QSerialPortInfo doesn't return ports and connection doesn't work (Windows 10)

[kkettinger]

Hello,

i'm currently developing an application with Qt Quick/QML using the QSerialPort und QSerialPortInfo library.

My backend class is written in C++ and connected to QML with signal and slots. In my .pro file i've added "QT += serialport" and my backend class (.cpp) looks like this:

#include <QSerialPort>
#include <QSerialPortInfo>

class {
QList<QSerialPortInfo> m_availablePorts;
...
void Backend::printAvailablePorts()
{
   emit debugOutput("Available ports:");
    for (const auto &info : m_availablePorts)
    {
        emit debugOutput(info.portName());
    }
}

The class hierarchy is like this:

1. Qt Quick/QML
2. Backend_Adapter (Used as component block in QML)
3. Backend_Worker (Thread which runs in Backend_Adapter for blocking operations, serial, ...)

After sending the signal, i get no listing of ports. Even when running the application as administrator no ports are listed. When trying to connect to the port using the port name (COM7 e.g.), i get "Access denied" as error-string.

The strange thing is, an application written as "Qt Core Application" (only console output) does list the ports and connection works as expected.

What i am missing here in Qt Quick/QML?

Thanks!

Best regards,
Kevin

[jsulm]

@kkettinger Are you sure you're not trying to open the port in your GUI app more than once?
Did you compare your console app to your GUI app - maybe there are some differences in how you handle COM ports?

[jsulm]

@kkettinger said in QSerialPortInfo doesn't return ports and connection doesn't work (Windows 10):

m_availablePorts

How do you initialise/fill it?

[kkettinger]

Oh wow, i forget the initialization.. thanks. With m_availablePorts = QSerialPortInfo::availablePorts(); the port listing it is working now.

For the connection part, i'm still getting "Access denied". The relevant code part:

QSerialPort m_serialPort;
...
void cBackendWorker::openSerialPort(
    QString serialPortName)
{
    // ---- Port configuration
    m_serialPort.setPortName(serialPortName);
    m_serialPort.setBaudRate(QSerialPort::Baud9600);

    emit writeDebugOutput(QString("Open serial port %1...").arg(m_serialPort.portName()));

    // ---- Opening port
    if (m_serialPort.open(QIODevice::ReadWrite) == false)
    {
        emit writeDebugOutput(QString("Failed to open port %1, error: %2").arg(m_serialPort.portName()).arg(m_serialPort.errorString()));
        return;
    }

    emit writeDebugOutput("Port successfully opened");
}

This is the output i get:

Without m_serialPort.open(), busy is "No" for COM7. Other ports yield the same error string.

Thanks.

[jsulm]

@kkettinger Do you start your app with elevated permissions?

[kkettinger]

Yes, I directly start the .exe from the build folder (after using windeployqt.exe) with administrative right (Rightclick -> Run as administrator).

The console app doesn't run with administrative right and still works though.

[jsulm]

@kkettinger Are you sure you're not trying to open the port in your GUI app more than once?
Did you compare your console app to your GUI app - maybe there are some differences in how you handle COM ports?

[kkettinger]

Thank you jsulm, that was it. I accidentally had another backend instance for testing in the main.cpp, directly before using qmlRegisterType<BackendAdapter::cBackendAdapter>("com.ex.backend", 1, 0, "Backend");. This way two worker instances were instantiated and two emit signals were sent. But ony one was used in qml and was connected to the debug window, so i only saw one output.