1.线程安全的消息排队机制
该例子演示ACE Condition包装和ACE Mutex包装的使用。代码从Message_Queue类中摘录，该类包含在Task类中。Message_Queue可被同步策略类型参数化，以获取所期望的并发控制级。缺省地，并发控制级是“线程安全”，如ACE Synch.h文件中的MT_Synch类所定义的：
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class MT_Synch
{
public:
typedef Condition<Mutex> CONDITION;
typedef Mutex MUTEX;
};
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如果MT_Synch被用于实例化Message_Queue，所有的公共方法都将是线程安全的，但同时也带来相应的开销。相反，如果Null_Synch类用于实例化Message_Queue，所有公共方法都不是线程安全的，同时也就没有额外的开销。Null_Synch也在Synch.h中定义，如下所示：
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class Null_Synch
{
public:
typedef Null_Condition<Null_Mutex> CONDITION;
typedef Null_Mutex MUTEX;
};
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ACE Message_Queue由一或多个通过prev_和next_指针链接在一起的Message_Block组成。这样的结构可以高效地操作任意大的消息，而不会导致巨大的内存拷贝开销。
Message_Queue是一种线程安全的消息排队机制。注意C++“traits”习语的使用将Condition和Mutex类型合并进了单个模板参数。
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template <class SYNCH = MT_Synch>
class Message_Queue
{
public:
// Default high and low water marks.
enum
{
// 0 is the low water mark.
DEFAULT_LWM = 0,
// 1 K is the high water mark.
DEFAULT_HWM = 4096,
// Message queue was active before activate() or deactivate().
WAS_ACTIVE = 1,
// Message queue was inactive before activate() or deactivate().
WAS_INACTIVE = 2
};
// Initialize a Message_Queue.(构造函数)
Message_Queue (size_t hwm = DEFAULT_HWM, size_t lwm = DEFAULT_LWM);
// Destroy a Message_Queue.
~Message_Queue (void);(析构函数)
/* Checks if queue is full/empty. */
int is_full (void) const;
int is_empty (void) const;
// Enqueue and dequeue a Message_Block *.
int enqueue_tail (Message_Block *new_item,Time_Value *tv = 0);
int enqueue_head (Message_Block *new_item,Time_Value *tv = 0);
int dequeue_head (Message_Block *&first_item, Time_Value *tv = 0);
// Deactivate the queue and wakeup all threads waiting on the queue so they can continue.
int deactivate (void);
// Reactivate the queue so that threads can enqueue and dequeue messages again.
int activate (void);
private:
// Routines that actually do the enqueueing and dequeueing (assumes locks are held).
int enqueue_tail_i (Message_Block *);
int enqueue_head_i (Message_Block *);
int enqueue_head_i (Message_Block *&);
// Check the boundary conditions.
int is_empty_i (void) const;
int is_full_i (void) const;
// Implement activate() and deactivate() methods (assumes locks are held).
int deactivate_i (void);
int activate_i (void);
//数据成员
// Pointer to head of Message_Block list.
Message_Block *head_;
// Pointer to tail of Message_Block list.
Message_Block *tail_;
// Lowest number before unblocking occurs.
int low_water_mark_;
// Greatest number of bytes before blocking.
int high_water_mark_;
// Current number of bytes in the queue.
int cur_bytes_;
// Current number of messages in the queue.
int cur_count_;
// Indicates that the queue is inactive.
int deactivated_;
// C++ wrapper synchronization primitives for controlling concurrent access.
SYNCH::MUTEX lock_;
SYNCH::CONDITION notempty_;
SYNCH::CONDITION notfull_;
};
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Message_Queue类的实现显示如下。Message_Queue的构造器创建一个空的消息列表，并初始化Condition对象。注意Mutex的lock_被它的缺省构造器自动创建。
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template <class SYNCH>
Message_Queue::Message_Queue (size_t hwm,size_t lwm)
: notfull_ (lock_), notempty_ (lock_)
{
// ...
