bufferevent 與 socket

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

很多時候，除了響應事件之外，應用還希望做一定的數據緩沖。比如說，寫入數據的時候，通常的運行模式是：

l?決定要向連接寫入一些數據，把數據放入到緩沖區中

l?等待連接可以寫入

l?寫入盡量多的數據

l?記住寫入了多少數據，如果還有更多數據要寫入，等待連接再次可以寫入

這種緩沖IO模式很通用，libevent為此提供了一種通用機制，即bufferevent。bufferevent由一個底層的傳輸端口（如套接字），一個讀取緩沖區和一個寫入緩沖區組成。與通常的事件在底層傳輸端口已經就緒，可以讀取或者寫入的時候執行回調不同的是，bufferevent在讀取或者寫入了足夠量的數據之后調用用戶提供的回調。

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bufferevent 的簡單范例

這里選取了 Libevent 的一個范例程序 hello-world.c 來看看 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> // 連接監聽器 #include <event2/listener.h> #include <event2/util.h> #include <event2/event.h> static const char MESSAGE[] = "Hello, World!\n"; static const int PORT = 9995; // 新連接到來時的回調 static void listener_cb(struct evconnlistener *, evutil_socket_t, struct sockaddr *, int socklen, void *); // 讀取回調 static void conn_writecb(struct bufferevent *, void *); // 事件回調 static void conn_eventcb(struct bufferevent *, short, void *); // 信號回調 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  // 首先構建 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);  // 創建監聽器  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;  }  // 中斷信號  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;  // 得到一個新的連接，通過連接 fd 構建一個 bufferevent  bev = bufferevent_socket_new(base, fd, BEV_OPT_CLOSE_ON_FREE);  if (!bev) {   fprintf(stderr, "Error constructing bufferevent!");   event_base_loopbreak(base);   return;  }  // 設置創建的 bufferevent 的回調函數  bufferevent_setcb(bev, NULL, conn_writecb, conn_eventcb, NULL);  bufferevent_enable(bev, EV_WRITE);  bufferevent_disable(bev, EV_READ);  // 寫入數據到 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");  // 停止事件循環  event_base_loopexit(base, &delay); }

研究 bufferevent 的關鍵代碼

這里只研究基于 socket 的 bufferevent。從上面 bufferevent 的使用可以看出，有幾個關鍵函數：

開始需要調用 bufferevent_socket_new 創建一個 bufferevent
調用 bufferevent_setcb 設置回調函數
調用 bufferevent_write 寫入數據
調用 bufferevent_free 釋放 bufferevent

bufferevent_socket_new 的源碼以及分析如下：

// base --- 新創建的 bufferevent 關聯的 base
// fd --- bufferevent 關聯的文件描述符
struct bufferevent *
bufferevent_socket_new(struct event_base *base, evutil_socket_t fd,int options) {  // bufferevent_private 結構體持有 bufferevent 的數據  struct bufferevent_private *bufev_p;  // bufev == &(bufev_p->bev);  // struct bufferevent 中存放的是不同類型的 bufferevent 所共有的部分  // struct bufferevent 是 struct bufferevent_private 的子集  struct bufferevent *bufev;  // windows 下如果啟用 IOCP 則構建異步 IO bufferevent #ifdef WIN32  if (base && event_base_get_iocp(base))   // 細節略   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 有不同類型，所以這里設計了 bufferevent_ops_socket  // 對于不同類型的 bufferevent 有不同的 bufferevent_ops_socket 對象  // bufferevent_ops_socket 包括函數指針和一些信息  if (bufferevent_init_common(bufev_p, base, &bufferevent_ops_socket,     options) < 0) {   mm_free(bufev_p);   return NULL;  }  bufev = &bufev_p->bev;  // 設置 EVBUFFER_FLAG_DRAINS_TO_FD，此選項和 evbuffer_add_file() 函數有關（詳見文檔）  evbuffer_set_flags(bufev->output, EVBUFFER_FLAG_DRAINS_TO_FD);  // 初始化 read 和 write event  // 一個 bufferevent（一個 fd）關聯兩個 event 對象 ev_read 和 ev_write  // ev_read --- socket 可讀或者超時  // ev_write --- socket 可寫或者超時  // 它們都未使用 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);  // 為輸出緩沖區設置回調  // 當輸出緩沖區被修改時調用 bufferevent_socket_outbuf_cb 回調函數  evbuffer_add_cb(bufev->output, bufferevent_socket_outbuf_cb, bufev);  // 防止輸入緩沖區和輸出緩沖區被意外修改  evbuffer_freeze(bufev->input, 0);  evbuffer_freeze(bufev->output, 1);  return bufev; }

