dbus
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;TRUEon success, andFALSEotherwise. 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 callingdbus_g_method_returnordbus_g_method_return_error.
-
D-Bus Tutorial
Version 0.5.0
Table of Contents
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.
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 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.
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.
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.
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.
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".
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.
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);
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.
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.
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.
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.
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.
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 the section called “Calling a Method - Behind the Scenes”) 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.
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.
The GLib binding is defined in the header file <dbus/dbus-glib.h>.
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.
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.
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 |
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.
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;
}
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.
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.
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.
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 asDBUS_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_GERRORas above, and will also have the codeDBUS_GERROR_REMOTE_EXCEPTION. In order to access the remote exception name, you must use a special accessor, such asdbus_g_error_has_nameordbus_g_error_get_name. The remote exception detailed message is accessible via the regular GErrormessagemember.
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);
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);
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);
/* 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);
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);
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);
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);
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 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:FILENAME > HEADER_NAME
/* 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 section called “GLib API: Implementing Objects”), 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.
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" ?>
<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.
Once you have written this XML, run dbus-binding-tool --mode=glib-server to generate a header file. For example: dbus-binding-tool --mode=glib-server my-object.xml > my-object-glue.h.FILENAME > HEADER_NAME.
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;TRUEon success, andFALSEotherwise. -
