QMutexLocker Class Reference

The QMutexLocker class is a convenience class that simplifies locking and unlocking mutexes. More...

#include <qmutex.h>

Public Functions
QMutex *	mutex () const
	Returns a pointer to the mutex that was locked in the constructor. More...
	QMutexLocker (QMutex *m)
	Constructs a QMutexLocker and locks mutex. More...
void	relock ()
	Relocks an unlocked mutex locker. More...
void	unlock ()
	Unlocks this mutex locker. More...
	~QMutexLocker ()
	Destroys the QMutexLocker and unlocks the mutex that was locked in the constructor. More...

Properties
quintptr	val

Detailed Description

The QMutexLocker class is a convenience class that simplifies locking and unlocking mutexes.

Note

This class or function is threadsafe.

Locking and unlocking a QMutex in complex functions and statements or in exception handling code is error-prone and difficult to debug. QMutexLocker can be used in such situations to ensure that the state of the mutex is always well-defined.

QMutexLocker should be created within a function where a QMutex needs to be locked. The mutex is locked when QMutexLocker is created. You can unlock and relock the mutex with unlock() and relock(). If locked, the mutex will be unlocked when the QMutexLocker is destroyed.

For example, this complex function locks a QMutex upon entering the function and unlocks the mutex at all the exit points:

int complexFunction(int flag)
{
    mutex.lock();

    int retVal = 0;

    switch (flag) {
    case 0:
    case 1:
        retVal = moreComplexFunction(flag);
        break;
    case 2:
        {
            int status = anotherFunction();
            if (status < 0) {
                mutex.unlock();
                return -2;
            }
            retVal = status + flag;
        }
        break;
    default:
        if (flag > 10) {
            mutex.unlock();
            return -1;
        }
        break;
    }

    mutex.unlock();
    return retVal;
}

This example function will get more complicated as it is developed, which increases the likelihood that errors will occur.

Using QMutexLocker greatly simplifies the code, and makes it more readable:

int complexFunction(int flag)
{
    QMutexLocker locker(&mutex);

    int retVal = 0;

    switch (flag) {
    case 0:
    case 1:
        return moreComplexFunction(flag);
    case 2:
        {
            int status = anotherFunction();
            if (status < 0)
                return -2;
            retVal = status + flag;
        }
        break;
    default:
        if (flag > 10)
            return -1;
        break;
    }

    return retVal;
}

Now, the mutex will always be unlocked when the QMutexLocker object is destroyed (when the function returns since locker is an auto variable).

The same principle applies to code that throws and catches exceptions. An exception that is not caught in the function that has locked the mutex has no way of unlocking the mutex before the exception is passed up the stack to the calling function.

QMutexLocker also provides a mutex() member function that returns the mutex on which the QMutexLocker is operating. This is useful for code that needs access to the mutex, such as QWaitCondition::wait(). For example:

class SignalWaiter
{
private:
    QMutexLocker locker;

public:
    SignalWaiter(QMutex *mutex)
        : locker(mutex)
    {
    }

    void waitForSignal()
    {
        ...
        while (!signalled)
            waitCondition.wait(locker.mutex());
        ...
    }
};

See also

QReadLocker, QWriteLocker, QMutex

Definition at line 101 of file qmutex.h.

Constructors and Destructors

QMutexLocker()

QMutexLocker::QMutexLocker ( QMutex *  mutex )

inlineexplicit

Constructs a QMutexLocker and locks mutex.

The mutex will be unlocked when the QMutexLocker is destroyed. If mutex is zero, QMutexLocker does nothing.

See also

QMutex::lock()

Definition at line 104 of file qmutex.h.

    {
        Q_ASSERT_X((reinterpret_cast<quintptr>(m) & quintptr(1u)) == quintptr(0),
                   "QMutexLocker", "QMutex pointer is misaligned");
        if (m) {
            m->lockInline();
            val = reinterpret_cast<quintptr>(m) | quintptr(1u);
        } else {
            val = 0;
        }
    }

~QMutexLocker()

QMutexLocker::~QMutexLocker ( )

inline

Destroys the QMutexLocker and unlocks the mutex that was locked in the constructor.

See also

QMutex::unlock()

Definition at line 115 of file qmutex.h.

{ unlock(); }

Functions

mutex()

QMutex * QMutexLocker::mutex ( ) const

inline

Returns a pointer to the mutex that was locked in the constructor.

Definition at line 140 of file qmutex.h.

Referenced by QSemaphore::acquire(), QThreadPrivate::finish(), QThreadPoolThread::run(), QThread::start(), QSemaphore::tryAcquire(), QThread::wait(), and QThreadPoolPrivate::waitForDone().

    {
        return reinterpret_cast<QMutex *>(val & ~quintptr(1u));
    }

relock()

void QMutexLocker::relock ( )

inline

Relocks an unlocked mutex locker.

See also

unlock()

Definition at line 125 of file qmutex.h.

