//// process_per_connection.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)//该实例演示如何如何为已完成的handler fork一个进程#include <boost/asio/io_service.hpp>#include <boost/asio/ip/tcp.hpp>#include <boost/asio/signal_set.hpp>#include <boost/asio/write.hpp>#include <boost/array.hpp>#include <boost/bind.hpp>#include <cstdlib>#include <iostream>#include <sys/types.h>#include <sys/wait.h>#include <unistd.h>using boost::asio::ip::tcp;class server{public:  server(boost::asio::io_service& io_service, unsigned short port)    : io_service_(io_service),      signal_(io_service, SIGCHLD),//调用boost::asio::signal_set构造函数，注册SIGCHLD信号，为子进程终止信号。      acceptor_(io_service, tcp::endpoint(tcp::v4(), port)),      socket_(io_service)  {    start_signal_wait();    start_accept();  }private:  void start_signal_wait()  {    signal_.async_wait(boost::bind(&server::handle_signal_wait, this));  }  void handle_signal_wait()  {    // Only the parent process should check for this signal. We can determine    // whether we are in the parent by checking if the acceptor is still open.    if (acceptor_.is_open())    {      // Reap completed child processes so that we don‘t end up with zombies.      int status = 0;      while (waitpid(-1, &status, WNOHANG) > 0) {}      start_signal_wait();    }  }  void start_accept()  {    acceptor_.async_accept(socket_,        boost::bind(&server::handle_accept, this, _1));  }  void handle_accept(const boost::system::error_code& ec)  {    if (!ec)    {      // Inform the io_service that we are about to fork. The io_service cleans      // up any internal resources, such as threads, that may interfere with      // forking.        //fork一个进程！需要向io_service进行告知。io_service要清楚资源。比如会干扰fork的进程。      io_service_.notify_fork(boost::asio::io_service::fork_prepare);      if (fork() == 0)      {        // Inform the io_service that the fork is finished and that this is the        // child process. The io_service uses this opportunity to create any        // internal file descriptors that must be private to the new process.          //告知io_service fork完毕，该进程是子进程。io_service将创建内部的文件描述符。        io_service_.notify_fork(boost::asio::io_service::fork_child);        // The child won‘t be accepting new connections, so we can close the        // acceptor. It remains open in the parent.        //子进程不再accept新的连接，故关闭。        acceptor_.close();        // The child process is not interested in processing the SIGCHLD signal.        //子进程不关心进程回收的过程，故取消对SIGCHLD的信号处理        signal_.cancel();        start_read();      }      else      {        // Inform the io_service that the fork is finished (or failed) and that        // this is the parent process. The io_service uses this opportunity to        // recreate any internal resources that were cleaned up during        // preparation for the fork.          //父进程的操作。关闭socket，因为该socket由子进程处理。父进程继续listen。        io_service_.notify_fork(boost::asio::io_service::fork_parent);        socket_.close();        start_accept();      }    }    else    {      std::cerr << "Accept error: " << ec.message() << std::endl;      start_accept();    }  }  void start_read()  {    socket_.async_read_some(boost::asio::buffer(data_),        boost::bind(&server::handle_read, this, _1, _2));  }  void handle_read(const boost::system::error_code& ec, std::size_t length)  {    if (!ec)      start_write(length);  }  void start_write(std::size_t length)  {    boost::asio::async_write(socket_, boost::asio::buffer(data_, length),        boost::bind(&server::handle_write, this, _1));  }  void handle_write(const boost::system::error_code& ec)  {    if (!ec)      start_read();  }  boost::asio::io_service& io_service_;  boost::asio::signal_set signal_;  tcp::acceptor acceptor_;  tcp::socket socket_;  boost::array<char, 1024> data_;};int main(int argc, char* argv[]){  try  {    if (argc != 2)    {      std::cerr << "Usage: process_per_connection <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() << std::endl;  }}