X-Git-Url: http://review.tizen.org/git/?a=blobdiff_plain;f=doc%2Fdbus-tutorial.xml;h=5c385f0ee6385a66fb6a566328d2965d8e80b8fb;hb=e9762e1542449601ff23b0c5fc8d6764e0898d3f;hp=3ee26daaf6823211bfb84fc3a9e8a4357dc7516d;hpb=f44c0367187416d81ee10ca038214bfeb41c05bc;p=platform%2Fupstream%2Fdbus.git diff --git a/doc/dbus-tutorial.xml b/doc/dbus-tutorial.xml index 3ee26da..5c385f0 100644 --- a/doc/dbus-tutorial.xml +++ b/doc/dbus-tutorial.xml @@ -6,50 +6,88 @@
- D-BUS Tutorial - Version 0.1 - 02 October 2003 + D-Bus Tutorial + Version 0.5.0 + 20 August 2006 Havoc Pennington Red Hat, Inc. -
- hp@pobox.com -
+
hp@pobox.com
+ + + + David + Wheeler + + + John + Palmieri + + Red Hat, Inc. +
johnp@redhat.com
+ + + + Colin + Walters + + Red Hat, Inc. +
walters@redhat.com
 + + Tutorial Work In Progress + + + This tutorial is not complete; it probably contains some useful information, but + also has plenty of gaps. Right now, you'll also need to refer to the D-Bus specification, + Doxygen reference documentation, and look at some examples of how other apps use D-Bus. + + + + Enhancing the tutorial is definitely encouraged - send your patches or suggestions to the + mailing list. If you create a D-Bus binding, please add a section to the tutorial for your + binding, if only a short section with a couple of examples. + + + + - What is D-BUS? + What is D-Bus? - D-BUS is a system for interprocess communication + D-Bus is a system for interprocess communication (IPC). Architecturally, it has several layers: - A library, libdbus, that allows two applications to connect - to each other and exchange messages. + A library, libdbus, that allows two + applications to connect to each other and exchange messages. - A message bus daemon executable, built on libdbus, that multiple - applications can connect to. The daemon can route messages - from one application to zero or more other applications. + A message bus daemon executable, built on + libdbus, that multiple applications can connect to. The daemon can + route messages from one application to zero or more other + applications. - Wrapper libraries based on particular application frameworks. - For example, libdbus-glib and libdbus-qt. There are also - bindings to languages such as Python. These wrapper libraries - are the API most people should use, as they simplify the - details of D-BUS programming. + Wrapper libraries or bindings + based on particular application frameworks. For example, libdbus-glib and + libdbus-qt. There are also bindings to languages such as + Python. These wrapper libraries are the API most people should use, + as they simplify the details of D-Bus programming. libdbus is + intended to be a low-level backend for the higher level bindings. + Much of the libdbus API is only useful for binding implementation. @@ -88,16 +126,21 @@ - D-BUS applications + D-Bus applications There are many, many technologies in the world that have "Inter-process communication" or "networking" in their stated purpose: MBUS, CORBA, DCE, DCOM, DCOP, XML-RPC, SOAP, and probably hundreds - more. Each of these is tailored for particular kinds of application. - D-BUS is designed for two specific cases: + url="http://www.w3.org/TR/SOAP/">SOAP, MBUS, Internet Communications Engine (ICE), + and probably hundreds more. + Each of these is tailored for particular kinds of application. + D-Bus is designed for two specific cases: @@ -119,9 +162,10 @@ 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 + 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. + in particular. D-Bus may or may not be appropriate for other + applications; the FAQ has some comparisons to other IPC systems. The problem solved by the systemwide or communication-with-the-OS case @@ -153,7 +197,7 @@ - D-BUS may happen to be useful for purposes other than the one it was + D-Bus may happen to be useful for purposes other than the one it was designed for. Its general properties that distinguish it from other forms of IPC are: @@ -177,7 +221,7 @@ Many implementation and deployment issues are specified rather - than left ambiguous. + than left ambiguous/configurable/pluggable. @@ -200,29 +244,32 @@ Concepts Some basic concepts apply no matter what application framework you're - using to write a D-BUS application. The exact code you write will be + using to write a D-Bus application. The exact code you write will be different for GLib vs. Qt vs. Python applications, however. + + + Here is a diagram (png svg) that may help you visualize the concepts + that follow. + - Objects and Object Paths + Native Objects and Object Paths - Each application using D-BUS contains objects, - which generally map to GObject, QObject, C++ objects, or Python objects - (but need not). An object is an instance rather - than a type. When messages are received over a D-BUS connection, they - are sent to a specific object, not to the application as a whole. + Your programming framework probably defines what an "object" is like; + usually with a base class. For example: java.lang.Object, GObject, QObject, + python's base Object, or whatever. Let's call this a native object. - To allow messages to specify their destination object, there has to be a - way to refer to an object. In your favorite programming language, this - is normally called a pointer or - reference. However, these references are - implemented as memory addresses relative to the address space of your - application, and