mutex lock实现reader writer lock

class rw_lock_t {

    int NoOfReaders;
    int NoOfWriters, NoOfWritersWaiting;
    pthread_mutex_t class_mutex;
    pthread_cond_t  reader_gate;
    pthread_cond_t  writer_gate;

public:

    rw_lock_t()
    : NoOfReaders(0), NoOfWriters(0), NoOfWritersWating(0),
      class_mutex(PTHREAD_MUTEX_INITIALIZER),
      reader_gate(PTHREAD_COND_INITIALIZER),
      writer_gate(PTHREAD_COND_INITIALIZER)
    {}
    ~rw_lock_t()
    {
        pthread_mutex_destroy(&class_mutex);
        pthread_cond_destroy(&reader_gate);
        pthread_cond_destroy(&writer_gate);
    }
    void r_lock()
    {
        pthread_mutex_lock(&class_mutex);
        //while(NoOfWriters>0 || NoOfWritersWaiting>0) //Writer Preference
        while(NoOfWriters>0)
        {
            pthread_cond_wait(&reader_gate, &class_mutex);
        }
        NoOfReaders++;        
        pthread_mutex_unlock(&class_mutex);
    }
    void w_lock()
    {
        pthread_mutex_lock(&class_mutex);
        NoOfWritersWaiting++;
        while(NoOfReaders>0 || NoOfWriters>0)
        {
            pthread_cond_wait(&writer_gate, &class_mutex);
        }
        NoOfWritersWaiting--; NoOfWriters++;
        pthread_mutex_unlock(&class_mutex);
    }
    void r_unlock()
    {
        pthread_mutex_lock(&class_mutex);
        NoOfReaders--;
        if(NoOfReaders==0 && NoOfWritersWaiting>0)
            pthread_cond_signal(&writer_gate);
        pthread_mutex_unlock(&class_mutex);
    }
    void w_unlock()
    {
        pthread_mutex_lock(&class_mutex);
        NoOfWriters--;
        if(NoOfWritersWaiting>0)
            pthread_cond_signal(&writer_gate);
        //else //Writer Preference - don't signal readers unless no writers
        pthread_cond_broadcast(&reader_gate);
        pthread_mutex_unlock(&class_mutex);
    }
};