents
126 use of generated bindings; any method which a client wants to gracefully fall
127 back from should be called using a generic D-Bus method invocation rather than
128 a specific generated binding.)
130 When API is broken, changed or removed, the service’s version number must be
131 bumped; for example, from $code(com.example.MyService1)
132 to $code(com.example.MyService2). If backwards compatibility is maintained in
133 the service by implementing both the old and new interfaces, the service can
134 own $em(both) well-known names and clients can use whichever they support.
136 As discussed in $link[>#annotations], new or deprecated APIs should be marked in
137 the interface XML using annotations.
139 Note, however, that supporting multiple interface versions simultaneously
140 requires that $em(object paths) are versioned as well, so objects $em(must not)
141 be put on the bus at the root path (‘/’). This is for technical reasons: signals
142 sent from a D-Bus service have the originating service name overwritten by its
143 unique name (e.g. $code(com.example.MyService2) is overwritten by $code(:1:15)).
144 If object paths are shared between objects implementing two versions of the
145 service’s interface, client programs cannot tell which object a signal has come
146 from. The solution is to include the version number in all object paths, for
147 example $code(/com/example/MyService1/Manager) instead of $code(/) or
148 $code(/com/example/MyService/Manager).
150 In summary, version numbers should be included in all service names, interface
151 names and object paths:
153 * $code(com.example.MyService1)
154 * $code(com.example.MyService1.InterestingInterface)
155 * $code(com.example.MyService1.OtherInterface)
156 * $code(/com/example/MyService1/Manager)
157 * $code(/com/example/MyService1/OtherObject)
162 D-Bus API design is broadly the same as C API design, but there are a few
163 additional points to bear in mind which arise both from D-Bus’ features
164 (explicit errors, signals and properties), and from its implementation as an IPC
167 D-Bus method calls are much more expensive than C function calls, typically
168 taking on the order of milliseconds to complete a round trip. Therefore, the
169 design should minimize the number of method calls needed to perform an
172 The type system is very expressive, especially compared to C’s, and APIs should
173 take full advantage of it.
175 Similarly, its support for signals and properties differentiates it from normal
176 C APIs, and well written D-Bus APIs make full use of these features where
177 appropriate. They can be coupled with standard interfaces defined in the D-Bus
178 specification to allow for consistent access to properties and objects in a
181 === Minimizing Round Trips
184 Each D-Bus method call is a round trip from a client program to a service and
185 back again, which is expensive — taking on the order of a millisecond. One of
186 the top priorities in D-Bus API design is to minimize the number of round trips
187 needed by clients. This can be achieved by a combination of providing specific
188 convenience APIs and designing APIs which operate on large data sets in a single
189 call, rather than requiring as many calls as elements in the data set.
191 Consider an address book API, $code(com.example.MyService1.AddressBook). It
192 might have an $code(AddContact(ss) → (u)) method to add a contact (taking name
193 and e-mail address parameters and returning a unique contact ID), and a
194 $code(RemoveContact(u)) method to remove one by ID. In the normal case of
195 operating on single contacts in the address book, these calls are optimal.
196 However, if the user wants to import a list of contacts, or delete multiple
197 contacts simultaneously, one call has to be made per contact — this could take
198 a long time for large contact lists.
200 Instead of the $code(AddContact) and $code(RemoveContact) methods, the interface
201 could have an $code(UpdateContacts(a(ss)au) → (au)) method, which takes an array
202 of structs containing the new contacts’ details, and an array of IDs of the
203 contacts to delete, and returns an array of IDs for the new contacts. This
204 reduces the number of round trips needed to one.
206 Adding convenience APIs to replace a series of common method calls with a single
207 method call specifically for that task is best done after the API has been
208 profiled and bottlenecks identified, otherwise it could lead to premature
209 optimization. One area where convenience methods can typically be added
210 is during initialization of a client, where multiple method calls are needed to
211 establish its state with the service.
213 === Taking Advantage of the Type System
216 D-Bus’ type system is similar to Python’s, but with a terser syntax which may be
217 unfamiliar to C developers. The key to using the type system effectively is
218 to expose as much structure in types as possible. In particular, sending
219 structured strings over D-Bus should be avoided, as they need to be built and
220 parsed; both are complex operations which are prone to bugs.
