blockingconcurrentqueue.h

 
// Provides an efficient blocking version of moodycamel::ConcurrentQueue.
// ©2015-2016 Cameron Desrochers. Distributed under the terms of the simplified
// BSD license, available at the top of concurrentqueue.h.
// Uses Jeff Preshing's semaphore implementation (under the terms of its
// separate zlib license, embedded below).
 
#ifndef DMLC_BLOCKINGCONCURRENTQUEUE_H_
#define DMLC_BLOCKINGCONCURRENTQUEUE_H_
 
#pragma once
 
#include "concurrentqueue.h"
#include <type_traits>
#include <cerrno>
#include <memory>
#include <chrono>
#include <ctime>
 
#if defined(_WIN32)
// Avoid including windows.h in a header; we only need a handful of
// items, so we'll redeclare them here (this is relatively safe since
// the API generally has to remain stable between Windows versions).
// I know this is an ugly hack but it still beats polluting the global
// namespace with thousands of generic names or adding a .cpp for nothing.
extern "C" {
    struct _SECURITY_ATTRIBUTES;
    __declspec(dllimport) void* __stdcall CreateSemaphoreW(_SECURITY_ATTRIBUTES* lpSemaphoreAttributes, long lInitialCount, long lMaximumCount, const wchar_t* lpName);
    __declspec(dllimport) int __stdcall CloseHandle(void* hObject);
    __declspec(dllimport) unsigned long __stdcall WaitForSingleObject(void* hHandle, unsigned long dwMilliseconds);
    __declspec(dllimport) int __stdcall ReleaseSemaphore(void* hSemaphore, long lReleaseCount, long* lpPreviousCount);
}
#elif defined(__MACH__)
#include <mach/mach.h>
#elif defined(__unix__)
#include <semaphore.h>
#endif
 
namespace dmlc {
 
namespace moodycamel
{
namespace details
{
    // Code in the mpmc_sema namespace below is an adaptation of Jeff Preshing's
    // portable + lightweight semaphore implementations, originally from
    // https://github.com/preshing/cpp11-on-multicore/blob/master/common/sema.h
    // LICENSE:
    // Copyright (c) 2015 Jeff Preshing
    //
    // This software is provided 'as-is', without any express or implied
    // warranty. In no event will the authors be held liable for any damages
    // arising from the use of this software.
    //
    // Permission is granted to anyone to use this software for any purpose,
    // including commercial applications, and to alter it and redistribute it
    // freely, subject to the following restrictions:
    //
    // 1. The origin of this software must not be misrepresented; you must not
    //  claim that you wrote the original software. If you use this software
    //  in a product, an acknowledgement in the product documentation would be
    //  appreciated but is not required.
    // 2. Altered source versions must be plainly marked as such, and must not be
    //  misrepresented as being the original software.
    // 3. This notice may not be removed or altered from any source distribution.
    namespace mpmc_sema
    {
#if defined(_WIN32)
        class Semaphore
        {
        private:
            void* m_hSema;
 
            Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
            Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
 
        public:
            Semaphore(int initialCount = 0)
            {
                assert(initialCount >= 0);
                const long maxLong = 0x7fffffff;
                m_hSema = CreateSemaphoreW(nullptr, initialCount, maxLong, nullptr);
            }
 
            ~Semaphore()
            {
                CloseHandle(m_hSema);
            }
 
            void wait()
            {
                const unsigned long infinite = 0xffffffff;
                WaitForSingleObject(m_hSema, infinite);
            }
 
            bool try_wait()
            {
                const unsigned long RC_WAIT_TIMEOUT = 0x00000102;
                return WaitForSingleObject(m_hSema, 0) != RC_WAIT_TIMEOUT;
            }
 
            bool timed_wait(std::uint64_t usecs)
            {
                const unsigned long RC_WAIT_TIMEOUT = 0x00000102;
                return WaitForSingleObject(m_hSema, (unsigned long)(usecs / 1000)) != RC_WAIT_TIMEOUT;
            }
 
            void signal(int count = 1)
            {
                ReleaseSemaphore(m_hSema, count, nullptr);
            }
        };
#elif defined(__MACH__)
        //---------------------------------------------------------
        // Semaphore (Apple iOS and OSX)
        // Can't use POSIX semaphores due to http://lists.apple.com/archives/darwin-kernel/2009/Apr/msg00010.html
        //---------------------------------------------------------
        class Semaphore
        {
        private:
            semaphore_t m_sema;
 
            Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
            Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
 
        public:
            Semaphore(int initialCount = 0)
            {
                assert(initialCount >= 0);
                semaphore_create(mach_task_self(), &m_sema, SYNC_POLICY_FIFO, initialCount);
            }
 
            ~Semaphore()
            {
                semaphore_destroy(mach_task_self(), m_sema);
            }
 
            void wait()
            {
                semaphore_wait(m_sema);
            }
 
            bool try_wait()
            {
                return timed_wait(0);
            }
 
            bool timed_wait(std::uint64_t timeout_usecs)
            {
                mach_timespec_t ts;
                ts.tv_sec = static_cast<unsigned int>(timeout_usecs / 1000000);
                ts.tv_nsec = (timeout_usecs % 1000000) * 1000;
 
                // added in OSX 10.10: https://developer.apple.com/library/prerelease/mac/documentation/General/Reference/APIDiffsMacOSX10_10SeedDiff/modules/Darwin.html
                kern_return_t rc = semaphore_timedwait(m_sema, ts);
 
                return rc != KERN_OPERATION_TIMED_OUT;
            }
 
            void signal()
            {
                semaphore_signal(m_sema);
            }
 
            void signal(int count)
            {
                while (count-- > 0)
                {
                    semaphore_signal(m_sema);
                }
            }
        };
#elif defined(__unix__)
        //---------------------------------------------------------
        // Semaphore (POSIX, Linux)
        //---------------------------------------------------------
        class Semaphore
        {
        private:
            sem_t m_sema;
 
            Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
            Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
 
        public:
            Semaphore(int initialCount = 0)
            {
                assert(initialCount >= 0);
                sem_init(&m_sema, 0, initialCount);
            }
 
            ~Semaphore()
            {
                sem_destroy(&m_sema);
            }
 
            void wait()
            {
                // http://stackoverflow.com/questions/2013181/gdb-causes-sem-wait-to-fail-with-eintr-error
                int rc;
                do {
                    rc = sem_wait(&m_sema);
                } while (rc == -1 && errno == EINTR);
            }
 
            bool try_wait()
            {
                int rc;
                do {
                    rc = sem_trywait(&m_sema);
                } while (rc == -1 && errno == EINTR);
                return !(rc == -1 && errno == EAGAIN);
            }
 
            bool timed_wait(std::uint64_t usecs)
            {
                struct timespec ts;
                const int usecs_in_1_sec = 1000000;
                const int nsecs_in_1_sec = 1000000000;
                clock_gettime(CLOCK_REALTIME, &ts);
                ts.tv_sec += usecs / usecs_in_1_sec;
                ts.tv_nsec += (usecs % usecs_in_1_sec) * 1000;
                // sem_timedwait bombs if you have more than 1e9 in tv_nsec
                // so we have to clean things up before passing it in
                if (ts.tv_nsec >= nsecs_in_1_sec) {
                    ts.tv_nsec -= nsecs_in_1_sec;
                    ++ts.tv_sec;
                }
 
                int rc;
                do {
                    rc = sem_timedwait(&m_sema, &ts);
                } while (rc == -1 && errno == EINTR);
                return !(rc == -1 && errno == ETIMEDOUT);
            }
 
            void signal()
            {
                sem_post(&m_sema);
            }
 
            void signal(int count)
            {
                while (count-- > 0)
                {
                    sem_post(&m_sema);
                }
            }
        };
#else
#error Unsupported platform! (No semaphore wrapper available)
#endif
 
        //---------------------------------------------------------
        // LightweightSemaphore
        //---------------------------------------------------------
        class LightweightSemaphore
        {
        public:
            typedef std::make_signed<std::size_t>::type ssize_t;
 
        private:
            std::atomic<ssize_t> m_count;
            Semaphore m_sema;
 
