bufferevent 与 socket

http://blog.sina.com.cn/s/blog_56dee71a0100qx4s.html

 

很多时候，除了响应事件之外，应用还希望做一定的数据缓冲。比如说，写入数据的时候，通常的运行模式是：

l 决定要向连接写入一些数据，把数据放入到缓冲区中

l 等待连接可以写入

l 写入尽量多的数据

l 记住写入了多少数据，如果还有更多数据要写入，等待连接再次可以写入

这种缓冲IO模式很通用，libevent为此提供了一种通用机制，即bufferevent。bufferevent由一个底层的传输端口（如套接字），一个读取缓冲区和一个写入缓冲区组成。与通常的事件在底层传输端口已经就绪，可以读取或者写入的时候执行回调不同的是，bufferevent在读取或者写入了足够量的数据之后调用用户提供的回调。

 

 

 

bufferevent 的简单范例

这里选取了 Libevent 的一个范例程序 hello-world.c 来看看 Libevent 的用法：

#include <string.h>
#include <errno.h> #include <stdio.h> #include <signal.h> #ifndef WIN32 #include <netinet/in.h> # ifdef _XOPEN_SOURCE_EXTENDED # include <arpa/inet.h> # endif #include <sys/socket.h> #endif // bufferevent #include <event2/bufferevent.h> // bufferevent 使用的 buffer #include <event2/buffer.h> // 连接监听器 #include <event2/listener.h> #include <event2/util.h> #include <event2/event.h> static const char MESSAGE[] = "Hello, World!\n"; static const int PORT = 9995; // 新连接到来时的回调 static void listener_cb(struct evconnlistener *, evutil_socket_t, struct sockaddr *, int socklen, void *); // 读取回调 static void conn_writecb(struct bufferevent *, void *); // 事件回调 static void conn_eventcb(struct bufferevent *, short, void *); // 信号回调 static void signal_cb(evutil_socket_t, short, void *); int main(int argc, char **argv) {  struct event_base *base;  struct evconnlistener *listener;  struct event *signal_event;  struct sockaddr_in sin; #ifdef WIN32  WSADATA wsa_data;  WSAStartup(0x0201, &wsa_data); #endif  // 首先构建 base  base = event_base_new();  if (!base) {   fprintf(stderr, "Could not initialize libevent!\n");   return 1;  }  memset(&sin, 0, sizeof(sin));  sin.sin_family = AF_INET;  sin.sin_port = htons(PORT);  // 创建监听器  listener = evconnlistener_new_bind(base, listener_cb, (void *)base,  LEV_OPT_REUSEABLE|LEV_OPT_CLOSE_ON_FREE, -1,  (struct sockaddr*)&sin,  sizeof(sin));  if (!listener) {   fprintf(stderr, "Could not create a listener!\n");   return 1;  }  // 中断信号  signal_event = evsignal_new(base, SIGINT, signal_cb, (void *)base);  if (!signal_event || event_add(signal_event, NULL)<0) {   fprintf(stderr, "Could not create/add a signal event!\n");   return 1;  }  event_base_dispatch(base);  evconnlistener_free(listener);  event_free(signal_event);  event_base_free(base);  printf("done\n");  return 0; } static void listener_cb(struct evconnlistener *listener, evutil_socket_t fd, struct sockaddr *sa, int socklen, void *user_data) {  struct event_base *base = user_data;  struct bufferevent *bev;  // 得到一个新的连接，通过连接 fd 构建一个 bufferevent  bev = bufferevent_socket_new(base, fd, BEV_OPT_CLOSE_ON_FREE);  if (!bev) {   fprintf(stderr, "Error constructing bufferevent!");   event_base_loopbreak(base);   return;  }  // 设置创建的 bufferevent 的回调函数  bufferevent_setcb(bev, NULL, conn_writecb, conn_eventcb, NULL);  bufferevent_enable(bev, EV_WRITE);  bufferevent_disable(bev, EV_READ);  // 写入数据到 bufferevent 中  bufferevent_write(bev, MESSAGE, strlen(MESSAGE)); } static void conn_writecb(struct bufferevent *bev, void *user_data) {  struct evbuffer *output = bufferevent_get_output(bev);  if (evbuffer_get_length(output) == 0) {   printf("flushed answer\n");   bufferevent_free(bev);  } } static void conn_eventcb(struct bufferevent *bev, short events, void *user_data) {  if (events & BEV_EVENT_EOF) {   printf("Connection closed.\n");  } else if (events & BEV_EVENT_ERROR) {   printf("Got an error on the connection: %s\n",   strerror(errno));/*XXX win32*/  }  /* None of the other events can happen here, since we haven't enabled  * timeouts */  bufferevent_free(bev); } static void signal_cb(evutil_socket_t sig, short events, void *user_data) {  struct event_base *base = user_data;  struct timeval delay = { 2, 0 };  printf("Caught an interrupt signal; exiting cleanly in two seconds.\n");  // 停止事件循环  event_base_loopexit(base, &delay); }