222 For APIs being used in constrained situations, enumerated values should be
223 transmitted as unsigned integers. For APIs which need to be extended by third
224 parties or which are used in more loosely coupled systems, enumerated values
225 should be strings in some defined format.
227 Transmitting values as integers means string parsing and matching can be
228 avoided, the messages are more compact, and typos can be more easily avoided by
229 developers (if, for example, C enums are used in the implementation).
231 Transmitting values as strings means additional values can be defined by third
232 parties without fear of conflicting over integer values; for instance by using
233 the same reverse-domain-name namespacing as D-Bus interfaces.
235 In both cases, the interface documentation should describe the meaning of each
236 value. It should state whether the type can be extended in future and, if so,
237 how the service and client should handle unrecognized values — typically by
238 considering them equal to an ‘unknown’ or ‘failure’ value. Conventionally, zero
239 is used as the ‘unknown’ value.
242 For example, instead of:
243 [code style="invalid" mime="application/xml"]
247 Status of the object.
248 Valid statuses: ‘unknown’, ‘ready’, ‘complete’.
250 <property name="Status" type="s" access="read" />
253 [code style="valid" mime="application/xml"]
257 Status of the object.
258 Valid statuses: 0 = Unknown, 1 = Ready, 2 = Complete.
259 Unrecognized statuses should be considered equal to Unknown.
261 <property name="Status" type="u" access="read" />
263 Similarly, enumerated values should be used instead of booleans, as they allow
264 extra values to be added in future, and there is no ambiguity about the sense of
268 For example, instead of:
269 [code style="invalid" mime="application/xml"]
272 @direction: %TRUE to move it up in the list, %FALSE to move it down
274 Move this address book up or down in the user’s list of address books.
275 Higher address books have their contacts displayed first in search
278 <method name="MoveAddressBook">
279 <arg name="direction" type="b" direction="in" />
282 Be more explicit than a boolean:
283 [code style="valid" mime="application/xml"]
286 @direction: 0 = Move it up in the list, 1 = Move it down
288 Move this address book up or down in the user’s list of address books.
289 Higher address books have their contacts displayed first in search
292 Unrecognized enum values for @direction will result in the address book
295 <method name="MoveAddressBook">
296 <arg name="direction" type="u" direction="in" />
299 Enumerated values should also be used instead of $em(human readable) strings,
300 which should not be sent over the bus if possible. The service and client could
301 be running in different locales, and hence interpret any human readable strings
302 differently, or present them to the user in the wrong language. Transmit an
303 enumerated value and convert it to a human readable string in the client.
305 In situations where a service has received a human readable string from
306 somewhere else, it should pass it on unmodified to the client, ideally with its
307 locale alongside. Passing human readable information to a client is better than
311 For example, instead of:
312 [code style="invalid" mime="application/xml"]
314 ProgressNotification:
315 @progress_message: Human readable progress message string.
317 Emitted whenever significant progress is made with some example
318 operation. The @progress_message can be displayed in a UI dialogue to
321 <signal name="ProgressNotification">
322 <arg name="progress_message" type="s" />
325 The progress should be reported as an enumerated value:
326 [code style="valid" mime="application/xml"]
328 ProgressNotification:
329 @progress_state: 0 = Preparing, 1 = In progress, 2 = Finished
331 Emitted whenever significant progress is made with some example
332 operation. The @progress_state is typically converted to a human readable
333 string and presented to the user. Unrecognized @progress_state values
334 should be treated as state 1, in progress.
336 <signal name="ProgressNotification">
337 <arg name="progress_state" type="u" />
340 D-Bus has none of the problems of signed versus unsigned integers which C has
341 (specifically, it does not do implicit sign conversion), so integer types should
342 always be chosen to be an appropriate size and signedness for the data they
343 could possibly contain. Typically, unsigned values are more frequently needed
346 Structures can be used almost anywhere in a D-Bus type, and arrays of structures
347 are particularly useful. Structures should be used wherever data fields are
348 related. Note, however, that structures are not extensible in future, so always
349 consider $link[>#extensibility].
