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/****************************************************************************
**
** Copyright (C) 2012 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Digia. For licensing terms and
** conditions see http://qt.digia.com/licensing. For further information
** use the contact form at http://qt.digia.com/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Digia gives you certain additional
** rights. These rights are described in the Digia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 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 the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
#include "qsemaphore.h"
#ifndef QT_NO_THREAD
#include "qmutex.h"
#include "qwaitcondition.h"
#include "qelapsedtimer.h"
#include "qdatetime.h"
QT_BEGIN_NAMESPACE
/*!
\class QSemaphore
\brief The QSemaphore class provides a general counting semaphore.
\threadsafe
\ingroup thread
A semaphore is a generalization of a mutex. While a mutex can
only be locked once, it's possible to acquire a semaphore
multiple times. Semaphores are typically used to protect a
certain number of identical resources.
Semaphores support two fundamental operations, acquire() and
release():
\list
\o acquire(\e{n}) tries to acquire \e n resources. If there aren't
that many resources available, the call will block until this
is the case.
\o release(\e{n}) releases \e n resources.
\endlist
There's also a tryAcquire() function that returns immediately if
it cannot acquire the resources, and an available() function that
returns the number of available resources at any time.
Example:
\snippet doc/src/snippets/code/src_corelib_thread_qsemaphore.cpp 0
A typical application of semaphores is for controlling access to
a circular buffer shared by a producer thread and a consumer
thread. The \l{threads/semaphores}{Semaphores} example shows how
to use QSemaphore to solve that problem.
A non-computing example of a semaphore would be dining at a
restaurant. A semaphore is initialized with the number of chairs
in the restaurant. As people arrive, they want a seat. As seats
are filled, available() is decremented. As people leave, the
available() is incremented, allowing more people to enter. If a
party of 10 people want to be seated, but there are only 9 seats,
those 10 people will wait, but a party of 4 people would be
seated (taking the available seats to 5, making the party of 10
people wait longer).
\sa QMutex, QWaitCondition, QThread, {Semaphores Example}
*/
class QSemaphorePrivate {
public:
inline QSemaphorePrivate(int n) : avail(n) { }
QMutex mutex;
QWaitCondition cond;
int avail;
};
/*!
Creates a new semaphore and initializes the number of resources
it guards to \a n (by default, 0).
\sa release(), available()
*/
QSemaphore::QSemaphore(int n)
{
Q_ASSERT_X(n >= 0, "QSemaphore", "parameter 'n' must be non-negative");
d = new QSemaphorePrivate(n);
}
/*!
Destroys the semaphore.
\warning Destroying a semaphore that is in use may result in
undefined behavior.
*/
QSemaphore::~QSemaphore()
{ delete d; }
/*!
Tries to acquire \c n resources guarded by the semaphore. If \a n
> available(), this call will block until enough resources are
available.
\sa release(), available(), tryAcquire()
*/
void QSemaphore::acquire(int n)
{
Q_ASSERT_X(n >= 0, "QSemaphore::acquire", "parameter 'n' must be non-negative");
QMutexLocker locker(&d->mutex);
while (n > d->avail)
d->cond.wait(locker.mutex());
d->avail -= n;
}
/*!
Releases \a n resources guarded by the semaphore.
This function can be used to "create" resources as well. For
example:
\snippet doc/src/snippets/code/src_corelib_thread_qsemaphore.cpp 1
\sa acquire(), available()
*/
void QSemaphore::release(int n)
{
Q_ASSERT_X(n >= 0, "QSemaphore::release", "parameter 'n' must be non-negative");
QMutexLocker locker(&d->mutex);
d->avail += n;
d->cond.wakeAll();
}
/*!
Returns the number of resources currently available to the
semaphore. This number can never be negative.
\sa acquire(), release()
*/
int QSemaphore::available() const
{
QMutexLocker locker(&d->mutex);
return d->avail;
}
/*!
Tries to acquire \c n resources guarded by the semaphore and
returns true on success. If available() < \a n, this call
immediately returns false without acquiring any resources.
Example:
\snippet doc/src/snippets/code/src_corelib_thread_qsemaphore.cpp 2
\sa acquire()
*/
bool QSemaphore::tryAcquire(int n)
{
Q_ASSERT_X(n >= 0, "QSemaphore::tryAcquire", "parameter 'n' must be non-negative");
QMutexLocker locker(&d->mutex);
if (n > d->avail)
return false;
d->avail -= n;
return true;
}
/*!
Tries to acquire \c n resources guarded by the semaphore and
returns true on success. If available() < \a n, this call will
wait for at most \a timeout milliseconds for resources to become
available.
Note: Passing a negative number as the \a timeout is equivalent to
calling acquire(), i.e. this function will wait forever for
resources to become available if \a timeout is negative.
Example:
\snippet doc/src/snippets/code/src_corelib_thread_qsemaphore.cpp 3
\sa acquire()
*/
bool QSemaphore::tryAcquire(int n, int timeout)
{
Q_ASSERT_X(n >= 0, "QSemaphore::tryAcquire", "parameter 'n' must be non-negative");
QMutexLocker locker(&d->mutex);
if (timeout < 0) {
while (n > d->avail)
d->cond.wait(locker.mutex());
} else {
QElapsedTimer timer;
timer.start();
while (n > d->avail) {
const qint64 elapsed = timer.elapsed();
if (timeout - elapsed <= 0
|| !d->cond.wait(locker.mutex(), timeout - elapsed))
return false;
}
}
d->avail -= n;
return true;
}
QT_END_NAMESPACE
#endif // QT_NO_THREAD
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