semaphore.h

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/***************************************************************************
 * include/stxxl/bits/common/semaphore.h
 *
 * Part of the STXXL. See http://stxxl.sourceforge.net
 *
 * Copyright (C) 2002 Roman Dementiev <[email protected]>
 * Copyright (C) 2013 Timo Bingmann <[email protected]>
 *
 * Distributed under the Boost Software License, Version 1.0.
 * (See accompanying file LICENSE_1_0.txt or copy at
 * http://www.boost.org/LICENSE_1_0.txt)
 **************************************************************************/

#ifndef STXXL_COMMON_SEMAPHORE_HEADER
#define STXXL_COMMON_SEMAPHORE_HEADER

#include <stxxl/bits/noncopyable.h>
#include <stxxl/bits/common/mutex.h>
#include <stxxl/bits/common/condition_variable.h>

STXXL_BEGIN_NAMESPACE

class semaphore : private noncopyable
{
 //! value of the semaphore
 int v;

 //! mutex for condition variable
 mutex m_mutex;

 //! condition variable
 condition_variable m_cond;

public:
 //! construct semaphore
 semaphore(int init_value = 1)
 : v(init_value)
 { }
 //! function increments the semaphore and signals any threads that are
 //! blocked waiting a change in the semaphore
 int operator ++ (int)
 {
 scoped_mutex_lock lock(m_mutex);
 int res = ++v;
 lock.unlock();
 m_cond.notify_one();
 return res;
 }
 //! function decrements the semaphore and blocks if the semaphore is <= 0
 //! until another thread signals a change
 int operator -- (int)
 {
 scoped_mutex_lock lock(m_mutex);
 while (v <= 0)
 m_cond.wait(lock);

 return --v;
 }
 //! function does NOT block but simply decrements the semaphore should not
 //! be used instead of down -- only for programs where multiple threads
 //! must up on a semaphore before another thread can go down, i.e., allows
 //! programmer to set the semaphore to a negative value prior to using it
 //! for synchronization.
 int decrement()
 {
 scoped_mutex_lock lock(m_mutex);
 return --v;
 }
#if 0
 //! function returns the value of the semaphore at the time the
 //! critical section is accessed. obviously the value is not guaranteed
 //! after the function unlocks the critical section.
 int get_value()
 {
 scoped_mutex_lock lock(m_mutex);
 return v;
 }
#endif
};

STXXL_END_NAMESPACE

#endif // !STXXL_COMMON_SEMAPHORE_HEADER