            bool waitWithPartialSpinning(std::int64_t timeout_usecs = -1)
            {
                ssize_t oldCount;
                // Is there a better way to set the initial spin count?
                // If we lower it to 1000, testBenaphore becomes 15x slower on my Core i7-5930K Windows PC,
                // as threads start hitting the kernel semaphore.
                int spin = 10000;
                while (--spin >= 0)
                {
                    oldCount = m_count.load(std::memory_order_relaxed);
                    if ((oldCount > 0) && m_count.compare_exchange_strong(oldCount, oldCount - 1, std::memory_order_acquire, std::memory_order_relaxed))
                        return true;
                    std::atomic_signal_fence(std::memory_order_acquire);     // Prevent the compiler from collapsing the loop.
                }
                oldCount = m_count.fetch_sub(1, std::memory_order_acquire);
                if (oldCount > 0)
                    return true;
                if (timeout_usecs < 0)
                {
                    m_sema.wait();
                    return true;
                }
                if (m_sema.timed_wait((std::uint64_t)timeout_usecs))
                    return true;
                // At this point, we've timed out waiting for the semaphore, but the
                // count is still decremented indicating we may still be waiting on
                // it. So we have to re-adjust the count, but only if the semaphore
                // wasn't signaled enough times for us too since then. If it was, we
                // need to release the semaphore too.
                while (true)
                {
                    oldCount = m_count.load(std::memory_order_acquire);
                    if (oldCount >= 0 && m_sema.try_wait())
                        return true;
                    if (oldCount < 0 && m_count.compare_exchange_strong(oldCount, oldCount + 1, std::memory_order_relaxed, std::memory_order_relaxed))
                        return false;
                }
            }
 
            ssize_t waitManyWithPartialSpinning(ssize_t max, std::int64_t timeout_usecs = -1)
            {
                assert(max > 0);
                ssize_t oldCount;
                int spin = 10000;
                while (--spin >= 0)
                {
                    oldCount = m_count.load(std::memory_order_relaxed);
                    if (oldCount > 0)
                    {
                        ssize_t newCount = oldCount > max ? oldCount - max : 0;
                        if (m_count.compare_exchange_strong(oldCount, newCount, std::memory_order_acquire, std::memory_order_relaxed))
                            return oldCount - newCount;
                    }
                    std::atomic_signal_fence(std::memory_order_acquire);
                }
                oldCount = m_count.fetch_sub(1, std::memory_order_acquire);
                if (oldCount <= 0)
                {
                    if (timeout_usecs < 0)
                        m_sema.wait();
                    else if (!m_sema.timed_wait((std::uint64_t)timeout_usecs))
                    {
                        while (true)
                        {
                            oldCount = m_count.load(std::memory_order_acquire);
                            if (oldCount >= 0 && m_sema.try_wait())
                                break;
                            if (oldCount < 0 && m_count.compare_exchange_strong(oldCount, oldCount + 1, std::memory_order_relaxed, std::memory_order_relaxed))
                                return 0;
                        }
                    }
                }
                if (max > 1)
                    return 1 + tryWaitMany(max - 1);
                return 1;
            }
 
        public:
            LightweightSemaphore(ssize_t initialCount = 0) : m_count(initialCount)
            {
                assert(initialCount >= 0);
            }
 
            bool tryWait()
            {
                ssize_t oldCount = m_count.load(std::memory_order_relaxed);
                while (oldCount > 0)
                {
                    if (m_count.compare_exchange_weak(oldCount, oldCount - 1, std::memory_order_acquire, std::memory_order_relaxed))
                        return true;
                }
                return false;
            }
 
            void wait()
            {
                if (!tryWait())
                    waitWithPartialSpinning();
            }
 
            bool wait(std::int64_t timeout_usecs)
            {
                return tryWait() || waitWithPartialSpinning(timeout_usecs);
            }
 
            // Acquires between 0 and (greedily) max, inclusive
            ssize_t tryWaitMany(ssize_t max)
            {
                assert(max >= 0);
                ssize_t oldCount = m_count.load(std::memory_order_relaxed);
                while (oldCount > 0)
                {
                    ssize_t newCount = oldCount > max ? oldCount - max : 0;
                    if (m_count.compare_exchange_weak(oldCount, newCount, std::memory_order_acquire, std::memory_order_relaxed))
                        return oldCount - newCount;
                }
                return 0;
            }
 
            // Acquires at least one, and (greedily) at most max
            ssize_t waitMany(ssize_t max, std::int64_t timeout_usecs)
            {
                assert(max >= 0);
                ssize_t result = tryWaitMany(max);
                if (result == 0 && max > 0)
                    result = waitManyWithPartialSpinning(max, timeout_usecs);
                return result;
            }
 
            ssize_t waitMany(ssize_t max)
            {
                ssize_t result = waitMany(max, -1);
                assert(result > 0);
                return result;
            }
 
            void signal(ssize_t count = 1)
            {
                assert(count >= 0);
                ssize_t oldCount = m_count.fetch_add(count, std::memory_order_release);
                ssize_t toRelease = -oldCount < count ? -oldCount : count;
                if (toRelease > 0)
                {
                    m_sema.signal((int)toRelease);
                }
            }
 
            ssize_t availableApprox() const
            {
                ssize_t count = m_count.load(std::memory_order_relaxed);
                return count > 0 ? count : 0;
            }
        };
    }   // end namespace mpmc_sema
}   // end namespace details
 
