by Harald Fernengel

Qt has had built-in D-Bus support via the QtDBus library since version 4.2. Here is a simple "Hello world" example that connects to your desktop's session bus and lists all registered service names:

#include <QtCore>
#include <QtDBus>
 
int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);
 
    QDBusConnection bus = QDBusConnection::sessionBus();
    QStringList serviceNames = bus.interface()->
                               registeredServiceNames();
    qDebug() << serviceNames;
 
    return 0;
}

The static function QDBusConnection::sessionBus() returns a handle to the session bus. The bus itself implements an interface which lets you query the existing services. Under the hood, a message will be sent to the bus, asking for all registered service names. The results are printed via qDebug().

Sending and Receiving

Let's send something to the bus. First, we create a new message &emdash; in our case, a signal.

QDBusMessage message = QDBusMessage::createSignal("/",
                           "com.mycompany.ping", "ping");

The first argument to createSignal() is the object path, describing where the sender object is located; in our case, we pass a single slash to refer to the root path. The second argument is the object interface; we choose com.mycompany.ping. In the third argument, we pass the name of the signal: ping.

We can add an arbitrary amount of arguments to our signals &emdash; in our case, we add one string containing a nice hello message.

message << "Hello, just wanted to say that I'm here.";

Finally, we send the message via the session bus.

bus.send(message);

Now that we know how to send signals to the bus, we also want to receive something. D-Bus and Qt connections are similar. Here is how we do connections within Qt code:

QObject::connect(<sender>, <signal>, <receiver>, <slot>);

And here is the syntax for D-Bus connections:

QDBusConnection::connect(<service name>, <object path>,
                         <interface name>, <signal name>, <receiver>, <slot>);

• is the name of the application. There is no equivalent in Qt since all signals and slots are connected within the same application.