libev教程一：libev簡單入門

最主要的還是看看ev_run這個部分程式碼。我們不打算仔細閱讀只是看看梗概然後大體分析一下資料結構應該怎麼樣的

void
ev_run (EV_P_ int flags)
{
  assert (("libev: ev_loop recursion during release detected", loop_done != EVBREAK_RECURSE));

  loop_done = EVBREAK_CANCEL;

  EV_INVOKE_PENDING; /* in case we recurse, ensure ordering stays nice and clean */ 

  do
    {
      if (expect_false (loop_done))
        break;

      /* update fd-related kernel structures */
      fd_reify (EV_A);

      /* calculate blocking time */
      {
        ev_tstamp waittime  = 0.;
        ev_tstamp sleeptime = 0.;

        /* remember old timestamp for io_blocktime calculation */
        ev_tstamp prev_mn_now = mn_now;

        /* update time to cancel out callback processing overhead */
        time_update (EV_A_ 1e100);

        if (expect_true (!(flags & EVRUN_NOWAIT || idleall || !activecnt)))
          {
            waittime = MAX_BLOCKTIME;

            if (timercnt)
              {
                ev_tstamp to = ANHE_at (timers [HEAP0]) - mn_now + backend_fudge;
                if (waittime > to) waittime = to;
              }

            /* don't let timeouts decrease the waittime below timeout_blocktime */
            if (expect_false (waittime < timeout_blocktime))
              waittime = timeout_blocktime;

            /* extra check because io_blocktime is commonly 0 */
            if (expect_false (io_blocktime))
              {
                sleeptime = io_blocktime - (mn_now - prev_mn_now);

                if (sleeptime > waittime - backend_fudge)
                  sleeptime = waittime - backend_fudge;

                if (expect_true (sleeptime > 0.))
                  {
                    ev_sleep (sleeptime);
                    waittime -= sleeptime;
                  }
              }
          }

        assert ((loop_done = EVBREAK_RECURSE, 1)); /* assert for side effect */
        backend_poll (EV_A_ waittime);
        assert ((loop_done = EVBREAK_CANCEL, 1)); /* assert for side effect */

        /* update ev_rt_now, do magic */
        time_update (EV_A_ waittime + sleeptime);
      }

      /* queue pending timers and reschedule them */
      timers_reify (EV_A); /* relative timers called last */

      EV_INVOKE_PENDING;
    }
  while (expect_true (
    activecnt
    && !loop_done
    && !(flags & (EVRUN_ONCE | EVRUN_NOWAIT))
  ));

  if (loop_done == EVBREAK_ONE)
    loop_done = EVBREAK_CANCEL;
}

我們可以總結一下大致步驟，其實和大部分的event loop寫出來差不多。

• 首先觸發那些已經pending的watchers.
• 判斷是否loop_done
• fd_reify.這個後面會單獨說。
• 計算出waittime並且進行必要的sleep.
• backend_poll開始輪詢,並且整理好pending事件
• timers_reify.這個和fd_reify不同
• 呼叫EV_INVOKE_PENDING來觸發pending的io事件

非常簡單。接下來我們看看fd_reify,backend_poll,timers_reify以及EV_INVOKE_PENDING.

下面是fd_reify程式碼片段.可以看出，這個部分就是在修改fd關注的events。

inline_size void
fd_reify (EV_P)
{
  int i;
  for (i = 0; i < fdchangecnt; ++i)
    {
      int fd = fdchanges [i];
      ANFD *anfd = anfds + fd;
      ev_io *w;

      unsigned char o_events = anfd->events;
      unsigned char o_reify  = anfd->reify;

      anfd->reify  = 0;

      /*if (expect_true (o_reify & EV_ANFD_REIFY)) probably a deoptimisation */
        {
          anfd->events = 0;

          for (w = (ev_io *)anfd->head; w; w = (ev_io *)((WL)w)->next)
            anfd->events |= (unsigned char)w->events;

          if (o_events != anfd->events)
            o_reify = EV__IOFDSET; /* actually |= */
        }

      if (o_reify & EV__IOFDSET)
        backend_modify (EV_A_ fd, o_events, anfd->events);
    }

  fdchangecnt = 0;
}

而這個fdchanges這個是在哪裡呼叫的呢。我們可以看到就是在ev_io_start這個部分。也就是說如果我們想要修改 fd關注事件的話，我們必須顯示地ev_io_stop掉然後修正之後重新ev_io_start.底層呼叫fd_change的話底層維護陣列fdchanges來儲存發生events變動的fd.