 
// This is a blocking version of the queue. It has an almost identical interface to
// the normal non-blocking version, with the addition of various wait_dequeue() methods
// and the removal of producer-specific dequeue methods.
template<typename T, typename Traits = ConcurrentQueueDefaultTraits>
class BlockingConcurrentQueue
{
private:
    typedef ::dmlc::moodycamel::ConcurrentQueue<T, Traits> ConcurrentQueue;
    typedef details::mpmc_sema::LightweightSemaphore LightweightSemaphore;
 
public:
    typedef typename ConcurrentQueue::producer_token_t producer_token_t;
    typedef typename ConcurrentQueue::consumer_token_t consumer_token_t;
 
    typedef typename ConcurrentQueue::index_t index_t;
    typedef typename ConcurrentQueue::size_t size_t;
    typedef typename std::make_signed<size_t>::type ssize_t;
 
    static const size_t BLOCK_SIZE = ConcurrentQueue::BLOCK_SIZE;
    static const size_t EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD = ConcurrentQueue::EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD;
    static const size_t EXPLICIT_INITIAL_INDEX_SIZE = ConcurrentQueue::EXPLICIT_INITIAL_INDEX_SIZE;
    static const size_t IMPLICIT_INITIAL_INDEX_SIZE = ConcurrentQueue::IMPLICIT_INITIAL_INDEX_SIZE;
    static const size_t INITIAL_IMPLICIT_PRODUCER_HASH_SIZE = ConcurrentQueue::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE;
    static const std::uint32_t EXPLICIT_CONSUMER_CONSUMPTION_QUOTA_BEFORE_ROTATE = ConcurrentQueue::EXPLICIT_CONSUMER_CONSUMPTION_QUOTA_BEFORE_ROTATE;
    static const size_t MAX_SUBQUEUE_SIZE = ConcurrentQueue::MAX_SUBQUEUE_SIZE;
 
public:
    // Creates a queue with at least `capacity` element slots; note that the
    // actual number of elements that can be inserted without additional memory
    // allocation depends on the number of producers and the block size (e.g. if
    // the block size is equal to `capacity`, only a single block will be allocated
    // up-front, which means only a single producer will be able to enqueue elements
    // without an extra allocation -- blocks aren't shared between producers).
    // This method is not thread safe -- it is up to the user to ensure that the
    // queue is fully constructed before it starts being used by other threads (this
    // includes making the memory effects of construction visible, possibly with a
    // memory barrier).
    explicit BlockingConcurrentQueue(size_t capacity = 6 * BLOCK_SIZE)
        : inner(capacity), sema(create<LightweightSemaphore>(), &BlockingConcurrentQueue::template destroy<LightweightSemaphore>)
    {
        assert(reinterpret_cast<ConcurrentQueue*>((BlockingConcurrentQueue*)1) == &((BlockingConcurrentQueue*)1)->inner && "BlockingConcurrentQueue must have ConcurrentQueue as its first member");
        if (!sema) {
            MOODYCAMEL_THROW(std::bad_alloc());
        }
    }
 
    BlockingConcurrentQueue(size_t minCapacity, size_t maxExplicitProducers, size_t maxImplicitProducers)
        : inner(minCapacity, maxExplicitProducers, maxImplicitProducers), sema(create<LightweightSemaphore>(), &BlockingConcurrentQueue::template destroy<LightweightSemaphore>)
    {
        assert(reinterpret_cast<ConcurrentQueue*>((BlockingConcurrentQueue*)1) == &((BlockingConcurrentQueue*)1)->inner && "BlockingConcurrentQueue must have ConcurrentQueue as its first member");
        if (!sema) {
            MOODYCAMEL_THROW(std::bad_alloc());
        }
    }
 
    // Disable copying and copy assignment
    BlockingConcurrentQueue(BlockingConcurrentQueue const&) MOODYCAMEL_DELETE_FUNCTION;
    BlockingConcurrentQueue& operator=(BlockingConcurrentQueue const&) MOODYCAMEL_DELETE_FUNCTION;
 
