QSerialPort only reads correctly after second open - c++

Question

I have a program that uses serial input. It's installed on quite a few machines with both Win7 and Win10. On some machines I have the strange issue that when opening the serial port at first it reads strange/incorrect values, mostly 0xff. When I close the port and reopen it, it works correctly.
m_port = new QSerialPort( info ); // some info from QSerialPortInfo::availablePorts();
if( m_port->open( QIODevice::ReadOnly ) )
{
m_port->setBaudRate( m_baudRate );
m_port->setDataBits( m_dataBits );
m_port->setParity( m_parity );
m_port->setStopBits( m_stopBits );
m_port->setFlowControl( QSerialPort::FlowControl::HardwareControl );
m_port->clear();
}
}
So am I just lucky that it works on like 90% of my installations and it's missing some explicit setting or might it be a bug in Qt? (5.6.0 msvc 2013)

Answer 1

Most likely what the problem here is that you're setting the settings on the serial port after you have opened it. Therefore, there's a small period of time where your settings could be in an odd state. It works the second time you open the port because the settings have been correctly set from the first time you opened the port.
QSerialPort will apply the serial port settings when open is called.
m_port = new QSerialPort( info ); // some info from QSerialPortInfo::availablePorts();
m_port->setBaudRate( m_baudRate );
m_port->setDataBits( m_dataBits );
m_port->setParity( m_parity );
m_port->setStopBits( m_stopBits );
m_port->setFlowControl( QSerialPort::FlowControl::HardwareControl );
if( m_port->open( QIODevice::ReadOnly ) )
{
m_port->clear();
}