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libevent中的bufferevent原理

以前的文章看过缓冲区buffer了，libevent用bufferevent来负责管理缓冲区与buffer读写事件。
今天就带大家看下evbuffer.c，使用bufferevent处理事件的数据，是buffer和event的综合。在最后用一个稍微综合的例子看下使用bufferevent的整个流程。
　　首先依旧看下bufferevent的结构。结构很清晰。源码版本1.4.14。

 1 struct bufferevent {
 2     struct event_base *ev_base;
 3 
 4     //读事件
 5     struct event ev_read;
 6     //写事件
 7     struct event ev_write;
 8     //读缓冲区，输入缓冲
 9     struct evbuffer *input;
10     //写缓冲区，输出缓冲
11     struct evbuffer *output;
12 
13     //读水位
14     struct event_watermark wm_read;
15     //写水位
16     struct event_watermark wm_write;
17 
18     //发生读触发用户设置的回调
19     evbuffercb readcb;
20     //发生写触发用户设置的回调
21     evbuffercb writecb;
22     //发生错误触发用户设置的回调
23     everrorcb errorcb;
24     //当前设置的回调函数传递的参数，和上面3个回调配合使用
25     void *cbarg;
26 
27     //设置读超时时间，默认为0
28     int timeout_read;    /* in seconds */
29     //设置写超时时间，默认为0
30     int timeout_write;    /* in seconds */
31 
32     //当前事件是否可用
33     short enabled;    /* events that are currently enabled */
34 };
35 //水位
36 struct event_watermark {
37     //低水位
38     size_t low;
39     //高水位
40     size_t high;
41 };

evbuffer中有2个缓冲区，一个是读缓冲区，一个写缓冲区。分别用来处理读写事件的数据。
evbuffer中有读水位和写水位，分别对应了读缓冲区和写缓冲区。
里面有个水位的概念。其实很好理解。水位有一个高水位，一个低水位。
如果水位达到高水位时，不能再往里面灌水了。如果水位达到低水位，不能再从中取水了。

读操作发生时：如果高于高水位，那就不能再读入数据了，等待数据被读掉然后再开始读入数据。低水位只做判断。低水位不为0，如果缓冲区低于低水位，可以继续直接读数据到缓冲区。
写操作发生时：如果写缓冲区数据长度小于等于低水位，触发用户写事件，通知用户。写数据高水位没用。因为写数据是把缓冲区的数据读出写到对应的文件描述符中，所以水位肯定是下降的。
我的理解：水位控制了信息的颗粒度，多少数据触发次用户事件。数据缓冲区降低了频繁申请内存带来的开销。

接着我们来看evbuffer.c中最重要的几个函数

1.bufferevent_new

进行一些初始化。最重要的是指定了eventbuffer内部读写事件的回调，bufferevent_readcb与bufferevent_writecb。当前也可以通过后面的bufferevent_setcb实现。

 1 struct bufferevent *
 2 bufferevent_new(int fd, evbuffercb readcb, evbuffercb writecb,
 3     everrorcb errorcb, void *cbarg)
 4 {
 5     struct bufferevent *bufev;
 6 
 7     //申请内存空间并且初始化，使用calloc
 8     if ((bufev = calloc(1, sizeof(struct bufferevent))) == NULL)
 9         return (NULL);
10 
11     if ((bufev->input = evbuffer_new()) == NULL) {
12         free(bufev);
13         return (NULL);
14     }
15 
16     if ((bufev->output = evbuffer_new()) == NULL) {
17         evbuffer_free(bufev->input);
18         free(bufev);
19         return (NULL);
20     }
21     //读事件关联回调，传递参数
22     event_set(&bufev->ev_read, fd, EV_READ, bufferevent_readcb, bufev);
23 
24     //写事件关联回调，传递参数
25     event_set(&bufev->ev_write, fd, EV_WRITE, bufferevent_writecb, bufev);
26 
27     //设置bufferevent的读、写和出错事件回调，并且传递cbarg参数。
28     bufferevent_setcb(bufev, readcb, writecb, errorcb, cbarg);
29 
30     /*
31      * Set to EV_WRITE so that using bufferevent_write is going to
32      * trigger a callback.  Reading needs to be explicitly enabled
33      * because otherwise no data will be available.
34      */
35     //开启可写，否则无法执行写入回调
36     bufev->enabled = EV_WRITE;
37 
38     return (bufev);
39 }