研究 bufferevent 的关键代码

这里只研究基于 socket 的 bufferevent。从上面 bufferevent 的使用可以看出，有几个关键函数：

1. 开始需要调用 bufferevent_socket_new 创建一个 bufferevent
2. 调用 bufferevent_setcb 设置回调函数
3. 调用 bufferevent_write 写入数据
4. 调用 bufferevent_free 释放 bufferevent

bufferevent_socket_new 的源码以及分析如下：

// base --- 新创建的 bufferevent 关联的 base
// fd --- bufferevent 关联的文件描述符 struct bufferevent * bufferevent_socket_new(struct event_base *base, evutil_socket_t fd, int options) {  // bufferevent_private 结构体持有 bufferevent 的数据  struct bufferevent_private *bufev_p;  // bufev == &(bufev_p->bev);  // struct bufferevent 中存放的是不同类型的 bufferevent 所共有的部分  // struct bufferevent 是 struct bufferevent_private 的子集  struct bufferevent *bufev;  // windows 下如果启用 IOCP 则构建异步 IO bufferevent #ifdef WIN32  if (base && event_base_get_iocp(base))   // 细节略   return bufferevent_async_new(base, fd, options); #endif  if ((bufev_p = mm_calloc(1, sizeof(struct bufferevent_private)))== NULL)   return NULL;  // 初始化 bufferevent_private  // 由于 bufferevent 有不同类型，所以这里设计了 bufferevent_ops_socket  // 对于不同类型的 bufferevent 有不同的 bufferevent_ops_socket 对象  // bufferevent_ops_socket 包括函数指针和一些信息  if (bufferevent_init_common(bufev_p, base, &bufferevent_ops_socket,     options) < 0) {   mm_free(bufev_p);   return NULL;  }  bufev = &bufev_p->bev;  // 设置 EVBUFFER_FLAG_DRAINS_TO_FD，此选项和 evbuffer_add_file() 函数有关（详见文档）  evbuffer_set_flags(bufev->output, EVBUFFER_FLAG_DRAINS_TO_FD);  // 初始化 read 和 write event  // 一个 bufferevent（一个 fd）关联两个 event 对象 ev_read 和 ev_write  // ev_read --- socket 可读或者超时  // ev_write --- socket 可写或者超时  // 它们都未使用 Edge triggered 方式  event_assign(&bufev->ev_read, bufev->ev_base, fd,  EV_READ|EV_PERSIST, bufferevent_readcb, bufev);  event_assign(&bufev->ev_write, bufev->ev_base, fd,  EV_WRITE|EV_PERSIST, bufferevent_writecb, bufev);  // 为输出缓冲区设置回调  // 当输出缓冲区被修改时调用 bufferevent_socket_outbuf_cb 回调函数  evbuffer_add_cb(bufev->output, bufferevent_socket_outbuf_cb, bufev);  // 防止输入缓冲区和输出缓冲区被意外修改  evbuffer_freeze(bufev->input, 0);  evbuffer_freeze(bufev->output, 1);  return bufev; }