thus can't be passed from one application to another. + The low-level D-Bus protocol, and corresponding libdbus API, does not care about native objects. + However, it provides a concept called an + object path. The idea of an object path is that + higher-level bindings can name native object instances, and allow remote applications + to refer to them. - To solve this, D-BUS introduces a name for each object. The name + The object path looks like a filesystem path, for example an object could be named /org/kde/kspread/sheets/3/cells/4/5. Human-readable paths are nice, but you are free to create an @@ -237,126 +284,145 @@ + + Methods and Signals + + + Each object has members; the two kinds of member + are methods and + signals. Methods are operations that can be + invoked on an object, with optional input (aka arguments or "in + parameters") and output (aka return values or "out parameters"). + Signals are broadcasts from the object to any interested observers + of the object; signals may contain a data payload. + + + + Both methods and signals are referred to by name, such as + "Frobate" or "OnClicked". + + + + Interfaces Each object supports one or more interfaces. Think of an interface as a named group of methods and signals, - just as it is in GLib or Qt. Interfaces define the + just as it is in GLib or Qt or Java. Interfaces define the type of an object instance. + + DBus identifies interfaces with a simple namespaced string, + something like org.freedesktop.Introspectable. + Most bindings will map these interface names directly to + the appropriate programming language construct, for example + to Java interfaces or C++ pure virtual classes. + - - - Message Types + + + Proxies - Messages are not all the same; in particular, D-BUS has - 4 built-in message types: - - - - Method call messages ask to invoke a method - on an object. - - - - - Method return messages return the results - of invoking a method. - - - - - Error messages return an exception caused by - invoking a method. - - - - - Signal messages are notifications that a given signal - has been emitted (that an event has occurred). - You could also think of these as "event" messages. - - - + A proxy object is a convenient native object created to + represent a remote object in another process. The low-level DBus API involves manually creating + a method call message, sending it, then manually receiving and processing + the method reply message. Higher-level bindings provide proxies as an alternative. + Proxies look like a normal native object; but when you invoke a method on the proxy + object, the binding converts it into a DBus method call message, waits for the reply + message, unpacks the return value, and returns it from the native method.. - A method call maps very simply to messages, then: you send a method call - message, and receive either a method return message or an error message - in reply. + In pseudocode, programming without proxies might look like this: + + Message message = new Message("/remote/object/path", "MethodName", arg1, arg2); + Connection connection = getBusConnection(); + connection.send(message); + Message reply = connection.waitForReply(message); + if (reply.isError()) { + + } else { + Object returnValue = reply.getReturnValue(); + } + - - - - Services - - Object paths, interfaces, and messages exist on the level of - libdbus and the D-BUS protocol; they are used even in the - 1-to-1 case with no message bus involved. + Programming with proxies might look like this: + + Proxy proxy = new Proxy(getBusConnection(), "/remote/object/path"); + Object returnValue = proxy.MethodName(arg1, arg2); + + + + + Bus Names - Services, on the other hand, are a property of the message bus daemon. - A service is simply a name mapped to - some application connected to the message bus daemon. - These names are used to specify the origin and destination - of messages passing through the message bus. When a name is mapped - to a particular application, the application is said to - own that service. + When each application connects to the bus daemon, the daemon immediately + assigns it a name, called the unique connection name. + A unique name begins with a ':' (colon) character. These names are never + reused during the lifetime of the bus daemon - that is, you know + a given name will always refer to the same application. + An example of a unique name might be + :34-907. The numbers after the colon have + no meaning other than their uniqueness. - On connecting to the bus daemon, each application immediately owns a - special name called the base service. A base - service begins with a ':' (colon) character; no other services are - allowed to begin with that character. Base services are special because - each one is unique. They are created dynamically, and are never re-used - during the lifetime of the same bus daemon. You know that a given - base service name will have the same owner at all times. - An example of a base service name might be :34-907. + When a name is mapped + to a particular application's connection, that application is said to + own that name. Applications may ask to own additional well-known - services. For example, you could write a specification to - define a service called com.mycompany.TextEditor. - Your definition could specify that to own this service, an application + names. For example, you could write a specification to + define a name called