void noinline
ev_io_start (EV_P_ ev_io *w)
{
  int fd = w->fd;

  if (expect_false (ev_is_active (w)))
    return;

  assert (("libev: ev_io_start called with negative fd", fd >= 0));
  assert (("libev: ev_io_start called with illegal event mask", !(w->events & ~(EV__IOFDSET | EV_READ | EV_WRITE))));

  EV_FREQUENT_CHECK;

  ev_start (EV_A_ (W)w, 1);
  array_needsize (ANFD, anfds, anfdmax, fd + 1, array_init_zero);
  wlist_add (&anfds[fd].head, (WL)w);

  fd_change (EV_A_ fd, w->events & EV__IOFDSET | EV_ANFD_REIFY);
  w->events &= ~EV__IOFDSET;

  EV_FREQUENT_CHECK;
}

inline_size void
fd_change (EV_P_ int fd, int flags)
{
  unsigned char reify = anfds [fd].reify;
  anfds [fd].reify |= flags;

  if (expect_true (!reify))
    {
      ++fdchangecnt;
      array_needsize (int, fdchanges, fdchangemax, fdchangecnt, EMPTY2);
      fdchanges [fdchangecnt - 1] = fd;
    }
}

backend_poll底層支援很多poll實現，我們這裡僅僅看ev_epoll.c就可以.程式碼在這裡面我們不列舉了，如果某個fd觸發事件的話那麼最終會呼叫fd_event(EV_A_,fd,event)來進行通知。所以我們看看fd_event.

inline_speed void
fd_event_nocheck (EV_P_ int fd, int revents)
{
  ANFD *anfd = anfds + fd;
  ev_io *w;

  for (w = (ev_io *)anfd->head; w; w = (ev_io *)((WL)w)->next)
    {
      int ev = w->events & revents;

      if (ev)
        ev_feed_event (EV_A_ (W)w, ev);
    }
}
void noinline
ev_feed_event (EV_P_ void *w, int revents)
{
  W w_ = (W)w;
  int pri = ABSPRI (w_);

  if (expect_false (w_->pending))
    pendings [pri][w_->pending - 1].events |= revents;
  else
    {
      w_->pending = ++pendingcnt [pri];
      array_needsize (ANPENDING, pendings [pri], pendingmax [pri], w_->pending, EMPTY2);
      // set the watcher and revents.
      pendings [pri][w_->pending - 1].w      = w_;
      pendings [pri][w_->pending - 1].events = revents;
    }
}

可以看到底層是一個ANFD的陣列，根據fd進行偏移。如果fd過大的話似乎會影響效能沒有hpserver裡面的demuxtable實現方式好。然後得到這個fd下面所有的watcher,然後在loop->pendings裡面記錄所有這些觸發的watcher.

其中HEAP0就是最小堆下標。如果repeat的話說明需要重複發生，那麼就會重新調整時間戳，如果不是repeat的話，那麼內部會呼叫ev_timer_stop這個方法將這個計時器移除。所有的定時任務都通過feed_reverse新增。feed_reverse 內部是維護一個動態陣列來儲存所有的定時器任務，然後在feed_reverse_done裡面遍歷這些任務來觸發這些定時器任務。

inline_size void
timers_reify (EV_P)
{
  EV_FREQUENT_CHECK;

  if (timercnt && ANHE_at (timers [HEAP0]) < mn_now)
    {
      do
        {
          ev_timer *w = (ev_timer *)ANHE_w (timers [HEAP0]);

          /*assert (("libev: inactive timer on timer heap detected", ev_is_active (w)));*/

          /* first reschedule or stop timer */
          if (w->repeat)
            {
              ev_at (w) += w->repeat;
              if (ev_at (w) < mn_now)
                ev_at (w) = mn_now;

              assert (("libev: negative ev_timer repeat value found while processing timers", w->repeat > 0.));

              ANHE_at_cache (timers [HEAP0]);
              downheap (timers, timercnt, HEAP0);
            }
          else
            ev_timer_stop (EV_A_ w); /* nonrepeating: stop timer */