}
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下面的代码检查队列是否为“空”（也就是，没有消息在其中）或“满”（也就是，在其中的字节的数目多于high_water_mark）。公共方法藉此来获取锁，而私有方法假定锁已被持有。
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template <class SYNCH> int
Message_Queue<SYNCH>::is_empty_i (void) const
{
return cur_bytes_ <= 0 && cur_count_ <= 0;
}
template <class SYNCH> int
Message_Queue<SYNCH>::is_full_i (void) const
{
return cur_bytes_ > high_water_mark_;
}
template <class SYNCH> int
Message_Queue<SYNCH>::is_empty (void) const
{
Guard monitor (lock_);
return is_empty_i ();
}
template <class SYNCH> int
Message_Queue<SYNCH>::is_full (void) const
{
Guard monitor (lock_);
return is_full_i();
}
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下面的方法用于启用和停用Message_Queue。deactivate方法停用队列，并唤醒所有等待在该队列上的线程，以使它们继续。没有消息被从队列中移除。其他任何被调用的方法都立即返回-1，且errno == ESHUTDOWN，直到队列被再次激活。这些信息允许调用者检测状态的变化。
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template <class SYNCH> int
Message_Queue<SYNCH>::deactivate (void)
{
Guard m (lock_);
return deactivate_i ();
}
template <class SYNCH> int
Message_Queue<SYNCH>::deactivate_i (void)
{
int current_status = deactivated_ ? WAS_INACTIVE : WAS_ACTIVE;
// Wake up all the waiters.==>使他们不在等待，直接放弃，呵呵
notempty_.broadcast();
notfull_.broadcast();
deactivated_ = 1;
return current_status;
}
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activate方法重新启用队列，以使线程能够再次让消息入队或出队。如果队列在调用前是不活动的，该方法返回WAS_INACTIVE；如果队列在调用前是活动的，就返回WAS_ACTIVE。
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template <class SYNCH> int
Message_Queue<SYNCH>::activate (void)
{
Guard m (lock_);
return activate_i ();
}
template <class SYNCH> int
Message_Queue<SYNCH>::activate_i (void)
{
int current_status = deactivated_ ? WAS_INACTIVE : WAS_ACTIVE;
deactivated_ = 0;
return current_status;
}
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enqueue_head方法在队列的前面插入一个新条目。像其他的入队和出队方法一样，如果tv参数为NULL，调用者将阻塞直到可以继续执行。否则，调用者将阻塞，等待*tv所指定的数量的时间。但是当队列被关闭、有信号发生，或tv中指定的时间过去了，阻塞的调用都会返回，且errno == EWOULDBLOCK。
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template <class SYNCH> int
Message_Queue<SYNCH>::enqueue_head
(Message_Block *new_item, Time_Value *tv)
{
Guard monitor(lock_);
if (deactivated_)//消息队列处在没有激活的状态
{
errno = ESHUTDOWN;
return -1;
}
// Wait while the queue is full.
while (is_full_i ())
{
// Release the lock_ and wait for timeout, signal, or space becoming available in the list.
if (notfull_.wait (tv) == -1)
{
if (errno == ETIME)
errno = EWOULDBLOCK;
return -1;
}
if (deactivated_)
{
errno = ESHUTDOWN;
return -1;
}
}
// Actually enqueue the message at the head of the list.
enqueue_head_i (new_item);
// Tell any blocked threads that the queue has a new item!
notempty_.signal ();
return 0;
}
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enqueue_tail方法在队列的末尾插入新条目。它返回的是队列中条目的数量。
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template <class SYNCH> int
Message_Queue<SYNCH>::enqueue_tail
(Message_Block *new_item, Time_Value *tv)
{
Guard<SYNCH::MUTEX> monitor (lock_);
if (deactivated_)
{
errno = ESHUTDOWN;
return -1;
}
// Wait while the queue is full.
while (is_full_i ())
{
// Release the lock_ and wait for timeout, signal, or space becoming available in the list.
if (notfull_.wait (tv) == -1)
{
if (errno == ETIME)
errno = EWOULDBLOCK;
return -1;
}
if (deactivated_)
{
errno = ESHUTDOWN;
return -1;
}
}
// Actually enqueue the message at the end of the list.
enqueue_tail_i (new_item);
// Tell any blocked threads that the queue has a new item!
notempty_.signal ();
return 0;
}
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dequeue_head方法移除队列中最前面的条目，并将它传回给调用者。该方法返回队列中所余条目的数目。
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template <class SYNCH> int
Message_Queue<SYNCH>::dequeue_head
(Message_Block *&first_item, Time_Value *tv)
{
Guard monitor (lock);
// Wait while the queue is empty.