其中 bufferevent_init_common 函数实现为：

int
bufferevent_init_common(struct bufferevent_private *bufev_private, struct event_base *base, const struct bufferevent_ops *ops, enum bufferevent_options options) {  struct bufferevent *bufev = &bufev_private->bev;  // 创建输入缓冲区  if (!bufev->input) {   if ((bufev->input = evbuffer_new()) == NULL)    return -1;  }  // 创建输出缓冲区  if (!bufev->output) {   if ((bufev->output = evbuffer_new()) == NULL) {    evbuffer_free(bufev->input);    return -1;   }  }  // 初始化 bufferevent 的引用计数  bufev_private->refcnt = 1;  bufev->ev_base = base;  /* Disable timeouts. */  // 清理超时时间  evutil_timerclear(&bufev->timeout_read);  evutil_timerclear(&bufev->timeout_write);  bufev->be_ops = ops;  /*  * Set to EV_WRITE so that using bufferevent_write is going to  * trigger a callback. Reading needs to be explicitly enabled  * because otherwise no data will be available.  */  // enabled 被 bufferevent_get_enabled 函数返回  // enabled 的值可以为 EV_WRITE EV_READ  bufev->enabled = EV_WRITE;  // bufferevent 相关线程初始化 #ifndef _EVENT_DISABLE_THREAD_SUPPORT  if (options & BEV_OPT_THREADSAFE) {   if (bufferevent_enable_locking(bufev, NULL) < 0) {    /* cleanup */    evbuffer_free(bufev->input);    evbuffer_free(bufev->output);    bufev->input = NULL;    bufev->output = NULL;    return -1;   }  } #endif  // 选项正确性检查  if ((options & (BEV_OPT_DEFER_CALLBACKS|BEV_OPT_UNLOCK_CALLBACKS))  == BEV_OPT_UNLOCK_CALLBACKS) {   event_warnx("UNLOCK_CALLBACKS requires DEFER_CALLBACKS");   return -1;  }  // defer callbacks 初始化  if (options & BEV_OPT_DEFER_CALLBACKS) {   if (options & BEV_OPT_UNLOCK_CALLBACKS)    event_deferred_cb_init(&bufev_private->deferred,    bufferevent_run_deferred_callbacks_unlocked,    bufev_private);   else    event_deferred_cb_init(&bufev_private->deferred,    bufferevent_run_deferred_callbacks_locked,    bufev_private);  }  bufev_private->options = options;  // 关联 bufferevent 和 buffer  evbuffer_set_parent(bufev->input, bufev);  evbuffer_set_parent(bufev->output, bufev);  return 0; }