2.bufferevent_readcb

读事件，最先接触到数据，读出数据然后写入缓冲区

首先看下bufferevent_readcb的流程图

 1 //读事件，最先接触到数据，读出数据然后写入缓冲区
 2 static void
 3 bufferevent_readcb(int fd, short event, void *arg)
 4 {
 5     struct bufferevent *bufev = arg;
 6     int res = 0;
 7     short what = EVBUFFER_READ;
 8     size_t len;
 9     int howmuch = -1;
10     //超时事件，报错
11     if (event == EV_TIMEOUT) {
12         what |= EVBUFFER_TIMEOUT;
13         goto error;
14     }
15 
16     /*
17      * If we have a high watermark configured then we don't want to
18      * read more data than would make us reach the watermark.
19      */
20     //查看高水位，如果缓冲区数据已经高于高水位，不应该再写入。
21     if (bufev->wm_read.high != 0) {
22         howmuch = bufev->wm_read.high - EVBUFFER_LENGTH(bufev->input);
23         /* we might have lowered the watermark, stop reading */
24         if (howmuch <= 0) {
25             struct evbuffer *buf = bufev->input;
26             //达到高水位，删除读入事件，不再读入数据到缓冲区
27             event_del(&bufev->ev_read);
28             //设置bufev->input变化需要调用的回调函数和回调参数
29             evbuffer_setcb(buf,
30                 bufferevent_read_pressure_cb, bufev);
31             return;
32         }
33     }
34     //没达到高水位，读取数据到input缓冲区中
35     res = evbuffer_read(bufev->input, fd, howmuch);
36     if (res == -1) {
37         //信号中断等一些原因，goto reschedule，可以继续。
38         if (errno == EAGAIN || errno == EINTR)
39             goto reschedule;
40         /* error case */
41         what |= EVBUFFER_ERROR;
42     } else if (res == 0) {
43         /* eof case */
44         what |= EVBUFFER_EOF;
45     }
46 
47     if (res <= 0)
48         goto error;
49     //读事件加入事件队列
50     bufferevent_add(&bufev->ev_read, bufev->timeout_read);
51 
52     /* See if this callbacks meets the water marks */
53     len = EVBUFFER_LENGTH(bufev->input);
54     if (bufev->wm_read.low != 0 && len < bufev->wm_read.low)
55         return;
56     //如果高水位不为0，并且缓冲区数据长度已经不小于高水位了，触发事件。
57     if (bufev->wm_read.high != 0 && len >= bufev->wm_read.high) {
58         //缓冲区数据已经不小于高水位，不能再进数据了，删除读缓冲区的读外部数据事件
59         struct evbuffer *buf = bufev->input;
60         event_del(&bufev->ev_read);
61 
62         /* Now schedule a callback for us when the buffer changes */
63         //缓冲区大小发生变化，触发回调
64         //设置回调函数和回调参数
65         evbuffer_setcb(buf, bufferevent_read_pressure_cb, bufev);
66     }
67 
68     /* Invoke the user callback - must always be called last */
69     //触发用户回调事件
70     if (bufev->readcb != NULL)
71         (*bufev->readcb)(bufev, bufev->cbarg);
72     return;
73 
74 reschedule:
75     //读事件加入事件队列，继续进行读取
76     bufferevent_add(&bufev->ev_read, bufev->timeout_read);
77     return;
78 
79  error:
80     (*bufev->errorcb)(bufev, what, bufev->cbarg);
81 }

3.bufferevent_writecb

写事件

 1 static void
 2 bufferevent_writecb(int fd, short event, void *arg)
 3 {
 4     //事件缓冲区管理
 5     struct bufferevent *bufev = arg;
 6     int res = 0;
 7     short what = EVBUFFER_WRITE;
 8 
 9     //超时事件，报错
10     if (event == EV_TIMEOUT) {
11         what |= EVBUFFER_TIMEOUT;
12         goto error;
13     }
14 
15     if (EVBUFFER_LENGTH(bufev->output)) {
16         //将缓冲区数据读出，写入到fd文件描述符对应的文件中
17         res = evbuffer_write(bufev->output, fd);
18         if (res == -1) {
19 #ifndef WIN32
20 /*todo. evbuffer uses WriteFile when WIN32 is set. WIN32 system calls do not
21  *set errno. thus this error checking is not portable*/
22             if (errno == EAGAIN ||
23             errno == EINTR ||
24             errno == EINPROGRESS)
25                 goto reschedule;
26             /* error case */
27             what |= EVBUFFER_ERROR;
28 
29 #else
30                 goto reschedule;
31 #endif
32 
33         } else if (res == 0) {
34             /* eof case */
35             what |= EVBUFFER_EOF;
36         }
37         if (res <= 0)
38             goto error;
39     }
40     //缓冲区不为0，写事件加入执行队列
41     if (EVBUFFER_LENGTH(bufev->output) != 0)
42         bufferevent_add(&bufev->ev_write, bufev->timeout_write);
43 
44     /*
45      * Invoke the user callback if our buffer is drained or below the
46      * low watermark.
47      */
48     //缓冲区数据长度低于低水位，用户写事件触发。
49     if (bufev->writecb != NULL &&
50         EVBUFFER_LENGTH(bufev->output) <= bufev->wm_write.low)
51         (*bufev->writecb)(bufev, bufev->cbarg);
52     return;
53 
54  reschedule:
55     if (EVBUFFER_LENGTH(bufev->output) != 0)
56         bufferevent_add(&bufev->ev_write, bufev->timeout_write);
57     return;
58 
59  error:
60     (*bufev->errorcb)(bufev, what, bufev->cbarg);
61 }

