Question

I know how to use Unix semaphores in C. Before using them I must call a constructor-ish function named sem_init and after using them I have to call a destructor-like function named sem_destroy.

I know I can keep doing this in C++ because of its backwards compatibility with C, but does C++ have a real object-oriented way to use semaphores?

Answer 1

If you really insist on using POSIX semaphores and not Boost, you can of course wrap sem_t in a class:

class Semaphore {
    sem_t sem;

  public:
    Semaphore(int shared, unsigned value)
    { sem_init(&sem, shared, value); }

    ~Semaphore() { sem_destroy(&sem); }

    int wait() { return sem_wait(&sem); }
    int try_wait() { return sem_trywait(&sem); }
    int unlock() { return sem_post(&sem); }
};

Exercise for the reader: You may want to add exceptions instead of C-style error codes and perhaps other features. Also, this class should be noncopyable. The easiest way to achieve that is inheriting from boost::noncopyable 😉

Edit: as @Ringding remarks, looping on EINTR would be a very wise thing to do.

int Semaphore::wait()
{
    int r;
    do {
        r = sem_wait(&sem);
    } while (r == -1 && errno == EINTR);
    return r;
}