Stop a QThread befor quitting application

ODБOï

Hi,

I have toubles stopping a QThread when i quit the application.

I designed a class ProgramParser that will parse very long text files.

I have another class ThreadSafeParser that is my interface to use a ProgramParser object in a dedicated QThread through Signal/Slots so my GUI is not blocked during parsers work

// ThreadSafeParser ctor
   parserThread = new QThread(this);
   parser = new ProgramParser();
   parser->moveToThread(parserThread);

So far, OK. Then I put an instance of ThreadSafeParser as contextProperty to use it from QML.
And connect QGuiApplication aboutToQuit signal to ThreadsafeParser s safeQuit slot.
if not, parser will still work even if i quit the app.

ThreadsafeParser parser;
engine.rootContext()->setContextProperty("parser",&parser);
QObject::connect(qApp,&QGuiApplication::aboutToQuit,&parser,&ThreadsafeParser::safeQuit);

this is the content of my safeQuit method

    parserThread->quit();
    parserThread->wait();

and this is my ThreadsafeParser destructor

    ~ThreadsafeParser(){
        parserThread->quit();
        parserThread->wait();
    }

with this method the thread is now stopped but applications will quit with code 1073741845

I also tryed to call deleteLater instead of my own safeQuit method, in this case work in progress will not even stop.

Please tell me what i'm doing wrong.

ODБOï

WORKING VERSION

class ProgramParser : public QObject
{
    Q_OBJECT
public:
    /// Ctor
    explicit ProgramParser(QObject *parent = nullptr);
public slots:
    void openFile(QString filePath,int ind);
    void startParsing();// long task
};

class ThreadsafeParser : public QObject
{
    Q_OBJECT
    QThread *parserThread;
    ProgramParser *parser;
public:
    explicit ThreadsafeParser(QObject *parent = nullptr);

    ~ThreadsafeParser(){
        qDebug()<<"QUITTING THE QTHREAD";
       // parserThread->quit();  // not needed 
        parserThread->deleteLater();
    }
signals:
void s_parse();
public slots:
    /// Demande de commancer à parser les fichiers
    void startParsing(){
        emit s_parse();
    }

};

ThreadsafeParser::ThreadsafeParser(QObject *parent) : QObject(parent)
{
    parserThread = new QThread();// QThread(this);
    parser = new ProgramParser;
    parser->moveToThread(parserThread); 

  // QObject::connect(parserThread, &QThread::finished, parser, &QObject::deleteLater);
    ///  forward  SIGNALs
    QObject::connect(this,&ThreadsafeParser::s_parse,parser,&ProgramParser::startParsing);
 
    parserThread->start();
}

//main.cpp

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);
   
    QQmlApplicationEngine engine;
  
    ThreadsafeParser safeParser;
     // no neeed to handle this
     //QObject::connect(qApp,&QGuiApplication::aboutToQuit,&safeParser,&ThreadsafeParser::deleteLater);

    engine.rootContext()->setContextProperty("parser",&safeParser);

    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();
}

Output in qt creator when i quit the app :

Y :  -178.062 // parse result
X :  65.4623 // parse result
Y :  -178.09  // parse result
QUITTING THE QTHREAD  // destructor called... 
10:54:45: C:/.../JobPlan exited with code 0

J.Hilk

@LeLev hi

Inside your save quit, set the pointer to a Nullptr and inside your destructor check against a Nullptr before attempting to quit and wait
I think, that should help

SGaist

Hi,

Out of curiosity, why do that from two different places ?

Unless something goes wrong, the destructor will be called anyway and again unless something wrong, aboutToQuit also, therefore it's a bit redundant.

ODБOï

@SGaist said in Stop a QThread befor quitting application:

why do that from two different places

i do from one or the other
i first tryed to call only deleteLater

QObject::connect(qApp,&QGuiApplication::aboutToQuit,&parser,&ThreadsafeParser::deleteLater );

and expected this to be called

~ThreadsafeParser(){
       parserThread->quit();
       parserThread->wait();
   }

but application is exiting with code 1073741845

then i have created my safeQuit method

QObject::connect(qApp,&QGuiApplication::aboutToQuit,&parser,&ThreadsafeParser::safeQuit );

...
void safeQuit(){
        parserThread->quit();
        parserThread->wait();
    }

in this case parser is still parsing if i quit app.

tomorrow i will try to set parserThread = nullptr as @J-Hilk suggested

~ThreadsafeParser(){
       parserThread->quit();
       parserThread->wait();
   
      parserThread = nullptr
   }

thx for the interest

SGaist

How is that ThreadsafeParser object handled ?

Because I just realised that you are potentially creating a double delete with your deleteLater call since it seems that parser is an object that's on the stack.

