//
// server.cpp
// ~~~~~~~~~~
//
// Copyright (c) 2003-2013 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
/*
*该文件编写了一个简单的echo服务器。采用异步I/O的形式。特点在于自己编写了内存池，和会话管理。
**/
#include <cstdlib>
#include <iostream>
#include <boost/aligned_storage.hpp>
#include <boost/array.hpp>
#include <boost/bind.hpp>
#include <boost/enable_shared_from_this.hpp>
#include <boost/noncopyable.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/asio.hpp>
using boost::asio::ip::tcp;
// Class to manage the memory to be used for handler-based custom allocation.
// It contains a single block of memory which may be returned for allocation
// requests. If the memory is in use when an allocation request is made, the
// allocator delegates allocation to the global heap.
//相当于内存池功能，避免大量异步I/O时内存new delete的开销。
class handler_allocator
: private boost::noncopyable
{
public:
//每次session只做了一次实例化。可以在该构造函数加入输出验证。
handler_allocator()
: in_use_(false)
{
}
void* allocate(std::size_t size)
{
if (!in_use_ && size < storage_.size)
{
in_use_ = true;
return storage_.address();
}
else
{
return ::operator new(size);
}
}
void deallocate(void* pointer)
{
if (pointer == storage_.address())
{
in_use_ = false;
}
else
{
::operator delete(pointer);
}
}
private:
// Storage space used for handler-based custom memory allocation.
//保证对齐的内存空间。
boost::aligned_storage<1024> storage_;
// Whether the handler-based custom allocation storage has been used.
bool in_use_;
};
// Wrapper class template for handler objects to allow handler memory
// allocation to be customised. Calls to operator() are forwarded to the
// encapsulated handler.
//函数对象，handle的再次封装，封装了内存分配和释放
template <typename Handler>
class custom_alloc_handler
{
//通过该构造函数可以发现，每次异步读写操作会实例化一次
public:
custom_alloc_handler(handler_allocator& a, Handler h)
: allocator_(a),
handler_(h)
{
}
//实际上由内部的handle执行。外部只是一层函数对象进行包装。
template <typename Arg1>
void operator()(Arg1 arg1)
{
handler_(arg1);
}
template <typename Arg1, typename Arg2>
void operator()(Arg1 arg1, Arg2 arg2)
{
handler_(arg1, arg2);
}
/*为handler默认的分配函数http://www.boost.org/doc/libs/1_55_0/doc/html/boost_asio/reference/asio_handler_allocate.html
*异步操作需要分配一个临时对象，这对象和handle相对应。这些对象默认实现方式是new delete
*所有handle相关的临时对象，将在调用该handle之前执行deallocated解分配。所以允许对对象的内存重用。
**/
friend void* asio_handler_allocate(std::size_t size,
custom_alloc_handler<Handler>* this_handler)
{
return this_handler->allocator_.allocate(size);//自定义分配对异步临时对象内存
}
//注意参数
friend void asio_handler_deallocate(void* pointer, std::size_t /*size*/,
custom_alloc_handler<Handler>* this_handler)
{
this_handler->allocator_.deallocate(pointer);//释放内存
}
private:
//注意这里是引用，而不是对象。
handler_allocator& allocator_;
Handler handler_;
};
// Helper function to wrap a handler object to add custom allocation.工厂类，模板参数独立于任何类型
template <typename Handler>
inline custom_alloc_handler<Handler> make_custom_alloc_handler(
handler_allocator& a, Handler h)
{
return custom_alloc_handler<Handler>(a, h);//返回的是一个函数对象。
}
//会话类
//有关enable_shared_from_this当类对象被 shared_ptr 管理时。需要在类自己定义的函数里把当前类对象作为参数传给其他函数时，
//不能返回裸指针这时需要传递一个 shared_ptr ，否则就不能保持
//shared_ptr 管理这个类对象的语义。http://www.2cto.com/kf/201212/175430.html http://blog.sina.com.cn/s/blog_62cd38470100g3dd.html
class session
: public boost::enable_shared_from_this<session>
{
public:
session(boost::asio::io_service& io_service)
: socket_(io_service)
{
}
tcp::socket& socket()
{
return socket_;
}
void start()
{
//当数据读取完成后，执行后面的handle。第二个参数为一个函数handle。这个make_custom_alloc_handler怎么使用模板的参数？
socket_.async_read_some(boost::asio::buffer(data_),
make_custom_alloc_handler(allocator_,
boost::bind(&session::handle_read,// bind会返回函数对象.主要函数成员是这样绑定的。要有个函数成员取地址和一个对象的占位符
shared_from_this(),
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred)));//占位符，从该参数传递错误码和已读个数。
}
void handle_read(const boost::system::error_code& error,
size_t bytes_transferred)
{
if (!error)//如果没有发生错误，那么就异步写
{
//sleep(10);经测试好像还是存在排队等待的现象
boost::asio::async_write(socket_,
boost::asio::buffer(data_, bytes_transferred),
make_custom_alloc_handler(allocator_,
boost::bind(&session::handle_write,
shared_from_this(),
boost::asio::placeholders::error)));
}
}
void handle_write(const boost::system::error_code& error)
{
//如果不发生错误，那么就执行异步写。
if (!error)
{
socket_.async_read_some(boost::asio::buffer(data_),
make_custom_alloc_handler(allocator_,
boost::bind(&session::handle_read,
shared_from_this(),
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred)));
}
}
private:
// The socket used to communicate with the client.
tcp::socket socket_;
// Buffer used to store data received from the client.缓冲区
boost::array<char, 1024> data_;
// The allocator to use for handler-based custom memory allocation.
//每次会话所分配的内存池
handler_allocator allocator_;
};
typedef boost::shared_ptr<session> session_ptr;
class server
{
public:
server(boost::asio::io_service& io_service, short port)
: io_service_(io_service),
acceptor_(io_service, tcp::endpoint(tcp::v4(), port))
{
session_ptr new_session(new session(io_service_));
acceptor_.async_accept(new_session->socket(),
boost::bind(&server::handle_accept, this, new_session,
boost::asio::placeholders::error));
}
void handle_accept(session_ptr new_session,
const boost::system::error_code& error)
{
if (!error)
{
new_session->start();
}
new_session.reset(new session(io_service_));//重置指针
acceptor_.async_accept(new_session->socket(),
boost::bind(&server::handle_accept, this, new_session,
boost::asio::placeholders::error));
}
private:
boost::asio::io_service& io_service_;
tcp::acceptor acceptor_;
};
int main(int argc, char* argv[])
{
try
{
if (argc != 2)
{
std::cerr << "Usage: server <port>\n";
return 1;
}
boost::asio::io_service io_service;
using namespace std; // For atoi.
server s(io_service, atoi(argv[1]));
io_service.run();
}
catch (std::exception& e)
{
std::cerr << "Exception: " << e.what() << "\n";
}
return 0;
}
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