[SOLVED] QSerialPort::waitForBytesWritten() returns false

[frankiefrank]

I upgraded from Qt 5.2.0 to 5.3 - now a piece of code that worked before does not work.

My serial port lives in a worker thread, running without an event loop. To write data to the port, I'm calling write() and then waitForBytesWritten() with a 50 msecs timeout.

It seems like the calls actually do "work" (the data is written). BUT QSerialPort reports failures.

The first waitForBytesWritten() returns false, the error code is 12 and the description is "Unknown error".
Every attempt after that still returns false, error code still 12 and description is "Overlapped I/O operation is in progress."

[frankiefrank]

Looks like it might be this bug:
https://bugreports.qt-project.org/browse/QTBUG-36758

It's a bit hard to follow the discussion there but am I right in understanding that this is only going to be fixed in 5.3.1? Should I go back to 5.2.0 or 5.2.1?

[kuzulis]

bq. in understanding that this is only going to be fixed in 5.3.1?

Yes. Will be in 5.3.1. But in any case you can build QtSerialPort from Git sources and do not worry. :)

[frankiefrank]

Thanks kuzulis. Up until now I only used the binaries (with the web installer).

Could you point me to what exactly I need to get from github? The branch named Stable?

[kuzulis]

bq. Could you point me to what exactly I need to get from the github?

I don't know nothing about GitHub, but from Gitorious it is enough to do:

@git clone git://gitorious.org/qt/qtserialport.git@

see Wiki.: https://qt-project.org/wiki/QtSerialPort

bq. The branch named Stable?

Yes. Need the stable branch. (also by default will be downloaded the stable branch, will be active).

Also you can download from the WEB: http://qt.gitorious.org/qt/qtserialport/archive-tarball/stable as written on Wiki.

[frankiefrank]

Thanks for the links kuzulis - but I'm still having issues.

After using the stable Qt5SerialPortd.dll - indeed my waitForBytesWritten works - but now waitForReadyRead does not.

To be exact, the first calls to waitForReadyRead correctly return false, with a TimeoutError (code 12) and the errorString() is Unknown Error.

As soon as one of the following things happens -
A. there is actually data to read.
B. I called write() and waitForBytesWritten()

• the next call to waitForReadyRead() returns false, with same error code 12 and error string "Overlapped I/O operation is in progress."

[kuzulis]

Can you please provide an simple complete code to reproduce a problem?

[frankiefrank]

Simplified as much as I can - this is just for the error for reading data. I didn't include the writing code and the threading related stuff besides the minimal to reproduce.

SerialPortWorkerThread.h
@
class SerialPortWorkerThread : public QThread
{
Q_OBJECT

public:
SerialPortWorkerThread(QObject *parent = 0);
~SerialPortWorkerThread();
virtual void run();

private:
bool abortThread;
bool openPort(QSerialPort &serialPort, QString &errorMsg);
};
@

SerialPortWorkerThread.cpp
@
#include "serialportworkerthread.h"
#include <QDebug>

SerialPortWorkerThread::SerialPortWorkerThread(QObject *parent)
: QThread(parent), abortThread(false)
{
}

SerialPortWorkerThread::~SerialPortWorkerThread()
{
if (isRunning())
abortThread = true;
}

void SerialPortWorkerThread::run()
{
QSerialPort serialPort;
QString error;

bool opened = openPort(serialPort, error);
if (!opened)
qDebug() << QString("Error opening port: %1").arg(error);

while (opened && !abortThread)
{
bool haveData = serialPort.waitForReadyRead(15);
if (!haveData)
{
QString error = serialPort.errorString();
int errorId = serialPort.error();
//! skip the UNKNOWN
if (!error.contains("Unknown error", Qt::CaseInsensitive))
qDebug() << QString("Error %1 - %2").arg(errorId).arg(error);
}
else {
qDebug() << QString("Data was read!");
QByteArray data = serialPort.readAll();
}
}

qDebug() << "Serial Port Worker Thread - we got out of while loop";
}

bool SerialPortWorkerThread::openPort(QSerialPort &serialPort, QString &errorMsg)
{
errorMsg.clear();
if (serialPort.isOpen())
serialPort.close();

serialPort.setPortName("COM20");
bool success = serialPort.open(QIODevice::ReadWrite);