352 For example, instead of several identically-indexed arrays containing
353 different properties of the same set of items:
354 [code style="invalid" mime="application/xml"]
357 @names: Array of contact names to add.
358 @emails: Corresponding array of contact e-mail addresses.
359 @ids: Returned array of the IDs of the new contacts. This will be the
360 same length as @names.
362 Add zero or more contacts to the address book, using their names and
363 e-mail addresses. @names and @emails must be the same length.
365 <method name="AddContacts">
366 <arg name="names" type="as" direction="in" />
367 <arg name="emails" type="as" direction="in" />
368 <arg name="ids" type="au" direction="out" />
371 The arrays can be combined into a single array of structures:
372 [code style="invalid" mime="application/xml"]
375 @details: Array of (contact name, contact e-mail address) to add.
376 @ids: Returned array of the IDs of the new contacts. This will be the
377 same length as @details.
379 Add zero or more contacts to the address book, using their names and
382 <method name="AddContacts">
383 <arg name="details" type="a(ss)" direction="in" />
384 <arg name="ids" type="au" direction="out" />
387 Note that D-Bus arrays are automatically transmitted with their length, so there
388 is no need to null-terminate them or encode their length separately.
391 FIXME: Mention maybe types and the extended kdbus/GVariant type system once
392 that’s stable and round-trip-ability is no longer a concern.
397 Some D-Bus APIs have very well-defined use cases, and will never need extension.
398 Others are used in more loosely coupled systems which may change over time, and
399 hence should be designed to be extensible from the beginning without the need
400 to break API in future. This is a trade off between having a more complex API,
401 and being able to easily extend it in future.
403 The key tool for extensibility in D-Bus is $code(a{sv}), the dictionary mapping
404 strings to variants. If well-defined namespaced strings are used as the
405 dictionary keys, arbitrary D-Bus peers can add whatever information they need
406 into the dictionary. Any other peer which understands it can query and retrieve
407 the information; other peers will ignore it.
409 The canonical example of an extensible API using $code(a{sv}) is
410 $link[>>http://telepathy.freedesktop.org/spec/](Telepathy). It uses $code(a{sv})
411 values as the final element in structures to allow them to be extended in
414 A secondary tool is the use of flag fields in method calls. The set of accepted
415 flags is entirely under the control of the interface designer and, as with
416 enumerated types, can be extended in future without breaking API. Adding more
417 flags allows the functionality of the method call to be tweaked.
419 === Using Signals, Properties and Errors
420 [id="using-the-features"]
422 D-Bus method calls are explicitly asynchronous due to the latency inherent in
423 IPC. This means that peers should not block on receiving a reply from a method
424 call; they should schedule other work (in a main loop) and handle the reply when
425 it is received. Even though method replies may take a while, the caller is
426 $em(guaranteed) to receive exactly one reply eventually. This reply could be the
427 return value from the method, an error from the method, or an error from the
428 D-Bus daemon indicating the service failed in some way (e.g. due to crashing).
430 Because of this, service implementations should be careful to always reply
431 exactly once to each method call. Replying at the end of a long-running
432 operation is correct — the client will patiently wait until the operation has
433 finished and the reply is received.
435 Note that D-Bus client bindings may implement synthetic timeouts of several
436 tens of seconds, unless explicitly disabled for a call. For very long-running
437 operations, you should disable the client bindings’ timeout and make it clear
438 in the client’s UI that the application has not frozen and is simply running a
441 An anti-pattern to avoid in this situation is to start a long-running operation
442 when a method call is received, then to never reply to the method call and
443 instead notify the client of completion of the operation via a signal. This
444 approach is incorrect as signal emissions do not have a one-to-one relationship
445 with method calls — the signal could theoretically be emitted multiple times, or
446 never, which the client would not be able to handle.
448 Similarly, use a D-Bus error reply to signify failure of an operation triggered
449 by a method call, rather than using a custom error code in the method’s
450 reply. This means that a reply always indicates success, and an error always
451 indicates failure — rather than a reply indicating either depending on its
452 parameters, and having to return dummy values in the other parameters. Using
453 D-Bus error replies also means such failures can be highlighted in debugging
454 tools, simplifying debugging.
