main and background thread example

[user4592357]

i wanna write a basic thread example. here's the idea:
i have the main event loop. before calling app.exec(), i create an object which itself creates an object and puts it in a separate thread for running. i need some way to tell the background thread that main thread has finished (to join()), so i have a bool to indicate that.

however, when the application exits, something goes wrong. i'm running it using visual studio and the debugging session won't stop. i don't understand what's wrong. here's the code:

#include <QApplication>
#include <QObject>
#include <QThread>
#include <iostream>
#include <thread>
#include <QtWidgets/QLabel>
#include <atomic>

std::atomic<bool> bMainThreadRunning { true };

class ThreadObj {
public:
	void work() {
		while(bMainThreadRunning) {
			++counter;
			std::this_thread::sleep_for(std::chrono::seconds(5));
		}
	}
private:
	int counter { 0 };
};

class Obj {
public:
	void start() {
		ThreadObj obj;
		m_thread = std::thread(&ThreadObj::work, &obj);
	}
	void onMainThreadFinished() {
		bMainThreadRunning = true;
		m_thread.join();
	}
private:
	std::thread m_thread;
};

int main(int argc, char *argv[])
{
	QApplication a(argc, argv);
	Obj o;
	o.start();

	QWidget w;
	w.show();

	auto ret = a.exec();
	o.onMainThreadFinished();

	return 0;
}

[SGaist]

Hi,

@user4592357 said in main and background thread example:

void onMainThreadFinished() {
bMainThreadRunning = true;
m_thread.join();
}

Shouldn't that be bMainThreadRunning = false; ?

[user4592357]

my bad, that's right, i tried to implement what's in my actual project in a snippet and that's what i got automatically.
but anyways that's not the result i expect. what i expect is, when the main thread finishes i need the background thread to be notified and finish immediately, but with this code it was for the interval to exit.
actually in my actual project i implemented that scenario, it's basically the same code but then i had to change the places where all of these objects are created and this is the result i get.

[SGaist]

Well, if you thread just started to sleep, you'll have to wait for your 5 seconds before it ends properly.

Do you really have that kind of loop in your application ? It could be reworked to use e.g. a QTimer in a worker object.

[user4592357]

i'll try using QTimer instead of this_thread::sleep_for(). but it still blows my mind how my previous implementation worked the way i wanted with this code.

and is everything else okay with this code (i.e. the use of atomic bool etc.)?

[user4592357]

i can't figure out how to do that. this is what i have right now:

#include <QtWidgets/QMainWindow>
#include <QApplication>
#include <QObject>
#include <QThread>
#include <iostream>
#include <thread>
#include <QtWidgets/QLabel>
#include <atomic>
#include <QTimer>

std::atomic<bool> bMainThreadRunning { true };

class ThreadObj : public QObject {
public:
	ThreadObj() : counter(0) {}
	void work() {
		auto timer = new QTimer;
		//timer.setInterval(3000);
		connect(timer, &QTimer::timeout, this, &ThreadObj::inc);
		timer->start(3000);
	}
	void inc() { 
		while(bMainThreadRunning)
			++counter;
	}
private:
	int counter { 0 };
};

class Obj {
public:
	void start() {
		ThreadObj obj;
		m_thread = std::thread(&ThreadObj::work, &obj);
	}
	void onMainThreadFinished() {
		bMainThreadRunning = false;
		m_thread.join();
	}
private:
	std::thread m_thread;
};

int main(int argc, char *argv[]) {
	QApplication a(argc, argv);
	Obj o;
	o.start();

	QWidget w;
	w.show();

	auto ret = a.exec();
	o.onMainThreadFinished();

	return 0;
}

[SGaist]

You forgot you used a std::thread. Why not use QThread since you are using Qt and a worker object ?

