X-Git-Url: http://review.tizen.org/git/?a=blobdiff_plain;f=doc%2Fdbus-tutorial.xml;h=5c385f0ee6385a66fb6a566328d2965d8e80b8fb;hb=085cabfbf4262087aaaa2cf3a567c9aaa0aec282;hp=10cfc79fa2296a488cedae9245c83adf91f77c2b;hpb=78b69c683ea27514c0787b2d1e2244d7182bc72d;p=platform%2Fupstream%2Fdbus.git diff --git a/doc/dbus-tutorial.xml b/doc/dbus-tutorial.xml index 10cfc79..5c385f0 100644 --- a/doc/dbus-tutorial.xml +++ b/doc/dbus-tutorial.xml @@ -6,45 +6,1662 @@
- D-BUS Tutorial - Version 0.1 - 29 September 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
 - - Introduction + + 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? - D-BUS blah blah blah + D-Bus is a system for interprocess communication + (IPC). Architecturally, it has several layers: + - foo + A library, libdbus, that allows two + applications to connect to each other and exchange messages. - bar + 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 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. + + + + + + + libdbus only supports one-to-one connections, just like a raw network + socket. However, rather than sending byte streams over the connection, you + send messages. Messages have a header identifying + the kind of message, and a body containing a data payload. libdbus also + abstracts the exact transport used (sockets vs. whatever else), and + handles details such as authentication. + + + + The message bus daemon forms the hub of a wheel. Each spoke of the wheel + is a one-to-one connection to an application using libdbus. An + application sends a message to the bus daemon over its spoke, and the bus + daemon forwards the message to other connected applications as + appropriate. Think of the daemon as a router. + + + + The bus daemon has multiple instances on a typical computer. The + first instance is a machine-global singleton, that is, a system daemon + similar to sendmail or Apache. This instance has heavy security + restrictions on what messages it will accept, and is used for systemwide + communication. The other instances are created one per user login session. + These instances allow applications in the user's session to communicate + with one another. + + + + The systemwide and per-user daemons are separate. Normal within-session + IPC does not involve the systemwide message bus process and vice versa. + + + + D-Bus applications + + There are many, many technologies in the world that have "Inter-process + communication" or "networking" in their stated purpose: CORBA, DCE, DCOM, DCOP, XML-RPC, 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: + + + + Communication between desktop applications in the same desktop + session; to allow integration of the desktop session as a whole, + and address issues of process lifecycle (when do desktop components + start and stop running). + + + + + Communication between the desktop session and the operating system, + where the operating system would typically include the kernel + and any system daemons or processes. + + + + + + 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 + carefully tailored to meet the needs of these desktop projects + 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 + is explained well by the following text from the Linux Hotplug project: +
+ + A gap in current Linux support is that policies with any sort of + dynamic "interact with user" component aren't currently + supported. For example, that's often needed the first time a network + adapter or printer is connected, and to determine appropriate places + to mount disk drives. It would seem that such actions could be + supported for any case where a responsible human can be identified: + single user workstations, or any system which is remotely + administered. + + + + This is a classic "remote sysadmin" problem, where in this case + hotplugging needs to deliver an event from one security domain + (operating system kernel, in this case) to another (desktop for + logged-in user, or remote sysadmin). Any effective response must go + the other way: the remote domain taking some action that lets the + kernel expose the desired device capabilities. (The action can often + be taken asynchronously, for example letting new hardware be idle + until a meeting finishes.) At this writing, Linux doesn't have + widely adopted solutions to such problems. However, the new D-Bus + work may begin to solve that problem. + +
+ + + 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: + + + + Binary protocol designed to be used asynchronously + (similar in spirit to the X Window System protocol). + + + + + Stateful, reliable connections held open over time. + + + + + The message bus is a daemon, not a "swarm" or + distributed architecture. + + + + + Many implementation and deployment issues are specified rather + than left ambiguous/configurable/pluggable. + + + + + Semantics are similar to the existing DCOP system, allowing + KDE to adopt it more easily. + + + + + Security features to support the systemwide mode of the + message bus. + + + + + + + + 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 + different for GLib vs. Qt vs. Python applications, however. + + + + Here is a diagram (png svg) that may help you visualize the concepts + that follow. + + + + Native Objects and Object Paths + + 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. + + + 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. + + + 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 + object named /com/mycompany/c5yo817y0c1y1c5b + if it makes sense for your application. + + + Namespacing object paths is smart, by starting them with the components + of a domain name you own (e.g. /org/kde). This + keeps different code modules in the same process from stepping + on one another's toes. + + + + + 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 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. + + + + + Proxies + + 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.. + + + 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(); + } + + + + Programming with proxies might look like this: + + Proxy proxy = new Proxy(getBusConnection(), "/remote/object/path"); + Object returnValue = proxy.MethodName(arg1, arg2); + + + + + + Bus Names + + + 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. + + + + 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 + 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. + + + + Applications could then send messages to this bus name, + object, and interface to execute method calls. + + + + 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. + + + + 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 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 + 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 + connection. + + + + 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. + + + + 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 + (though you can override this address with an environment variable). + + + + 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. This is an unusual case. + + + + + + Big Conceptual Picture + + + Pulling all these concepts together, to specify a particular + method call on a particular object instance, a number of + nested components have to be named: + + Address -> [Bus Name] -> Path -> Interface -> Method + + 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, 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 + 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. - blah blah blah + 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"> + <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> + - blah blah blah + 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. + + + 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 + + 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). + + + + + Qt API: Using Remote Objects + + + The Qt bindings are not yet documented. + + + + + + Qt API: Implementing Objects + + The Qt bindings are not yet documented.