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 returnsFALSEfor an error, the error parameter must be initalized withg_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);
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;TRUEon success, andFALSEotherwise. 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 callingdbus_g_method_returnordbus_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 achar **to aconst 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, theGError *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 aGError *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)
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).
对D-Bus Tutorial 进行了一些翻译加上自己的一些理解。
有很多种IPC或者网络通信系统，如：CORBA, DCE, DCOM, DCOP, XML-RPC, SOAP, MBUS, Internet Communications Engine (ICE)等等，可能会有数百种，dbus的目的主要是下面两点：
1.在同一个桌面会话中，进行桌面应用程序之间的通讯
2.桌面程序与内核或者守护进程的通信。
Dbus是一套进程通信体系，它有以下几层：
1.libdbus库，提供给各个应用程序调用，使应用程序具有通信和数据交换的能力，两个应用程序可以直接进行通信，就像是一条socket通道，两个程序之间建立通道之后，就可以通讯了。
2.消息守护进程，在libdbus的基础上创建，可以管理多个应用程序之间的通信。每个应用程序都和消息守护进程建立dbus的链接，然后由消息守护进程进行消息的分派。
3.各种包装库，有libdbus-glib，libdbus-qt等等，目的是将dbus的底层api进行一下封装。
下面有一张图可以很方便说明dbus的体系结构。
dbus中的消息由一个消息头（标识是哪一种消息）和消息数据组成，比socket的流式数据更方便一些。bus daemon 就像是一个路由器，与各个应用程序进行连接，分派这些消息。bus daemon 在一台机器上有多个实例，第一个实例是全局的实例，类似于sendmail和或者apache，这个实例有很严格的安全限制，只接受一些特定的系统消息，用于系统通信。其他bus daemon是一些会话，用于用户登录之后，在当前会话(session)中进行的通讯。系统的bus daemon 和会话的bus daemon 是分开的，彼此不会互相影响，会话bus daemon 不会去调用系统的bus daemon 。
Native Objects and Object Paths
在不同的编程语言中，都定义了一些“对象”，如java中的java.lang.Object，GLIB中的GObject，QT中的QObject等等。D-BUS的底层接口，和libdbus API相关，是没有这些对象的概念的，它提供的是一种叫对象路径（object path），用于让高层接口绑定到各个对象中去，允许远端应用程序指向它们。object path就像是一个文件路径，可以叫做/org/kde/kspread/sheets/3/cells/4/5等。
Methods and Signals
每个对象都有一些成员，两种成员:方法(methods)和信号(signals)，在对象中，方法可以被调用。信号会被广播，感兴趣的对象可以处理这个信号，同时信号中也可以带有相关的数据。每一个方法或者信号都可以用一个名字来命名，如”Frobate” 或者 “OnClicked”。
Interfaces
每个对象都有一个或者多个接口，一个接口就是多个方法和信号的集合。dbus使用简单的命名空间字符串来表示接口，如org.freedesktop.Introspectable。可以说dbus接口相当于C++中的纯虚类。
Proxies
代理对象用于模拟在另外的进程中的远端对象，代理对象像是一个正常的普通对象。d-bus的底层接口必须手动创建方法调用的消息，然后发送，同时必须手动接受和处理返回的消息。高层接口可以使用代理来替换这些，当调用代理对象的方法时，代理内部会转换成dbus的方法调用，等待消息返回，对返回结果解包，返回给相应的方法。可以看看下面的例子，使用dbus底层接口编写的代码：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();
}
使用代理对象编写的代码：Proxy proxy = new Proxy(getBusConnection(), "/remote/object/path");
Object returnValue = proxy.MethodName(arg1, arg2);
客户端代码减少很多。
Bus Names
当一个应用程序连接上bus daemon时，daemon会分配一个唯一的名字给它。以冒号（:）开始，这些名字在daemon的生命周期中是不会改变的，可以认为这些名字就是一个 IP地址。当这个名字映射到应用程序的连接上时，应用程序可以说拥有这个名字。同时应用可以声明额外的容易理解的名字，比如可以取一个名字 com.mycompany.TextEditor，可以认为这些名字就是一个域名。其他应用程序可以往这个名字发送消息，执行各种方法。
名字还有第二个重要的用途，可以用于跟踪应用程序的生命周期。当应用退出（或者崩溃）时，与bus的连接将被OS内核关掉，bus将会发送通知，告诉剩余的应用程序，该程序已经丢失了它的名字。名字还可以检测应用是否已经启动，这往往用于只能启动一个实例的应用。
Addresses
使用d-bus的应用程序既可以是server也可以是client，server监听到来的连接，client连接到server，一旦连接建立，消息就可以流转。如果使用dbus daemon，所有的应用程序都是client，daemon监听所有的连接，应用程序初始化连接到daemon。
dbus地址指明server将要监听的地方，client将要连接的地方，例如，地址：unix:path=/tmp/abcdef表明 server将在/tmp/abcdef路径下监听unix域的socket，client也将连接到这个socket。一个地址也可以指明是TCP /IP的socket，或者是其他的。
当使用bus daemon时，libdbus会从环境变量中（DBUS_SESSION_BUS_ADDRESS）自动认识“会话daemon”的地址。如果是系统 daemon，它会检查指定的socket路径获得地址，也可以使用环境变量（DBUS_SESSION_BUS_ADDRESS）进行设定。