//! connection error - just quit here
if (!success) {
int errorId = serialPort.error();
QString errorDescription = serialPort.errorString();
errorMsg = tr("Could not open port: (errorId=%1) %2").arg(errorId).arg(errorDescription);

return false;
}

//! Try to apply settings one after another, storing potential error message
success = serialPort.setBaudRate(QSerialPort::Baud9600);
if (!success)
errorMsg = tr("Could not set baud rate.");

if (success)
success = serialPort.setDataBits(QSerialPort::Data8);
if (!success)
errorMsg = tr("Could not set data bits number.");

if (success)
success = serialPort.setParity(QSerialPort::NoParity);
if (!success)
errorMsg = tr("Could not set parity check option.");

if (success)
success = serialPort.setStopBits(QSerialPort::OneStop);
if (!success)
errorMsg = tr("Could not set stop bits.");

if (success)
success = serialPort.setFlowControl(QSerialPort::NoFlowControl);
if (!success)
errorMsg = tr("Could not set flow control mode.");

//! If necessary, close
if (!success)
serialPort.close();

return success;
}
@

Then I create a regular UI application, with a SerialPortWorkerThread* member and in the constructor I call:

@
serialPortWorker = new SerialPortWorkerThread(this);
serialPortWorker->start();
@

[kuzulis]

Ohh.. it is not simple example.. :)
Though yes, I found some problems with waitForReadyRead() in own research. I will try to prepare a patch. :(

[frankiefrank]

Thank you for confirming. Should I do anything in the Qt Bug Tracker or would you update any existing issues?

[kuzulis]

You can try a patch, see issue: https://bugreports.qt-project.org/browse/QTBUG-39314

[frankiefrank]

Thanks kuzulis. I will try this.

I hate to hijack you to another thread (which is way more creative than hijacking a thread) - but is there a chance you could help out over here?
http://qt-project.org/forums/viewthread/43156/

(if not, I'll work it out).

[frankiefrank]

Unfortunately still doesn't work.

I took qtserialport-stable from git and manually replaced the qserialport_win.cpp with the patched version. I then cleaned and rebuilt, with 5.3.0.

In my test project I removed
@QT += serialport@
and added
@
INCLUDEPATH += "C:/qtserialport-stable/src/serialport"
LIBS += "C:/build-qtserialport-Desktop_Qt_5_3_0_MSVC2010_OpenGL_32bit-Debug/lib/Qt5SerialPortd.lib"
@

For the first time data should be read, I get
"Error 12 - Overlapped I/O operation is in progress."
"Error 12 - The handle is invalid."

After that, the code returns one or two of the "overlapped" errors each time data is sent to the port and should be read.

[kuzulis]

[quote]
For the first time data should be read, I get
“Error 12 – Overlapped I/O operation is in progress.”
“Error 12 – The handle is invalid.”
[/quote]

Hmm.. This is strange problem.. Can you please provide a more simple example without threads? E.g. try to modify an "creadersync" example..

[kuzulis]

UPD: I just checked a patch with a modified "creadersync" example: http://paste.kde.org/pg353hzw4

I got a such output:

@
waitForReadyRead failed, 12, error: Unknown error
waitForReadyRead failed, 12, error: Unknown error
Data successfully received from port:
q
waitForReadyRead failed, 12, error: Протекает наложенное событие ввода/вывода.
waitForReadyRead failed, 12, error: Протекает наложенное событие ввода/вывода.
Data successfully received from port:
qqqqqqqq
Data successfully received from port:
qqqqqqqq
Data successfully received from port:
qqqqqqqq
Data successfully received from port:
qqqqqqqq
Data successfully received from port:
qqqqqqqq
Data successfully received from port:
qqqqqqqq
Data successfully received from port:
q
waitForReadyRead failed, 12, error: Протекает наложенное событие ввода/вывода.
@

Where "Протекает наложенное событие ввода/вывода." is an "Overlapped I/O operation is in progress."