456 Clients should handle all possible standard and documented D-Bus errors for each
457 method call. IPC inherently has more potential failures than normal C function
458 calls, and clients should be prepared to handle all of them gracefully.
460 === Using Standard Interfaces
461 [id="standard-interfaces"]
463 Use standard D-Bus interfaces where possible.
466 [id="interface-properties"]
468 The D-Bus specification defines the
469 $link[>>http://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-properties]($code(org.freedesktop.DBus.Properties))
470 interface, which should be used by all objects to notify clients of changes
471 to their property values, with the $code(PropertiesChanged) signal. This signal
472 eliminates the need for individual $code($var(PropertyName)Changed) signals, and
473 allows multiple properties to be notified in a single signal emission, reducing
474 IPC round trips. Similarly, the $code(Get) and $code(Set) methods can be used to
475 manipulate properties on an object, eliminating redundant
476 $code(Get$var(PropertyName)) and $code(Set$var(PropertyName)) methods.
479 For example, consider an object implementing an interface
480 $code(com.example.MyService1.SomeInterface) with methods:
482 * $code(GetName() → (s))
483 * $code(SetName(s) → ())
484 * $code(GetStatus() → (u))
485 * $code(RunOperation(ss) → (u))
488 * $code(NameChanged(u))
489 * $code(StatusChanged(u))
491 The interface could be cut down to a single method:
493 * $code(RunOperation(ss) → (u))
494 The object could then implement the $code(org.freedesktop.DBus.Properties)
495 interface and define properties:
497 * $code(Name) of type $code(s), read–write
498 * $code(Status) of type $code(u), read-only
500 The $code(NameChanged) and $code(StatusChanged) signals would be replaced by
501 $code(org.freedesktop.DBus.Properties.PropertiesChanged).
504 [id="interface-object-manager"]
506 The specification also defines the
507 $link[>>http://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-objectmanager]($code(org.freedesktop.DBus.ObjectManager))
508 interface, which should be used whenever a service needs to expose a variable
509 number of objects of the same class in a flat or tree-like structure, and
510 clients are expected to be interested in most or all of the objects. For
511 example, this could be used by an address book service which exposes multiple
512 address books, each as a separate object. The $code(GetManagedObjects) method
513 allows the full object tree to be queried, returning all the objects’ properties
514 too, eliminating the need for further IPC round trips to query the properties.
516 If clients are not expected to be interested in most of the exposed objects,
517 $code(ObjectManager) should $em(not) be used, as it will send all of the objects
518 to each client anyway, wasting bus bandwidth. A file manager, therefore, should
519 not expose the entire file system hierarchy using $code(ObjectManager).
522 For example, consider an object implementing an interface
523 $code(com.example.MyService1.AddressBookManager) with methods:
525 * $code(GetAddressBooks() → (ao))
528 * $code(AddressBookAdded(o))
529 * $code(AddressBookRemoved(o))
531 If the manager object is at path
532 $code(/com/example/MyService1/AddressBookManager), each address book is a
534 $code(/com/example/MyService1/AddressBookManager/SomeAddressBook).
536 The interface could be eliminated, and the
537 $code(org.freedesktop.DBus.ObjectManager) interface implemented on the manager
540 Calls to $code(GetAddressBooks) would become calls to $code(GetManagedObjects)
541 and emissions of the $code(AddressBookAdded) and $code(AddressBookRemoved)
542 signals would become emissions of $code(InterfacesAdded) and
543 $code(InterfacesRemoved).
545 === Naming Conventions
546 [id="naming-conventions"]
548 All D-Bus names, from service names through to method parameters, follow a set
549 of conventions. Following these conventions makes APIs more natural to use and
550 consistent with all other services on the system.
552 Services use reverse-domain-name notation, based on the domain name of the
553 project providing the service (all in lower case), plus a unique name for the
554 service (in camel case).
557 For example, version 2 of an address book application called $code(ContactDex)
558 provided by a software vendor whose website is $code(chocolateteapot.com)
559 would use service name $code(com.chocolateteapot.ContactDex2).
561 Almost all names use camel case with no underscores, as in the examples below.
562 Method and signal parameters are an exception, using
563 $code(lowercase_with_underscores). Type information is never encoded in the
564 parameter name (i.e. $em(not)
565 $link[>>http://en.wikipedia.org/wiki/Hungarian_notation](Hungarian notation)).
