src_corelib_thread_qmutex.cpp

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
 
int number = 6;
 
void method1()
{
    number *= 5;
    number /= 4;
}
 
void method2()
{
    number *= 3;
    number /= 2;
}
 
 
// method1()
number *= 5;        // number is now 30
number /= 4;        // number is now 7
 
// method2()
number *= 3;        // number is now 21
number /= 2;        // number is now 10
 
 
// Thread 1 calls method1()
number *= 5;        // number is now 30
 
// Thread 2 calls method2().
//
// Most likely Thread 1 has been put to sleep by the operating
// system to allow Thread 2 to run.
number *= 3;        // number is now 90
number /= 2;        // number is now 45
 
// Thread 1 finishes executing.
number /= 4;        // number is now 11, instead of 10
 
 
QMutex mutex;
int number = 6;
 
void method1()
{
    mutex.lock();
    number *= 5;
    number /= 4;
    mutex.unlock();
}
 
void method2()
{
    mutex.lock();
    number *= 3;
    number /= 2;
    mutex.unlock();
}
 
 
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;
}
 
 
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;
}
 
 
class SignalWaiter
{
private:
    QMutexLocker<QMutex> locker;
 
public:
    SignalWaiter(QMutex *mutex)
        : locker(mutex)
    {
    }
 
    void waitForSignal()
    {
        ...
        while (!signalled)
            waitCondition.wait(locker.mutex());
        ...
    }
};