com.mycompany.TextEditor. + Your definition could specify that to own this name, an application should have an object at the path /com/mycompany/TextFileManager supporting the - interface org.freedesktop.FileHandler. + interface org.freedesktop.FileHandler. - Applications could then send messages to this service, + Applications could then send messages to this bus name, object, and interface to execute method calls. - You could think of the base service names as IP addresses, and the - well-known services as domain names. So + You could think of the unique names as IP addresses, and the + well-known names as domain names. So com.mycompany.TextEditor might map to something like :34-907 just as mycompany.com maps to something like 192.168.0.5. - Services have a second important use, other than routing messages. They + Names have a second important use, other than routing messages. They are used to track lifecycle. When an application exits (or crashes), its connection to the message bus will be closed by the operating system kernel. The message bus then sends out notification messages telling - remaining applications that the application's services have lost their + remaining applications that the application's names have lost their owner. By tracking these notifications, your application can reliably monitor the lifetime of other applications. + + Bus names can also be used to coordinate single-instance applications. + If you want to be sure only one + com.mycompany.TextEditor application is running for + example, have the text editor application exit if the bus name already + has an owner. + + Addresses - Applications using D-BUS are either servers or clients. A server + Applications using D-Bus are either servers or clients. A server listens for incoming connections; a client connects to a server. Once the connection is established, it is a symmetric flow of messages; the client-server distinction only matters when setting up the @@ -364,18 +430,24 @@ - A D-BUS address specifies where a server will + If you're using the bus daemon, as you probably are, your application + will be a client of the bus daemon. That is, the bus daemon listens + for connections and your application initiates a connection to the bus + daemon. + + + + A D-Bus address specifies where a server will listen, and where a client will connect. For example, the address unix:path=/tmp/abcdef specifies that the server will listen on a UNIX domain socket at the path /tmp/abcdef and the client will connect to that socket. An address can also specify TCP/IP sockets, or any other - transport defined in future iterations of the D-BUS specification. + transport defined in future iterations of the D-Bus specification. - When using D-BUS with a message bus, the bus daemon is a server - and all other applications are clients of the bus daemon. + When using D-Bus with a message bus daemon, libdbus automatically discovers the address of the per-session bus daemon by reading an environment variable. It discovers the systemwide bus daemon by checking a well-known UNIX domain socket path @@ -383,10 +455,10 @@ - If you're using D-BUS without a bus daemon, it's up to you to + If you're using D-Bus without a bus daemon, it's up to you to define which application will be the server and which will be the client, and specify a mechanism for them to agree on - the server's address. + the server's address. This is an unusual case. @@ -399,68 +471,1197 @@ method call on a particular object instance, a number of nested components have to be named: - Address -> [Service] -> Path -> Interface -> Method + Address -> [Bus Name] -> Path -> Interface -> Method - The service is in brackets to indicate that it's optional -- you only - provide a service name to route the method call to the right application + The bus name is in brackets to indicate that it's optional -- you only + provide a name to route the method call to the right application when using the bus daemon. If you have a direct connection to another - application, services aren't used. + application, bus names aren't used; there's no bus daemon. The interface is also optional, primarily for historical reasons; DCOP does not require specifying the interface, instead simply forbidding duplicate method names - on the same object instance. D-BUS will thus let you + on the same object instance. D-Bus will thus let you omit the interface, but if your method name is ambiguous it is undefined which method will be invoked. - + + + + + Messages - Behind the Scenes + + D-Bus works by sending messages between processes. If you're using + a sufficiently high-level binding, you may never work with messages directly. + + + There are 4 message types: + + + + Method call messages ask to invoke a method + on an object. + + + + + Method return messages return the results + of invoking a method. + + + + + Error messages return an exception caused by + invoking a method. + + + + + Signal messages are notifications that a given signal + has been emitted (that an event has occurred). + You could also think of these as "event" messages. + + + + + + A method call maps very simply to messages: you send a method call + message, and receive either a method return message or an error message + in reply. + + + Each message has a header, including fields, + and a body, including arguments. You can think + of the header as the routing information for the message, and