570 * $code(com.example.MyService1)
572 * $code(com.example.MyService1.SomeInterface)
573 - Object Path (Root Object)
574 * $code(/com/example/MyService1)
575 - Object Path (Child Object)
576 * $code(/com/example/MyService1/SomeChild)
577 - Object Path (Grandchild Object)
578 * $code(/com/example/MyService1/AnotherChild/Grandchild1)
580 * $code(com.example.MyService1.SomeInterface.MethodName)
582 * $code(com.example.MyService1.SomeInterface.SignalName)
584 * $code(com.example.MyService1.SomeInterface.PropertyName)
586 See also: $link[>#api-versioning].
589 [id="code-generation"]
591 Rather than manually implementing both the server and client sides of a D-Bus
592 interface, it is often easier to write the interface XML description and use a
594 $link[>>https://developer.gnome.org/gio/stable/gdbus-codegen.html]($cmd(gdbus-codegen))
595 to generate type-safe C APIs, then build the implementation using those. This
596 avoids the tedious and error-prone process of writing code to build and read
597 D-Bus parameter variants for each method call.
599 Use of code generators is beyond the scope of this guide; for more information,
601 $link[>>https://developer.gnome.org/gio/stable/gdbus-codegen.html]($cmd(gdbus-codegen)
607 Annotations may be added to the interface XML to expose metadata on the API
608 which can be used by documentation or code generation tools to modify their
609 output. Some standard annotations are given in the
610 $link[>>http://dbus.freedesktop.org/doc/dbus-specification.html#introspection-format](D-Bus
611 specification), but further annotations may be defined by specific tools.
613 For example, $cmd(gdbus-codegen) defines several useful annotations, listed on
616 The following annotations are the most useful:
619 - $code(org.freedesktop.DBus.Deprecated)
620 * Mark a symbol as deprecated. This should be used whenever the API is changed,
621 specifying the version introducing the deprecation, the reason for
622 deprecation, and a replacement symbol to use.
623 - $code(org.gtk.GDBus.Since)
624 * Mark a symbol as having been added to the API after the initial release. This
625 should include the version the symbol was first added in.
626 - $code(org.gtk.GDBus.C.Name) and $code(org.freedesktop.DBus.GLib.CSymbol)
627 * Both used interchangeably to hint at a C function name to use when generating
628 code for a symbol. Use of this annotation can make generated bindings a lot
629 more pleasant to use.
630 - $code(org.freedesktop.DBus.Property.EmitsChangedSignal)
631 * Indicate whether a property is expected to emit change signals. This can
632 affect code generation, but is also useful documentation, as client programs
633 then know when to expect property change notifications and when they have to
639 Also just like C APIs, D-Bus APIs must be documented. There are several methods
640 for documenting the interfaces, methods, properties and signals in a D-Bus
641 interface XML file, each unfortunately with their own downsides. You should
642 choose the method which best matches the tooling and workflow you are using.
646 XML comments containing documentation in the
647 $link[>>https://developer.gnome.org/gtk-doc-manual/stable/documenting_syntax.html.en](gtk-doc
648 format) is the recommended format for use with
649 $link[>>https://developer.gnome.org/gio/stable/gdbus-codegen.html]($cmd(gdbus-codegen)).
650 Using $cmd(gdbus-codegen), these comments can be extracted, converted to DocBook
651 format and included in the project’s API manual. For example:
655 Documentation Comments in D-Bus Interface XML
657 Example gtk-doc–style documentation comments in the introspection XML for
658 the $code(org.freedesktop.DBus.Properties) interface.
659 [code mime="application/xml"]
661 org.freedesktop.DBus.Properties:
662 @short_description: Standard property getter/setter interface
664 Interface for all objects which expose properties on the bus, allowing
665 those properties to be got, set, and signals emitted to notify of changes
666 to the property values.
668 <interface name="org.freedesktop.DBus.Properties">
671 @interface_name: Name of the interface the property is defined on.
672 @property_name: Name of the property to get.
673 @value: Property value, wrapped in a variant.
