Semaphore.hpp

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//---------------------------------------------------------
// For conditions of distribution and use, see
// https://github.com/preshing/cpp11-on-multicore/blob/master/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.
*/
 
#ifndef __CPP11OM_SEMAPHORE_H__
#define __CPP11OM_SEMAPHORE_H__
 
#include <atomic>
#include <cassert>
 
 
#if defined(_WIN32)
//---------------------------------------------------------
// Semaphore (Windows)
//---------------------------------------------------------
 
#include <windows.h>
#undef min
#undef max
 
class Semaphore
{
private:
    HANDLE m_hSema;
 
    Semaphore(const Semaphore& other) = delete;
    Semaphore& operator=(const Semaphore& other) = delete;
 
public:
    Semaphore(int initialCount = 0)
    {
        assert(initialCount >= 0);
        m_hSema = CreateSemaphore(NULL, initialCount, MAXLONG, NULL);
    }
 
    ~Semaphore()
    {
        CloseHandle(m_hSema);
    }
 
    void wait()
    {
        WaitForSingleObject(m_hSema, INFINITE);
    }
 
    void signal(int count = 1)
    {
        ReleaseSemaphore(m_hSema, count, NULL);
    }
};
 
 
#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
//---------------------------------------------------------
 
#include <mach/mach.h>
 
class Semaphore
{
private:
    semaphore_t m_sema;
 
    Semaphore(const Semaphore& other) = delete;
    Semaphore& operator=(const Semaphore& other) = delete;
 
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);
    }
 
    void signal()
    {
        semaphore_signal(m_sema);
    }
 
    void signal(int count)
    {
        while (count-- > 0)
        {
            semaphore_signal(m_sema);
        }
    }
};
 
 
#elif defined(__unix__)
//---------------------------------------------------------
// Semaphore (POSIX, Linux)
//---------------------------------------------------------
 
#include <semaphore.h>
 
class Semaphore
{
private:
    sem_t m_sema;
 
    Semaphore(const Semaphore& other) = delete;
    Semaphore& operator=(const Semaphore& other) = delete;
 
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);
    }
 
    void signal()
    {
        sem_post(&m_sema);
    }
 
    void signal(int count)
    {
        while (count-- > 0)
        {
            sem_post(&m_sema);
        }
    }
};
 
 
#else
 
#error Unsupported platform!
 
#endif
 
 
//---------------------------------------------------------
// LightweightSemaphore
//---------------------------------------------------------
class LightweightSemaphore
{
private:
    std::atomic<int> m_count;
    Semaphore m_sema;
 
    void waitWithPartialSpinning()
    {
        int 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--)
        {
            oldCount = m_count.load(std::memory_order_relaxed);
            if ((oldCount > 0) && m_count.compare_exchange_strong(oldCount, oldCount - 1, std::memory_order_acquire))
                return;
            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)
        {
            m_sema.wait();
        }
    }
 
public:
    LightweightSemaphore(int initialCount = 0) : m_count(initialCount)
    {
        assert(initialCount >= 0);
    }
 
    bool tryWait()
    {
        int oldCount = m_count.load(std::memory_order_relaxed);
        return (oldCount > 0 && m_count.compare_exchange_strong(oldCount, oldCount - 1, std::memory_order_acquire));
    }
 
    void wait()
    {
        if (!tryWait())
            waitWithPartialSpinning();
    }
 
    void signal(int count = 1)
    {
        int oldCount = m_count.fetch_add(count, std::memory_order_release);
        int toRelease = -oldCount < count ? -oldCount : count;
        if (toRelease > 0)
        {
            m_sema.signal(toRelease);
        }
    }
 
    int getCount() {
        return m_count;
    }
};
 
 
typedef LightweightSemaphore DefaultSemaphoreType;
 
 
#endif // __CPP11OM_SEMAPHORE_H__