Thus, everything is fine. It seems, the error description has incorrect text (seems, the last GetLastError() is decrypted), possibly it needs to be fix in the future (but it isn't critical). To be guided by the error code number =12 (that there is QSerialPort::TimeoutError), and this is correct code.

PS: If to be honest, for me do not like your error text: "The handle is invalid.” Seems, that your code has some problems.. You trying to manage on closed device.. or something else..

[frankiefrank]

So you're saying I should ignore the Overlapped I/O string and just treat this is an incorrect error string? OK - that's better.

I see that indeed now with the patch and if I ignore this specific error, it should eliminate the problems I had in my mini project.

About the "The handle is invalid" - I got this error when my waitForReadyRead() calls followed a write() and waitForBytesWritten() calls. I indeed didn't get that error in your modified CReaderSync.

BUT I did manage to reproduce it based on your code with a few steps and without any threads :)

Create a new Qt Widgets Application and copy all of your suggested CReaderSync code to the constructor of the MainWindow.

• Modification 1: open the port as QIODevice::ReadWrite.

• Modification 2: Add the following code (picked up from CWriterSync and slightly reformatted output string) before starting the while loop - so it happens only once.

@
QByteArray writeData(QString::number(QDateTime::currentMSecsSinceEpoch()).toLatin1());
qint64 bytesWritten = serialPort.write(writeData);

if (bytesWritten == -1) {
qDebug() << QObject::tr("Failed to write the data to port, error: %1 - %2").arg(serialPort.error()).arg(serialPort.errorString()) << endl;
return;
} else if (bytesWritten != writeData.size()) {
qDebug() << QObject::tr("Failed to write all the data to port, error: %1 - %2").arg(serialPort.error()).arg(serialPort.errorString()) << endl;
return;
} else if (!serialPort.waitForBytesWritten(5000)) {
qDebug() << QObject::tr("Operation timed out or an error occurred for port, error: %1 - %2").arg(serialPort.error()).arg(serialPort.errorString()) << endl;
return;
}
@

• Modification 3: I removed the return values (since I threw code into constructor) and changed standardoutput to qDebug() (to see messages in the Application Output in Qt Creator). Copied a copy of the patched Qt5SerialPortd.dll into the shadow build folder. Made it compile.

What I get here is the mentioned:
"waitForReadyRead failed, 12, error: Overlapped I/O operation is in progress."
"waitForReadyRead failed, 12, error: The handle is invalid."

[kuzulis]

Hmm.. I can not reproduce:

bq. “waitForReadyRead failed, 12, error: The handle is invalid.”

with our modifications (If I correctly understand): http://paste.kde.org/pt8uhw1xp

Quite possibly that problems with the driver of your device or something else. Because someone trying to have access to your device... I.e. someone earlier set this error, maybe you trying to access to other I/O resource? (file, pipe and so on)?

In any case, don't pay attention to the text of this error in case the timeout works correctly (is fired after 10 sec). :)

[frankiefrank]

You mean this error is not necessarily related to the specific QSerialPort I'm accessing? I'm not sure how that part works.

Anyway, I think this solves the issue for me - so thank you! Will the patch be integrated into the stable branch of QSerialPort and will be released as a part of 5.3.1?

[kuzulis]

You mean this error is not necessarily related to the specific QSerialPort I’m accessing?

Yes, because someone can do something on some I/O resources in context of current thread (maybe you have an some other code in your app, not only the constructor, as you mention??). And this maybe lead to the access to invalid descriptor of some resource (not necessarily that it will serial port).. I just assumption..

QSerialPort and will be released as a part of 5.3.1?

Yes, most likely in 5.3.1 if reviewers don't block this patch. You can vote or add comments for this patch here: https://bugreports.qt-project.org/browse/QTBUG-39314

[kuzulis]

Many thanks for your vote. :)

BTW: Is ready an other patch to display an correct error message for TimeoutError: https://codereview.qt-project.org/#change,86406

You can also try it. :)