bufferevent 创建成功之后，fd 上存在数据可读则调用 bufferevent_readcb

// fd 可读
static void bufferevent_readcb(evutil_socket_t fd, short event, void *arg) {  struct bufferevent *bufev = arg;  struct bufferevent_private *bufev_p =  EVUTIL_UPCAST(bufev, struct bufferevent_private, bev);  struct evbuffer *input;  int res = 0;  short what = BEV_EVENT_READING;  ev_ssize_t howmuch = -1, readmax=-1;  _bufferevent_incref_and_lock(bufev);  // 如果超时了  if (event == EV_TIMEOUT) {   /* Note that we only check for event==EV_TIMEOUT. If   * event==EV_TIMEOUT|EV_READ, we can safely ignore the   * timeout, since a read has occurred */   what |= BEV_EVENT_TIMEOUT;   goto error;  }  input = bufev->input;  /*  * If we have a high watermark configured then we don't want to  * read more data than would make us reach the watermark.  */  // 是否设置了输入缓冲区的最大大小  if (bufev->wm_read.high != 0) {   howmuch = bufev->wm_read.high - evbuffer_get_length(input);   /* we somehow lowered the watermark, stop reading */   // 缓冲区中数据过多   if (howmuch <= 0) {    // 暂停 bufferevent 的数据读取    // 具体的做法是移除 read event（ev_read）    bufferevent_wm_suspend_read(bufev);    goto done;   }  }  // 获取可读最大大小  // 和限速有关，如果不限速，则为 MAX_TO_READ_EVER(16384) 也就是 16K  readmax = _bufferevent_get_read_max(bufev_p);  if (howmuch < 0 || howmuch > readmax) /* The use of -1 for "unlimited"      * uglifies this code. XXXX */   howmuch = readmax;  // 如果读取暂停  if (bufev_p->read_suspended)   goto done;  // 输入缓冲区可读  evbuffer_unfreeze(input, 0);  // 读取 fd 上的数据  res = evbuffer_read(input, fd, (int)howmuch); /* XXXX evbuffer_read would do better to take and return ev_ssize_t */  // 输入缓冲区禁止读取  evbuffer_freeze(input, 0);  // 读取数据失败  if (res == -1) {   // 获取到错误   int err = evutil_socket_geterror(fd);   // EINTR、EAGAIN   // Windows 下为 WSAEWOULDBLOCK、WSAEINTR   if (EVUTIL_ERR_RW_RETRIABLE(err))    goto reschedule;   // 如果错误是不可重试的，报错   /* error case */   what |= BEV_EVENT_ERROR;  // eof  } else if (res == 0) {   /* eof case */   what |= BEV_EVENT_EOF;  }  if (res <= 0)   goto error;  _bufferevent_decrement_read_buckets(bufev_p, res);  /* Invoke the user callback - must always be called last */  // 判断是否需要调用回调  if (evbuffer_get_length(input) >= bufev->wm_read.low)   _bufferevent_run_readcb(bufev);  goto done; reschedule:  goto done; error:  // 出错后暂停读取数据  bufferevent_disable(bufev, EV_READ);  // 通过事件回调通知出错  _bufferevent_run_eventcb(bufev, what); done:  _bufferevent_decref_and_unlock(bufev); }

这里比较关键的函数为 evbuffer_read：

int
evbuffer_read(struct evbuffer *buf, evutil_socket_t fd, int howmuch) {  struct evbuffer_chain **chainp;  int n;  int result; #ifdef USE_IOVEC_IMPL  int nvecs, i, remaining; #else  struct evbuffer_chain *chain;  unsigned char *p; #endif  EVBUFFER_LOCK(buf);  // buffer 是否可读  if (buf->freeze_end) {   result = -1;   goto done;  }  // 获取当前 socket 可读的字节数  n = get_n_bytes_readable_on_socket(fd);  if (n <= 0 || n > EVBUFFER_MAX_READ)   n = EVBUFFER_MAX_READ;  // 尽可能多的读取  if (howmuch < 0 || howmuch > n)   howmuch = n;  // 一种实现 #ifdef USE_IOVEC_IMPL  /* Since we can use iovecs, we're willing to use the last  * NUM_READ_IOVEC chains. */  // 调整缓冲区（细节略）  if (_evbuffer_expand_fast(buf, howmuch, NUM_READ_IOVEC) == -1) {   result = -1;   goto done;  } else {   IOV_TYPE vecs[NUM_READ_IOVEC]; #ifdef _EVBUFFER_IOVEC_IS_NATIVE   nvecs = _evbuffer_read_setup_vecs(buf, howmuch, vecs,   NUM_READ_IOVEC, &chainp, 1); #else   /* We aren't using the native struct iovec. Therefore,   we are on win32. */   struct evbuffer_iovec ev_vecs[NUM_READ_IOVEC];   nvecs = _evbuffer_read_setup_vecs(buf, howmuch, ev_vecs, 2,   &chainp, 1);   for (i=0; i < nvecs; ++i)    WSABUF_FROM_EVBUFFER_IOV(&vecs[i], &ev_vecs[i]); #endif #ifdef WIN32   {    // 读取到的数据字节数    DWORD bytesRead;    DWORD flags=0;    // windows 下进行 recv    if (WSARecv(fd, vecs, nvecs, &bytesRead, &flags, NULL, NULL)) {     /* The read failed. It might be a close,     * or it might be an error. */     // 这里使用 WSARecv 时需要注意 WSAECONNABORTED 表示了连接关闭了     if (WSAGetLastError() == WSAECONNABORTED)      n = 0;     else      n = -1;    } else     n = bytesRead;   } #else   // 非 windows 平台 read   n = readv(fd, vecs, nvecs); #endif  }  // 使用另外一种实现 #else /*!USE_IOVEC_IMPL*/  /* If we don't have FIONREAD, we might waste some space here */  /* XXX we _will_ waste some space here if there is any space left  * over on buf->last. */  if ((chain = evbuffer_expand_singlechain(buf, howmuch)) == NULL) {   result = -1;   goto done;  }  /* We can append new data at this point */  p = chain->buffer + chain->misalign + chain->off;  // read #ifndef WIN32  n = read(fd, p, howmuch); #else  n = recv(fd, p, howmuch, 0); #endif #endif /* USE_IOVEC_IMPL */  if (n == -1) {   result = -1;   goto done;  }  if (n == 0) {   result = 0;   goto done;  } #ifdef USE_IOVEC_IMPL  remaining = n;  for (i=0; i < nvecs; ++i) {   ev_ssize_t space = (ev_ssize_t) CHAIN_SPACE_LEN(*chainp);   if (space < remaining) {    (*chainp)->off += space;    remaining -= (int)space;   } else {    (*chainp)->off += remaining;    buf->last_with_datap = chainp;    break;   }   chainp = &(*chainp)->next;  } #else  chain->off += n;  advance_last_with_data(buf); #endif  buf->total_len += n;  buf->n_add_for_cb += n;  /* Tell someone about changes in this buffer */  // 调用回调  evbuffer_invoke_callbacks(buf);  result = n; done:  EVBUFFER_UNLOCK(buf);  return result; }