675 Retrieves the value of the property at @property_name on
676 @interface_name on this object. If @interface_name is an empty string,
677 all interfaces will be searched for @property_name; if multiple
678 properties match, the result is undefined.
680 If @interface_name or @property_name do not exist, a
681 #org.freedesktop.DBus.Error.InvalidArgs error is returned.
684 <arg type="s" name="interface_name" direction="in"/>
685 <arg type="s" name="property_name" direction="in"/>
686 <arg type="v" name="value" direction="out"/>
691 @interface_name: Name of the interface the properties changed on.
692 @changed_properties: Map of property name to updated value for the
694 @invalidated_properties: List of names of other properties which have
695 changed, but whose updated values are not notified.
697 Emitted when one or more properties change values on @interface_name.
698 A property may be listed in @changed_properties or
699 @invalidated_properties depending on whether the service wants to
700 broadcast the property’s new value. If a value is large or infrequently
701 used, the service might not want to broadcast it, and will wait for
702 clients to request it instead.
704 <signal name="PropertiesChanged">
705 <arg type="s" name="interface_name"/>
706 <arg type="a{sv}" name="changed_properties"/>
707 <arg type="as" name="invalidated_properties"/>
712 FIXME: This is only present to fix
713 $link[>>https://github.com/projectmallard/mallard-ducktype/issues/2].
717 Documentation can also be added as annotation elements in the XML. This format
718 is also supported by $cmd(gdbus-codegen), but gtk-doc comments are preferred.
723 Documentation Annotations in D-Bus Interface XML
725 Example GDBus documentation annotations in the introspection XML for
726 the $code(org.freedesktop.DBus.Properties) interface.
727 [code mime="application/xml"]
728 <interface name="org.freedesktop.DBus.Properties">
729 <annotation name="org.gtk.GDBus.DocString" value="Interface for all
730 objects which expose properties on the bus, allowing those properties to
731 be got, set, and signals emitted to notify of changes to the property
733 <annotation name="org.gtk.GDBus.DocString.Short"
734 value="Standard property getter/setter interface"/>
737 <annotation name="org.gtk.GDBus.DocString" value="Retrieves the value of
738 the property at @property_name on @interface_name on this object. If
739 @interface_name is an empty string, all interfaces will be searched
740 for @property_name; if multiple properties match, the result is
743 If @interface_name or @property_name do not exist, a
744 #org.freedesktop.DBus.Error.InvalidArgs error is returned."/>
746 <arg type="s" name="interface_name" direction="in">
747 <annotation name="org.gtk.GDBus.DocString"
748 value="Name of the interface the property is defined on."/>
751 <arg type="s" name="property_name" direction="in">
752 <annotation name="org.gtk.GDBus.DocString"
753 value="Name of the property to get."/>
756 <arg type="v" name="value" direction="out">
757 <annotation name="org.gtk.GDBus.DocString"
758 value="Property value, wrapped in a variant."/>
762 <signal name="PropertiesChanged">
763 <annotation name="org.gtk.GDBus.DocString" value="Emitted when one or
764 more properties change values on @interface_name. A property may be
765 listed in @changed_properties or @invalidated_properties depending on
766 whether the service wants to broadcast the property’s new value. If a
767 value is large or infrequently used, the service might not want to
768 broadcast it, and will wait for clients to request it instead."/>
770 <arg type="s" name="interface_name">
771 <annotation name="org.gtk.GDBus.DocString"
772 value="Name of the interface the properties changed on."/>
775 <arg type="a{sv}" name="changed_properties">
776 <annotation name="org.gtk.GDBus.DocString"
777 value="Map of property name to updated value for the changed
781 <arg type="as" name="invalidated_properties">
782 <annotation name="org.gtk.GDBus.DocString"
783 value="List of names of other properties which have changed, but
784 whose updated values are not notified."/>
790 FIXME: This is only present to fix
791 $link[>>https://github.com/projectmallard/mallard-ducktype/issues/2].
