线程间使用条件变量同步正确方式

线程间同步标准的使用方式如下：

thread1:
    pthread_mutex_lock(&mutex);
    while (!condition)
          pthread_cond_wait(&cond, &mutex);
    func_1();/* do something that requires holding the mutex and condition is true */
    pthread_mutex_unlock(&mutex);


thread2:
    pthread_mutex_lock(&mutex);
    func_2();/* do something that might make condition true */
    pthread_cond_signal(&cond);
    pthread_mutex_unlock(&mutex);

关于条件变量使用的一些陷阱参照：http://www.cppblog.com/Solstice/archive/2013/09/09/203094.html

pthread_cond_signal():  唤醒等待cond的线程
pthread_cond_wait():    释放mutex 睡眠等待cond条件发生，pthread_cond_wait()通常和mutex绑定在一起.

在pthread_cond_wait中,为保证信号不会丢失 释放mutex和进入cond的等待队列是原子的操作. 当线程收到
信号时在wakeup前需要重新获得mutex. 如果发送线程还没有释放mutex，则当前线程继续等待直到获得mutex
该函数才返回.

简单实现一个线程安全队列：

template <class T>
class Queue{
  private:
    pthread_cond_t cond;
    pthread_mutex_t mutex;
    std::queue<T> items;
  public:
  Queue();
  ~Queue();
  bool empty();
  int size();
  int push(const T item);
  int pop(T *data);
};


template <class T>
Queue<T>::Queue(){
  pthread_cond_init(&cond, NULL);
  pthread_mutex_init(&mutex, NULL);
}

template <class T>
Queue<T>::~Queue(){
  pthread_cond_destroy(&cond);
  pthread_mutex_destroy(&mutex);
}

template <class T>
bool Queue<T>::empty(){
  bool ret = false;
  if(pthread_mutex_lock(&mutex) != 0){
    return -1;
  }
  ret = items.empty();
  pthread_mutex_unlock(&mutex);
  return ret;
}

template <class T>
int Queue<T>::size(){
  int ret = -1;
  if(pthread_mutex_lock(&mutex) != 0){
    return -1;
  }
  ret = items.size();
  pthread_mutex_unlock(&mutex);
  return ret;
}

template <class T>
int Queue<T>::push(const T item){
  if(pthread_mutex_lock(&mutex) != 0){
    return -1;
  }
  {
    items.push(item);
  }
  pthread_mutex_unlock(&mutex);
  pthread_cond_signal(&cond);
  return 1;
}

template <class T>
int Queue<T>::pop(T *data){
  if(pthread_mutex_lock(&mutex) != 0){
    return -1;
  }
  {
    while(items.empty()){
      if(pthread_cond_wait(&cond, &mutex) != 0){
        return -1;
      }
    }
    *data = items.front();
    items.pop();
  }
  if(pthread_mutex_unlock(&mutex) != 0){
    return -1;
  }
		
  return 1;
}

转载请注明出处，谢谢。。