读完了 bufferevent_readcb 接下来再看看 bufferevent_writecb：

// fd 可写
static void bufferevent_writecb(evutil_socket_t fd, short event, void *arg) {  struct bufferevent *bufev = arg;  struct bufferevent_private *bufev_p =  EVUTIL_UPCAST(bufev, struct bufferevent_private, bev);  int res = 0;  short what = BEV_EVENT_WRITING;  int connected = 0;  ev_ssize_t atmost = -1;  _bufferevent_incref_and_lock(bufev);  // 如果超时了  if (event == EV_TIMEOUT) {   /* Note that we only check for event==EV_TIMEOUT. If   * event==EV_TIMEOUT|EV_WRITE, we can safely ignore the   * timeout, since a read has occurred */   what |= BEV_EVENT_TIMEOUT;   goto error;  }  // 正在连接  if (bufev_p->connecting) {   // 连接是否完成   int c = evutil_socket_finished_connecting(fd);   /* we need to fake the error if the connection was refused   * immediately - usually connection to localhost on BSD */   if (bufev_p->connection_refused) {   bufev_p->connection_refused = 0;   c = -1;   }   // 如果还没有连接完成，则不处理   if (c == 0)    goto done;   // 如果连接完成   bufev_p->connecting = 0;   // 如果连接出错   if (c < 0) {    // 移除 bufferevent 的事件    event_del(&bufev->ev_write);    event_del(&bufev->ev_read);    // 事件回调，告知连接出错    _bufferevent_run_eventcb(bufev, BEV_EVENT_ERROR);    goto done;   // 如果连接成功   } else {    connected = 1; #ifdef WIN32    // 是否为异步 IO bufferevent（IOCP）    if (BEV_IS_ASYNC(bufev)) {     event_del(&bufev->ev_write);     bufferevent_async_set_connected(bufev);     _bufferevent_run_eventcb(bufev,       BEV_EVENT_CONNECTED);     goto done;    } #endif    // 通知连接成功    _bufferevent_run_eventcb(bufev,      BEV_EVENT_CONNECTED);    // 如果不可写    if (!(bufev->enabled & EV_WRITE) ||    bufev_p->write_suspended) {     // 移除 ev_write     event_del(&bufev->ev_write);     goto done;    }   }  }  // 获取可写最大大小  // 和限速有关，如果不限速，则为 MAX_TO_WRITE_EVER(16384) 也就是 16K  atmost = _bufferevent_get_write_max(bufev_p);  // 如果不可写  if (bufev_p->write_suspended)   goto done;  // 如果输出缓冲区存在数据  if (evbuffer_get_length(bufev->output)) {   evbuffer_unfreeze(bufev->output, 1);   // 写入尽可能多的数据到 fd   res = evbuffer_write_atmost(bufev->output, fd, atmost);   evbuffer_freeze(bufev->output, 1);   if (res == -1) {    int err = evutil_socket_geterror(fd);    // 如果写入数据出错时可重试    if (EVUTIL_ERR_RW_RETRIABLE(err))     goto reschedule;    // 真正出错则设置    what |= BEV_EVENT_ERROR;   // 连接断开   } else if (res == 0) {    /* eof case    XXXX Actually, a 0 on write doesn't indicate    an EOF. An ECONNRESET might be more typical.    */    what |= BEV_EVENT_EOF;   }   if (res <= 0)    goto error;   _bufferevent_decrement_write_buckets(bufev_p, res);  }  // 如果缓冲区所有数据都被写入完了  if (evbuffer_get_length(bufev->output) == 0) {   // 清除 ev_write（无需继续写入数据了）   event_del(&bufev->ev_write);  }  /*  * Invoke the user callback if our buffer is drained or below the  * low watermark.  */  // 尝试调用写入回调函数  if ((res || !connected) &&  evbuffer_get_length(bufev->output) <= bufev->wm_write.low) {   _bufferevent_run_writecb(bufev);  }  goto done; reschedule:  // 如果无法写入，但是输出缓冲区已经为空时  if (evbuffer_get_length(bufev->output) == 0) {   // 无需继续写入数据了   event_del(&bufev->ev_write);  }  goto done; error:  // 出错时告知上层  bufferevent_disable(bufev, EV_WRITE);  _bufferevent_run_eventcb(bufev, what); done:  _bufferevent_decref_and_unlock(bufev); }

