Send and receive a signal between 2 Qt applications.

I am running ubuntu linux 10.10.

Well, my program runs only on one instance, so what I need is to open my application's project file while the application is already running. By clicking this project file, the new program instance realize that there is already one running and it shows a message that the program is already open. What I want is to open the project file with the already running process. So, I want
1) If the program is already running and
2) there are arguments (a project file has been opened)
to open the project file with the already running process.

So, the new instance may send the URL of the project file to the already running process and the already running process open it. I have not idea how to do this. Maybe System-Wide Signal-Slot ?

Sorry if this is not a Newbie question. Fell free to move it to the Programming talk.

If you use QtSingleApplication, you can use isRunning and sendMessage methods of that class. isRunning will tell you if another copy is running in memory, whilst sendMessage will allow you to talk to that instance. The already running instance will receive the signal messageReceived.

docs: http://doc.qt.nokia.com/solutions/4/...plication.html

You can download QtSingleApplication from the GIT repository: http://qt.gitorious.org/qt-solutions

Hey thanks, I'm not using QtSingleApplication!

You might want to look into Inter Process Communication (IPC)

A popular choice is a TCP socket.
Or shared memory.
Depending on the OS, there's also DBus

I am using pyQt but maybe this can help you to find a solution also in C++.
I am implementing my SingleInstance Applications with the help of QSharedMemory, QLocalSocket and QLocalServer

Code:

---

#import std
import os, sys
 
# import stuff for ipc
import getpass, pickle
 
# Import Qt modules
from PyQt4.QtGui import QApplication
from PyQt4.QtCore import QSharedMemory, QIODevice, SIGNAL
from PyQt4.QtNetwork import QLocalServer, QLocalSocket
 
class SingletonApp(QApplication):
 
    timeout = 1000
 
    def __init__(self, argv, application_id=None):
        QApplication.__init__(self, argv)
 
        self.socket_filename = unicode(os.path.expanduser("~/.ipc_%s" 
                                                          % self.generate_ipc_id()) )
        self.shared_mem = QSharedMemory()
        self.shared_mem.setKey(self.socket_filename)
 
        if self.shared_mem.attach():
            self.is_running = True
            return
 
        self.is_running = False
        if not self.shared_mem.create(1):
            print >>sys.stderr, "Unable to create single instance"
            return
        # start local server
        self.server = QLocalServer(self)
        # connect signal for incoming connections
        self.connect(self.server, SIGNAL("newConnection()"), self.receive_message)
        # if socket file exists, delete it
        if os.path.exists(self.socket_filename):
            os.remove(self.socket_filename)
        # listen
        self.server.listen(self.socket_filename)
 
    def __del__(self):
        self.shared_mem.detach()
        if not self.is_running:
            if os.path.exists(self.socket_filename):
                os.remove(self.socket_filename)
 
 
    def generate_ipc_id(self, channel=None):
        if channel is None:
            channel = os.path.basename(sys.argv[0])
        return "%s_%s" % (channel, getpass.getuser())
 
    def send_message(self, message):
        if not self.is_running:
            raise Exception("Client cannot connect to IPC server. Not running.")
        socket = QLocalSocket(self)
        socket.connectToServer(self.socket_filename, QIODevice.WriteOnly)
        if not socket.waitForConnected(self.timeout):
            raise Exception(str(socket.errorString()))
        socket.write(pickle.dumps(message))
        if not socket.waitForBytesWritten(self.timeout):
            raise Exception(str(socket.errorString()))
        socket.disconnectFromServer()
 
    def receive_message(self):
        socket = self.server.nextPendingConnection()
        if not socket.waitForReadyRead(self.timeout):
            print >>sys.stderr, socket.errorString()
            return
        byte_array = socket.readAll()
        self.handle_new_message(pickle.loads(str(byte_array)))
 
    def handle_new_message(self, message):
        print "Received:", message
 
# Create a class for our main window
class Main(QtGui.QMainWindow):
    def __init__(self):
         QtGui.QMainWindow.__init__(self)
         # This is always the same
         self.ui=Ui_MainWindow()
         self.ui.setupUi(self)
 
if __name__ == "__main__":
    app = SingletonApp(sys.argv)
    if app.is_running:
        # send arguments to running instance
        app.send_message(sys.argv)
    else:    
        MyApp = Main()
        MyApp.show()
        sys.exit(app.exec_())

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Thank you all for the interest. I am interested in DBUS (because there is QDbus lib), but I'll see other solutions as well! Thank you :)

---

Added after 1 12 minutes:

I am tryin' got figure it out with QDBus. I was able to send a DBus message(I was able to see the "HELLO" string with dbus-monitor) with this code:

Code:

---

QDBusMessage msg = QDBusMessage::createSignal("/", "open.album", "message");
    msg << "HELLO";
    QDBusConnection::sessionBus().send(msg);

---

Where open.album.xml is

Code:

---

<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
 <node>
   <interface name="open.album">
     <signal name="message">
       <arg name="album" type="s" direction="out"/>
     </signal>
     <signal name="action">
       <arg name="album" type="s" direction="out"/>
     </signal>
   </interface>
 </node>

---

Ok, all right till now, But I've no Idea how will I tell the other application to listen to the specific interface for a specific string and do things with this string!
Any help?

Ok, it is simple.

Code:

---

//this set it ready to receive a message to a specific chanell, once received, album_slot() will run
   QDBusConnection::sessionBus().connect(QString(),QString(), "open.album", "MY_message", this, SLOT(album_slot(QString)));
//these 3 lines create a signal and send it to DBus
QDBusMessage msg = QDBusMessage::createSignal("/", "open.album", "MY_message");
msg="text"
QDBusConnection::sessionBus().send(msg);
...
...
void album_slot(const QString &text){
if(text=="text") etc
}

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