the body as the payload. + Header fields might include the sender bus name, destination bus name, method or signal name, + and so forth. One of the header fields is a type signature describing the + values found in the body. For example, the letter "i" means "32-bit integer" so the signature + "ii" means the payload has two 32-bit integers. + + + + + Calling a Method - Behind the Scenes + + + A method call in DBus consists of two messages; a method call message sent from process A to process B, + and a matching method reply message sent from process B to process A. Both the call and the reply messages + are routed through the bus daemon. The caller includes a different serial number in each call message, and the + reply message includes this number to allow the caller to match replies to calls. + + + + The call message will contain any arguments to the method. + The reply message may indicate an error, or may contain data returned by the method. + + + + A method invocation in DBus happens as follows: + + + + The language binding may provide a proxy, such that invoking a method on + an in-process object invokes a method on a remote object in another process. If so, the + application calls a method on the proxy, and the proxy + constructs a method call message to send to the remote process. + + + + + For more low-level APIs, the application may construct a method call message itself, without + using a proxy. + + + + + In either case, the method call message contains: a bus name belonging to the remote process; the name of the method; + the arguments to the method; an object path inside the remote process; and optionally the name of the + interface that specifies the method. + + + + + The method call message is sent to the bus daemon. + + + + + The bus daemon looks at the destination bus name. If a process owns that name, + the bus daemon forwards the method call to that process. Otherwise, the bus daemon + creates an error message and sends it back as the reply to the method call message. + + + + + The receiving process unpacks the method call message. In a simple low-level API situation, it + may immediately run the method and send a method reply message to the bus daemon. + When using a high-level binding API, the binding might examine the object path, interface, + and method name, and convert the method call message into an invocation of a method on + a native object (GObject, java.lang.Object, QObject, etc.), then convert the return + value from the native method into a method reply message. + + + + + The bus daemon receives the method reply message and sends it to the process that + made the method call. + + + + + The process that made the method call looks at the method reply and makes use of any + return values included in the reply. The reply may also indicate that an error occurred. + When using a binding, the method reply message may be converted into the return value of + of a proxy method, or into an exception. + + + + + + + The bus daemon never reorders messages. That is, if you send two method call messages to the same recipient, + they will be received in the order they were sent. The recipient is not required to reply to the calls + in order, however; for example, it may process each method call in a separate thread, and return reply messages + in an undefined order depending on when the threads complete. Method calls have a unique serial + number used by the method caller to match reply messages to call messages. + + + + + + Emitting a Signal - Behind the Scenes + + + A signal in DBus consists of a single message, sent by one process to any number of other processes. + That is, a signal is a unidirectional broadcast. The signal may contain arguments (a data payload), but + because it is a broadcast, it never has a "return value." Contrast this with a method call + (see ) where the method call message has a matching method reply message. + + + + The emitter (aka sender) of a signal has no knowledge of the signal recipients. Recipients register + with the bus daemon to receive signals based on "match rules" - these rules would typically include the sender and + the signal name. The bus daemon sends each signal only to recipients who have expressed interest in that + signal. + + + + A signal in DBus happens as follows: + + + + A signal message is created and sent to the bus daemon. When using the low-level API this may be + done manually, with certain bindings it may be done for you by the binding when a native object + emits a native signal or event. + + + + + The signal message contains the name of the interface that specifies the signal; + the name of the signal; the bus name of the process sending the signal; and + any arguments + + + + + Any process on the message bus can register "match rules" indicating which signals it + is interested in. The bus has a list of registered match rules. + + + + + The bus daemon examines the signal and determines which processes are interested in it. + It sends the signal message to these processes. + + + + + Each process receiving the signal decides what to do with it; if using a binding, + the binding may choose to emit a native signal on a proxy object. If using the + low-level API, the process may just look at the signal sender and name and decide + what to do based on that. + + + + + + + + + Introspection + + + D-Bus objects may support the interface org.freedesktop.DBus.Introspectable. + This