while (is_empty_i ())
{
// Release the lock_ and wait for timeout, signal, or a new message being placed in the list.
if (notempty_.wait (tv) == -1)
{
if (errno == ETIME)
errno = EWOULDBLOCK;
return -1;
}
if (deactivated_)
{
errno = ESHUTDOWN;
return -1;
}
}
// Actually dequeue the first message.
dequeue_head_i (first_item);
// Tell any blocked threads that the queue is no longer full.
notfull_.signal ();
return 0;
}
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2.使用消息队列(ACE_Message_Queue)实现的生产者和消费者模型
ACE 中的每个任务(ACE_Task)都有一个底层消息队列(ACE_Message_Queue)。这个消息队列被用作任务间通信的一种方法。当一个任务想要与另一任务“谈话”时，它创建一个消息，并将此消息放入它想要与之谈话的任务的消息队列。接收任务通常用 getq()从消息队列里获取消息。如果队列中没有数据可用，它就进入休眠状态。如果有其他任务将消息插入它的队列，它就会苏醒过来，从队列中拾取数据并处理它。因而，在这种情况下，接收任务将从发送任务那里接收消息，并以应用特定的方式作出反馈。（上面所说的前提是在ACE_MT_SYNCH 下）如果是ACE_Null_Synch就没有这种相应的唤醒机制了！！
经典的生产者－消费者问题的实现。生产者任务生成数据，将它发送给消费者任务。消费者任务随后消费这个数据。使用 ACE_Task 构造，我们可将生产者和消费者看作是不同的 ACE_Task 类型的对象。这两种任务使用底层消息队列进行通信。
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#pragma once
#pragma comment(lib,"ACEd.lib")
#include "ace/OS.h"
#include "ace/Task.h"
#include "ace/Message_Block.h"
//The Consumer Task.
class Consumer: public ACE_Task<ACE_MT_SYNCH>
{
public:
int open(void*)
{
ACE_DEBUG((LM_DEBUG, "(%t) Consumer Task opened \n"));
//Activate the Task
activate(THR_NEW_LWP,1);
return 0;
}
//The Service Processing routine
int svc(void)
{
//Get ready to receive message from Producer
ACE_Message_Block * mb =0;
do
{
if (mb != 0)
{
mb->release();//消费
mb = 0;
}
//Get message from underlying queue
getq(mb);
ACE_DEBUG((LM_DEBUG,"(%t) Got message: %d from remote task\n",*mb->rd_ptr()));
}while(*mb->rd_ptr()<5);
if (mb != 0)
{
mb->release();//消费
mb = 0;
}
return 0;
}
int close(u_long)
{
ACE_DEBUG((LM_DEBUG,"Consumer closes down \n"));
return 0;
}
};
class Producer:public ACE_Task<ACE_MT_SYNCH>
{
public:
Producer(Consumer * consumer):consumer_(consumer), data_(0)
{
mb_= new ACE_Message_Block((char*)&data_,sizeof(data_));//生产
}
int open(void*)
{
ACE_DEBUG((LM_DEBUG, "(%t) Producer task opened \n"));
//Activate the Task
activate(THR_NEW_LWP,1);
return 0;
}
//The Service Processing routine
int svc(void)
{
while(data_<6)
{
//Send message to consumer
ACE_DEBUG((LM_DEBUG, "(%t)Sending message: %d to remote task\n",data_));
consumer_->putq(mb_);
//Go to sleep for a sec.
ACE_OS::sleep(1);
data_++;
mb_= new ACE_Message_Block((char*)&data_,sizeof(data_));//生产
}
return 0;
}
int close(u_long)
{
ACE_DEBUG((LM_DEBUG,"Producer closes down \n"));
return 0;
}
private:
char data_;
Consumer * consumer_;
ACE_Message_Block * mb_;
};
int main(int argc, char * argv[])
{
Consumer *consumer = new Consumer();
Producer * producer = new Producer(consumer);
producer->open(0);
consumer->open(0);
//Wait for all the tasks to exit.
ACE_Thread_Manager::instance()->wait();
getchar();
return 0;
}
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程序的运行结果是：
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