          EV_FREQUENT_CHECK;
          feed_reverse (EV_A_ (W)w);
        }
      while (timercnt && ANHE_at (timers [HEAP0]) < mn_now);

      feed_reverse_done (EV_A_ EV_TIMER);
    }
}

這個巨集最終呼叫的函式就是下面這個，遍歷所有的pendings事件並且逐一觸發。

void noinline
ev_invoke_pending (EV_P)
{
  int pri;

  for (pri = NUMPRI; pri--; )
    while (pendingcnt [pri])
      {
        ANPENDING *p = pendings [pri] + --pendingcnt [pri];

        p->w->pending = 0;
        EV_CB_INVOKE (p->w, p->events);
        EV_FREQUENT_CHECK;
      }
}

#ifndef _COMMON_H_
#define _COMMON_H_

#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#include <strings.h>
#include <cstdlib>
#include <cstdio>
#include <cstddef>
#include <string>

namespace common{

#define D(exp,fmt,...) do {                     \
        if(!(exp)){                             \
            fprintf(stderr,fmt,##__VA_ARGS__);  \
            abort();                            \
        }                                       \
    }while(0)

static void setnonblock(int fd){
    fcntl(fd,F_SETFL,fcntl(fd,F_GETFL) | O_NONBLOCK);
}
static void setreuseaddr(int fd){
    int ok=1;
    setsockopt(fd,SOL_SOCKET,SO_REUSEADDR,&ok,sizeof(ok));
}

static void setaddress(const char* ip,int port,struct sockaddr_in* addr){
    bzero(addr,sizeof(*addr));
    addr->sin_family=AF_INET;
    inet_pton(AF_INET,ip,&(addr->sin_addr));
    addr->sin_port=htons(port);
}

static std::string address_to_string(struct sockaddr_in* addr){
    char ip[128];
    inet_ntop(AF_INET,&(addr->sin_addr),ip,sizeof(ip));
    char port[32];
    snprintf(port,sizeof(port),"%d",ntohs(addr->sin_port));
    std::string r;
    r=r+"("+ip+":"+port+")";    
    return r;
}

static int new_tcp_server(int port){
    int fd=socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
    D(fd>0,"socket failed(%m)\n");
    setnonblock(fd);
    setreuseaddr(fd);
    sockaddr_in addr;
    setaddress("0.0.0.0",port,&addr);
    bind(fd,(struct sockaddr*)&addr,sizeof(addr));
    listen(fd,64); // backlog = 64
    return fd;
}

static int new_tcp_client(const char* ip,int port){
    int fd=socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
    setnonblock(fd);
    sockaddr_in addr;
    setaddress(ip,port,&addr);
    connect(fd,(struct sockaddr*)(&addr),sizeof(addr));
    return fd;
}

}; // namespace common

#endif // _COMMON_H_

#include "ev.h"
#include "common.h"

static void do_connected(struct ev_loop* reactor,ev_io* w,int events){
    close(w->fd);
    ev_break(reactor,EVBREAK_ALL);
}

int main(){
    struct ev_loop* reactor=ev_loop_new(EVFLAG_AUTO);
    int fd=common::new_tcp_client("127.0.0.1",34567);
    ev_io io;
    ev_io_init(&io,&do_connected,fd,EV_WRITE);
    ev_io_start(reactor,&io);
    ev_run(reactor,0);
    close(fd);
    ev_loop_destroy(reactor);
    return 0;
}

#include "ev.h"

#include "common.h"

static void do_accept(struct ev_loop* reactor,ev_io* w,int events){
    struct sockaddr_in addr;
    socklen_t addr_size=sizeof(addr);
    int conn=accept(w->fd,(struct sockaddr*)&addr,&addr_size);
    std::string r=common::address_to_string(&addr);
    fprintf(stderr,"accept %s\n",r.c_str());
    close(conn);
}

int main(){
    struct ev_loop* reactor=ev_loop_new(EVFLAG_AUTO);
    int fd=common::new_tcp_server(34567);
    ev_io w;
    ev_io_init(&w,do_accept,fd,EV_READ);
    ev_io_start(reactor,&w);
    ev_run(reactor,0);
    close(fd);
    ev_loop_destroy(reactor);    
}

http://wangjunle23.blog.163.com/blog/static/11783817120124308920321/