《linux多執行緒服務端程式設計 使用muduo c++網路庫》的第8章是從0開始講述一個網路庫的實現，比較適合初學者入門。

 在本書的第8章中是實現了以下幾個類：eventloop類，poller類，channel類

首先分析channel類的定義：

#ifndef MUDUO_NET_CHANNEL_H
#define MUDUO_NET_CHANNEL_H

#include <boost/function.hpp>
#include <boost/noncopyable.hpp>

namespace muduo
{

class EventLoop;

///
/// A selectable I/O channel.
///
/// This class doesn't own the file descriptor.
/// The file descriptor could be a socket,
/// an eventfd, a timerfd, or a signalfd
class Channel : boost::noncopyable
{
 public:
  typedef boost::function<void()> EventCallback;

  Channel(EventLoop* loop, int fd);

  void handleEvent();
  void setReadCallback(const EventCallback& cb)
  { readCallback_ = cb; }
  void setWriteCallback(const EventCallback& cb)
  { writeCallback_ = cb; }
  void setErrorCallback(const EventCallback& cb)
  { errorCallback_ = cb; }

  int fd() const { return fd_; }
  int events() const { return events_; }
  void set_revents(int revt) { revents_ = revt; }
  bool isNoneEvent() const { return events_ == kNoneEvent; }

  void enableReading() { events_ |= kReadEvent; update(); }
  // void enableWriting() { events_ |= kWriteEvent; update(); }
  // void disableWriting() { events_ &= ~kWriteEvent; update(); }
  // void disableAll() { events_ = kNoneEvent; update(); }

  // for Poller
  int index() { return index_; }
  void set_index(int idx) { index_ = idx; }

  EventLoop* ownerLoop() { return loop_; }

 private:
  void update();

  static const int kNoneEvent;
  static const int kReadEvent;
  static const int kWriteEvent;

  EventLoop* loop_;
  const int  fd_;
  int        events_;
  int        revents_;
  int        index_; // used by Poller.

  EventCallback readCallback_;
  EventCallback writeCallback_;
  EventCallback errorCallback_;
};

}
#endif  // MUDUO_NET_CHANNEL_H
 

fd_是channel類的私有變數，一個channel物件是跟檔案控制代碼fd直接掛鉤的。channel類的建構函式是需要loop 和 fd兩個引數，因此每一個關注的fd都會建立一個專門的channel物件來對其負責。

接下來看看poller類的定義：

class Poller : boost::noncopyable
{
 public:
  typedef std::vector<Channel*> ChannelList;

  Poller(EventLoop* loop);
  ~Poller();

  /// Polls the I/O events.
  /// Must be called in the loop thread.
  Timestamp poll(int timeoutMs, ChannelList* activeChannels);

  /// Changes the interested I/O events.
  /// Must be called in the loop thread.
  void updateChannel(Channel* channel);

  void assertInLoopThread() { ownerLoop_->assertInLoopThread(); }

 private:
  void fillActiveChannels(int numEvents,
                          ChannelList* activeChannels) const;

  typedef std::vector<struct pollfd> PollFdList;
  typedef std::map<int, Channel*> ChannelMap;

  EventLoop* ownerLoop_;
  PollFdList pollfds_;
  ChannelMap channels_;
};

}
#endif  // MUDUO_NET_POLLER_H
 

poller類的實現

#include "Poller.h"

#include "Channel.h"
#include "logging/Logging.h"

#include <assert.h>
#include <poll.h>

using namespace muduo;