interface has one method Introspect which takes no arguments and returns + an XML string. The XML string describes the interfaces, methods, and signals of the object. + See the D-Bus specification for more details on this introspection format. + + GLib API: Using Remote Objects + - + The GLib binding is defined in the header file + <dbus/dbus-glib.h>. + + + + D-Bus - GLib type mappings + + The heart of the GLib bindings for D-Bus is the mapping it + provides between D-Bus "type signatures" and GLib types + (GType). The D-Bus type system is composed of + a number of "basic" types, along with several "container" types. + + + Basic type mappings + + Below is a list of the basic types, along with their associated + mapping to a GType. + + + + + D-Bus basic type + GType + Free function + Notes + + + + + BYTE + G_TYPE_UCHAR + + + + BOOLEAN + G_TYPE_BOOLEAN + + + + INT16 + G_TYPE_INT + + Will be changed to a G_TYPE_INT16 once GLib has it + + UINT16 + G_TYPE_UINT + + Will be changed to a G_TYPE_UINT16 once GLib has it + + INT32 + G_TYPE_INT + + Will be changed to a G_TYPE_INT32 once GLib has it + + UINT32 + G_TYPE_UINT + + Will be changed to a G_TYPE_UINT32 once GLib has it + + INT64 + G_TYPE_GINT64 + + + + UINT64 + G_TYPE_GUINT64 + + + + DOUBLE + G_TYPE_DOUBLE + + + + STRING + G_TYPE_STRING + g_free + + + OBJECT_PATH + DBUS_TYPE_G_PROXY + g_object_unref + The returned proxy does not have an interface set; use dbus_g_proxy_set_interface to invoke methods + + + + + As you can see, the basic mapping is fairly straightforward. + + + + Container type mappings + + The D-Bus type system also has a number of "container" + types, such as DBUS_TYPE_ARRAY and + DBUS_TYPE_STRUCT. The D-Bus type system + is fully recursive, so one can for example have an array of + array of strings (i.e. type signature + aas). + + + However, not all of these types are in common use; for + example, at the time of this writing the author knows of no + one using DBUS_TYPE_STRUCT, or a + DBUS_TYPE_ARRAY containing any non-basic + type. The approach the GLib bindings take is pragmatic; try + to map the most common types in the most obvious way, and + let using less common and more complex types be less + "natural". + + + First, D-Bus type signatures which have an "obvious" + corresponding built-in GLib type are mapped using that type: + + + + + D-Bus type signature + Description + GType + C typedef + Free function + Notes + + + + + as + Array of strings + G_TYPE_STRV + char ** + g_strfreev + + + v + Generic value container + G_TYPE_VALUE + GValue * + g_value_unset + The calling conventions for values expect that method callers have allocated return values; see below. + + + + + + + The next most common recursive type signatures are arrays of + basic values. The most obvious mapping for arrays of basic + types is a GArray. Now, GLib does not + provide a builtin GType for + GArray. However, we actually need more than + that - we need a "parameterized" type which includes the + contained type. Why we need this we will see below. + + + The approach taken is to create these types in the D-Bus GLib + bindings; however, there is nothing D-Bus specific about them. + In the future, we hope to include such "fundamental" types in GLib + itself. + + + + + D-Bus type signature + Description + GType + C typedef + Free function + Notes + + + + + ay + Array of bytes + DBUS_TYPE_G_BYTE_ARRAY + GArray * + g_array_free + + + + au + Array of uint + DBUS_TYPE_G_UINT_ARRAY + GArray * + g_array_free + + + + ai + Array of int + DBUS_TYPE_G_INT_ARRAY + GArray * + g_array_free + + + + ax + Array of int64 + DBUS_TYPE_G_INT64_ARRAY + GArray * + g_array_free + + + + at + Array of uint64 + DBUS_TYPE_G_UINT64_ARRAY + GArray * + g_array_free + + + + ad + Array of double + DBUS_TYPE_G_DOUBLE_ARRAY + GArray * + g_array_free + + + + ab + Array of boolean + DBUS_TYPE_G_BOOLEAN_ARRAY + GArray * + g_array_free + + + + + + + + D-Bus also includes a special type DBUS_TYPE_DICT_ENTRY which + is only valid in arrays. It's intended to be mapped to a "dictionary" + type by bindings. The obvious GLib mapping here is GHashTable. Again, + however, there is no builtin GType for a GHashTable. + Moreover, just like for arrays, we need a parameterized type so that + the bindings can communiate which types are contained in the hash table. + + + At present, only strings are supported. Work is in progress to + include more types. + + + + + D-Bus type signature + Description + GType + C typedef + Free function + Notes + + + + + a{ss} + Dictionary mapping strings to strings + DBUS_TYPE_G_STRING_STRING_HASHTABLE + GHashTable * + g_hash_table_destroy + + + + + + + + + Arbitrarily recursive type mappings + + Finally, it is possible users will want to write or invoke D-Bus + methods which have arbitrarily complex type signatures not + directly supported by these bindings. For this case, we have a + DBusGValue which acts as a kind of special + variant value which may be iterated over manually. The + GType associated is + DBUS_TYPE_G_VALUE. + + + TODO insert usage of DBUS_TYPE_G_VALUE here. + + + + + A sample program + Here is a D-Bus program using the GLib bindings. + +int +main (int argc, char **argv) +{ + DBusGConnection *connection; + GError *error; + DBusGProxy *proxy; + char **name_list; + char **name_list_ptr; + + g_type_init (); + + error = NULL; + connection = dbus_g_bus_get (DBUS_BUS_SESSION, + &error); + if (connection == NULL) + { + g_printerr ("Failed to open connection to bus: %s\n", + error->message); + g_error_free (error); + exit (1); + } + + /* Create a proxy object for the "bus driver" (name "org.freedesktop.DBus") */ + + proxy = dbus_g_proxy_new_for_name (connection, + DBUS_SERVICE_DBUS, + DBUS_PATH_DBUS, + DBUS_INTERFACE_DBUS); + + /* Call ListNames method, wait for reply */ + error = NULL; + if (!dbus_g_proxy_call (proxy, "ListNames", &error, G_TYPE_INVALID, + G_TYPE_STRV, &name_list, G_TYPE_INVALID)) + { + /* Just do demonstrate remote exceptions versus regular GError */ + if (error->domain == DBUS_GERROR && error->code == DBUS_GERROR_REMOTE_EXCEPTION) + g_printerr ("Caught remote method exception %s: %s", + dbus_g_error_get_name (error), + error->message); + else + g_printerr ("Error: %s\n", error->message); + g_error_free (error); + exit (1); + } + + /* Print the results */ + + g_print ("Names on the message bus:\n"); + + for (name_list_ptr = name_list; *name_list_ptr; name_list_ptr++) + { + g_print (" %s\n", *name_list_ptr); + } + g_strfreev (name_list); + + g_object_unref (proxy); + + return 0; +} + + + + Program initalization + + A connection to the bus is acquired using + dbus_g_bus_get. Next, a proxy + is created for the object "/org/freedesktop/DBus" with + interface org.freedesktop.DBus + on the service org.freedesktop.DBus. + This is a proxy for the message bus itself. + + + + Understanding method invocation + + You have a number of choices for method invocation. First, as + used above, dbus_g_proxy_call sends a + method call to the remote object, and blocks until a reply is + recieved. The outgoing arguments are specified in the varargs + array, terminated with G_TYPE_INVALID. + Next, pointers to return values are specified, followed again + by G_TYPE_INVALID. + + + To invoke a method asynchronously, use + dbus_g_proxy_begin_call. This returns a + DBusGPendingCall object; you may then set a + notification function using + dbus_g_pending_call_set_notify. + + + + Connecting to object signals + + You may connect to signals using + dbus_g_proxy_add_signal and + dbus_g_proxy_connect_signal. You must + invoke dbus_g_proxy_add_signal to specify + the signature of your signal handlers; you may then invoke + dbus_g_proxy_connect_signal multiple times. + + + Note that it will often be the case that there is no builtin + marshaller for the type signature of a remote signal. In that + case, you must generate a marshaller yourself by using + glib-genmarshal, and then register + it using dbus_g_object_register_marshaller. + + + + Error handling and remote exceptions + + All of the GLib binding methods such as + dbus_g_proxy_end_call return a + GError. This GError can + represent two different things: + + + + An internal D-Bus error, such as an out-of-memory + condition, an I/O error, or a network timeout. Errors + generated by the D-Bus library itself have the domain + DBUS_GERROR, and a corresponding code + such as DBUS_GERROR_NO_MEMORY. It will + not be typical for applications to handle these errors + specifically. + + + + + A remote D-Bus exception, thrown by the peer, bus, or + service. D-Bus remote exceptions have both a textual + "name" and a "message". The GLib bindings store this + information in the GError, but some + special rules apply. + + + The set error will have the domain + DBUS_GERROR as above, and will also + have the code + DBUS_GERROR_REMOTE_EXCEPTION. In order + to access the remote exception name, you must use a + special accessor, such as + dbus_g_error_has_name or + dbus_g_error_get_name. The remote + exception detailed message is accessible via the regular + GError message member. + + + + + + + More examples of method invocation + + Sending an integer and string, receiving an array of bytes + + + GArray *arr; + + error = NULL; + if (!dbus_g_proxy_call (proxy, "Foobar", &error, + G_TYPE_INT, 42, G_TYPE_STRING, "hello", + G_TYPE_INVALID, + DBUS_TYPE_G_UCHAR_ARRAY, &arr, G_TYPE_INVALID)) + { + /* Handle error */ + } + g_assert (arr != NULL); + printf ("got back %u values", arr->len); + + + + + Sending a GHashTable + + + GHashTable *hash = g_hash_table_new (g_str_hash, g_str_equal); + guint32 ret; + + g_hash_table_insert (hash, "foo", "bar"); + g_hash_table_insert (hash, "baz", "whee"); + + error = NULL; + if (!dbus_g_proxy_call (proxy, "HashSize", &error, + DBUS_TYPE_G_STRING_STRING_HASH, hash, G_TYPE_INVALID, + G_TYPE_UINT, &ret, G_TYPE_INVALID)) + { + /* Handle error */ + } + g_assert (ret == 2); + g_hash_table_destroy (hash); + + + + + Receiving a boolean and a string + + + gboolean boolret; + char *strret; + + error = NULL; + if (!dbus_g_proxy_call (proxy, "GetStuff", &error, + G_TYPE_INVALID, + G_TYPE_BOOLEAN, &boolret, + G_TYPE_STRING, &strret, + G_TYPE_INVALID)) + { + /* Handle error */ + } + printf ("%s %s", boolret ? "TRUE" : "FALSE", strret); + g_free (strret); + + + + + Sending two arrays of strings + + + /* NULL terminate */ + char *strs_static[] = {"foo", "bar", "baz", NULL}; + /* Take pointer to array; cannot pass array directly */ + char **strs_static_p = strs_static; + char **strs_dynamic; + + strs_dynamic = g_new (char *, 4); + strs_dynamic[0] = g_strdup ("hello"); + strs_dynamic[1] = g_strdup ("world"); + strs_dynamic[2] = g_strdup ("!"); + /* NULL terminate */ + strs_dynamic[3] = NULL; + + error = NULL; + if (!dbus_g_proxy_call (proxy, "TwoStrArrays", &error, + G_TYPE_STRV, strs_static_p, + G_TYPE_STRV, strs_dynamic, + G_TYPE_INVALID, + G_TYPE_INVALID)) + { + /* Handle