QMutexLock 自动解锁机制

QMutexLock 自动解锁机制

QMutexLocker 是一个便利类，它可以自动对QMutex加锁与解锁。因为QMutexLocker 申请的这个lock变量在这个函数退出时，自动的调用析构函数来解锁。这样可以防止在程序编写的过程中，不同的地方有多个return的情况，在发生return的时候，没有解锁，导致程序死锁。

QMutex类提供线程间的访问序列化。
QMutex的目的是保护一个对象、数据结构或代码片段，
这样每次只有一个线程可以访问它(这类似于Java synchronized关键字)。
通常最好将互斥对象与QMutexLocker一起使用，因为这样可以很容易地确保一致地执行锁定和解锁。

（2）QMutexLocker上锁，解锁的原理：在该局部变量被创建的时候上锁，当所在函数运行完毕后该QMutexLocker局部变量在栈中销毁掉，根据他自己的机制也就相对应的解锁了。注意，如果该局部变量在中间被打断，那么QMutexLocker上的锁就不会被解锁掉，因为该函数没有被完整的是执行完。QMutexLocker所创建的局部变量也没有被正确销毁销毁，可能就和QMutexLocker他自己本身的机制不服也就不会解锁。

1

http://blog.debao.me/2013/08/how-to-use-qthread-in-the-right-way-part-1/

A short history
Long long ago, subclass QThread and reimplement its run() function is the only recommended way of using QThread. This is rather intuitive and easy to used. But when SLOTS and Qt event loop are used in the worker thread, some users do it wrong. So Bradley T. Hughes, one of the Qt core developers, recommend that use worker objects by moving them to the thread using QObject::moveToThread . Unfortunately, some users went on a crusade against the former usage. So Olivier Goffart, one of the former Qt core developers, tell the subclass users: You were not doing so wrong. Finally, we can find both usages in the documentation of QThread.

QThread::run() is the thread entry point
From the Qt Documentation, we can see that

A QThread instance represents a thread and provides the means to start() a thread, which will then execute the reimplementation of QThread::run(). The run() implementation is for a thread what the main() entry point is for the application.

As QThread::run() is the thread entry point, it is rather intuitive to use the Usage 1.

Usage 1-0
To run some code in a new thread, subclass QThread and reimplement its run() function.

For example

#include

class Thread : public QThread
{
private:
void run()
{
qDebug()<<"From worker thread: "<<currentThreadId();
}
};

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
qDebug()<<"From main thread: "<<QThread::currentThreadId();

Thread t;
QObject::connect(&t, SIGNAL(finished()), &a, SLOT(quit()));

t.start();
return a.exec();

}
The output more or less look like:

From main thread: 0x15a8
From worker thread: 0x128c
Usage 1-1
As QThread::run() is the thread entry point, so it easy to undersand that, all the codes that are not get called in the run() function directly won’t be executed in the worker thread.

In the following example, the member variable m_stop will be accessed by both stop() and run(). Consider that the former will be executed in main thread while the latter is executed in worker thread, mutex or other facility is needed.

#if QT_VERSION>=0x050000
#include
#else
#include
#endif

class Thread : public QThread
{
Q_OBJECT

public:
Thread():m_stop(false)
{}

public slots:
void stop()
{
qDebug()<<"Thread::stop called from main thread: "<<currentThreadId();
QMutexLocker locker(&m_mutex);
m_stop=true;
}

private:
QMutex m_mutex;
bool m_stop;

void run()
{
    qDebug()<<"From worker thread: "<<currentThreadId();
    while (1) {
        {
        QMutexLocker locker(&m_mutex);
        if (m_stop) break;
        }
        msleep(10);
    }
}

};

#include “main.moc”
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
qDebug()<<"From main thread: "<<QThread::currentThreadId();
QPushButton btn(“Stop Thread”);
Thread t;

QObject::connect(&btn, SIGNAL(clicked()), &t, SLOT(stop()));
QObject::connect(&t, SIGNAL(finished()), &a, SLOT(quit()));

t.start();
btn.show();
return a.exec();

}
The output is more or less like

From main thread: 0x13a8
From worker thread: 0xab8
Thread::stop called from main thread: 0x13a8
You can see that the Thread::stop() is executed in the main thread.

Usage 1-2 (Wrong Usage)
Though above examples are easy to understand, but it’s not so intuitive when event system(or queued-connection) is introduced in worker thread.

For example, what should we do if we want to do something periodly in the worker thread?

Create a QTimer in the Thread::run()
Connect the timeout signal to the slot of Thread
#include

class Thread : public QThread
{
Q_OBJECT
private slots:
void onTimeout()
{
qDebug()<<"Thread::onTimeout get called from? : "<<QThread::currentThreadId();
}

private:
void run()
{
qDebug()<<"From worker thread: "<<currentThreadId();
QTimer timer;
connect(&timer, SIGNAL(timeout()), this, SLOT(onTimeout()));
timer.start(1000);

    exec();
}

};

#include “main.moc”

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
qDebug()<<"From main thread: "<<QThread::currentThreadId();

Thread t;
t.start();

return a.exec();

}
At first glance, the code seems fine. When the thread starts executing, we setup a QTimer thats going to run in the current thread’s event queue. We connect the onTimeout() to the timeout signal. Then we except it works in the worker thread?