Referenced by QMetaObject::activate(), QNetworkManagerEngine::activationFinished(), QNetworkManagerEngine::activeConnectionPropertiesChanged(), QIcdEngine::addConfiguration(), QConnmanEngine::addServiceConfiguration(), QIcdEngine::asyncUpdateConfigurationsSlot(), QConnmanEngine::configurationChange(), QIcdEngine::connectionStateSignalsSlot(), QNativeWifiEngine::connectToId(), QCoreWlanEngine::disconnectFromId(), QGenericEngine::doRequestUpdate(), QEventLoop::exec(), QThread::exec(), QThreadPrivate::finish(), QScanThread::foundNetwork(), QIcdEngine::icdServiceOwnerChanged(), initDefaultPaths(), QNetworkManagerEngine::initialize(), QIcdEngine::initialize(), QNetworkManagerEngine::interfacePropertiesChanged(), QCoreWlanEngine::networksChanged(), QNlaEngine::networksChanged(), QDeclarativePixmapReader::processJobs(), QDeclarativeXmlQueryEngine::processJobs(), QNetworkManagerEngine::removeAccessPoint(), QConnmanEngine::removeConfiguration(), QWindowsFileSystemWatcherEngine::removePaths(), QIcdEngine::requestUpdate(), QThreadPoolPrivate::reset(), QThreadPoolThread::run(), QWindowsFileSystemWatcherEngineThread::run(), QNativeWifiEngine::scanComplete(), QCoreApplicationPrivate::sendPostedEvents(), QThreadPoolPrivate::startFrontRunnable(), and QThread::~QThread().

    {
        if (val) {
            if ((val & quintptr(1u)) == quintptr(0u)) {
                mutex()->lockInline();
                val |= quintptr(1u);
            }
        }
    }

unlock()

void QMutexLocker::unlock ( )

inline

Unlocks this mutex locker.

You can use relock() to lock it again. It does not need to be locked when destroyed.

See also

relock()

Definition at line 117 of file qmutex.h.

Referenced by QMetaObject::activate(), QNetworkManagerEngine::activationFinished(), QNetworkManagerEngine::activeConnectionPropertiesChanged(), QIcdEngine::addConfiguration(), QCoreApplication::addLibraryPath(), QConnmanEngine::addServiceConfiguration(), QIcdEngine::addSessionConfiguration(), QIcdEngine::asyncUpdateConfigurationsSlot(), QTextCodec::availableCodecs(), QTextCodec::availableMibs(), QConnmanEngine::configurationChange(), QIcdEngine::connectionStateSignalsSlot(), QCoreWlanEngine::connectToId(), QNativeWifiEngine::connectToId(), QIcdEngine::deleteConfiguration(), QUrl::detach(), QCoreWlanEngine::disconnectFromId(), QNativeWifiEngine::disconnectFromId(), QGenericEngine::doRequestUpdate(), QBBEngine::doRequestUpdate(), QCoreWlanEngine::doRequestUpdate(), QIcdEngine::doRequestUpdate(), QDeclarativeListModelWorkerAgent::event(), QEventLoop::exec(), QThread::exec(), QThreadPrivate::finish(), QScanThread::foundNetwork(), QIcdEngine::getIcdInitialState(), QDeclarativeEnginePrivate::getImageFromProvider(), QDeclarativeEnginePrivate::getImageProviderType(), QDeclarativeEnginePrivate::getPixmapFromProvider(), QIcdEngine::icdServiceOwnerChanged(), initDefaultPaths(), QNetworkManagerEngine::initialize(), QIcdEngine::initialize(), QNetworkManagerEngine::interfacePropertiesChanged(), QUrl::isParentOf(), QCoreWlanEngine::networksChanged(), QNlaEngine::networksChanged(), QNetworkManagerEngine::newAccessPoint(), QNetworkManagerEngine::newConnection(), QDBusConnectionPrivate::processFinishedCall(), QDeclarativePixmapReader::processJobs(), QDeclarativeXmlQueryEngine::processJobs(), QNetworkManagerEngine::removeAccessPoint(), QBBEngine::removeConfiguration(), QConnmanEngine::removeConfiguration(), QNetworkManagerEngine::removeConnection(), QWindowsFileSystemWatcherEngine::removePaths(), QPollingFileSystemWatcherEngine::removePaths(), QCoreApplication::removePostedEvents(), QNativeWifiEngine::requestUpdate(), QIcdEngine::requestUpdate(), QThreadPoolPrivate::reset(), QThreadPoolThread::run(), QWindowsFileSystemWatcherEngineThread::run(), QFileInfoGatherer::run(), QNativeWifiEngine::scanComplete(), QCoreApplicationPrivate::sendPostedEvents(), QCoreApplication::setLibraryPaths(), QThreadPoolPrivate::startFrontRunnable(), QNetworkSessionPrivateImpl::syncStateWithInterface(), QNetworkManagerEngine::updateAccessPoint(), QBBEngine::updateConfiguration(), QNetworkConfigurationManagerPrivate::updateConfigurations(), QNetworkManagerEngine::updateConnection(), QFileInfoGatherer::~QFileInfoGatherer(), and QThread::~QThread().

    {
        if ((val & quintptr(1u)) == quintptr(1u)) {
            val &= ~quintptr(1u);
            mutex()->unlockInline();
        }
    }

Properties

val

quintptr QMutexLocker::val

private

Definition at line 152 of file qmutex.h.

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The documentation for this class was generated from the following files:

• /src/corelib/thread/qmutex.h
• /src/corelib/thread/qmutex.cpp