/******************************************************************************** Copyright (C) 1992-2006 Trolltech ASA. All rights reserved.**** This file is part of the QtCore module of the Qt Toolkit.**** This file may be used under the terms of the GNU General Public** License version 2.0 as published by the Free Software Foundation** and appearing in the file LICENSE.GPL included in the packaging of** this file.  Please review the following information to ensure GNU** General Public Licensing requirements will be met:** http://www.trolltech.com/products/qt/opensource.html**** If you are unsure which license is appropriate for your use, please** review the following information:** http://www.trolltech.com/products/qt/licensing.html or contact the** sales department at sales@trolltech.com.**** This file is provided AS IS with NO WARRANTY OF ANY KIND, INCLUDING THE** WARRANTY OF DESIGN, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.******************************************************************************/#include "qplatformdefs.h"#include "qreadwritelock.h"#ifndef QT_NO_THREAD#include "qmutex.h"#include "qwaitcondition.h"struct QReadWriteLockPrivate{    QReadWriteLockPrivate()    : accessCount(0), waitingReaders(0), waitingWriters(0) {}        QMutex mutex;    QWaitCondition readerWait;    QWaitCondition writerWait;       int accessCount;    int waitingReaders;    int waitingWriters;};/*! \class QReadWriteLock    \brief The QReadWriteLock class provides read-write locking.    \threadsafe    \ingroup thread    \ingroup environment    A read-write lock is a synchronization tool for protecting    resources that can be accessed for reading and writing. This type    of lock is useful if you want to allow multiple threads to have    simultaneous read-only access, but as soon as one thread wants to    write to the resource, all other threads must be blocked until    the writing is complete.    In many cases, QReadWriteLock is a direct competitor to QMutex.    QReadWriteLock is a good choice if there are many concurrent    reads and writing occurs infrequently.    Example:    \code        QReadWriteLock lock;        void ReaderThread::run()        {            ...            lock.lockForRead();            read_file();            lock.unlock();            ...        }        void WriterThread::run()        {            ...            lock.lockForWrite();            write_file();            lock.unlock();            ...        }    \endcode    To ensure that writers aren't blocked forever by readers, readers    attempting to obtain a lock will not succeed if there is a blocked    writer waiting for access, even if the lock is currently only    accessed by other readers. Also, if the lock is accessed by a    writer and another writer comes in, that writer will have    priority over any readers that might also be waiting.    \sa QReadLocker, QWriteLocker, QMutex, QSemaphore*//*!    Constructs a QReadWriteLock object.    \sa lockForRead(), lockForWrite()*/QReadWriteLock::QReadWriteLock()    :d(new QReadWriteLockPrivate()){}/*!    Destroys the QReadWriteLock object.    \warning Destroying a read-write lock that is in use may result    in undefined behavior.*/QReadWriteLock::~QReadWriteLock(){    delete d;}/*!    Locks the lock for reading. This function will block the current    thread if another thread has locked for writing.    \sa unlock() lockForWrite() tryLockForRead()*/void QReadWriteLock::lockForRead(){    QMutexLocker lock(&d->mutex);    while (d->accessCount < 0 || d->waitingWriters) {        ++d->waitingReaders;        d->readerWait.wait(&d->mutex);        --d->waitingReaders;     }    ++d->accessCount;    Q_ASSERT_X(d->accessCount > 0, "QReadWriteLock::lockForRead()", "Overflow in lock counter");}/*!    Attempts to lock for reading. If the lock was obtained, this    function returns true, otherwise it returns false instead of    waiting for the lock to become available, i.e. it does not block.    The lock attempt will fail if another thread has locked for    writing.    If the lock was obtained, the lock must be unlocked with unlock()    before another thread can successfully lock it.    \sa unlock() lockForRead()*/bool QReadWriteLock::tryLockForRead(){   QMutexLocker lock(&d->mutex);        if (d->accessCount < 0)        return false;        ++d->accessCount;    Q_ASSERT_X(d->accessCount > 0, "QReadWriteLock::lockForRead()", "Overflow in lock counter");        return true;} /*!    Locks the lock for writing. This function will block the current    thread if any thread has locked for reading or writing.    \sa unlock() lockForRead() tryLockForWrite() */void QReadWriteLock::lockForWrite(){    QMutexLocker lock(&d->mutex);    while (d->accessCount != 0) {        ++d->waitingWriters;        d->writerWait.wait(&d->mutex);        --d->waitingWriters;    }    d->accessCount = -1;}/*!    Attempts to lock for writing. If the lock was obtained, this    function returns true; otherwise, it returns false immediately.    