    // Moving is supported, but note that it is *not* a thread-safe operation.
    // Nobody can use the queue while it's being moved, and the memory effects
    // of that move must be propagated to other threads before they can use it.
    // Note: When a queue is moved, its tokens are still valid but can only be
    // used with the destination queue (i.e. semantically they are moved along
    // with the queue itself).
    BlockingConcurrentQueue(BlockingConcurrentQueue&& other) MOODYCAMEL_NOEXCEPT
        : inner(std::move(other.inner)), sema(std::move(other.sema))
    { }
 
    inline BlockingConcurrentQueue& operator=(BlockingConcurrentQueue&& other) MOODYCAMEL_NOEXCEPT
    {
        return swap_internal(other);
    }
 
    // Swaps this queue's state with the other's. Not thread-safe.
    // Swapping two queues does not invalidate their tokens, however
    // the tokens that were created for one queue must be used with
    // only the swapped queue (i.e. the tokens are tied to the
    // queue's movable state, not the object itself).
    inline void swap(BlockingConcurrentQueue& other) MOODYCAMEL_NOEXCEPT
    {
        swap_internal(other);
    }
 
private:
    BlockingConcurrentQueue& swap_internal(BlockingConcurrentQueue& other)
    {
        if (this == &other) {
            return *this;
        }
 
        inner.swap(other.inner);
        sema.swap(other.sema);
        return *this;
    }
 
public:
    // Enqueues a single item (by copying it).
    // Allocates memory if required. Only fails if memory allocation fails (or implicit
    // production is disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE is 0,
    // or Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
    // Thread-safe.
    inline bool enqueue(T const& item)
    {
        if (details::likely(inner.enqueue(item))) {
            sema->signal();
            return true;
        }
        return false;
    }
 
    // Enqueues a single item (by moving it, if possible).
    // Allocates memory if required. Only fails if memory allocation fails (or implicit
    // production is disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE is 0,
    // or Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
    // Thread-safe.
    inline bool enqueue(T&& item)
    {
        if (details::likely(inner.enqueue(std::move(item)))) {
            sema->signal();
            return true;
        }
        return false;
    }
 
    // Enqueues a single item (by copying it) using an explicit producer token.
    // Allocates memory if required. Only fails if memory allocation fails (or
    // Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
    // Thread-safe.
    inline bool enqueue(producer_token_t const& token, T const& item)
    {
        if (details::likely(inner.enqueue(token, item))) {
            sema->signal();
            return true;
        }
        return false;
    }
 
    // Enqueues a single item (by moving it, if possible) using an explicit producer token.
    // Allocates memory if required. Only fails if memory allocation fails (or
    // Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
    // Thread-safe.
    inline bool enqueue(producer_token_t const& token, T&& item)
    {
        if (details::likely(inner.enqueue(token, std::move(item)))) {
            sema->signal();
            return true;
        }
        return false;
    }
 
    // Enqueues several items.
    // Allocates memory if required. Only fails if memory allocation fails (or
    // implicit production is disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE
    // is 0, or Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
    // Note: Use std::make_move_iterator if the elements should be moved instead of copied.
    // Thread-safe.
    template<typename It>
    inline bool enqueue_bulk(It itemFirst, size_t count)
    {
        if (details::likely(inner.enqueue_bulk(std::forward<It>(itemFirst), count))) {
            sema->signal((LightweightSemaphore::ssize_t)(ssize_t)count);
            return true;
        }
        return false;
    }
 
    // Enqueues several items using an explicit producer token.
    // Allocates memory if required. Only fails if memory allocation fails
    // (or Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
    // Note: Use std::make_move_iterator if the elements should be moved
    // instead of copied.
    // Thread-safe.
    template<typename It>
    inline bool enqueue_bulk(producer_token_t const& token, It itemFirst, size_t count)
    {
        if (details::likely(inner.enqueue_bulk(token, std::forward<It>(itemFirst), count))) {
            sema->signal((LightweightSemaphore::ssize_t)(ssize_t)count);
            return true;
        }
        return false;
    }
 
    // Enqueues a single item (by copying it).
    // Does not allocate memory. Fails if not enough room to enqueue (or implicit
    // production is disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE
    // is 0).
    // Thread-safe.
    inline bool try_enqueue(T const& item)
    {
        if (inner.try_enqueue(item)) {
            sema->signal();
            return true;
        }
        return false;
    }
 