示例

下面看一个改造过的服务器和客户端的例子。（当前你可以直接使用test中的regress.c例子，我这边因为libevent本来就是用来解决网络问题的，所以自己就用了这个例子）

server.c

我的编译命令：gcc -g -Wall -I/usr/local/include -o server server.c -L/usr/local/lib -levent

服务端监听所有socket。端口5555。这里我们为了演示：evbuffer读缓冲区对应水位设置为高水位10，低水位0。

  1 /*
  2 * libevent echo server example using buffered events.
  3 */
  4 
  5 #include <sys/types.h>
  6 #include <sys/socket.h>
  7 #include <netinet/in.h>
  8 #include <arpa/inet.h>
  9 
 10 /* Required by event.h. */
 11 #include <sys/time.h>
 12 
 13 #include <stdlib.h>
 14 #include <stdio.h>
 15 #include <string.h>
 16 #include <fcntl.h>
 17 #include <unistd.h>
 18 #include <errno.h>
 19 #include <err.h>
 20 
 21 /* Libevent. */
 22 #include <event.h>
 23 
 24 /* Port to listen on. */
 25 #define SERVER_PORT 5555
 26 
 27 /**
 28 * A struct for client specific data, also includes pointer to create
 29 * a list of clients.
 30 */
 31 struct client {
 32     /* The clients socket. */
 33     int fd;
 34 
 35     /* The bufferedevent for this client. */
 36     struct bufferevent *buf_ev;
 37 };
 38 
 39 /**
 40 * Set a socket to non-blocking mode.
 41 */
 42 //用于设置非阻塞
 43 int
 44 setnonblock(int fd)
 45 {
 46     int flags;
 47 
 48     flags = fcntl(fd, F_GETFL);
 49     if (flags < 0)
 50         return flags;
 51     flags |= O_NONBLOCK;
 52     if (fcntl(fd, F_SETFL, flags) < 0)
 53         return -1;
 54 
 55     return 0;
 56 }
 57 
 58 /**
 59 * Called by libevent when there is data to read.
 60 */
 61 void
 62 buffered_on_read(struct bufferevent *bev, void *arg)
 63 {
 64     /* Write back the read buffer. It is important to note that
 65     * bufferevent_write_buffer will drain the incoming data so it
 66     * is effectively gone after we call it. */
 67     char msg[4096];
 68 
 69     size_t len = bufferevent_read(bev, msg, sizeof(msg));
 70 
 71     msg[len] = '';
 72     printf("recv the client msg: %s
", msg);
 73 
 74     char reply_msg[4096] = "I have recvieced the msg: ";
 75     strcat(reply_msg + strlen(reply_msg), msg);
 76     bufferevent_write(bev, reply_msg, strlen(reply_msg));
 77 
 78 }
 79 
 80 /**
 81 * Called by libevent when the write buffer reaches 0.  We only
 82 * provide this because libevent expects it, but we don't use it.
 83 */
 84 //当写缓冲区达到低水位时触发调用，我们这边不用
 85 void
 86 buffered_on_write(struct bufferevent *bev, void *arg)
 87 {
 88     
 89 }
 90 
 91 /**
 92 * Called by libevent when there is an error on the underlying socket
 93 * descriptor.
 94 */
 95 void
 96 buffered_on_error(struct bufferevent *bev, short what, void *arg)
 97 {
 98     struct client *client = (struct client *)arg;
 99 
100     if (what & EVBUFFER_EOF) {
101         /* Client disconnected, remove the read event and the
102         * free the client structure. */
103         printf("Client disconnected.
");
104     }
105     else {
106         warn("Client socket error, disconnecting.
");
107     }
108     bufferevent_free(client->buf_ev);
109     close(client->fd);
110     free(client);
111 }
112 
113 /**
114 * This function will be called by libevent when there is a connection
115 * ready to be accepted.
116 */
117 void
118 on_accept(int fd, short ev, void *arg)
119 {
120     int client_fd;
121     struct sockaddr_in client_addr;
122     socklen_t client_len = sizeof(client_addr);
123     struct client *client;
124 
125     client_fd = accept(fd, (struct sockaddr *)&client_addr, &client_len);
126     if (client_fd < 0) {
127         warn("accept failed");
128         return;
129     }
130 
131     /* Set the client socket to non-blocking mode. */
132     if (setnonblock(client_fd) < 0)
133         warn("failed to set client socket non-blocking");
134 
135     /* We've accepted a new client, create a client object. */
136     client = calloc(1, sizeof(*client));
137     if (client == NULL)
