Semaphore implementation

Question

I was wondering if there was a way to implement semaphore in C++ (or C#), any libraries that'd help. I tried using OpenMP but I had no way of actually blocking threads, instead I had to busy wait on 'em which lead to deadlocks if/when I hadn't enough number of threads. So First I'm looking for a some library that would let me block/spawn/kill my threads.
Secondly, are there any libraries out there that already implement semaphores?
And finally, when I was introduced to the context of semaphores I found it very useful (maybe I'm wrong?) but I don't see many libraries (if at all) implementing it. I'm familiar with OpenMP, looked around Intel's TBB, C# threads. But in none of these I don't see semaphores explicitly. So are semaphores not as practical as I think? Or is it that they're hard to implement? Or is it me not being aware?
P.S.
Can semaphores be implemented cross-platform? Since they're probably related to OS.

Answer 1

First advice use boost. All the hard work has been done.

If you want to see how it is implemented it should look like this (though this is a rough sketch I am sure with some research ti can be optimized). Basically a semaphore is built from three things:

• A count
• A condition variable (that provides the suspend)
• A mutex which provides the exclusiveness to modify the count and wait on the condition.

Here is the simple version:

#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 2

In C++, for the way to block threads I would recommend you to use condition variables rather than semaphores. In C#, monitors might be more appropriate.

Even for a rather simple case of Producer-Consumer problem, a semaphore-based solution is harder to do right: doing semaphore increments & decrements in a wrong order could result in problems. On the contrary, a condition-variable-based solution would not have such issues: condition variables are used with a lock (mutex) and the right order of operations is imposed automatically; so after wakeup, a thread already has the lock acquired.

See also my asnwer to When should I use semaphores? where I give another example of condition variable being in my opinion more appropriate for a problem often solved with semaphores.

And to address another your question, I think higher liability to erroneous use and higher complexity of solutions (comparing to alternatives) are the reason why semaphores are not provided by some threading packages. For TBB, I can say that for sure. Thread support in C++11 (designed after Boost.Thread) does not have it too; see Anthony Williams' answer why.

Answer 3

Are there any libraries out there that already implement this?
For C++ there are multiple multithreading libraries, which provide Semaphore implementations:

• Posix
• Poco Thread
• Tiny Threads

Also, You can also implement Semaphores using Boost. Check this out.

Answer 4

In .NET, there exists an implementation within the BCL: System.Threading.Semaphore.

For native code on Windows, have a look at the CreateSemaphore Function. If you are targeting Linux, then you can find a semaphore implementation of the Vienna University of Technology here (which I have already used before and works).