c++ - boost::interprocess 互斥量和条件变量

Question

我正在查看两个进程共享互斥锁和条件变量的 Boost 示例代码:

https://www.boost.org/doc/libs/1_57_0/doc/html/interprocess/synchronization_mechanisms.html

但我不明白这里的互斥条件变量设计是如何工作的。

初始流程调用:

for(int i = 0; i < NumMsg; ++i){
     scoped_lock<interprocess_mutex> lock(data->mutex);    // Take mutex
     if(data->message_in){
        data->cond_full.wait(lock);                        // Wait
     }
     if(i == (NumMsg-1))
        std::sprintf(data->items, "%s", "last message");
     else
        std::sprintf(data->items, "%s_%d", "my_trace", i);

     //Notify to the other process that there is a message
     data->cond_empty.notify_one();                        // Notify

     //Mark message buffer as full
     data->message_in = true;
  }

第二个进程调用:

  bool end_loop = false;
  do{
     scoped_lock<interprocess_mutex> lock(data->mutex);     // Take mutex
     if(!data->message_in){
        data->cond_empty.wait(lock);                        // Wait
     }
     if(std::strcmp(data->items, "last message") == 0){
        end_loop = true;
     }
     else{
        //Print the message
        std::cout << data->items << std::endl;
        //Notify the other process that the buffer is empty
        data->message_in = false;
        data->cond_full.notify_one();                       // Notify         
     }
  }
  while(!end_loop);

要调用 wait() 或 notify() 任一进程都必须持有共享互斥锁，因此如果一个进程在 wait() 上另一个肯定不能调用notify()？

Answer 1

wait 在等待时释放互斥锁，这样其他线程就可以获取互斥锁并执行通知。另见关于 https://www.boost.org/doc/libs/1_57_0/doc/html/interprocess/synchronization_mechanisms.html#interprocess.synchronization_mechanisms.conditions.conditions_whats_a_condition 的描述.