C++0x has no semaphores? How to synchronize thread

Question

Is it true that C++0x will come without semaphores? There are already some questions on Stack Overflow regarding the use of semaphores. I use them (posix semaphores) all the time to let a thread wait for some event in another thread:

void thread0(...)
{
  doSomething0();

  event1.wait();

  ...
}

void thread1(...)
{
  doSomething1();

  event1.post();

  ...
}

If I would do that with a mutex:

void thread0(...)
{
  doSomething0();

  event1.lock(); event1.unlock();

  ...
}

void thread1(...)
{
  event1.lock();

  doSomethingth1();

  event1.unlock();

  ...
}

Problem: It's ugly and it's not guaranteed that thread1 locks the mutex first (Given that the same thread should lock and unlock a mutex, you also can't lock event1 before thread0 and thread1 started).

So since boost doesn't have semaphores either, what is the simplest way to achieve the above?

Answer 1

I found the shared_ptr and weak_ptr, a long with a list, did the job I needed. My issue was, I had several clients wanting to interact with a host's internal data. Typically, the host updates the data on it's own, however, if a client requests it, the host needs to stop updating until no clients are accessing the host data. At the same time, a client could ask for exclusive access, so that no other clients, nor the host, could modify that host data.

How I did this was, I created a struct:

struct UpdateLock
{
    typedef std::shared_ptr< UpdateLock > ptr;
};

Each client would have a member of such:

UpdateLock::ptr m_myLock;

Then the host would have a weak_ptr member for exclusivity, and a list of weak_ptrs for non-exclusive locks:

std::weak_ptr< UpdateLock > m_exclusiveLock;
std::list< std::weak_ptr< UpdateLock > > m_locks;

There is a function to enable locking, and another function to check if the host is locked:

UpdateLock::ptr LockUpdate( bool exclusive );       
bool IsUpdateLocked( bool exclusive ) const;

I test for locks in LockUpdate, IsUpdateLocked, and periodically in the host's Update routine. Testing for a lock is as simple as checking if the weak_ptr's expired, and removing any expired from the m_locks list (I only do this during the host update), I can check if the list is empty; at the same time, I get automatic unlocking when a client resets the shared_ptr they are hanging onto, which also happens when a client gets destroyed automatically.

The over all effect is, since clients rarely need exclusivity (typically reserved for additions and deletions only), most of the time a request to LockUpdate( false ), that is to say non-exclusive, succeeds so long as (! m_exclusiveLock). And a LockUpdate( true ), a request for exclusivity, succeeds only when both (! m_exclusiveLock) and (m_locks.empty()).

A queue could be added to mitigate between exclusive and non-exclusive locks, however, I have had no collisions thus far, so I intend to wait until that happens to add the solution (mostly so I have a real-world test condition).

So far this is working well for my needs; I can imagine the need to expand this, and some issues that might arise over expanded use, however, this was quick to implement, and required very little custom code.

Answer 2

Also can be useful RAII semaphore wrapper in threads:

class ScopedSemaphore
{
public:
    explicit ScopedSemaphore(Semaphore& sem) : m_Semaphore(sem) { m_Semaphore.Wait(); }
    ScopedSemaphore(const ScopedSemaphore&) = delete;
    ~ScopedSemaphore() { m_Semaphore.Notify(); }

   ScopedSemaphore& operator=(const ScopedSemaphore&) = delete;

private:
    Semaphore& m_Semaphore;
};

Usage example in multithread app:

boost::ptr_vector<std::thread> threads;
Semaphore semaphore;

for (...)
{
    ...
    auto t = new std::thread([..., &semaphore]
    {
        ScopedSemaphore scopedSemaphore(semaphore);
        ...
    }
    );
    threads.push_back(t);
}

for (auto& t : threads)
    t.join();

Answer 3

In case someone is interested in the atomic version, here is the implementation. The performance is expected better than the mutex & condition variable version.

class semaphore_atomic
{
public:
    void notify() {
        count_.fetch_add(1, std::memory_order_release);
    }

    void wait() {
        while (true) {
            int count = count_.load(std::memory_order_relaxed);
            if (count > 0) {
                if (count_.compare_exchange_weak(count, count-1, std::memory_order_acq_rel, std::memory_order_relaxed)) {
                    break;
                }
            }
        }
    }

    bool try_wait() {
        int count = count_.load(std::memory_order_relaxed);
        if (count > 0) {
            if (count_.compare_exchange_strong(count, count-1, std::memory_order_acq_rel, std::memory_order_relaxed)) {
                return true;
            }
        }
        return false;
    }
private:
    std::atomic_int count_{0};
};