bufferevent_writecb 中比较关键的一个函数为 evbuffer_write_atmost：

int
evbuffer_write_atmost(struct evbuffer *buffer, evutil_socket_t fd, ev_ssize_t howmuch) {  int n = -1;  EVBUFFER_LOCK(buffer);  // 是否不可写  if (buffer->freeze_start) {   goto done;  }  // 写尽量多的数据  if (howmuch < 0 || (size_t)howmuch > buffer->total_len)   howmuch = buffer->total_len;  // 如果有数据需要写  if (howmuch > 0) {   // 使用 evbuffer_write_sendfile 写数据 #ifdef USE_SENDFILE   struct evbuffer_chain *chain = buffer->first;   if (chain != NULL && (chain->flags & EVBUFFER_SENDFILE))    n = evbuffer_write_sendfile(buffer, fd, howmuch);   else { #endif #ifdef USE_IOVEC_IMPL   // 使用 evbuffer_write_iovec 写数据   n = evbuffer_write_iovec(buffer, fd, howmuch); #elif defined(WIN32)   /* XXX(nickm) Don't disable this code until we know if   * the WSARecv code above works. */   void *p = evbuffer_pullup(buffer, howmuch);   // windows 下 send   n = send(fd, p, howmuch, 0); #else   void *p = evbuffer_pullup(buffer, howmuch);   // 其他平台 write   n = write(fd, p, howmuch); #endif #ifdef USE_SENDFILE   } #endif  }  if (n > 0)   // 如果写入的数据大于 0 则缓冲区对应移除相关数据   evbuffer_drain(buffer, n); done:  EVBUFFER_UNLOCK(buffer);  return (n); }

