[SOLVED] QSerialPort fails opening with "File exists" error message

pschuster

Hi all,

I am developing a custom embedded device. This device runs Buildroot linux with Qt 5.3. It features an AVR-MCU (Arduino 1284p) that is connected to the main processor (that runs Linux and Qt) via a serial connection (RX, TX). The Arduino-MCU simple reads the serial port for commands, executes them (i.e. read temperature sensor) and prints the result.

In the linux shell I can run cat /dev/ttyS3 to read the data the Arduino has written and I can send bytes by calling echo 'Hello Arduino' >> /dev/ttyS3 for example. That works fine.

If I want to use QSerialPort by calling:

foreach (const QSerialPortInfo &info, QSerialPortInfo::availablePorts())
    qDebug() << info.portName();

m_serialPort = new QSerialPort("ttyS3");
m_serialPort->setBaudRate(QSerialPort::Baud9600);
m_serialPort->setFlowControl(QSerialPort::NoFlowControl);
m_serialPort->setParity(QSerialPort::NoParity);
m_serialPort->setDataBits(QSerialPort::Data8);
m_serialPort->setStopBits(QSerialPort::OneStop);
if (m_serialPort->open(QIODevice::ReadWrite))
{
    qDebug() << "Serial Port opened ttyS3";

    this->connect(m_serialPort,SIGNAL(readyRead()),SLOT(serialDataReceived()));
}
else
{
    qDebug() << "Failed to open ttyS3 serial port";
    qDebug() << m_serialPort->errorString();
    qDebug() << m_serialPort->error();
}

I receive the error message "File exists" (errorString) and the error-ID is 2. I am running the Qt application as root. http://superuser.com/questions/569255/how-to-quickly-determine-which-process-if-any-listens-to-a-serial-port lists a few linux commands that identify the process that has the serial port in use. Non of the examples (lsof /dev/ttyS3 nor fuser -n file /dev/ttyS3) lists a process. So I am pretty sure the serial port is not in use.

If I try the source code with ttyS2 everything works fine (besides the fact that nothing useful is connected to ttyS2 of course ;-)).

Anyone has an idea what's going wrong?

Thanks a lot.
Phillip

SGaist

Hi and welcome to devnet,

Just an idea: did you check the permissions on /dev/ttyS3 ?

kuzulis

Most likely that "File exists" goes from the tty-locker (from QLockFile), because the ttyS3-lock file exists. It is probable that this issue belongs to this bug: https://bugreports.qt.io/browse/QTBUG-36983

So, you can:

Try to use a latest version of QtSerialPort
Just find out the lock-file and to remove it before opening.
That is better - it is do not start the application with a root privileges. You should try to create an user, and to add his to "dialout" grop or similar (depends on your base distro).

pschuster

AWESEOME! Thank you very much. 2) did the trick. I found a file /var/lock/LCK..ttyS3. After removing it the code posted above works fine. I will now try 1) or implement a patch myself.

Thanks a lot. You made my day.
Phillip