Boost Interprocess mutexes and condition variables

Question

I'm looking at this Boost example code for two processes sharing a mutex and condition variable between them:

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

but I don't understand how the mutex-condition variable design here can work.

The initial process calls:

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;
  }

and the second process calls:

  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);

To call wait() or notify() either process must hold the shared mutex, so if one process is on wait() the other surely cannot call notify()?

Answer 1

wait releases the mutex while waiting, so the other thread can acquire the mutex and perform the notify. Also see the description on https://www.boost.org/doc/libs/1_57_0/doc/html/interprocess/synchronization_mechanisms.html#interprocess.synchronization_mechanisms.conditions.conditions_whats_a_condition.