The lock attempt will fail if any thread has locked for reading or    writing.    If the lock was obtained, the lock must be unlocked with unlock()    before another thread can successfully lock it.    \sa unlock() lockForWrite()*/bool QReadWriteLock::tryLockForWrite(){    QMutexLocker lock(&d->mutex);        if (d->accessCount != 0)        return false;        d->accessCount = -1;    return true;}/*!    Unlocks the lock.    Attempting to unlock a lock that is not locked is an error, and will result    in program termination.    \sa lockForRead() lockForWrite() tryLockForRead() tryLockForWrite()*/void QReadWriteLock::unlock(){    QMutexLocker lock(&d->mutex);        Q_ASSERT_X(d->accessCount != 0, "QReadWriteLock::unlock()", "Cannot unlock an unlocked lock");      if ((d->accessCount > 0 && --d->accessCount == 0) || (d->accessCount == -1 && ++d->accessCount == 0)) {        if (d->waitingWriters) {            d->writerWait.wakeOne();        } else if (d->waitingReaders) {            d->readerWait.wakeAll();        }   }      }/*!    \class QReadLocker    \brief The QReadLocker class is a convenience class that    simplifies locking and unlocking read-write locks for read access.    \threadsafe    \ingroup thread    \ingroup environment    The purpose of QReadLocker (and QWriteLocker) is to simplify    QReadWriteLock locking and unlocking. Locking and unlocking    statements or in exception handling code is error-prone and    difficult to debug. QReadLocker can be used in such situations    to ensure that the state of the lock is always well-defined.    Here's an example that uses QReadLocker to lock and unlock a    read-write lock for reading:    \code        QReadWriteLock lock;        QByteArray readData()        {            QReadLocker locker(&lock);            ...            return data;        }    \endcode    It is equivalent to the following code:    \code        QReadWriteLock lock;        QByteArray readData()        {            locker.lockForRead();            ...            locker.unlock();            return data;        }    \endcode    The QMutexLocker documentation shows examples where the use of a    locker object greatly simplifies programming.    \sa QWriteLocker, QReadWriteLock*//*!    \fn QReadLocker::QReadLocker(QReadWriteLock *lock)    Constructs a QReadLocker and locks \a lock for reading. The lock    will be unlocked when the QReadLocker is destroyed. If \c lock is    zero, QReadLocker does nothing.    \sa QReadWriteLock::lockForRead()*//*!    \fn QReadLocker::~QReadLocker()    Destroys the QReadLocker and unlocks the lock that was passed to    the constructor.    \sa QReadWriteLock::unlock()*//*!    \fn void QReadLocker::unlock()    Unlocks the lock associated with this locker.    \sa QReadWriteLock::unlock()*//*!    \fn void QReadLocker::relock()    Relocks an unlocked lock.    \sa unlock()*//*!    \fn QReadWriteLock *QReadLocker::readWriteLock() const    Returns a pointer to the read-write lock that was passed    to the constructor.*//*!    \class QWriteLocker    \brief The QWriteLocker class is a convenience class that    simplifies locking and unlocking read-write locks for write access.    \threadsafe    \ingroup thread    \ingroup environment    The purpose of QWriteLocker (and QReadLocker is to simplify    QReadWriteLock locking and unlocking. Locking and unlocking    statements or in exception handling code is error-prone and    difficult to debug. QWriteLocker can be used in such situations    to ensure that the state of the lock is always well-defined.    Here's an example that uses QWriteLocker to lock and unlock a    read-write lock for writing:    \code        QReadWriteLock lock;        void writeData(const QByteArray &data)        {            QWriteLocker locker(&lock);            ...        }    \endcode    It is equivalent to the following code:    \code        QReadWriteLock lock;        void writeData(const QByteArray &data)        {            locker.lockForWrite();            ...            locker.unlock();        }    \endcode    The QMutexLocker documentation shows examples where the use of a    locker object greatly simplifies programming.    \sa QReadLocker, QReadWriteLock*//*!    \fn QWriteLocker::QWriteLocker(QReadWriteLock *lock)    Constructs a QWriteLocker and locks \a lock for writing. The lock    will be unlocked when the QWriteLocker is destroyed. If \c lock is    zero, QWriteLocker does nothing.    \sa QReadWriteLock::lockForWrite()*//*!    \fn QWriteLocker::~QWriteLocker()    Destroys the QWriteLocker and unlocks the lock that was passed to    the constructor.    \sa QReadWriteLock::unlock()*//*!    \fn void QWriteLocker::unlock()    Unlocks the lock associated with this locker.    \sa QReadWriteLock::unlock()*//*!    \fn void QWriteLocker::relock()    Relocks an unlocked lock.    \sa unlock()*//*!    \fn QReadWriteLock *QWriteLocker::readWriteLock() const    Returns a pointer to the read-write lock that was passed    to the constructor.*/#endif // QT_NO_THREAD