thread-safe blocking queue

gistfile1.cpp

class ThreadQueue {
  private:
    ThreadQueue& operator=(ThreadQueue&);
    
    list<int> data;
    pthread_mutex lock;
    pthread_mutex empty_lock;
  public:
    ThreadQueue() {
      pthread_mutex_init(&lock,NULL);
      thread_mutex_init(&empty_lock,NULL);
    }
    
    int push(int d) {
      pthread_mutex_lock(&lock);
      data.push_back(d);
      pthread_mutex_unlock(&lock);
      
      // unlock has no side effect when it's not locked
      pthread_mutex_unlock(&empty_lock);
      return 0;
    }
    
    int pop() {
      
      while(1) {
        pthread_mutex_lock(&empty_lock);
        pthread_mutex_lock(&lock);
        if(!data.empty()) {
          // ok, get two lock, and data is not empty, break while loop to do real pop
          break;
        } 
        //cancel all lock, retry again!
        pthread_mutex_unlock(&lock);
        pthread_mutex_unlock(&empty_lock);
        // sleep 0 second used to force this thread release the cpu
        sleep(0);
      }
      
      int val = data.pop_front();
      int is_empty = data.empty();
      pthread_mutex_unlock(&lock);
      
      if(!is_empty) {
        pthread_mutex_unlock(&empty_lock);
      }
      
      return val;
    }
};