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Posted 15 Mar 2019
Licenced MIT

Fast Semaphore

, 15 Mar 2019
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Fast semaphore

I received an interesting piece of code during my recent discussion about the blocking queue: a fast semaphore implementation. The fast_semaphore class uses a regular C++ semaphore as fallback for waiting and unblocking the thread(s) while doing its magic using an atomic variable and memory fences of the new C++ memory model (see here and here).

I present to you, my shortened version of, Fast-Semaphore by Joe Seigh; C++ Implementation by Chris Thomasson:

#pragma once

#include <mutex>
#include <atomic>
#include <condition_variable>
#include <cassert>

class semaphore
    semaphore(int count) noexcept
    : m_count(count) { assert(count > -1); }

    void post() noexcept
            std::unique_lock<std::mutex> lock(m_mutex);

    void wait() noexcept
        std::unique_lock<std::mutex> lock(m_mutex);
        m_cv.wait(lock, [&]() { return m_count != 0; });

    int m_count;
    std::mutex m_mutex;
    std::condition_variable m_cv;

class fast_semaphore
    fast_semaphore(int count) noexcept
    : m_count(count), m_semaphore(0) {}

    void post()
        int count = m_count.fetch_add(1, std::memory_order_relaxed);
        if (count < 0)

    void wait()
        int count = m_count.fetch_sub(1, std::memory_order_relaxed);
        if (count < 1)

    std::atomic<int> m_count;
    semaphore m_semaphore;


This article, along with any associated source code and files, is licensed under The MIT License


About the Author

Martin Vorbrodt
Software Developer (Senior)
United States United States
No Biography provided

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Questionmhm Pin
Goran Mitrovic15-Mar-19 13:56
memberGoran Mitrovic15-Mar-19 13:56 

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