    // Enqueues a single item (by moving it, if possible).
    // Does not allocate memory (except for one-time implicit producer).
    // Fails if not enough room to enqueue (or implicit production is
    // disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE is 0).
    // Thread-safe.
    inline bool try_enqueue(T&& item)
    {
        if (inner.try_enqueue(std::move(item))) {
            sema->signal();
            return true;
        }
        return false;
    }
 
    // Enqueues a single item (by copying it) using an explicit producer token.
    // Does not allocate memory. Fails if not enough room to enqueue.
    // Thread-safe.
    inline bool try_enqueue(producer_token_t const& token, T const& item)
    {
        if (inner.try_enqueue(token, item)) {
            sema->signal();
            return true;
        }
        return false;
    }
 
    // Enqueues a single item (by moving it, if possible) using an explicit producer token.
    // Does not allocate memory. Fails if not enough room to enqueue.
    // Thread-safe.
    inline bool try_enqueue(producer_token_t const& token, T&& item)
    {
        if (inner.try_enqueue(token, std::move(item))) {
            sema->signal();
            return true;
        }
        return false;
    }
 
    // Enqueues several items.
    // Does not allocate memory (except for one-time implicit producer).
    // Fails if not enough room to enqueue (or implicit production is
    // disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE is 0).
    // Note: Use std::make_move_iterator if the elements should be moved
    // instead of copied.
    // Thread-safe.
    template<typename It>
    inline bool try_enqueue_bulk(It itemFirst, size_t count)
    {
        if (inner.try_enqueue_bulk(std::forward<It>(itemFirst), count)) {
            sema->signal((LightweightSemaphore::ssize_t)(ssize_t)count);
            return true;
        }
        return false;
    }
 
    // Enqueues several items using an explicit producer token.
    // Does not allocate memory. Fails if not enough room to enqueue.
    // Note: Use std::make_move_iterator if the elements should be moved
    // instead of copied.
    // Thread-safe.
    template<typename It>
    inline bool try_enqueue_bulk(producer_token_t const& token, It itemFirst, size_t count)
    {
        if (inner.try_enqueue_bulk(token, std::forward<It>(itemFirst), count)) {
            sema->signal((LightweightSemaphore::ssize_t)(ssize_t)count);
            return true;
        }
        return false;
    }
 
 
    // Attempts to dequeue from the queue.
    // Returns false if all producer streams appeared empty at the time they
    // were checked (so, the queue is likely but not guaranteed to be empty).
    // Never allocates. Thread-safe.
    template<typename U>
    inline bool try_dequeue(U& item)
    {
        if (sema->tryWait()) {
            while (!inner.try_dequeue(item)) {
                continue;
            }
            return true;
        }
        return false;
    }
 
    // Attempts to dequeue from the queue using an explicit consumer token.
    // Returns false if all producer streams appeared empty at the time they
    // were checked (so, the queue is likely but not guaranteed to be empty).
    // Never allocates. Thread-safe.
    template<typename U>
    inline bool try_dequeue(consumer_token_t& token, U& item)
    {
        if (sema->tryWait()) {
            while (!inner.try_dequeue(token, item)) {
                continue;
            }
            return true;
        }
        return false;
    }
 
    // Attempts to dequeue several elements from the queue.
    // Returns the number of items actually dequeued.
    // Returns 0 if all producer streams appeared empty at the time they
    // were checked (so, the queue is likely but not guaranteed to be empty).
    // Never allocates. Thread-safe.
    template<typename It>
    inline size_t try_dequeue_bulk(It itemFirst, size_t max)
    {
        size_t count = 0;
        max = (size_t)sema->tryWaitMany((LightweightSemaphore::ssize_t)(ssize_t)max);
        while (count != max) {
            count += inner.template try_dequeue_bulk<It&>(itemFirst, max - count);
        }
        return count;
    }
 
    // Attempts to dequeue several elements from the queue using an explicit consumer token.
    // Returns the number of items actually dequeued.
    // Returns 0 if all producer streams appeared empty at the time they
    // were checked (so, the queue is likely but not guaranteed to be empty).
    // Never allocates. Thread-safe.
    template<typename It>
    inline size_t try_dequeue_bulk(consumer_token_t& token, It itemFirst, size_t max)
    {
        size_t count = 0;
        max = (size_t)sema->tryWaitMany((LightweightSemaphore::ssize_t)(ssize_t)max);
        while (count != max) {
            count += inner.template try_dequeue_bulk<It&>(token, itemFirst, max - count);
        }
        return count;
    }
 
