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qsemaphore.cpp

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** This file is part of the QtCore module of the Qt Toolkit.
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#include "qsemaphore.h"

#ifndef QT_NO_THREAD
#include "qmutex.h"
#include "qwaitcondition.h"

/*!
    \class QSemaphore
    \brief The QSemaphore class provides a general counting semaphore.

    \threadsafe

    \ingroup thread
    \ingroup environment

    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:

    \code
        QSemaphore sem(5);      // sem.available() == 5

        sem.acquire(3);         // sem.available() == 2
        sem.acquire(2);         // sem.available() == 0
        sem.release(5);         // sem.available() == 5
        sem.release(5);         // sem.available() == 10

        sem.tryAcquire(1);      // sem.available() == 9, returns true
        sem.tryAcquire(250);    // sem.available() == 9, returns false
    \endcode

    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()
*/
00106 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.
*/
00118 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()
*/
00128 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:

    \code
        QSemaphore sem(5);      // a semaphore that guards 5 resources
        sem.acquire(5);         // acquire all 5 resources
        sem.release(5);         // release the 5 resources
        sem.release(10);        // "create" 10 new resources
    \endcode

    \sa acquire(), available()
*/
00152 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()
*/
00166 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:

    \code
        QSemaphore sem(5);      // sem.available() == 5
        sem.tryAcquire(250);    // sem.available() == 5, returns false
        sem.tryAcquire(3);      // sem.available() == 2, returns true
    \endcode

    \sa acquire()
*/
00187 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;
}
#endif // QT_NO_THREAD

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