当dbus中不使用daemon时，需要定义哪一个应用是server，哪一个应用是client，同时要指明server的地址，这不是很通常的做法。
Big Conceptual Picture
要在指定的对象中调用指定的方法，需要知道的参数如下：
Address -> [Bus Name] -> Path -> Interface -> Method
bus name是可选的，除非是希望把消息送到特定的应用中才需要。interface也是可选的，有一些历史原因，DCOP不需要指定接口，因为DCOP在同一个对象中禁止同名的方法。
Messages – Behind the Scenes
如果使用dbus的高层接口，就可以不用直接操作这些消息。DBUS有四种类型的消息：
1.方法调用（method call） 在对象上执行一个方法
2.方法返回（method return）返回方法执行的结果
3.错误（error）调用方法产生的异常
4.信号（signal）通知指定的信号发生了，可以想象成“事件”。
要执行 D-BUS 对象的方法，需要向对象发送一个方法调用消息。它将完成一些处理并返回一个方法返回消息或者错误消息。信号的不同之处在于它们不返回任何内容：既没有“信号返回”消息，也没有任何类型的错误消息。
每个消息都有一个消息头，包含多个字段，有一个消息体，包含多个参数。可以认为消息头是消息的路由信息，消息体作为一个载体。消息头里面的字段包含发送的bus name，目标bus name，方法或者信号名字等，同时消息头里面定义的字段类型规定了消息体里面的数据格式。例如：字符“i”代表了”32-bit integer”，“ii”就代表了消息体里面有两个”32-bit integer”。
Calling a Method – Behind the Scenes
在dbus中调用一个方法包含了两条消息，进程A向进程B发送方法调用消息，进程B向进程A发送应答消息。所有的消息都由daemon进行分派，每个调用的消息都有一个不同的序列号，返回消息包含这个序列号，以方便调用者匹配调用消息与应答消息。调用消息包含一些参数，应答消息可能包含错误标识，或者包含方法的返回数据。
方法调用的一般流程：
1.使用不同语言绑定的dbus高层接口，都提供了一些代理对象，调用其他进程里面的远端对象就像是在本地进程中的调用一样。应用调用代理上的方法，代理将构造一个方法调用消息给远端的进程。
2.在DBUS的底层接口中，应用需要自己构造方法调用消息（method call message），而不能使用代理。
3.方法调用消息里面的内容有：目的进程的bus name，方法的名字，方法的参数，目的进程的对象路径，以及可选的接口名称。
4.方法调用消息是发送到bus daemon中的。
5.bus daemon查找目标的bus name，如果找到，就把这个方法发送到该进程中，否则，daemon会产生错误消息，作为应答消息给发送进程。
6.目标进程解开消息，在dbus底层接口中，会立即调用方法，然后发送方法的应答消息给daemon。在dbus高层接口中，会先检测对象路径，接口，方法名称，然后把它转换成对应的对象（如GObject，QT中的QObject等）的方法，然后再将应答结果转换成应答消息发给daemon。
7.bus daemon接受到应答消息，将把应答消息直接发给发出调用消息的进程。
8.应答消息中可以包容很多返回值，也可以标识一个错误发生，当使用绑定时，应答消息将转换为代理对象的返回值，或者进入异常。
bus daemon不对消息重新排序，如果发送了两条消息到同一个进程，他们将按照发送顺序接受到。接受进程并需要按照顺序发出应答消息，例如在多线程中处理这些消息，应答消息的发出是没有顺序的。消息都有一个序列号可以与应答消息进行配对。
Emitting a Signal – Behind the Scenes
在dbus中一个信号包含一条信号消息，一个进程发给多个进程。也就是说，信号是单向的广播。信号可以包含一些参数，但是作为广播，它是没有返回值的。
信号触发者是不了解信号接受者的，接受者向daemon注册感兴趣的信号，注册规则是”match rules”，记录触发者名字和信号名字。daemon只向注册了这个信号的进程发送信号。
信号的一般流程如下：
1.当使用dbus底层接口时，信号需要应用自己创建和发送到daemon，使用dbus高层接口时，可以使用相关对象进行发送，如Glib里面提供的信号触发机制。
2.信号包含的内容有：信号的接口名称，信号名称，发送进程的bus name，以及其他参数。
3.任何进程都可以依据”match rules”注册相关的信号，daemon有一张注册的列表。
4.daemon检测信号，决定哪些进程对这个信号感兴趣，然后把信号发送给这些进程。
5.每个进程收到信号后，如果是使用了dbus高层接口，可以选择触发代理对象上的信号。如果是dbus底层接口，需要检查发送者名称和信号名称，然后决定怎么做。
Glib绑定接口在"dbus/dbus-glib.h" 头文件中定义。
dbus和glib的数据类型映射如下：
D-Bus basic type GType Free function Notes BYTEG_TYPE_UCHARBOOLEANG_TYPE_BOOLEANINT16G_TYPE_INTWill be changed to a G_TYPE_INT16once
GLib has itUINT16G_TYPE_UINTWill be changed to a G_TYPE_UINT16once
GLib has itINT32G_TYPE_INTWill be changed to a G_TYPE_INT32once
GLib has itUINT32G_TYPE_UINTWill be changed to a G_TYPE_UINT32once
GLib has itINT64G_TYPE_GINT64UINT64G_TYPE_GUINT64DOUBLEG_TYPE_DOUBLESTRINGG_TYPE_STRINGg_freeOBJECT_PATHDBUS_TYPE_G_PROXYg_object_unrefThe returned proxy does not have an interface set; use dbus_g_proxy_set_interfaceto invoke methods
Container type mappings
dbus数据也有包容器类型，像DBUS_TYPE_ARRAY 和 DBUS_TYPE_STRUCT，dbus的数据类型可以是嵌套的，如有一个数组，内容是字符串的数组集合。
但是，并不是所有的类型都有普通的使用，DBUS_TYPE_STRUCT应该可以包容非基本类型的数据类型。glib绑定尝试使用比较明显的方式进行声明。
D-Bus type signature Description GType C typedef Free function Notes asArray of strings G_TYPE_STRVchar **g_strfreevvGeneric value container G_TYPE_VALUEGValue *g_value_unsetThe calling conventions for values expect that method callers have
allocated return values; see below.