error */ + } + g_strfreev (strs_dynamic); + + + + + Sending a boolean, receiving an array of strings + + + char **strs; + char **strs_p; + gboolean blah; + + error = NULL; + blah = TRUE; + if (!dbus_g_proxy_call (proxy, "GetStrs", &error, + G_TYPE_BOOLEAN, blah, + G_TYPE_INVALID, + G_TYPE_STRV, &strs, + G_TYPE_INVALID)) + { + /* Handle error */ + } + for (strs_p = strs; *strs_p; strs_p++) + printf ("got string: \"%s\"", *strs_p); + g_strfreev (strs); + + + + + Sending a variant + + + GValue val = {0, }; + + g_value_init (&val, G_TYPE_STRING); + g_value_set_string (&val, "hello world"); + + error = NULL; + if (!dbus_g_proxy_call (proxy, "SendVariant", &error, + G_TYPE_VALUE, &val, G_TYPE_INVALID, + G_TYPE_INVALID)) + { + /* Handle error */ + } + g_assert (ret == 2); + g_value_unset (&val); + + + + + Receiving a variant + + + GValue val = {0, }; + + error = NULL; + if (!dbus_g_proxy_call (proxy, "GetVariant", &error, G_TYPE_INVALID, + G_TYPE_VALUE, &val, G_TYPE_INVALID)) + { + /* Handle error */ + } + if (G_VALUE_TYPE (&val) == G_TYPE_STRING) + printf ("%s\n", g_value_get_string (&val)); + else if (G_VALUE_TYPE (&val) == G_TYPE_INT) + printf ("%d\n", g_value_get_int (&val)); + else + ... + g_value_unset (&val); + + + + + + + Generated Bindings + + By using the Introspection XML files, convenient client-side bindings + can be automatically created to ease the use of a remote DBus object. + + + Here is a sample XML file which describes an object that exposes + one method, named ManyArgs. + +<?xml version="1.0" encoding="UTF-8" ?> +<node name="/com/example/MyObject"> + <interface name="com.example.MyObject"> + <method name="ManyArgs"> + <arg type="u" name="x" direction="in" /> + <arg type="s" name="str" direction="in" /> + <arg type="d" name="trouble" direction="in" /> + <arg type="d" name="d_ret" direction="out" /> + <arg type="s" name="str_ret" direction="out" /> + </method> + </interface> +</node> + + + + Run dbus-binding-tool --mode=glib-client + FILENAME > + HEADER_NAME to generate the header + file. For example: dbus-binding-tool --mode=glib-client + my-object.xml > my-object-bindings.h. This will generate + inline functions with the following prototypes: + +/* This is a blocking call */ +gboolean +com_example_MyObject_many_args (DBusGProxy *proxy, const guint IN_x, + const char * IN_str, const gdouble IN_trouble, + gdouble* OUT_d_ret, char ** OUT_str_ret, + GError **error); + +/* This is a non-blocking call */ +DBusGProxyCall* +com_example_MyObject_many_args_async (DBusGProxy *proxy, const guint IN_x, + const char * IN_str, const gdouble IN_trouble, + com_example_MyObject_many_args_reply callback, + gpointer userdata); + +/* This is the typedef for the non-blocking callback */ +typedef void +(*com_example_MyObject_many_args_reply) +(DBusGProxy *proxy, gdouble OUT_d_ret, char * OUT_str_ret, + GError *error, gpointer userdata); + + The first argument in all functions is a DBusGProxy + *, which you should create with the usual + dbus_g_proxy_new_* functions. Following that are the + "in" arguments, and then either the "out" arguments and a + GError * for the synchronous (blocking) function, or + callback and user data arguments for the asynchronous (non-blocking) + function. The callback in the asynchronous function passes the + DBusGProxy *, the returned "out" arguments, an + GError * which is set if there was an error otherwise + NULL, and the user data. + + + As with the server-side bindings support (see ), the exact behaviour of the client-side + bindings can be manipulated using "annotations". Currently the only + annotation used by the client bindings is + org.freedesktop.DBus.GLib.NoReply, which sets the + flag indicating that the client isn't expecting a reply to the method + call, so a reply shouldn't be sent. This is often used to speed up + rapid method calls where there are no "out" arguments, and not knowing + if the method succeeded is an acceptable compromise to half the traffic + on the bus. + + GLib API: Implementing Objects - + At the moment, to expose a GObject via D-Bus, you must + write XML by hand which describes the methods exported + by the object. In the future, this manual step will + be obviated by the upcoming GLib introspection support. - + + Here is a sample XML file which describes an object that exposes + one method, named ManyArgs. + +<?xml version="1.0" encoding="UTF-8" ?> - - Qt API: Using Remote Objects +<node name="/com/example/MyObject"> + + <interface name="com.example.MyObject"> + <annotation name="org.freedesktop.DBus.GLib.CSymbol" value="my_object"/> + <method name="ManyArgs"> + <!