一直对ACE reactor framework中的handle_output困惑不解，而且个人觉得select是个很恶心的函数。

直到前段时间工作上需要用到网络编程，又把C++网络编程卷2找出来看看，写了几个小程序做实验才渐渐明白了reactor中都handle_output的触发条件。回想以前抱着书看了很久就是懒得动手写代码，结果总是一知半解的，看来“实践出真知”一点都不假。

关于handle_output的小结：

当ACE_WFMO_Reactor回调到handle_output时你可以认为一直可写，直到写操作返回EWOULDBLOCK为止。如果socket一直都可以写，ACE_WFMO_Reactor只会调用一次handle_output。因为socket本身的状态没有发生改变，ACE_WFMO_Reactor不会将它放在待分派的事件源中。那么下一次调用handle_output会是什么时候呢？答案是socket从不可写状态变为可写。这就是所谓的“边缘触发方式”。

再来看看ACE_Select_Reactor。Select_Reactor基于BSD Socket的select系统函数，使用的是“水平触发方式”。也就是说，如果socket可写，ACE_Select_Reactor就会不停地调用handle_output，可以说它是比较勤快和公平的，呵呵。

下面是个简单的测试程序：echo服务器。你可以通过宏USE_SELECT_REACTOR来决定使用WFMO_Reactor还是Select_Reactor。

// Reactive_Server.cpp : Defines the entry point for the console application. // #include "stdafx.h" #include "ace/Event_Handler.h" #include "ace/SOCK_Acceptor.h" #include "ace/SOCK_Stream.h" #include "ace/Reactor.h" #include "ace/OS.h" #define USE_SELECT_REACTOR #ifdef USE_SELECT_REACTOR #include "ace/Select_Reactor.h" #endif #include <iostream> class Acceptor : public ACE_Event_Handler { private: ~Acceptor() {} public: virtual int open(const ACE_INET_Addr & addr); /// overrides from ACE_Event_Handler virtual int handle_input(ACE_HANDLE handle = ACE_INVALID_HANDLE); virtual int handle_close(ACE_HANDLE handle,ACE_Reactor_Mask close_mask); virtual ACE_HANDLE get_handle() const; private: ACE_SOCK_Acceptor m_acceptor; }; class Client_Handler : public ACE_Event_Handler { private: ~Client_Handler() {} public: Client_Handler(); virtual int open(); /// overrides from ACE_Event_Handler virtual int handle_input(ACE_HANDLE handle = ACE_INVALID_HANDLE); virtual int handle_output(ACE_HANDLE handle = ACE_INVALID_HANDLE); virtual int handle_close(ACE_HANDLE handle,ACE_Reactor_Mask close_mask); virtual ACE_HANDLE get_handle() const; ACE_SOCK_Stream & peer (); private: ACE_SOCK_Stream m_stream; char m_buffer[8]; }; int _tmain(int argc,_TCHAR* argv[]) { ACE_INET_Addr addr("0.0.0.0:1080"); #ifdef USE_SELECT_REACTOR ACE_Select_Reactor select_reactor; ACE_Reactor reactor(&select_reactor); ACE_Reactor::instance(&reactor,0); #endif Acceptor * acceptor = new Acceptor(); int ret = acceptor->open(addr); if (ret == -1) return -1; return ACE_Reactor::instance()->run_event_loop(); } /// /// Implementation of class Acceptor /// int Acceptor::open(const ACE_INET_Addr & addr) { int ret = m_acceptor.open(addr); if (ret == -1) return -1; ret = ACE_Reactor::instance()->register_handler(this,ACCEPT_MASK); return ret; } int Acceptor::handle_input(ACE_HANDLE handle) { ACE_INET_Addr remotePeer; Client_Handler * client_handler = new Client_Handler(); int ret = m_acceptor.accept(client_handler->peer(),&remotePeer); if (ret == -1) { client_handler->handle_close(ACE_INVALID_HANDLE,NULL_MASK); return -1; } cout << "Received connection from " << remotePeer.get_host_name() << ":" << remotePeer.get_port_number() << endl; return client_handler->open(); } int Acceptor::handle_close(ACE_HANDLE handle,ACE_Reactor_Mask close_mask) { m_acceptor.close(); delete this; return -1; // return value ignored by reactor. } ACE_HANDLE Acceptor::get_handle() const { return this->m_acceptor.get_handle(); } /// /// Implementation of class Client_Handler /// Client_Handler::Client_Handler() { ACE_OS::memset(m_buffer,sizeof(m_buffer)); } int Client_Handler::open() { assert (m_stream.get_handle() != ACE_INVALID_HANDLE); ACE_Time_Value timeout(1); const char welcome [] = "Welcome to echo server"; m_stream.enable(ACE_NONBLOCK); size_t size = m_stream.send(welcome,sizeof(welcome),&timeout); if (size == 0) return -1; else if (size == -1) { assert (errno == EWOULDBLOCK); } m_stream.disable(ACE_NONBLOCK); return ACE_Reactor::instance()->register_handler(this,READ_MASK | WRITE_MASK); } int Client_Handler::handle_input(ACE_HANDLE handle) { ACE_OS::memset(m_buffer,sizeof(m_buffer)); m_stream.enable(ACE_NONBLOCK); size_t size = m_stream.recv(m_buffer,sizeof(m_buffer)); if (size == 0) // connection loss return -1; else if (size == -1) // block { if (errno == EWOULDBLOCK) return 0; else return -1; } // give us chance to reply to client ACE_Reactor::instance()->schedule_wakeup(this,WRITE_MASK); if (size == sizeof(m_buffer)) // there may be more data available return 1; return 0; } int Client_Handler::handle_output(ACE_HANDLE handle) { // just echo size_t size = sizeof(m_buffer[0]) * (ACE_OS::strlen(m_buffer) + 1); if (size > 1) { size_t num_bytes_sent = m_stream.send(m_buffer,size); if (num_bytes_sent == 0) // connection loss return -1; else if (num_bytes_sent == -1) // block { assert (errno == EWOULDBLOCK); } if (num_bytes_sent == size) { ACE_Reactor::instance()->cancel_wakeup(this,WRITE_MASK); } else { // only part of the buffer are sent,:( char * temp = new char[size - num_bytes_sent]; assert (temp != 0); ACE_OS::memcpy(temp,m_buffer + num_bytes_sent,size - num_bytes_sent); ACE_OS::memset(m_buffer,sizeof(m_buffer)); ACE_OS::memcpy(m_buffer,temp,size - num_bytes_sent); delete [] temp; } return 0; } else { ACE_Reactor::instance()->cancel_wakeup(this,WRITE_MASK); return 0; } } int Client_Handler::handle_close(ACE_HANDLE handle,ACE_Reactor_Mask close_mask) { if (close_mask == WRITE_MASK) { return 0; } else { ACE_INET_Addr remotePeer; m_stream.get_remote_addr(remotePeer); cout << "Disconnected: " << remotePeer.get_host_name() << ":" << remotePeer.get_port_number() << endl; m_stream.close(); delete this; return -1; // return value ignored by reactor } } ACE_HANDLE Client_Handler::get_handle() const { return m_stream.get_handle(); } ACE_SOCK_Stream & Client_Handler::peer () { return this->m_stream; }