problem using QSerialPort

[mzimmers]

Hi all -

I've got an app that uses a COM port for initial configuration of a device that then joins a Wifi network. I'm using the QSerialPort class for this configuration. I'm connecting a readyRead signal for input, but it never triggers. I'm also getting this error message:

serial port error QSerialPort::NoError encountered.
QObject: Cannot create children for a parent that is in a different thread.
(Parent is QSerialPort(0x16d13d8), parent's thread is QThread(0x161cb20), current thread is QThread(0x16e58d0)

I'm wondering if I'm doing something wrong with my threads. Here's a code snippet:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    worker = new Worker(widget);
    thread = new QThread;
    worker->moveToThread(thread);

    widget->show();
    thread->start();
    rc = a.exec();
...
Worker::Worker(Widget *widget, QObject *parent) : QObject (parent)
{
    m_serialPort = new SerialPort(this);
...
SerialPort::SerialPort(QObject *parent) : QObject (parent)
{
    QSerialPort m_serialPort;
    connect(&m_serialPort, &QSerialPort::readyRead, this, &SerialPort::readSerial);

My slot SerialPort::readSerial() never gets called. I've verified that there's incoming data using a port monitor. Any ideas what I've done wrong here?

Thanks...

[Kent-Dorfman]

What happens if you defer creation of the Serial port object until AFTER you've moved Worker to the other thread?

[mzimmers]

@Kent-Dorfman how do I do that?

[Kent-Dorfman]

dont construct it in the constructor, but send a signal to the thread later to construct and start the serial object.

[jsulm]

@mzimmers Is there a reason why you're using a thread? QSerialPort has an asynchronous API and there is usually no need to use threads for that.

[kshegunov]

@mzimmers said in problem using QSerialPort:

SerialPort::SerialPort(QObject *parent) : QObject (parent)
{
    QSerialPort m_serialPort;
    connect(&m_serialPort, &QSerialPort::readyRead, this, &SerialPort::readSerial);

This doesn't look right. Are you sure this is what you wrote?

[J.Hilk]

@mzimmers
IIRC you're using a static build? so its probably not the latest Qt Version on what are you running?

If it isn't the error @kshegunov pointed out

[mzimmers]

@jsulm I'm not using a thread exclusively for the serial port work; my worker thread essentially does all my non-UI stuff(encoding/decoding messages, etc.)
@kshegunov sorry; I was engaging in a bit of shorthand. m_serialPort is a member variable of SerialPort. The connect is correct, though.
@J-Hilk I'm currently using (non-static) 5.12.5.

Last night it occurred to me that long ago, I'd come across a problem with transferring objects to new threads, in which some sub-objects don't get transferred properly.

I might try Dorfman's idea, though I'd prefer not to have to do that.

Thanks...

[J.Hilk]

@mzimmers said in problem using QSerialPort:

@J-Hilk I'm currently using (non-static) 5.12.5.

And there is your problem
https://bugreports.qt.io/browse/QTBUG-78086

please update

[mzimmers]

@J-Hilk bang! Good eye. Thanks for catching that. I'll update immediately.

[SGaist]

Hi,

IIRC, your QSerialPort object won't get moved if your don't parent it.

[KroMignon]

@mzimmers said in problem using QSerialPort:

Good eye. Thanks for catching that. I'll update immediately.

Any way, your initialization seems not correct to me:

SerialPort::SerialPort(QObject *parent) : QObject (parent)
{
    QSerialPort m_serialPort;
    connect(&m_serialPort, &QSerialPort::readyRead, this, &SerialPort::readSerial);
...
}

should be, I think:

SerialPort::SerialPort(QObject *parent)
     : QObject (parent)
     , m_serialPort(new QSerialPort(this))
{
    connect(m_serialPort, &QSerialPort::readyRead, this, &SerialPort::readSerial);
...
}

[mzimmers]

@SGaist ah yes, that's what it was. Thanks for refreshing my memory.

Edit:

Which member variables does this apply to? For example, I also have a member QByteArray; does that need to be parented too?

Thanks...

[KroMignon]

@mzimmers said in problem using QSerialPort:

Which member variables does this apply to?

Each member which base class is QObject.

[mzimmers]

@KroMignon thanks...so a QByteArray object wouldn't be affected then.

It looks like the mechanics of updating have changed since I last used them. Is there a pre-built 5.14 for Windows available somewhere on github? EDIT: looks like the answer is "no;" I guess everyone builds their own now. Given my recent troubles trying to build a static Qt, I hope the directions in the 5.14 README are accurate.

I'm going to mark this solved, in anticipation that when I get a new Qt built, it'll work. Thanks for everyone's input on this.

[SGaist]

You need now to use your Qt account to access the pre-built binaries (the same you use to post here)

[mzimmers]

@SGaist thanks. Good thing they still exist, because I'm continuing to have nothing but trouble in my attempts to build the libraries. I really should report my problems with this to someone; any suggestions?

[SGaist]

The bug tracker is your friend :-)

[mzimmers]

@SGaist done. Thanks.

[aha_1980]

Hi @mzimmers,

would you mind letting us know, where?

Thanks and regards