-- This is optional, and in this case is redunundant --> + <annotation name="org.freedesktop.DBus.GLib.CSymbol" value="my_object_many_args"/> + <arg type="u" name="x" direction="in" /> + <arg type="s" name="str" direction="in" /> + <arg type="d" name="trouble" direction="in" /> + <arg type="d" name="d_ret" direction="out" /> + <arg type="s" name="str_ret" direction="out" /> + </method> + </interface> +</node> + + - + This XML is in the same format as the D-Bus introspection XML + format. Except we must include an "annotation" which give the C + symbols corresponding to the object implementation prefix + (my_object). In addition, if particular + methods symbol names deviate from C convention + (i.e. ManyArgs -> + many_args), you may specify an annotation + giving the C symbol. - - - - Qt API: Implementing Objects - + Once you have written this XML, run dbus-binding-tool --mode=glib-server FILENAME > HEADER_NAME. to + generate a header file. For example: dbus-binding-tool --mode=glib-server my-object.xml > my-object-glue.h. + + + Next, include the generated header in your program, and invoke + dbus_g_object_class_install_info in the class + initializer, passing the object class and "object info" included in the + header. For example: + + dbus_g_object_type_install_info (COM_FOO_TYPE_MY_OBJECT, &com_foo_my_object_info); + + This should be done exactly once per object class. + + + To actually implement the method, just define a C function named e.g. + my_object_many_args in the same file as the info + header is included. At the moment, it is required that this function + conform to the following rules: + + + + The function must return a value of type gboolean; + TRUE on success, and FALSE + otherwise. + + + + + The first parameter is a pointer to an instance of the object. + + + + + Following the object instance pointer are the method + input values. + + + + + Following the input values are pointers to return values. + + + + + The final parameter must be a GError **. + If the function returns FALSE for an + error, the error parameter must be initalized with + g_set_error. + + + + + + Finally, you can export an object using dbus_g_connection_register_g_object. For example: + + dbus_g_connection_register_g_object (connection, + "/com/foo/MyObject", + obj); + - + + Server-side Annotations + + There are several annotations that are used when generating the + server-side bindings. The most common annotation is + org.freedesktop.DBus.GLib.CSymbol but there are other + annotations which are often useful. + + + org.freedesktop.DBus.GLib.CSymbol + + + This annotation is used to specify the C symbol names for + the various types (interface, method, etc), if it differs from the + name DBus generates. + + + + + org.freedesktop.DBus.GLib.Async + + + This annotation marks the method implementation as an + asynchronous function, which doesn't return a response straight + away but will send the response at some later point to complete + the call. This is used to implement non-blocking services where + method calls can take time. + + + When a method is asynchronous, the function prototype is + different. It is required that the function conform to the + following rules: + + + + The function must return a value of type gboolean; + TRUE on success, and FALSE + otherwise. TODO: the return value is currently ignored. + + + + + The first parameter is a pointer to an instance of the object. + + + + + Following the object instance pointer are the method + input values. + + + + + The final parameter must be a + DBusGMethodInvocation *. This is used + when sending the response message back to the client, by + calling dbus_g_method_return or + dbus_g_method_return_error. + + + + + + + + org.freedesktop.DBus.GLib.Const + + This attribute can only be applied to "out" + <arg> nodes, and specifies that the + parameter isn't being copied when returned. For example, this + turns a 's' argument from a char ** to a + const char **, and results in the argument not + being freed by DBus after the message is sent. + + + + + org.freedesktop.DBus.GLib.ReturnVal + + + This attribute can only be applied to "out" + <arg> nodes, and alters the expected + function signature. It currently can be set to two values: + "" or "error". The + argument marked with this attribute is not returned via a + pointer argument, but by the function's return value. If the + attribute's value is the empty string, the GError + * argument is also omitted so there is no standard way + to return an error value. This is very useful for interfacing + with existing code, as it is possible to match existing APIs. + If the attribute's value is "error", then the + final argument is a GError * as usual. + + + Some examples to demonstrate the usage. This introspection XML: + +<method name="Increment"> + <arg type="u" name="x" /> + <arg type="u" direction="out" /> +</method> + + Expects the following function declaration: + +gboolean +my_object_increment (MyObject *obj, gint32 x, gint32 *ret, GError **error); + + + + This introspection XML: + +<method name="IncrementRetval"> + <arg type="u" name="x" /> + <arg type="u" direction="out" > + <annotation name="org.freedesktop.DBus.GLib.ReturnVal" value=""/> + </arg> +</method> + + Expects the following function declaration: + +gint32 +my_object_increment_retval (MyObject *obj, gint32 x) + + + + This introspection XML: + +<method name="IncrementRetvalError"> + <arg type="u" name="x" /> + <arg type="u" direction="out" > + <annotation name="org.freedesktop.DBus.GLib.ReturnVal" value="error"/> + </arg> +</method> + + Expects the following function declaration: + +gint32 +my_object_increment_retval_error (MyObject *obj, gint32 x, GError **error) + + + + + + + + - Python API: Using Remote Objects + Python API - + The Python API, dbus-python, is now documented separately in + the dbus-python tutorial (also available in doc/tutorial.txt, + and doc/tutorial.html if built with python-docutils, in the dbus-python + source distribution). - - Python API: Implementing Objects + + Qt API: Using Remote Objects + The Qt bindings are not yet documented. + + + Qt API: Implementing Objects + + The Qt bindings are not yet documented. + +