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QProcess: Start, interact and keep running when Application exits



  • Hi guys,

    I need to start a openVpn-connection in linux.
    Therefore I want to use QProcess to start openvpn, pass some arguments, enter a passphrase, and get some info about the connection status.

    As soon as I quit my application I need to keep the openvpn-connection active, so I have to detach from that process. Any idea how to do it?

    When I use QProcess::startDetached, I can no longer interact with it.


  • Lifetime Qt Champion

    @themts said in QProcess: Start, interact and keep running when Application exits:

    As soon as I quit my application I need to keep the openvpn-connection active

    This doesn't make sense to me. How do you know that the connection is (not) open when you start your application the next time?



  • QProcess p;
    p.start("pidof", QStringList({ processName}), QIODevice::ReadOnly);


  • Lifetime Qt Champion

    But how do you can interact with the already started process in the case it's already running?



  • @themts
    You must use startDetached() if you want a child process you spawn to outlive its parent.

    I don't know what you mean by " I can no longer interact with it". But if you intend to "re-connect" to an already-running process in future --- however you recognise that situation --- you cannot use channels like stdin/out/err, you must use some kind of IPC, like sockets/named pipes/shared memory.



  • @Christian-Ehrlicher
    I no longer interact. I‘m just displaying „connected“ and the user is able to disconnect now.

    The situation is like that:
    For remote support, the user can click „start remote-support“. This starts a vpn connection. While establishing, it is showing „connecting“, on error it shows „failed: reason“, on connected it shows „connected“ and is displaying the remote IP.

    When I connect to that machine now, sometimes I have to quit the application. Of course I don’t want to interrupt the vpn-connection in this case.


  • Lifetime Qt Champion

    You can start it detached and output the stdout/stderr of the app to a file which can be read in later on by your own app.



  • @JonB
    To start the vpn-connection, I need to enter a passphrase (that way steeling the cert is not enough to establish a connection).
    So normally I would start the process and use stdin, stdout,… to enter the passphrase and get all the connection-information openvpn is giving me.
    When I use startDetached, I cannot use stdin,… anymore.


  • Lifetime Qt Champion

    Then write a wrapper application where you can communicate via some IPC mechanism which handles your vpn connection. You can start this helper app as detached from your real app then.



  • @themts
    Then exactly as @Christian-Ehrlicher has just written. You will need to write your own wrapper program around starting your VPN connection, which is what you will invoke from QProcess::startDetached(). That can talk stdin/out/err to your VPN process and can talk IPC to your Qt program both initially and at future times.



  • I also had this in mind as plan-b if I cannot detach from a running process.
    I think I‘ll just go that way.

    Thanks guys



  • @themts
    Assuming you do write a separate Qt "VPN spawner" application, you then have a choice if it helps: you can start that startDetached(), or if you prefer for communication with it you can start it with normal start() and have it do the startDetached() when it starts the VPN process.


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