其中 bufferevent_init_common 函數實現為：

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;  // 創建輸入緩沖區  if (!bufev->input) {   if ((bufev->input = evbuffer_new()) == NULL)    return -1;  }  // 創建輸出緩沖區  if (!bufev->output) {   if ((bufev->output = evbuffer_new()) == NULL) {    evbuffer_free(bufev->input);    return -1;   }  }  // 初始化 bufferevent 的引用計數  bufev_private->refcnt = 1;  bufev->ev_base = base;  /* Disable timeouts. */  // 清理超時時間  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 函數返回  // enabled 的值可以為 EV_WRITE EV_READ  bufev->enabled = EV_WRITE;  // bufferevent 相關線程初始化 #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  // 選項正確性檢查  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;  // 關聯 bufferevent 和 buffer  evbuffer_set_parent(bufev->input, bufev);  evbuffer_set_parent(bufev->output, bufev);  return 0; }

bufferevent 創建成功之后，fd 上存在數據可讀則調用 bufferevent_readcb

// fd 可讀
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);  // 如果超時了  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.  */  // 是否設置了輸入緩沖區的最大大小  if (bufev->wm_read.high != 0) {   howmuch = bufev->wm_read.high - evbuffer_get_length(input);   /* we somehow lowered the watermark, stop reading */   // 緩沖區中數據過多   if (howmuch <= 0) {    // 暫停 bufferevent 的數據讀取    // 具體的做法是移除 read event（ev_read）    bufferevent_wm_suspend_read(bufev);    goto done;   }  }  // 獲取可讀最大大小  // 和限速有關，如果不限速，則為 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;  // 如果讀取暫停  if (bufev_p->read_suspended)   goto done;  // 輸入緩沖區可讀  evbuffer_unfreeze(input, 0);  // 讀取 fd 上的數據  res = evbuffer_read(input, fd, (int)howmuch); /* XXXX evbuffer_read would do better to take and return ev_ssize_t */  // 輸入緩沖區禁止讀取  evbuffer_freeze(input, 0);  // 讀取數據失敗  if (res == -1) {   // 獲取到錯誤   int err = evutil_socket_geterror(fd);   // EINTR、EAGAIN   // Windows 下為 WSAEWOULDBLOCK、WSAEINTR   if (EVUTIL_ERR_RW_RETRIABLE(err))    goto reschedule;   // 如果錯誤是不可重試的，報錯   /* 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 */  // 判斷是否需要調用回調  if (evbuffer_get_length(input) >= bufev->wm_read.low)   _bufferevent_run_readcb(bufev);  goto done; reschedule:  goto done; error:  // 出錯后暫停讀取數據  bufferevent_disable(bufev, EV_READ);  // 通過事件回調通知出錯  _bufferevent_run_eventcb(bufev, what); done:  _bufferevent_decref_and_unlock(bufev); }

這里比較關鍵的函數為 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 是否可讀  if (buf->freeze_end) {   result = -1;   goto done;  }  // 獲取當前 socket 可讀的字節數  n = get_n_bytes_readable_on_socket(fd);  if (n <= 0 || n > EVBUFFER_MAX_READ)   n = EVBUFFER_MAX_READ;  // 盡可能多的讀取  if (howmuch < 0 || howmuch > n)   howmuch = n;  // 一種實現 #ifdef USE_IOVEC_IMPL  /* Since we can use iovecs, we're willing to use the last  * NUM_READ_IOVEC chains. */  // 調整緩沖區（細節略）  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   {    // 讀取到的數據字節數    DWORD bytesRead;    DWORD flags=0;    // windows 下進行 recv    if (WSARecv(fd, vecs, nvecs, &bytesRead, &flags, NULL, NULL)) {     /* The read failed. It might be a close,     * or it might be an error. */     // 這里使用 WSARecv 時需要注意 WSAECONNABORTED 表示了連接關閉了     if (WSAGetLastError() == WSAECONNABORTED)      n = 0;     else      n = -1;    } else     n = bytesRead;   } #else   // 非 windows 平臺 read   n = readv(fd, vecs, nvecs); #endif  }  // 使用另外一種實現 #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;