Cannot read QSerialPort without event loop

[Hetsh]

Hi all,

i am writing a library which uses QSerialPort and has no event loop. Sending data works fine, but i cannot receive any. QSerialPort does not report any errors.
According to the documentation all QIODevices should work without an event loop, so i am missing something. But what am i missing?
Using QT 5.9.3 on Win7 x64 with MinGW. My code:

...
	if(!port.open(QIODevice::ReadOnly))
	{
		switch (port.error())
		{
			case QSerialPort::DeviceNotFoundError:
			{
				qCritical("Port with name \"%s\" not found", port.portName().toLocal8Bit().constData());
				return PORT_NOT_EXIST;
			}

			case QSerialPort::PermissionError:
			{
				qCritical("No permission to open serial port");
				return PORT_NO_ACCESS;
			}

			case QSerialPort::OpenError:
			{
				qCritical("Port is already in use");
				return PORT_BUSY;
			}

			default:
			{
				qCritical("Unknown error while opening the port");
				return PORT_OPEN_UNKNOWN;
			}
		}
	}

	std::thread thread(readPort, &port);
	thread.join();

	port.close();
}

void readPort(QSerialPort* port)
{
	for(qint64 startTime = QDateTime::currentSecsSinceEpoch()+10;QDateTime::currentSecsSinceEpoch() < startTime;std::this_thread::sleep_for(std::chrono::milliseconds(100)))
	{
		QByteArray data = port->read(100);
		if(!data.isEmpty())
		{
			qInfo("Read data %s", data.constData());
		}
	}
}

[Leonardo]

The documentation for QSerialPort says:

QSerialPort provides a set of functions that suspend the calling thread until certain signals are emitted. These functions can be used to implement blocking serial ports:

waitForReadyRead() blocks calls until new data is available for reading.
waitForBytesWritten() blocks calls until one payload of data has been written to the serial port.

I think you should try waitForReadyRead() instead of implementing your own pooling.

[aha_1980]

@Leonardo

I think you should try waitForReadyRead() instead of implementing your own pooling.

he can try, but I think the problem stays the same: you need an event loop.

that said, I have written a DLL (closed source, sorry) for serial communication. if that was called from a Qt program, all was fine.

for non Qt programs, I changed it in a way to let QtSerialPort run in a QThread (a bit difficult), und created an event loop in the DLL if QCoreApplication::instance was NULL.

maybe @kuzulis has some more comments?

[Leonardo]

The same documentation page for QSerialPort states that:

A blocking serial port does not require an event loop and typically leads to simpler code.

So I assumed that waitForReadyRead() might somehow handle this internally. If there's a QCoreApplication instance, maybe he could also call processEvents() inside his "for" to emulate a event loop.

[mrjj]

Oh ?!
So even
bool QIODevice::waitForReadyRead(int msecs)
says
"This function can operate without an event loop."

Its not in a way we except ?

[aha_1980]

@mrjj @Leonardo Oops, you may be right. Thinking again, my DLL may have used the signal driven approach - I cannot check this now.

@Hetsh: waitForReadyRead may be worth a try.

[Hetsh]

Thank you all for your input. Using waitForReadyRead did unfortunately not change anything. I think instantiating QCoreApplication as long as it does not exist will be the best short term solution for this.
Should this be reported as bug?

[aha_1980]

@Hetsh said in Cannot read QSerialPort without event loop:

Should this be reported as bug?

Yes, please do (and add a link here). That way we dig deeper into the issue.

Thanks.

[kuzulis]

There are no any bug. You need in QCoreApplication instance anyway. This even not an QtSerialPort issue.

[aha_1980]

@kuzulis @mrjj @Leonardo @Hetsh

I prepared a documentation update to clarify the status quo: https://codereview.qt-project.org/216669

[aha_1980]

Hi @Hetsh,

As @kuzulis correctly said (in the doc review), there are two examples that work without event loop:

http://doc.qt.io/qt-5/qtserialport-creadersync-example.html
http://doc.qt.io/qt-5/qtserialport-cwritersync-example.html

So you will need a QCoreApplication, but not an event loop.

[Hetsh]

@aha_1980 The example works, but it is using the waitFor methods in the "main" thread, which i cannot do.
@kuzulis I instantiated QCoreApplication both in the main thread and in the polling thread without any success.

I did a quick test on linux and it works without a QCoreApplication instance or event loop.

Back on windows I implemented the workaround from @aha_1980 by instantiating QCoreApplication in another thread and starting an event loop there. An important step is to move the port to the QCoreApplications thread. It won't work otherwise: https://pastebin.com/3sDcq3Hv

I filed a bug report.

Thank you all for your help :-)

[kuzulis]

Did creadersync example works for you?

[Hetsh]

@kuzulis yes it did

[kuzulis]

I have answered in your BR, and closed it as invalid, because you use QSP wrong.

PS: It works even w/o of QCoreApplication instrance and w/o of event loop. But needs to use the QCoreApplication instance to avoid some future/others problems (it suggested everywhere in Qt, AFAIK).

[Hetsh]

@kuzulis Thank you. It works perfectly now :-)

TLDR: My mistake was to not instantiate and open the QSerialPort in the thread i was reading from.