Qt Bluetooth stuck when connecting to Classic Bluetooth Device

Question

I am currently trying to read out Bluetooth Signals from peripherals using classic BT to control my Qt application. Later on I might use a small BT joystick but for now I am trying to connect to the buttons on my paired headphones.

I have previously

1. ..scanned for remote devices via the QBluetoothDeviceDiscoveryAgent, which returned the QBluetoothDeviceInfo of my headphones.
2. ...scanned for the services on that device using the QBluetoothServiceDiscoveryAgent, which returned QBluetoothServiceInfo with QBluetoothUuid::AV_RemoteControl

3. ...added a socket which should connect to the device, like so:

   socket0 = new QBluetoothSocket(QBluetoothServiceInfo::L2capProtocol);
   
   connect(socket0, &QBluetoothSocket::stateChanged, this , &BluetoothController::socketStateChanged);
   connect(socket0, &QBluetoothSocket::readyRead, this, &BluetoothController::readSocket);
   connect(socket0, &QBluetoothSocket::connected, this, &BluetoothController::serverConnected);
   connect(socket0, &QBluetoothSocket::disconnected, this, &BluetoothController::serverDisconnected);
   connect(socket0, QOverload<QBluetoothSocket::SocketError>::of(&QBluetoothSocket::error),
       this, &BluetoothController::serverError);
   
   
   QBluetoothAddress address = info.device().address();
   QIODevice::ReadOnly);
   socket0->connectToService(address, QBluetoothUuid::AV_RemoteControl, QIODevice::ReadOnly);
   

   At this point the socket state changes to ConnectingState, does not through an error, but also does not not trigger my readyRead() function.

I am new to Bluetooth and may be misunderstanding the concept of how connecting exactly works, so any help will be greatly appreciated.

PS.: I am working on Linux Ubuntu 18.04 and the application log also puts out:

qt.bluetooth.bluez: Missing CAP_NET_ADMIN permission. Cannot determine whether a found address is of random or public type.

Answer 1

You are trying to run your software as a non-root user. The Bluetooth protocol stack for Linux checks two capabilities, which are required for operations like ones mentioned in your error log — CAP_NET_RAW and CAP_NET_ADMIN. Capabilities are an alternative to "all or nothing" approach with privileged user (root). This gives more fine-grained control over permissions to unprivileged user so it can gain only a part of root's privileges. For more details, read man 7 capabilities.

There're several techniques for leveraging capabilities, one of them are file capabilities. You can achieve the desired effect with setcap(8) like this:

sudo setcap 'cap_net_raw,cap_net_admin+eip' yourapplication