[user4592357]

i did that. what i need is, when in background thread something weird happens, i tell the main thread to show a message box. then if at some time the "good" state of background thread is restored, main thread closes the message box.

here's what i have now. actually the application works. but at some points it crashes. looking at the crash log i can say that the reason is the background thread but i don't see how my implementation is wrong. i appended the crash log after code.

class BgThread : public QThread {
	Q_OBJECT
public:
	explicit BgThread (const int &nSeconds, QObject *parent = nullptr) : QThread(parent), interval(1000 * nSeconds) /* to milliseconds */  {}

	void onStopBgThread() { running = false; }

signals:
	void somethingWentWrong();
        void restoreGoodState();

private slots:
	void performWork() {
             if(/* something went wrong */) {
		emit somethingWentWrong();
		good_state = false;
	     } else if(!good_state) { // means going from bad state to good state
		emit restoreGoodState();
		good_state = true;
	     }
        }

private:
	void run() override {
            QTimer timer;
	    connect(&timer, SIGNAL(timeout()), this, SLOT(performWork()), Qt::DirectConnection);
	    timer.start(interval);

            exec();
            quit();
            wait();
        }

private:
	int interval;
        bool running { true };
        bool good_state { true };
};

class MainApp {
public:
    void start_bg_thread(const int seconds) {
       	thread = new BgThread { seconds, this };
        connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
	connect(thread, SIGNAL(somethingWentWrong()), SLOT(onSomethingWentWrong()));
        connect(thread, SIGNAL(restoreGoodState()), SLOT(onRestoreGoodState()));
	thread->start();
    }

    void stop_bg_thread() {
	thread->onStopBgThread();
    }

public slots:
    void onSomethingWentWrong() {
	if(!msgBox) {
		msgBox= new QMessageBox(window());
		msgBox->setIcon(QMessageBox::Warning);
		msgBox->setWindowTitle("window title");
		msgBox->setText("message box text");
		const auto pExitBtn = msgBox->addButton(tr("Exit"), QMessageBox::AcceptRole);
		connect(pExitBtn, SIGNAL(clicked()), qApp, SLOT(quit()));
	}
	msgBox->exec();	
    }

    void onRestoreGoodState() {
	msgBox->close();
    }
private:
    BgThread *thread { nullptr };
    QMessageBox *msgBox;
};

this is basically it. and somewhere in the app init process i call

MainApp app;
app.start_bg_thread(5);

the crash log is something like this (stacktrace):

do_system () from /lib64/libc.so.6
system () from /lib64/libc.so.6
// call signal handler
...
pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
QThread::msleep(unsigned long) ()
start_thread () from /lib64/libpthread.so.0
clone () from /lib64/libc.so.6
waitpid () from /lib64/libc.so.6
do_system () from /lib64/libc.so.6
system () from /lib64/libc.so.6
// call signal handler
BgThread::onStopBgThread ()
MainApp::qt_metacall(QMetaObject::Call, int, void**) ()
QMetaObject::activate(QObject*, QMetaObject const*, int, void**) ()
QCoreApplication::exec()
main ()

[SGaist]

Did you check the memory used by you application ?

[user4592357]

@SGaist

what do you mean by check? and no, how should i?

[SGaist]

Using the top command for example. Valgrind etc.

[user4592357]

@SGaist

okay, thanks.
one last question, as you can see, on timer's timeout i execute a function. is it possible stop the timer (so the function won't be executed), and then restart it again?

[SGaist]

Yes:
QTimer::stop
QTimer::start

[user4592357]

@SGaist

hi again,
i ran valgrind and it says there's a memory leak on exec() which is in run() overridden method. so what's wrong?

[VRonin]

@user4592357 said in main and background thread example:

ThreadObj obj;
m_thread = std::thread(&ThreadObj::work, &obj);

obj is allocated on the stack, it will go out of scope and delete itself

[user4592357]

@VRonin

hi, thanks for the reply.
the code has been modified since the first post (see my last code-post), so it's not the problem

[VRonin]

https://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/

[user4592357]

@VRonin

i've read similar articles, saying "you shouldn't subclass QThread". in my case i need to do that.

[SGaist]

Out of curiosity, what are you doing that requires to subclass QThread ?

[user4592357]

@SGaist

nothing requires it. it's just that the whole implementation is done and i don't wanna change everything