But, the result of the example is

From main thread: 0x13a4
From worker thread: 0x1330
Thread::onTimeout get called from?: 0x13a4
Thread::onTimeout get called from?: 0x13a4
Thread::onTimeout get called from?: 0x13a4
Oh, No!!! They get called in the main thread instead of the work thread.

Very interesting, isn’t it? (We will discuss what happened behined this in next blog)

How to solve this problem
In order to make the this SLOT works in the worker thread, some one pass the Qt::DirectConnection to the connect() function,

    connect(&timer, SIGNAL(timeout()), this, SLOT(onTimeout()), Qt::DirectConnection);

and some other add following line to the thread constructor.

    moveToThread(this)

Both of them work as expected. But …

The second usage is wrong,

Even though this seems to work, it’s confusing, and not how QThread was designed to be used(all of the functions in QThread were written and intended to be called from the creating thread, not the thread that QThread starts)

In fact, according to above statements, the first workaround is wrong too. As onTimeout() which is a member of our Thread object, get called from the creating thread too.

Both of them are bad uasge?! what should we do?

Usage 1-3
As none of the member of QThread object are designed to be called from the worker thread. So we must create an independent worker object if we want to use SLOTS.

#include

class Worker : public QObject
{
Q_OBJECT
private slots:
void onTimeout()
{
qDebug()<<"Worker::onTimeout get called from?: "<<QThread::currentThreadId();
}
};

class Thread : public QThread
{
Q_OBJECT

private:
void run()
{
qDebug()<<"From work thread: "<<currentThreadId();
QTimer timer;
Worker worker;
connect(&timer, SIGNAL(timeout()), &worker, SLOT(onTimeout()));
timer.start(1000);

    exec();
}

};

#include “main.moc”

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
qDebug()<<"From main thread: "<<QThread::currentThreadId();

Thread t;
t.start();

return a.exec();

}
The result of the application is

From main thread: 0x810
From work thread: 0xfac
Worker::onTimeout get called from?: 0xfac
Worker::onTimeout get called from?: 0xfac
Worker::onTimeout get called from?: 0xfac
Problem solved now!

Though this works perfect, but you may have notice that, when event loop QThread::exec() is used in the worker thread, the code in the QThread::run() seems has nothing to do with QThread itself.

So can we move the object creation out of the QThread::run(), and at the same time, the slots of they will still be called by the QThread::run()?

Usage 2-0
If we only want to make use of QThread::exec(), which has been called by QThread::run() by default, there will be no need to subclass the QThread any more.

Create a Worker object
Do signal and slot connections
Move the Worker object to a sub-thread
Start thread
#include

class Worker : public QObject
{
Q_OBJECT
private slots:
void onTimeout()
{
qDebug()<<"Worker::onTimeout get called from?: "<<QThread::currentThreadId();
}
};

#include “main.moc”

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
qDebug()<<"From main thread: "<<QThread::currentThreadId();

QThread t;
QTimer timer;
Worker worker;

QObject::connect(&timer, SIGNAL(timeout()), &worker, SLOT(onTimeout()));
timer.start(1000);

timer.moveToThread(&t);
worker.moveToThread(&t);

t.start();

return a.exec();

}
The result is:

From main thread: 0x1310
Worker::onTimeout get called from?: 0x121c
Worker::onTimeout get called from?: 0x121c
Worker::onTimeout get called from?: 0x121c
As expected, the slot doesn’t run in the main thread.

In this example, both of the QTimer and Worker are moved to the sub-thread. In fact, moving QTimer to sub-thread is not required.

Usage 2-1
Simply remove the line timer.moveToThread(&t); from above example will work as expected too.

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
qDebug()<<"From main thread: "<<QThread::currentThreadId();

QThread t;
QTimer timer;
Worker worker;

QObject::connect(&timer, SIGNAL(timeout()), &worker, SLOT(onTimeout()));
timer.start(1000);

// timer.moveToThread(&t);
worker.moveToThread(&t);

t.start();

return a.exec();

}
The difference is that:

In last example,

The signal timeout() is emitted from sub-thread
As timer and worker live in the same thread, their connection type is direct connection.
The slot get called in the same thead in which signal get emitted.
While in this example,

The signal timeout() emitted from main thread,
As timer and worker live in different threads, their connection type is queued connection.
The slot get called in its living thread, which is the sub-thread.
Thanks to a mechanism called queued connections, it is safe to connect signals and slots across different threads. If all the across threads communication are done though queued connections, the usual multithreading precautions such as QMutex will no longer need to be taken.

In short
Subclass QThread and reimplement its run() function is intuitive and there are still many perfectly valid reasons to subclass QThread, but when event loop is used in worker thread, it’s not easy to do it in the right way.
Use worker objects by moving them to the thread is easy to use when event loop exists, as it has hidden the details of event loop and queued connection.
Reference
http://blog.qt.digia.com/blog/2010/06/17/youre-doing-it-wrong/
http://woboq.com/blog/qthread-you-were-not-doing-so-wrong.html
http://ilearnstuff.blogspot.com/2012/08/when-qthread-isnt-thread.html