QIODevice::readAll hangs

[Chruetli]

Hi
(using Qt 6.5.3)
after increasing network traffic, reading from a socket results in an infinite loop.
I inherited the following code:

  while (threadRun) {
    while ((m_pDevice->bytesAvailable() == 0) && (threadRun)) {
      msleep(50);
    }
    if (threadRun) {
      msleep(20);
      QByteArray protoAr = m_pDevice->readAll();
      processXml(protoAr);
    } 
  } 

and some else we have:

m_pDevice = new QTcpSocket(this);

Means we are reading XML content from a TCP socket (~4 messages per second, ~120 Bytes each) and processing it. I know that incomplete reads can occur but that's currently not (yet) a problem.

After some time we notice that the task hangs in a loop and consumes 100% CPU.
gdb shows the following backtrace:

The implementation of readAll is shown here https://codebrowser.dev/qt6/qtbase/src/corelib/io/qiodevice.cpp.html#_ZN9QIODevice7readAllEv
from there we jump to read https://codebrowser.dev/qt6/qtbase/src/corelib/io/qiodevice.cpp.html#_ZN16QIODevicePrivate4readEPcxb and finally to QRingBuffer::read https://codebrowser.dev/qt6/qtbase/src/corelib/tools/qringbuffer.cpp.html#_ZN11QRingBuffer4readEPcx

qint64 QRingBuffer::read(char *data, qint64 maxLength)
{
    const qint64 bytesToRead = qMin(size(), maxLength);
    qint64 readSoFar = 0;
    while (readSoFar < bytesToRead) {
        const qint64 bytesToReadFromThisBlock = qMin(bytesToRead - readSoFar,
                                                     nextDataBlockSize());
        if (data)
            memcpy(data + readSoFar, readPointer(), bytesToReadFromThisBlock);
        readSoFar += bytesToReadFromThisBlock;
        free(bytesToReadFromThisBlock);
    }
    return readSoFar;
}

Any idea what's going wrong?

Thanks for helping!

[Chruetli]

@Christian-Ehrlicher : Of course you are right, but as this is a very low level routine i was reluctant to make wide changes. Nevertheless I tried it now and surprisingly little code remains. (Still have to remove Thread from the class name)

connect(m_pDevice.get(), &QIODevice::readyRead, this, &xmlReaderThread::readData);
...

void xmlReaderThread::readData() {
    QByteArray protoAr = m_pDevice->readAll();
    processXml(protoAr);
}

Looks better now?

[Christian Ehrlicher]

Even though I know the answer but: Use signals and slots (and no need for a thread at all) - all of the above is completely unneeded with Qt...

[Chruetli]

@Christian-Ehrlicher : Of course you are right, but as this is a very low level routine i was reluctant to make wide changes. Nevertheless I tried it now and surprisingly little code remains. (Still have to remove Thread from the class name)

connect(m_pDevice.get(), &QIODevice::readyRead, this, &xmlReaderThread::readData);
...

void xmlReaderThread::readData() {
    QByteArray protoAr = m_pDevice->readAll();
    processXml(protoAr);
}

Looks better now?

[Christian Ehrlicher]

@Chruetli said in QIODevice::readAll hangs:

Looks better now

Much better. But could not read out if the problem still persists. If so print out the bytesAvailable and only try to read that much bytes.

[Chruetli]

@Christian-Ehrlicher said in QIODevice::readAll hangs:

Much better. But could not read out if the problem still persists. If so print out the bytesAvailable and only try to read that much bytes.

Me neither... I need my hardware first to try it.

[Christian Ehrlicher]

@Chruetli said in QIODevice::readAll hangs:

I need my hardware first to try it.

:)

I'm not aware of a bug report wrt this problem. Was m_pDevice created in the correct thread?

[Chruetli]

@Christian-Ehrlicher said in QIODevice::readAll hangs:

I'm not aware of a bug report wrt this problem. Was m_pDevice created in the correct thread?

Yes, at least this looks fine! Just for interest, what's the problem with this? Do you have me a pointer?

Btw. what's the difference between readAll() and read(bytesAvailable());?

[Christian Ehrlicher]

@Chruetli said in QIODevice::readAll hangs:

Do you have me a pointer?

Accessing a QTcpSocket (or anything else) from different threads will not work / results in undefined behavior.

Btw. what's the difference between readAll() and read(bytesAvailable());?

Nothing in reality, just wanted to see if it makes a difference for you by any chance.

[JonB]

@Chruetli said in QIODevice::readAll hangs:

Btw. what's the difference between readAll() and read(bytesAvailable());?

Timing, I imagine! :)

[Chruetli]

I tested it with my hardware an now it works! Thanks for helping me out!