信号量实现

Question

我想知道是否有一种方法可以在C++ (或C#)中实现信号量，任何可以提供帮助的库。我试着使用OpenMP，但我没有办法真正阻塞线程，相反，我不得不忙于等待它们，当我没有足够数量的线程时，这会导致死锁。所以首先，我在寻找一个可以让我阻塞/产生/杀死我的线程的库。

其次，有没有已经实现了信号量的库？

最后，当我被介绍到信号量的上下文时，我发现它非常有用(也许我错了？)但我没有看到很多库(如果有的话)实现它。我熟悉OpenMP，看过英特尔的TBB，C#线程。但在所有这些函数中，我都没有明确地看到信号量。那么信号量不像我想的那么实用吗？或者是因为它们很难实现？或者是我没有意识到？

附注：

信号量可以跨平台实现吗？因为它们可能与操作系统有关。

Answer 1

有没有已经实现了这个功能的库？

对于C++，有多个多线程库，它们提供信号量实现：

• Posix
• Poco Thread
• Tiny Threads

此外，您还可以使用Boost实现信号量。查看this。

Answer 2

第一个建议是使用boost。所有艰苦的工作都已经完成了。

如果你想看看它是如何实现的，它应该是这样的(虽然这是一个粗略的草图，但我相信通过一些研究可以优化它)。基本上，信号量是由三件事构建的：

• A count
• 条件变量(提供挂起)
• 提供独占性以修改计数并等待条件的互斥锁。

下面是简单的版本：

#include <pthread.h>

// Need an exception safe locking class.
struct MutexLocker
{
    MutexLocker(pthread_mutex_t& m) :mutex(m)
    { if (pthread_mutex_lock(&mutex) != 0)      {throw int(1); }}
    ~MutexLocker()
    { if (pthread_mutex_unlock(&mutex) != 0)    {throw int(1); }}
    private:
        pthread_mutex_t&    mutex;
};

class Semaphore
{
    public:
        Semaphore(int initCount = 0)
            : count(initCount)
            , waitCount(0)
        {
            if (pthread_mutex_init(&mutex, NULL) != 0)
            {   throw int(1);
            }

            if (pthread_cond_init(&cond, NULL) != 0)
            {   pthread_mutex_destroy(&mutex);
                throw int(2);
            }
        }

        void wait()
        {
            MutexLocker locker(mutex);

            while(count == 0)
            {
                ++waitCount;
                if (pthread_cond_wait(&cond, &mutex) != 0)
                {   throw int(2);
                }

                // A call to pthread_cond_wait() unlocks the mutex and suspends the thread.
                // It does not busy wait the thread is suspended.
                //
                // When a condition variable receives apthread_cond_signal() a random thread
                // is un-suspended. But it is not released from the call to wait
                // until the mutex can be reacquired by the thread.
                //
                // Thus we get here only after the mutex has been locked.
                //
                // You need to use a while loop above because of this potential situation.
                //      Thread A:  Suspended waiting on condition variable.
                //      Thread B:  Working somewhere else.
                //      Thread C:  calls signal() below (incrementing count to 1)
                //                 This results in A being awakened but it can not exit pthread_cond_wait()
                //                 until it requires the mutex with a lock. While it tries to
                //                 do that thread B finishes what it was doing and calls wait()
                //                 Thread C has incremented the count to 1 so thread B does not
                //                 suspend but decrements the count to zero and exits.
                //                 Thread B now aquires the mutex but the count has been decremented to
                //                 zero so it must immediately re-suspend on the condition variable.


                // Note a thread will not be released from wait until
                // it receives a signal and the mustex lock can be re-established.

                --waitCount;
            }

            --count;
        }

        void signal()
        {

            // You could optimize this part with interlocked increment.
            MutexLocker locker(mutex);
            ++count;

            // This Comment based on using `interlocked increment` rather than mutex.
            //
            // As this part does not modify anything you don;t actually need the lock.
            // Potentially this will release more threads than you need (as you don't
            // have exclusivity on reading waitCount but that will not matter as the
            // wait() method does and any extra woken threads will be put back to sleep.

            // If there are any waiting threads let them out.
            if (waitCount > 0)
            {   if  (pthread_cond_signal(&cond) != 0)
                {   throw int(2);
                }
            }
        }
    private:
        unsigned int        count;
        unsigned int        waitCount;
        pthread_mutex_t     mutex;
        pthread_cond_t      cond;
};

Answer 3

在.NET中，BCL：System.Threading.Semaphore中存在一个实现。

对于Windows上的本机代码，请查看CreateSemaphore Function。如果您的目标是Linux，那么您可以找到维也纳理工大学here的信号量实现(我以前已经使用过它，并且工作正常)。