ASIO例子中的，守护进程初始化

// daemon.cpp
// 该例子演示结合ASIO和POSIX标准系统的fork系统调用，产生一个守护进程。
//时间服务器？
// Copyright (c) 2003-2014 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)
//

#include <boost/asio/io_service.hpp>
#include <boost/asio/ip/udp.hpp>
#include <boost/asio/signal_set.hpp>
#include <boost/array.hpp>
#include <boost/bind.hpp>
#include <ctime>
#include <iostream>
#include <syslog.h>
#include <unistd.h>

using boost::asio::ip::udp;
//定义个UDP时间服务器类
class udp_daytime_server
{
public:
  udp_daytime_server(boost::asio::io_service& io_service)
    : socket_(io_service, udp::endpoint(udp::v4(), 13))
  {
      //实例化后立刻开始接收
    start_receive();
  }

private:
  void start_receive()
  {
    socket_.async_receive_from(
        boost::asio::buffer(recv_buffer_), remote_endpoint_,
        boost::bind(&udp_daytime_server::handle_receive, this, _1));
  }

  void handle_receive(const boost::system::error_code& ec)
  {
    if (!ec || ec == boost::asio::error::message_size)
    {
      using namespace std; // For time_t, time and ctime;
      time_t now = time(0);
      std::string message = ctime(&now);

      boost::system::error_code ignored_ec;
      socket_.send_to(boost::asio::buffer(message),
          remote_endpoint_, 0, ignored_ec);
    }

    start_receive();
  }

  udp::socket socket_;
  udp::endpoint remote_endpoint_;
  boost::array<char, 1> recv_buffer_;
};

int main()
{
  try
  {
    boost::asio::io_service io_service;

    // Initialise the server before becoming a daemon. If the process is
    // started from a shell, this means any errors will be reported back to the
    // user.
    //如果程序是通过shell启动的，那么所有错误将返回给用户。
    udp_daytime_server server(io_service);

    // Register signal handlers so that the daemon may be shut down. You may
    // also want to register for other signals, such as SIGHUP to trigger a
    // re-read of a configuration file.
    //注册信号handlers。以便退出，停止。
    boost::asio::signal_set signals(io_service, SIGINT, SIGTERM);
    signals.async_wait(
        boost::bind(&boost::asio::io_service::stop, &io_service));

    // Inform the io_service that we are about to become a daemon. The
    // io_service cleans up any internal resources, such as threads, that may
    // interfere with forking.
    //通知io_service，要将进程进行daemon化，io_service将清除任意内部资源？比如说线程，将会影响fork
    io_service.notify_fork(boost::asio::io_service::fork_prepare);

    // Fork the process and have the parent exit. If the process was started
    // from a shell, this returns control to the user. Forking a new process is
    // also a prerequisite for the subsequent call to setsid().
    //fork该进程后，父进程退出。如果该进程是从shell启动的，那么对于用户来说将是立刻返回。
    //fork一个新的进程也是随后调用setsid的先决条件。setsid创建一个新的会话。
    //这些步骤是守护进程编程规则。详情见APUE第十三章。
    if (pid_t pid = fork())
    {
      if (pid > 0)
      {
        // We‘re in the parent process and need to exit.
        //
        // When the exit() function is used, the program terminates without
        // invoking local variables‘ destructors. Only global variables are
        // destroyed. As the io_service object is a local variable, this means
        // we do not have to call:
        //
        //   io_service.notify_fork(boost::asio::io_service::fork_parent);
        //
        // However, this line should be added before each call to exit() if
        // using a global io_service object. An additional call:
        //
        //   io_service.notify_fork(boost::asio::io_service::fork_prepare);
        //
        // should also precede the second fork().
          /*
              此处为父进程逻辑，当exit函数调用后。程序将不会调用局部变量的析构函数，而全局变量的析构函数会被调用。
          因为io_service是局部对象，所以这就意味着我们没有必要调用
          io_service.notify_fork(boost::asio::io_service::fork_parent);
          但是，当使用io_service作为全局对象的时候，是有必要在每次调用exit之前调用以上一行代码。
          另外需要在第二次fork前调用o_service.notify_fork(boost::asio::io_service::fork_prepare);
          */
        exit(0);
      }
      else
      {
        syslog(LOG_ERR | LOG_USER, "First fork failed: %m");
        return 1;
      }
    }

    // Make the process a new session leader. This detaches it from the
    // terminal.
    //开启新的会话，并作为会话leader，与终端分离。
    setsid();

    // A process inherits its working directory from its parent. This could be
    // on a mounted filesystem, which means that the running daemon would
    // prevent this filesystem from being unmounted. Changing to the root
    // directory avoids this problem.
    //为了防止守护进程所在的文件系统未被挂载，故更改到根目录。
    chdir("/");

    // The file mode creation mask is also inherited from the parent process.
    // We don‘t want to restrict the permissions on files created by the
    // daemon, so the mask is cleared.
    //为了不想限制守护进程创建的文件的权限，故umask设为0
    umask(0);

    // A second fork ensures the process cannot acquire a controlling terminal.
    //第二次fork保证该进程无法获取终端，因为不是leader？
    if (pid_t pid = fork())
    {
      if (pid > 0)
      {
        exit(0);
      }
      else
      {
        syslog(LOG_ERR | LOG_USER, "Second fork failed: %m");
        return 1;
      }
    }

    // Close the standard streams. This decouples the daemon from the terminal
    // that started it.
    //关闭标准输入输出出错，与终端分开。
    close(0);
    close(1);
    close(2);

    // We don‘t want the daemon to have any standard input.
    //不需要任何标准输入。为什么这么用？
    if (open("/dev/null", O_RDONLY) < 0)
    {
      syslog(LOG_ERR | LOG_USER, "Unable to open /dev/null: %m");
      return 1;
    }

    // Send standard output to a log file.
    //重定向标准输出至log文件
    const char* output = "/tmp/asio.daemon.out";
    const int flags = O_WRONLY | O_CREAT | O_APPEND;
    const mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
    if (open(output, flags, mode) < 0)
    {
      syslog(LOG_ERR | LOG_USER, "Unable to open output file %s: %m", output);
      return 1;
    }

    // Also send standard error to the same log file.
    if (dup(1) < 0)
    {
      syslog(LOG_ERR | LOG_USER, "Unable to dup output descriptor: %m");
      return 1;
    }

    // Inform the io_service that we have finished becoming a daemon. The
    // io_service uses this opportunity to create any internal file descriptors
    // that need to be private to the new process.
    //告知io_service已变成守护进程。
    io_service.notify_fork(boost::asio::io_service::fork_child);

    // The io_service can now be used normally.
    syslog(LOG_INFO | LOG_USER, "Daemon started");
    io_service.run();
    syslog(LOG_INFO | LOG_USER, "Daemon stopped");
  }
  catch (std::exception& e)
  {
    syslog(LOG_ERR | LOG_USER, "Exception: %s", e.what());
    std::cerr << "Exception: " << e.what() << std::endl;
  }
}