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qmutexpool.cpp

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#include "qatomic.h"
#include "qmutexpool_p.h"

#ifndef QT_NO_THREAD

// qt_global_mutexpool is here for backwards compatability only,
// use QMutexpool::instance() in new clode.
Q_CORE_EXPORT QMutexPool *qt_global_mutexpool = 0;
Q_GLOBAL_STATIC_WITH_ARGS(QMutexPool, globalMutexPool, (true))

/*!
    \class QMutexPool qmutexpool_p.h
    \brief The QMutexPool class provides a pool of QMutex objects.

    \internal

    \ingroup thread

    QMutexPool is a convenience class that provides access to a fixed
    number of QMutex objects.

    Typical use of a QMutexPool is in situations where it is not
    possible or feasible to use one QMutex for every protected object.
    The mutex pool will return a mutex based on the address of the
    object that needs protection.

    For example, consider this simple class:

    \code
    class Number {
    public:
        Number(double n) : num (n) { }

        void setNumber(double n) { num = n; }
        double number() const { return num; }

    private:
        double num;
    };
    \endcode

    Adding a QMutex member to the Number class does not make sense,
    because it is so small. However, in order to ensure that access to
    each Number is protected, you need to use a mutex. In this case, a
    QMutexPool would be ideal.

    Code to calculate the square of a number would then look something
    like this:

    \code
    void calcSquare(Number *num)
    {
        QMutexLocker locker(mutexpool.get(num));
        num.setNumber(num.number() * num.number());
    }
    \endcode

    This function will safely calculate the square of a number, since
    it uses a mutex from a QMutexPool. The mutex is locked and
    unlocked automatically by the QMutexLocker class. See the
    QMutexLocker documentation for more details.
*/

/*!
    Constructs  a QMutexPool, reserving space for \a size QMutexes. If
    \a recursive is true, all QMutexes in the pool will be recursive
    mutexes; otherwise they will all be non-recursive (the default).

    The QMutexes are created when needed, and deleted when the
    QMutexPool is destructed.
*/
QMutexPool::QMutexPool(bool recursive, int size)
    : count(size), recurs(recursive)
{
    mutexes = new QMutex*[count];
    for (int index = 0; index < count; ++index) {
        mutexes[index] = 0;
    }
}

/*!
    Destructs a QMutexPool. All QMutexes that were created by the pool
    are deleted.
*/
00128 QMutexPool::~QMutexPool()
{
    QMutexLocker locker(&mutex);
    for (int index = 0; index < count; ++index) {
        delete mutexes[index];
        mutexes[index] = 0;
    }
    delete [] mutexes;
    mutexes = 0;
}

/*!
    Returns the global QMutexPool instance.
*/
00142 QMutexPool *QMutexPool::instance()
{
    return globalMutexPool();
}

/*!
    Returns a QMutex from the pool. QMutexPool uses the value \a address
    to determine which mutex is returned from the pool.
*/
00151 QMutex *QMutexPool::get(const void *address)
{
    Q_ASSERT_X(address != 0, "QMutexPool::get()", "'address' argument cannot be zero");
    int index = int((ulong(address) >> (sizeof(address) >> 1)) % count);

    if (!mutexes[index]) {
        // mutex not created, create one

        QMutexLocker locker(&mutex);
        // we need to check once again that the mutex hasn't been created, since
        // 2 threads could be trying to create a mutex at the same index...
        if (!mutexes[index])
            mutexes[index] = new QMutex(recurs ? QMutex::Recursive : QMutex::NonRecursive);
    }

    return mutexes[index];
}

/*!
    Returns a QMutex from the global mutex pool.
*/
00172 QMutex *QMutexPool::globalInstanceGet(const void *address)
{
    QMutexPool * const globalInstance = globalMutexPool();
    if (globalInstance == 0)
        return 0;
    return globalInstance->get(address);
}

#endif // QT_NO_THREAD

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