796 Each D-Bus service must install a $file(.service) file describing its service
797 name and the command to run to start the service. This allows for service
799 $link[>>http://dbus.freedesktop.org/doc/dbus-specification.html#message-bus-starting-services](D-Bus
802 Service files must be named after the service’s well-known name, for example
803 file $file(com.example.MyService1.service) for well-known name
804 $code(com.example.MyService1). Files must be installed in
805 $file($var($$(datadir))/dbus-1/services) for the session bus and
806 $file($var($$(datadir))/dbus-1/system-services) for the system bus. Note, however,
807 that services on the system bus almost always need a
808 $link[>#security-policies](security policy) as well.
811 [id="security-policies"]
813 At a high level, the D-Bus security model is:
815 * There is a system bus, and zero or more session buses.
816 * Any process may connect to the system bus. The system bus limits which can own
817 names or send method calls, and only processes running as privileged users can
818 receive unicast messages not addressed to them. Every process may receive
820 * Each session bus has an owner (a user). Only its owner may connect; on
821 general-purpose Linux, a session bus is not treated as a privilege boundary,
822 so there is no further privilege separation between processes on it.
824 Full coverage of securing D-Bus services is beyond the scope of this guide,
825 however there are some steps which you can take when designing an API to ease
826 security policy implementation.
828 D-Bus security policies are written as XML files in
829 $file($var($$(datadir)/dbus-1/system.d)),
830 $file($var($$(datadir)/dbus-1/session.d)),
831 $file($var($$(sysconfdir)/dbus-1/system.d)) and
832 $file($var($$(sysconfdir)/dbus-1/session.d)) and use an allow/deny model, where
833 each message (method call, signal emission, etc.) can be allowed or denied
834 according to the sum of all policy rules which match it. Each $code(<allow>) or
835 $code(<deny>) rule in the policy should have the $code(own),
836 $code(send_destination) or $code(receive_sender) attribute set.
838 When designing an API, bear in mind the need to write and install such a
839 security policy, and consider splitting up methods or providing more restricted
840 versions which accept constrained parameters, so that they can be exposed with
841 less restrictive security policies if needed by less trusted clients. Since
842 dbus-daemon 1.10, security policies should be installed to
843 $file($var($$(datadir))) rather than $(file($var($$(sysconfdir))); the latter
844 is intended for system administators.
846 Secondly, the default D-Bus security policy for the system bus is restrictive
847 enough to allow sensitive data, such as passwords, to be safely sent over the
848 bus in unicast messages (including unicast signals); so there is no need to
849 complicate APIs by implementing extra security. Note, however, that sensitive
850 data must $em(not) be sent in broadcast signals, as they can be seen by all
851 peers on the bus. The default policy for the session bus is not restrictive, but
852 it is typically not a security boundary.
857 Debugging services running on D-Bus can be tricky, especially if they are
858 launched via service activation and hence in an environment out of your control.
860 Three main tools are available: D-Bus Monitor, Bustle and D-Feet.
865 $link[>>http://dbus.freedesktop.org/doc/dbus-monitor.1.html]($cmd(dbus-monitor))
866 is a core D-Bus tool, and allows eavesdropping on the session or system bus,
867 printing all messages it sees. The messages may be filtered using a standard
868 $link[>>http://dbus.freedesktop.org/doc/dbus-specification.html#message-bus-routing-match-rules](D-Bus
869 match rule) to make the stream more manageable.
871 Previous versions of D-Bus have required the security policy for the system bus
872 to be manually relaxed to allow eavesdropping on all messages. This meant that
873 debugging it was difficult and insecure. The latest versions of D-Bus add
874 support for monitor-only connections for the root user, which means that
875 $cmd(dbus-monitor) can be run as root to painlessly monitor all messages on the
876 system bus without modifying its security policy.
881 $link[>>http://willthompson.co.uk/bustle/](Bustle) is a graphical version of
882 $cmd(dbus-monitor), with a UI focused on profiling D-Bus performance by plotting
883 messages on a timeline. It is ideal for finding bottlenecks in IPC performance
884 between a service and client.
889 $link[>>https://wiki.gnome.org/Apps/DFeet](D-Feet) is an introspection tool for
890 D-Bus, which displays all peers on the bus graphically, with their objects,
891 interfaces, methods, signals and properties listed for examination. It is useful
892 for debugging all kinds of issues related to presence of services on the bus
893 and the objects they expose.