/**************************************************************************** ** ** Copyright (C) 1992-2007 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://trolltech.com/products/qt/licenses/licensing/opensource/ ** ** If you are unsure which license is appropriate for your use, please ** review the following information: ** http://trolltech.com/products/qt/licenses/licensing/licensingoverview ** or contact the sales department at sales@trolltech.com. ** ** In addition, as a special exception, Trolltech gives you certain ** additional rights. These rights are described in the Trolltech GPL ** Exception version 1.0, which can be found at ** http://www.trolltech.com/products/qt/gplexception/ and in the file ** GPL_EXCEPTION.txt in this package. ** ** In addition, as a special exception, Trolltech, as the sole copyright ** holder for Qt Designer, grants users of the Qt/Eclipse Integration ** plug-in the right for the Qt/Eclipse Integration to link to ** functionality provided by Qt Designer and its related libraries. ** ** Trolltech reserves all rights not expressly granted herein. ** ** Trolltech ASA (c) 2007 ** ** 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 "qwaitcondition.h" #include "qmutex.h" #include "qatomic.h" #include "qmutex_p.h" #include "qstring.h" #include #ifndef QT_NO_THREAD static void report_error(int code, const char *where, const char *what) { if (code != 0) qWarning("%s: %s failure: %s", where, what, qPrintable(qt_error_string(code))); } struct QWaitConditionPrivate { pthread_mutex_t mutex; pthread_cond_t cond; int waiters; int wakeups; }; /*! \class QWaitCondition \brief The QWaitCondition class provides a condition variable for synchronizing threads. \threadsafe \ingroup thread \ingroup environment QWaitCondition allows a thread to tell other threads that some sort of condition has been met. One or many threads can block waiting for a QWaitCondition to set a condition with wakeOne() or wakeAll(). Use wakeOne() to wake one randomly selected condition or wakeAll() to wake them all. For example, let's suppose that we have three tasks that should be performed whenever the user presses a key. Each task could be split into a thread, each of which would have a \l{QThread::run()}{run()} body like this: \code forever { mutex.lock(); keyPressed.wait(&mutex); do_something(); mutex.unlock(); } \endcode Here, the \c keyPressed variable is a global variable of type QWaitCondition. A fourth thread would read key presses and wake the other three threads up every time it receives one, like this: \code forever { getchar(); keyPressed.wakeAll(); } \endcode The order in which the three threads are woken up is undefined. Also, if some of the threads are still in \c do_something() when the key is pressed, they won't be woken up (since they're not waiting on the condition variable) and so the task will not be performed for that key press. This issue can be solved using a counter and a QMutex to guard it. For example, here's the new code for the worker threads: \code forever { mutex.lock(); keyPressed.wait(&mutex); ++count; mutex.unlock(); do_something(); mutex.lock(); --count; mutex.unlock(); } \endcode Here's the code for the fourth thread: \code forever { getchar(); mutex.lock(); // Sleep until there are no busy worker threads while (count > 0) { mutex.unlock(); sleep(1); mutex.lock(); } keyPressed.wakeAll(); mutex.unlock(); } \endcode The mutex is necessary because the results of two threads attempting to change the value of the same variable simultaneously are unpredictable. Wait conditions are a powerful thread synchronization primitive. The \l{threads/waitconditions}{Wait Conditions} example shows how to use QWaitCondition as an alternative to QSemaphore for controlling access to a circular buffer shared by a producer thread and a consumer thread. \sa QMutex, QSemaphore, QThread, {Wait Conditions Example} */ /*! Constructs a new wait condition object. */ QWaitCondition::QWaitCondition() { d = new QWaitConditionPrivate; report_error(pthread_mutex_init(&d->mutex, NULL), "QWaitCondition", "mutex init"); report_error(pthread_cond_init(&d->cond, NULL), "QWaitCondition", "cv init"); d->waiters = d->wakeups = 0; } /*! Destroys the wait condition object. */ QWaitCondition::~QWaitCondition() { report_error(pthread_cond_destroy(&d->cond), "QWaitCondition", "cv destroy"); report_error(pthread_mutex_destroy(&d->mutex), "QWaitCondition", "mutex destroy"); delete d; } /*! Wakes one thread waiting on the wait condition. The thread that is woken up depends on the operating system's scheduling policies, and cannot be controlled or predicted. If you want to wake up a specific thread, the solution is typically to use different wait conditions and have different threads wait on different conditions. \sa wakeAll() */ void QWaitCondition::wakeOne() { report_error(pthread_mutex_lock(&d->mutex), "QWaitCondition::wakeOne()", "mutex lock"); d->wakeups = qMin(d->wakeups + 1, d->waiters); report_error(pthread_cond_signal(&d->cond), "QWaitCondition::wakeOne()", "cv signal"); report_error(pthread_mutex_unlock(&d->mutex), "QWaitCondition::wakeOne()", "mutex unlock"); } /*! Wakes all threads waiting on the wait condition. The order in which the threads are woken up depends on the operating system's scheduling policies and cannot be controlled or predicted. \sa wakeOne() */ void QWaitCondition::wakeAll() { report_error(pthread_mutex_lock(&d->mutex), "QWaitCondition::wakeAll()", "mutex lock"); d->wakeups = d->waiters; report_error(pthread_cond_broadcast(&d->cond), "QWaitCondition::wakeAll()", "cv broadcast"); report_error(pthread_mutex_unlock(&d->mutex), "QWaitCondition::wakeAll()", "mutex unlock"); } /*! Releases the locked \a mutex and wait on the wait condition. The \a mutex must be initially locked by the calling thread. If \a mutex is not in a locked state, this function returns immediately. If \a mutex is a recursive mutex, this function returns immediately. The \a mutex will be unlocked, and the calling thread will block until either of these conditions is met: \list \o Another thread signals it using wakeOne() or wakeAll(). This function will return true in this case. \o \a time milliseconds has elapsed. If \a time is \c ULONG_MAX (the default), then the wait will never timeout (the event must be signalled). This function will return false if the wait timed out. \endlist The mutex will be returned to the same locked state. This function is provided to allow the atomic transition from the locked state to the wait state. \sa wakeOne(), wakeAll() */ bool QWaitCondition::wait(QMutex *mutex, unsigned long time) { if (! mutex) return false; if (mutex->d->recursive) { qWarning("QWaitCondition: cannot wait on recursive mutexes"); return false; } report_error(pthread_mutex_lock(&d->mutex), "QWaitCondition::wait()", "mutex lock"); ++d->waiters; mutex->unlock(); int code; forever { if (time != ULONG_MAX) { struct timeval tv; gettimeofday(&tv, 0); timespec ti; ti.tv_nsec = (tv.tv_usec + (time % 1000) * 1000) * 1000; ti.tv_sec = tv.tv_sec + (time / 1000) + (ti.tv_nsec / 1000000000); ti.tv_nsec %= 1000000000; code = pthread_cond_timedwait(&d->cond, &d->mutex, &ti); } else { code = pthread_cond_wait(&d->cond, &d->mutex); } if (code == 0 && d->wakeups == 0) { // many vendors warn of spurios wakeups from // pthread_cond_wait(), especially after signal delivery, // even though POSIX doesn't allow for it... sigh continue; } break; } Q_ASSERT_X(d->waiters > 0, "QWaitCondition::wait", "internal error (waiters)"); --d->waiters; if (code == 0) { Q_ASSERT_X(d->wakeups > 0, "QWaitCondition::wait", "internal error (wakeups)"); --d->wakeups; } report_error(pthread_mutex_unlock(&d->mutex), "QWaitCondition::wait()", "mutex unlock"); mutex->lock(); if (code && code != ETIMEDOUT) report_error(code, "QWaitCondition::wait()", "cv wait"); return (code == 0); } #endif // QT_NO_THREAD