Poller::Poller(EventLoop* loop)
  : ownerLoop_(loop)
{
}

Poller::~Poller()
{
}

Timestamp Poller::poll(int timeoutMs, ChannelList* activeChannels)
{
  // XXX pollfds_ shouldn't change
  int numEvents = ::poll(&*pollfds_.begin(), pollfds_.size(), timeoutMs);
  Timestamp now(Timestamp::now());
  if (numEvents > 0) {
    LOG_TRACE << numEvents << " events happended";
    fillActiveChannels(numEvents, activeChannels);
  } else if (numEvents == 0) {
    LOG_TRACE << " nothing happended";
  } else {
    LOG_SYSERR << "Poller::poll()";
  }
  return now;
}

void Poller::fillActiveChannels(int numEvents,
                                ChannelList* activeChannels) const
{
  for (PollFdList::const_iterator pfd = pollfds_.begin();
      pfd != pollfds_.end() && numEvents > 0; ++pfd)
  {
    if (pfd->revents > 0)
    {
      --numEvents;
      ChannelMap::const_iterator ch = channels_.find(pfd->fd);
      assert(ch != channels_.end());
      Channel* channel = ch->second;
      assert(channel->fd() == pfd->fd);
      channel->set_revents(pfd->revents);
      // pfd->revents = 0;
      activeChannels->push_back(channel);
    }
  }
}

void Poller::updateChannel(Channel* channel)
{
  assertInLoopThread();
  LOG_TRACE << "fd = " << channel->fd() << " events = " << channel->events();
  if (channel->index() < 0) {
    // a new one, add to pollfds_
    assert(channels_.find(channel->fd()) == channels_.end());
    struct pollfd pfd;
    pfd.fd = channel->fd();
    pfd.events = static_cast<short>(channel->events());
    pfd.revents = 0;
    pollfds_.push_back(pfd);
    int idx = static_cast<int>(pollfds_.size())-1;
    channel->set_index(idx);
    channels_[pfd.fd] = channel;
  } else {
    // update existing one
    assert(channels_.find(channel->fd()) != channels_.end());
    assert(channels_[channel->fd()] == channel);
    int idx = channel->index();
    assert(0 <= idx && idx < static_cast<int>(pollfds_.size()));
    struct pollfd& pfd = pollfds_[idx];
    assert(pfd.fd == channel->fd() || pfd.fd == -1);
    pfd.events = static_cast<short>(channel->events());
    pfd.revents = 0;
    if (channel->isNoneEvent()) {
      // ignore this pollfd
      pfd.fd = -1;
    }
  }
}
 

poller物件是有pollfdlist,channels這樣的私有成員。在poller::updatechannel中，會把channel物件中的fd抽取出來，組成pollfd結構體，然後新增到pollfds_陣列中。並且會對pollfdlist,channels等私有成員進行相應的修改，所以poller::update是將channel和poll聯絡起來的關鍵函式。

而在poller::poll中則會呼叫poll系統呼叫來完成關心事件的IO多路複用，並且將當前有活動的事件新增到activechannels中。

最後在eventloop::loop()中會呼叫相應事件的處理方法：

void EventLoop::loop()
{
  assert(!looping_);
  assertInLoopThread();
  looping_ = true;
  quit_ = false;

  while (!quit_)
  {
    activeChannels_.clear();
    poller_->poll(kPollTimeMs, &activeChannels_);
    for (ChannelList::iterator it = activeChannels_.begin();
        it != activeChannels_.end(); ++it)
    {
      (*it)->handleEvent();
    }
  }

  LOG_TRACE << "EventLoop " << this << " stop looping";
  looping_ = false;
}

通過以上幾個簡單的類，即可實現最簡單的reactor模式。

再來看簡單的測試程式：

#include "Channel.h"
#include "EventLoop.h"

#include <stdio.h>
#include <sys/timerfd.h>

muduo::EventLoop* g_loop;

void timeout()
{
  printf("Timeout!\n");
  g_loop->quit();
}

int main()
{
  muduo::EventLoop loop;
  g_loop = &loop;

  int timerfd = ::timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK | TFD_CLOEXEC);
  muduo::Channel channel(&loop, timerfd);
  channel.setReadCallback(timeout);
  channel.enableReading();

  struct itimerspec howlong;
  bzero(&howlong, sizeof howlong);
  howlong.it_value.tv_sec = 5;
  ::timerfd_settime(timerfd, 0, &howlong, NULL);

  loop.loop();

  ::close(timerfd);
}

上述程式中是將timerfd交由eventloop管理，並且編寫其handler函式timeout()。eventloop會完成事件的檢測和分發。