代码读到这里，对于 bufferevent 的创建、socket 读写已经有了一定的了解，下面再看看 bufferevent_write，此函数实际上只是直接向输出缓冲区写入数据，缓冲区写入数据后，会调用回调 bufferevent_socket_outbuf_cb（创建 bufferevent 时设置的），此回调工作内容比较简单，主要就是将 ev_write 注册到 base 中去：

static void bufferevent_socket_outbuf_cb(struct evbuffer *buf, const struct evbuffer_cb_info *cbinfo, void *arg) {  struct bufferevent *bufev = arg;  struct bufferevent_private *bufev_p =  EVUTIL_UPCAST(bufev, struct bufferevent_private, bev);   // evbuffer 中有数据  if (cbinfo->n_added &&   // bufferevent 可以写入数据  (bufev->enabled & EV_WRITE) &&   // 检测 ev_write 是否已经是 pending 状态或者已经被调度   // 这里无需重复注册 ev_write  !event_pending(&bufev->ev_write, EV_WRITE, NULL) &&   // bufferevent 是否已经禁止写入  !bufev_p->write_suspended) {   /* Somebody added data to the buffer, and we would like to   * write, and we were not writing. So, start writing. */   // 注册 ev_write 写入数据   if (be_socket_add(&bufev->ev_write, &bufev->timeout_write) == -1) {   /* Should we log this? */   }  } }

最后来看看释放过程：

void
bufferevent_free(struct bufferevent *bufev) {  BEV_LOCK(bufev);  // 清理回调  bufferevent_setcb(bufev, NULL, NULL, NULL, NULL);  // 此函数似乎啥也没做  _bufferevent_cancel_all(bufev);  // 真正的清理发生在 bufferevent 引用计数为 0 时  _bufferevent_decref_and_unlock(bufev); } int _bufferevent_decref_and_unlock(struct bufferevent *bufev) {  struct bufferevent_private *bufev_private =  EVUTIL_UPCAST(bufev, struct bufferevent_private, bev);  struct bufferevent *underlying;  EVUTIL_ASSERT(bufev_private->refcnt > 0);  // 引用计数减 1  if (--bufev_private->refcnt) {   BEV_UNLOCK(bufev);   return 0;  }  // 如果 bufferevent 引用技术为 0 了  // 获取底层 bufferevent  underlying = bufferevent_get_underlying(bufev);  /* Clean up the shared info */  if (bufev->be_ops->destruct)   // 调用 be_socket_destruct   // 清理 ev_read 和 ev_write   // 关闭 socket   bufev->be_ops->destruct(bufev);  /* XXX what happens if refcnt for these buffers is > 1?  * The buffers can share a lock with this bufferevent object,  * but the lock might be destroyed below. */  /* evbuffer will free the callbacks */  // 释放缓冲区  evbuffer_free(bufev->input);  evbuffer_free(bufev->output);  // 如果使用了限速，则进行相关清理  if (bufev_private->rate_limiting) {   if (bufev_private->rate_limiting->group)    bufferevent_remove_from_rate_limit_group_internal(bufev,0);   if (event_initialized(&bufev_private->rate_limiting->refill_bucket_event))    event_del(&bufev_private->rate_limiting->refill_bucket_event);   event_debug_unassign(&bufev_private->rate_limiting->refill_bucket_event);   mm_free(bufev_private->rate_limiting);   bufev_private->rate_limiting = NULL;  }  event_debug_unassign(&bufev->ev_read);  event_debug_unassign(&bufev->ev_write);  BEV_UNLOCK(bufev);  if (bufev_private->own_lock)   EVTHREAD_FREE_LOCK(bufev_private->lock,   EVTHREAD_LOCKTYPE_RECURSIVE);  /* Free the actual allocated memory. */  mm_free(((char*)bufev) - bufev->be_ops->mem_offset);  /* Release the reference to underlying now that we no longer need the  * reference to it. We wait this long mainly in case our lock is  * shared with underlying.  *  * The 'destruct' function will also drop a reference to underlying  * if BEV_OPT_CLOSE_ON_FREE is set.  *  * XXX Should we/can we just refcount evbuffer/bufferevent locks?  * It would probably save us some headaches.  */  if (underlying)   bufferevent_decref(underlying);  return 1; }

更多详细的内容还需要更进一步阅读源码。