 
 
    // Blocks the current thread until there's something to dequeue, then
    // dequeues it.
    // Never allocates. Thread-safe.
    template<typename U>
    inline void wait_dequeue(U& item)
    {
        sema->wait();
        while (!inner.try_dequeue(item)) {
            continue;
        }
    }
 
    // Blocks the current thread until either there's something to dequeue
    // or the timeout (specified in microseconds) expires. Returns false
    // without setting `item` if the timeout expires, otherwise assigns
    // to `item` and returns true.
    // Using a negative timeout indicates an indefinite timeout,
    // and is thus functionally equivalent to calling wait_dequeue.
    // Never allocates. Thread-safe.
    template<typename U>
    inline bool wait_dequeue_timed(U& item, std::int64_t timeout_usecs)
    {
        if (!sema->wait(timeout_usecs)) {
            return false;
        }
        while (!inner.try_dequeue(item)) {
            continue;
        }
        return true;
    }
 
    // Blocks the current thread until either there's something to dequeue
    // or the timeout expires. Returns false without setting `item` if the
    // timeout expires, otherwise assigns to `item` and returns true.
    // Never allocates. Thread-safe.
    template<typename U, typename Rep, typename Period>
    inline bool wait_dequeue_timed(U& item, std::chrono::duration<Rep, Period> const& timeout)
    {
        return wait_dequeue_timed(item, std::chrono::duration_cast<std::chrono::microseconds>(timeout).count());
    }
 
    // Blocks the current thread until there's something to dequeue, then
    // dequeues it using an explicit consumer token.
    // Never allocates. Thread-safe.
    template<typename U>
    inline void wait_dequeue(consumer_token_t& token, U& item)
    {
        sema->wait();
        while (!inner.try_dequeue(token, item)) {
            continue;
        }
    }
 
    // Blocks the current thread until either there's something to dequeue
    // or the timeout (specified in microseconds) expires. Returns false
    // without setting `item` if the timeout expires, otherwise assigns
    // to `item` and returns true.
    // Using a negative timeout indicates an indefinite timeout,
    // and is thus functionally equivalent to calling wait_dequeue.
    // Never allocates. Thread-safe.
    template<typename U>
    inline bool wait_dequeue_timed(consumer_token_t& token, U& item, std::int64_t timeout_usecs)
    {
        if (!sema->wait(timeout_usecs)) {
            return false;
        }
        while (!inner.try_dequeue(token, item)) {
            continue;
        }
        return true;
    }
 
    // Blocks the current thread until either there's something to dequeue
    // or the timeout expires. Returns false without setting `item` if the
    // timeout expires, otherwise assigns to `item` and returns true.
    // Never allocates. Thread-safe.
    template<typename U, typename Rep, typename Period>
    inline bool wait_dequeue_timed(consumer_token_t& token, U& item, std::chrono::duration<Rep, Period> const& timeout)
    {
        return wait_dequeue_timed(token, item, std::chrono::duration_cast<std::chrono::microseconds>(timeout).count());
    }
 
    // Attempts to dequeue several elements from the queue.
    // Returns the number of items actually dequeued, which will
    // always be at least one (this method blocks until the queue
    // is non-empty) and at most max.
    // Never allocates. Thread-safe.
    template<typename It>
    inline size_t wait_dequeue_bulk(It itemFirst, size_t max)
    {
        size_t count = 0;
        max = (size_t)sema->waitMany((LightweightSemaphore::ssize_t)(ssize_t)max);
        while (count != max) {
            count += inner.template try_dequeue_bulk<It&>(itemFirst, max - count);
        }
        return count;
    }
 
    // Attempts to dequeue several elements from the queue.
    // Returns the number of items actually dequeued, which can
    // be 0 if the timeout expires while waiting for elements,
    // and at most max.
    // Using a negative timeout indicates an indefinite timeout,
    // and is thus functionally equivalent to calling wait_dequeue_bulk.
    // Never allocates. Thread-safe.
    template<typename It>
    inline size_t wait_dequeue_bulk_timed(It itemFirst, size_t max, std::int64_t timeout_usecs)
    {
        size_t count = 0;
        max = (size_t)sema->waitMany((LightweightSemaphore::ssize_t)(ssize_t)max, timeout_usecs);
        while (count != max) {
            count += inner.template try_dequeue_bulk<It&>(itemFirst, max - count);
        }
        return count;
    }
 