同时定义了新的数组类型集合。
D-Bus type signature Description GType C typedef Free function Notes ayArray of bytes DBUS_TYPE_G_BYTE_ARRAYGArray *g_array_free auArray of uint DBUS_TYPE_G_UINT_ARRAYGArray *g_array_free aiArray of int DBUS_TYPE_G_INT_ARRAYGArray *g_array_free axArray of int64 DBUS_TYPE_G_INT64_ARRAYGArray *g_array_free atArray of uint64 DBUS_TYPE_G_UINT64_ARRAYGArray *g_array_free adArray of double DBUS_TYPE_G_DOUBLE_ARRAYGArray *g_array_free abArray of boolean DBUS_TYPE_G_BOOLEAN_ARRAYGArray *g_array_free
定义了字典类型
| 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 |
client端编写
我们的程序在使用dbus的时候，首先需要连接上dbus，使用dbus_g_bus_get获得dbus连接。然后可以创建代理对象。
需要调用方法的时候，可以有两种方式：1.同步调用，使用dbus_g_proxy_call发送方法请求到远端对象，dbus会阻塞等待远端对象的回应，输出参数里将会带有相应的回应数据,以G_TYPE_INVALID作为终止符。2.异步调用，使用 dbus_g_proxy_begin_call，它将返回一个DBusGPendingCall对象，可以使用 dbus_g_pending_call_set_notify连接到自己的处理函授中。
可以使用dbus_g_proxy_add_signal 和 dbus_g_proxy_connect_signal来连接信号，dbus_g_proxy_add_signal用来声明信号处理函数，属于必须被调用的接口，dbus_g_proxy_connect_signal可以调用多次。
Generated Bindings
使用内置的xml文件，可以很方便地自动创建出易于使用的dbus代理对象。如下的一个xml文件描述了了一个方法：
<?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 >
“in”标识输入参数，“out”标识输出参数。
使用dbus-binding-tool工具来生成头文件，如dbus-binding-tool –mode=glib-client my-object.xml > my-object-bindings.h，会产生如下的内联函数原型：
/* 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);
所有函数的第一个参数都是DBusGProxy对象，一般是使用dbus_g_proxy_new_*函数创建出来的。客户端发送方法请求可以增加标记，目前只有org.freedesktop.DBus.GLib.NoReply标记，dbus可以不要回应消息，没有“out”参数，这样运算速度会快一点。
server端的编写
在GLib中，通过dbus表现出GObject，必须写XML文件描述这个对象的方法等属性。像上一篇文章中提到的例子：
<?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 >
一旦写完XML，运行dbus-binding-tool工具，如 dbus-binding-tool –mode=glib-server my-object.xml > my-object-glue.h.
然后在本地代码中include产生的头文件，调用dbus_g_object_class_install_info进行类的初始化，传递对象和对象信息进去，如 dbus_g_object_type_install_info (COM_FOO_TYPE_MY_OBJECT, &com_foo_my_object_info);每个对象类都需要这样做。
为了执行方法，需要定义一个C函数，如my_object_many_args，需要遵守的规则如下：
1.函数返回gboolean，true表示成功，false标识失败。
2.第一个参数必须是对象实例的指针。
3.跟在实例指针后面的参数是方法的输入参数。
4.输入参数后面是输出参数。
5.最后一个参数必须是GError **，如果函数返回失败，必须使用g_set_error填充该错误参数。
如下的xml文件
<method name="Increment">
<arg type="u" name="x" />
<arg type="u" direction="out" />
</method>
对应的函数定义为：
gboolean
my_object_increment (MyObject *obj, gint32 x, gint32 *ret, GError **error);
最后可以使用dbus_g_connection_register_g_object输出一个对象，如
dbus_g_connection_register_g_object (connection,”/com/foo/MyObject”, obj);
server端的声明（Annotations）：
org.freedesktop.DBus.GLib.CSymbol
org.freedesktop.DBus.GLib.Async
org.freedesktop.DBus.GLib.Const
org.freedesktop.DBus.GLib.ReturnVal
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