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!