QSerialPort read and write in the same QThread

romain.donze

Hi everyone,

So, by following this example (writing data and then waiting for data to read)
https://doc.qt.io/qt-5/qtserialport-blockingmaster-example.html
I managed to make a QThread that send data through serial port whenever I want

And by following this example (waiting for data to read and then writing data)
https://doc.qt.io/qt-5/qtserialport-blockingslave-example.html
I managed to make a QThread that receive data from serial port whenever there some

Now here is my question. I would like to "merge" those two thread into one, so I have a QTread who can write data and also read data without blocking anything. Understand that data could be sent at any time while data could also arrive at any time too.

Really hope someone can help me on this one

Christian Ehrlicher

Don't use the blocking approach and use signals/slots instead.

romain.donze

ok that's what I though afterward.

But how can I do this? Is there some example? Using QSerialPort of course

aha_1980

@romain-donze

E.g. the Terminal example?

Regards

romain.donze

@aha_1980 yeah but it dows not seems to use QThread.

Or could I just throw it in one?

aha_1980

@romain-donze I'm pretty sure you don't need a thread.

But yes, it could be put in one.

Regards

romain.donze

@aha_1980 hey so after a few try i am a bit lost here.

So firstly I know I must use a thread for my use case (I use this thread to transfer complete files via xbee module using serial port so it takes some time)

But I ran in some issues.

Since my thread must run indefinitly, I put a while(1){} in it and I am doing all my stuff in this loop. But unfortunatly, it always use 100% of the cpu. Do you have any idea on how to put the thread in sleep mode and wake it when I make a write request or when a QSerialPort::readyRead occurs?

thanks for your help

addebito

@romain-donze look this code.
I hope you find it useful.

    m_serial->clear(QSerialPort::AllDirections);
    
    // write a request
    const QByteArray requestData = QString("your request\r\n").toUtf8();
    m_serial->write(requestData);
    
    if (m_serial->waitForBytesWritten(m_waitTimeout))
    {
        // wait and read the response
        if (m_serial->waitForReadyRead(m_waitTimeout))
        {
            QByteArray responseData = m_serial->readAll();
            while (m_serial->waitForReadyRead(100))
                responseData += m_serial->readAll();

        }
    }

aha_1980

@romain-donze

So firstly I know I must use a thread for my use case (I use this thread to transfer complete files via xbee module using serial port so it takes some time)

As said multiple times, that is no reason to need a thread.

Regards

romain.donze

@addebito yeah I have already seen something like this in the masterthread example for QSerialPort but the thing is that data could arrive at any moment and not only after a specifi write

romain.donze

@aha_1980 ok but I don't understand how this could not block my current thread (e.g. gui thread) if I have to send a big file that could take minutes to send...

How would I use waitForBytesWritten() to not block my code

aha_1980

@romain-donze said in QSerialPort read and write in the same QThread:

How would I use waitForBytesWritten() to not block my code

You would not use it at all. You would use the bytesWritten() signal.

Do note however, that this does not mean the bytes are actually transmitted. It only means, the bytes are transferred from Qt's buffer to the operating system buffer.

Serial devices are slow. You can calculate the needed time approximately with the baud rate the device is set to.

Regards

romain.donze

@aha_1980 ok so is there some examples on how to use it?

addebito

@romain-donze you can swap the code, before you wait to receive data with a specific timeout and after you can send a reply.
You can manage your other asyncronous write in the main loop with a Semaphor or Event for example.

ollarch

Hi,

Use the "readyRead" SIGNAL to receive the data.
In your write method you could write the data into chunks (N bytes). After write a chunk, call QApplication::processEvents() that forces the calling thread to process the events and so it will receive the data if it is there.

jsulm

@romain-donze said in QSerialPort read and write in the same QThread:

so is there some examples on how to use it?

https://doc.qt.io/qt-5/qtserialport-examples.html

romain.donze

@aha_1980 just so you know, I did some test with my uart device plugged to my board and I am receiving data evry 2ms (I am receiving a string that looks like that "#Z%d,%.2f\r")

In my readData() function, I do this (this function is connected to the readyRead signal)

void mUart::readData()
{
    QByteArray singleData = mSerial->readAll();
    mBufferData = mBufferData + QString::fromUtf8(singleData);

    QStringList dataList;

    if(mBufferData.contains('\r'))
    {
        dataList = mBufferData.split('\r');

        foreach (mTrame, dataList)
        {
            emit this->dataReceived(mTrame);
            qDebug() << mTrame;
        }
    }
}

the mUart class is move to a thread and even so, I am using more than 100% of my cpu... Is ther something I do wrong?

J.Hilk

@romain-donze said in QSerialPort read and write in the same QThread:

the mUart class is move to a thread and even so, I am using more than 100% of my cpu... Is ther something I do wrong?

of course, just because you moved the invite loop into a thread, doesn't mean it uses less CPU!

jsulm

This post is deleted!

romain.donze

@J-Hilk yes I know, but as you see, as I explained, I receive a lot of data and I think that is why my readData function is constantly called... so I don't think it's a good thing to let it in the main thread

P.S. sometimes my app crashes. Maybe because I have to many data to read? ):