Cannot read QSerialPort without event loop

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.

bast236

@kuzulis I am coding a Qt non-GUI DLL plugin that exports functions to do several things, including reading serial ports. These functions are called by an external .exe program (not using Qt, not coded by me) using a standardized API.

I have all sorts of bugs when using QSerialPort, as described in https://forum.qt.io/topic/137566/qserialport-with-no-gui-qobject-starttimer-timers-can-only-be-used-with-threads-started-with-qthread/12.

How to use QSerialPort without threads, without QCoreApplication, without event-loop, but just blocking IO calls?

kuzulis

As an option, you can create a separate thread in your DLL, e.g. QThread. Then, create the QSerialPort and then move it to that thread... Maybe it will help... An idea is that QSerialPort should get a QThread as a parent.

AFAIK, it is not simple to use the Qt classes in the separate DLL's. It belongs not only to QSerialPort, it belongs also to other QIODevice and other classes. e.g. that classess can use the QTimer internally, which does not work without of an event loop... So, it is hard... I have not ideas.