QSerialPort in another thread. Doesn't report errors...

[zeneslev]

Hi all. I am developing a GUI which will communicate to some hardware over a virtual serial port (USB to serial cable). I want to detect when the cable is disconnected. I have loaded the terminal example in QT creator and when I open the virtual serial port and then unplug it from the computer, an error is triggered, which is the correct behavior. I want my program to do the same thing.

The thing is, I want to make a class that runs in a separate thread which contains a QSerialPort object and handles communication using this QSerialPort object. I want to connect the error signal from the QSerialPort class to my own error handler in my new class. The problem I have is that the error signal is never triggered with the right signal (QSerialPort::ResourceError)! In the terminal example program, this signal is correctly generated.

I think this is some sort of thread affinity issue. If I instead make the QSerialPort object a member of the mainwindow class and then connect the serial port object's error signal to a member slot of mainwindow, everything works A-OK when I rip the USB cable from my computer. Any ideas?

[zeneslev]

Also, this is my first time using QThreads. My mainwindow constructor does the following to set up the SSACom2 object (the object that has a Qserialport and handles connection stuff at a higher level): cThread is member of my MainWindow class.

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
cThread = new QThread(this);

scom = new SSAComm2(NULL);

scom->doSetup(cThread);
scom->moveToThread(cThread);

cThread->start();

}

[JKSH]

Hi,

Can you show us the code inside SSAComm2::doSetup()?

[zeneslev]

Yes. ssacomm2.cpp:

#include "ssacomm2.h"
#include <QDebug>

SSAComm2::SSAComm2(QObject *parent) : QObject(parent)
{

}

SSAComm2::~SSAComm2()
{

}

void SSAComm2::doSetup(QThread *cThread)
{
connect(cThread, SIGNAL(started()), this, SLOT(doWork()));
thisThread = cThread;
port = new QSerialPort(this);
port->setPortName("COM3");
port->open(QIODevice::ReadWrite);
connect(port, SIGNAL(error(QSerialPort::SerialPortError)), this,
SLOT(handleError(QSerialPort::SerialPortError)));
qDebug() << "SSA Comm2 starting!";
}

void SSAComm2::doWork()
{
while(1);

return;

}

void SSAComm2::handleError(QSerialPort::SerialPortError error)
{
if (error == QSerialPort::ResourceError) {
qDebug() << "Error! Unplugged Cable!!!"; //This never executes when I unplug the USB cable!!!!
}
}

[JKSH]

Hi,

You have 2 problems:

@zeneslev said:

void SSAComm2::doSetup(QThread *cThread)
{
    ...
    port = new QSerialPort(this);
    port->setPortName("COM3");
    port->open(QIODevice::ReadWrite);
    ...
}

1st problem: You created and opened the serial port in the main thread.

You must call the QSerialPort constructor and open() in your worker thread.

@zeneslev said:

void SSAComm2::doWork()
{
    while(1);

    return;
}

2nd problem: You have an infinite loop in doWork().

When a thread is running an infinite loop, it cannot cannot process any signals because the infinite loop blocks the event loop.

[zeneslev]

@JKSH

Thank you! I get the correct behavior now. My initial understanding about how threading works was incorrect. I was under the impression that the thread will end when doWork finishes, but that isn't the case. Thanks for taking a look!