    // Attempts to dequeue several elements from the queue.
    // Returns the number of items actually dequeued, which can
    // be 0 if the timeout expires while waiting for elements,
    // and at most max.
    // Never allocates. Thread-safe.
    template<typename It, typename Rep, typename Period>
    inline size_t wait_dequeue_bulk_timed(It itemFirst, size_t max, std::chrono::duration<Rep, Period> const& timeout)
    {
        return wait_dequeue_bulk_timed<It&>(itemFirst, max, std::chrono::duration_cast<std::chrono::microseconds>(timeout).count());
    }
 
    // Attempts to dequeue several elements from the queue using an explicit consumer token.
    // Returns the number of items actually dequeued, which will
    // always be at least one (this method blocks until the queue
    // is non-empty) and at most max.
    // Never allocates. Thread-safe.
    template<typename It>
    inline size_t wait_dequeue_bulk(consumer_token_t& token, It itemFirst, size_t max)
    {
        size_t count = 0;
        max = (size_t)sema->waitMany((LightweightSemaphore::ssize_t)(ssize_t)max);
        while (count != max) {
            count += inner.template try_dequeue_bulk<It&>(token, itemFirst, max - count);
        }
        return count;
    }
 
    // Attempts to dequeue several elements from the queue using an explicit consumer token.
    // Returns the number of items actually dequeued, which can
    // be 0 if the timeout expires while waiting for elements,
    // and at most max.
    // Using a negative timeout indicates an indefinite timeout,
    // and is thus functionally equivalent to calling wait_dequeue_bulk.
    // Never allocates. Thread-safe.
    template<typename It>
    inline size_t wait_dequeue_bulk_timed(consumer_token_t& token, It itemFirst, size_t max, std::int64_t timeout_usecs)
    {
        size_t count = 0;
        max = (size_t)sema->waitMany((LightweightSemaphore::ssize_t)(ssize_t)max, timeout_usecs);
        while (count != max) {
            count += inner.template try_dequeue_bulk<It&>(token, itemFirst, max - count);
        }
        return count;
    }
 
    // Attempts to dequeue several elements from the queue using an explicit consumer token.
    // Returns the number of items actually dequeued, which can
    // be 0 if the timeout expires while waiting for elements,
    // and at most max.
    // Never allocates. Thread-safe.
    template<typename It, typename Rep, typename Period>
    inline size_t wait_dequeue_bulk_timed(consumer_token_t& token, It itemFirst, size_t max, std::chrono::duration<Rep, Period> const& timeout)
    {
        return wait_dequeue_bulk_timed<It&>(token, itemFirst, max, std::chrono::duration_cast<std::chrono::microseconds>(timeout).count());
    }
 
 
    // Returns an estimate of the total number of elements currently in the queue. This
    // estimate is only accurate if the queue has completely stabilized before it is called
    // (i.e. all enqueue and dequeue operations have completed and their memory effects are
    // visible on the calling thread, and no further operations start while this method is
    // being called).
    // Thread-safe.
    inline size_t size_approx() const
    {
        return (size_t)sema->availableApprox();
    }
 
 
    // Returns true if the underlying atomic variables used by
    // the queue are lock-free (they should be on most platforms).
    // Thread-safe.
    static bool is_lock_free()
    {
        return ConcurrentQueue::is_lock_free();
    }
 
 
private:
    template<typename U>
    static inline U* create()
    {
        auto p = (Traits::malloc)(sizeof(U));
        return p != nullptr ? new (p) U : nullptr;
    }
 
    template<typename U, typename A1>
    static inline U* create(A1&& a1)
    {
        auto p = (Traits::malloc)(sizeof(U));
        return p != nullptr ? new (p) U(std::forward<A1>(a1)) : nullptr;
    }
 
    template<typename U>
    static inline void destroy(U* p)
    {
        if (p != nullptr) {
            p->~U();
        }
        (Traits::free)(p);
    }
 
private:
    ConcurrentQueue inner;
    std::unique_ptr<LightweightSemaphore, void (*)(LightweightSemaphore*)> sema;
};
 
 
template<typename T, typename Traits>
inline void swap(BlockingConcurrentQueue<T, Traits>& a, BlockingConcurrentQueue<T, Traits>& b) MOODYCAMEL_NOEXCEPT
{
    a.swap(b);
}
 
}   // end namespace moodycamel
}  // namespace dmlc
 
#endif  // DMLC_BLOCKINGCONCURRENTQUEUE_H_