ODБOï

@SGaist said in Stop a QThread befor quitting application:

How is that ThreadsafeParser object handled ?

i create one in my main cpp thats all

ThreadsafeParser parser;
engine.rootContext()->setContextProperty("parser",&parser);
QObject::connect(qApp,&QGuiApplication::aboutToQuit,&parser,&ThreadsafeParser::safeQuit);

ODБOï

@SGaist said in Stop a QThread befor quitting application:

parser is an object that's on the stack

no, it is on heap, in my first post

parserThread = new QThread(this);
   parser = new ProgramParser();
   parser->moveToThread(parserThread);

SGaist

Then, no it's on the stack as I was talking about your ThreadsafeParser object which is also called parser (this is way using clear variable names is very important).

ODБOï

This post is deleted!

ODБOï

Hi,
I came back to the official example (i read it long time ago and was able to design multiple multithreaded application since. Now for some reason i'm stuck ): https://doc.qt.io/qt-5/qthread.html#details

I do exactly the same thing, My ProgramParser is the Worker of the example.
And my ThreadsafeParser is the Controller.

then i my main i just connect app's quit signal to deleteLater slot of My 'Controller' aka ThreadsafeParser

QObject::connect(qApp,&QGuiApplication::aboutToQuit,&safeParser,&ThreadsafeParser::deleteLater);

so this must be called right ?

~ThreadsafeParser(){
    qDebug()<<"QUITTING THE QTHREAD";
    parserThread.quit();
    parserThread.wait();
}

and since my parserThread 's finished is connected to my ProgramParser (Worker) deleteLater

QObject::connect(&parserThread, &QThread::finished, parser, &QObject::deleteLater);

I expect this to quit cleanly, but when i quit , the work is still in progress

edit

i changed the destructor to

~ThreadsafeParser(){
         parserThread->deleteLater();
    }

and putted parserThread on heap instead of stack
and passed ProgramParser *parser; to member instead of creating it in ThreadsafeParser's ctor

now QThread is stopped (it looks like) but application don't quit cleanly, i got : app crashed message.

Can this be related to the compiler i use, i did the exact same thing more than 10 times and it worked perfectly with MinGw compilers, for this project i have to use MSVC17 32bit
This is the only difference i see

ODБOï

WORKING VERSION

class ProgramParser : public QObject
{
    Q_OBJECT
public:
    /// Ctor
    explicit ProgramParser(QObject *parent = nullptr);
public slots:
    void openFile(QString filePath,int ind);
    void startParsing();// long task
};

class ThreadsafeParser : public QObject
{
    Q_OBJECT
    QThread *parserThread;
    ProgramParser *parser;
public:
    explicit ThreadsafeParser(QObject *parent = nullptr);

    ~ThreadsafeParser(){
        qDebug()<<"QUITTING THE QTHREAD";
       // parserThread->quit();  // not needed 
        parserThread->deleteLater();
    }
signals:
void s_parse();
public slots:
    /// Demande de commancer à parser les fichiers
    void startParsing(){
        emit s_parse();
    }

};

ThreadsafeParser::ThreadsafeParser(QObject *parent) : QObject(parent)
{
    parserThread = new QThread();// QThread(this);
    parser = new ProgramParser;
    parser->moveToThread(parserThread); 

  // QObject::connect(parserThread, &QThread::finished, parser, &QObject::deleteLater);
    ///  forward  SIGNALs
    QObject::connect(this,&ThreadsafeParser::s_parse,parser,&ProgramParser::startParsing);
 
    parserThread->start();
}

//main.cpp

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);
   
    QQmlApplicationEngine engine;
  
    ThreadsafeParser safeParser;
     // no neeed to handle this
     //QObject::connect(qApp,&QGuiApplication::aboutToQuit,&safeParser,&ThreadsafeParser::deleteLater);

    engine.rootContext()->setContextProperty("parser",&safeParser);

    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();
}

Output in qt creator when i quit the app :

Y :  -178.062 // parse result
X :  65.4623 // parse result
Y :  -178.09  // parse result
QUITTING THE QTHREAD  // destructor called... 
10:54:45: C:/.../JobPlan exited with code 0

ODБOï

... I only removed :

my own deletion handling (QObject::connect(qApp,&QGuiApplication::aboutToQuit,&safeParser,&ThreadsafeParser::deleteLater))
the parserThread->quit() line in my destructor
and the this i was passing to parserThread : (parserThread = new QThread(~~this~~);)
now this works perfectly. Exit with code 0.

 //QObject::connect(qApp,&QGuiApplication::aboutToQuit,&safeParser,&ThreadsafeParser::deleteLater); // no need
 // parserThread->quit();  //no need this in the destructor

I will update the la code i pasted.

@J-Hilk and @SGaist thank you