138         err(1, "malloc failed");
139     client->fd = client_fd;
140 
141     /* Create the buffered event.
142     *
143     * The first argument is the file descriptor that will trigger
144     * the events, in this case the clients socket.
145     *
146     * The second argument is the callback that will be called
147     * when data has been read from the socket and is available to
148     * the application.
149     *
150     * The third argument is a callback to a function that will be
151     * called when the write buffer has reached a low watermark.
152     * That usually means that when the write buffer is 0 length,
153     * this callback will be called.  It must be defined, but you
154     * don't actually have to do anything in this callback.
155     *
156     * The fourth argument is a callback that will be called when
157     * there is a socket error.  This is where you will detect
158     * that the client disconnected or other socket errors.
159     *
160     * The fifth and final argument is to store an argument in
161     * that will be passed to the callbacks.  We store the client
162     * object here.
163     */
164     client->buf_ev = bufferevent_new(client_fd, buffered_on_read,
165         buffered_on_write, buffered_on_error, client);
166     client->buf_ev->wm_read.high = 10;
167     client->buf_ev->wm_read.low = 0;
168     /* We have to enable it before our callbacks will be
169     * called. */
170     bufferevent_enable(client->buf_ev, EV_READ);
171 
172     printf("Accepted connection from %s
",
173         inet_ntoa(client_addr.sin_addr));
174 }
175 
176 int
177 main(int argc, char **argv)
178 {
179     int listen_fd;
180     struct sockaddr_in listen_addr;
181     struct event ev_accept;
182     int reuseaddr_on;
183 
184     /* Initialize libevent. */
185     event_init();
186 
187     /* Create our listening socket. */
188     listen_fd = socket(AF_INET, SOCK_STREAM, 0);
189     if (listen_fd < 0)
190         err(1, "listen failed");
191     memset(&listen_addr, 0, sizeof(listen_addr));
192     listen_addr.sin_family = AF_INET;
193     listen_addr.sin_addr.s_addr = INADDR_ANY;
194     listen_addr.sin_port = htons(SERVER_PORT);
195     if (bind(listen_fd, (struct sockaddr *)&listen_addr,
196         sizeof(listen_addr)) < 0)
197         err(1, "bind failed");
198     if (listen(listen_fd, 5) < 0)
199         err(1, "listen failed");
200     reuseaddr_on = 1;
201     setsockopt(listen_fd, SOL_SOCKET, SO_REUSEADDR, &reuseaddr_on,
202         sizeof(reuseaddr_on));
203 
204     /* Set the socket to non-blocking, this is essential in event
205     * based programming with libevent. */
206     if (setnonblock(listen_fd) < 0)
207         err(1, "failed to set server socket to non-blocking");
208 
209     /* We now have a listening socket, we create a read event to
210     * be notified when a client connects. */
211     event_set(&ev_accept, listen_fd, EV_READ | EV_PERSIST, on_accept, NULL);
212     event_add(&ev_accept, NULL);
213 
214     /* Start the event loop. */
215     event_dispatch();
216 
217     return 0;
218 }

client.c

读键盘输入，发送到服务端，服务端再返回，客户端回显。

gcc -g -Wall -I/usr/local/include -o client client.c -L/usr/local/lib -levent

  1 #include<sys/types.h>
  2 #include<sys/socket.h>
  3 #include<netinet/in.h>
  4 #include<arpa/inet.h>
  5 #include<errno.h>
  6 #include<unistd.h>
  7 
  8 #include <stdlib.h>
  9 #include <stdio.h>
 10 #include <string.h>
 11 #include <fcntl.h>
 12 #include <err.h>
 13 
 14 #include<event.h>
 15 
 16 #define SERVER_PORT 5555
 17 
 18 
 19 //服务端信息
 20 struct server {
 21     /* The server socket. */
 22     int fd;
 23 
 24     /* The bufferedevent for this server. */
 25     struct bufferevent *buf_ev;
 26 };
 27 
 28 //全局server数据
 29 struct server *serv;
 30 
 31 //设置文件状态标记
 32 int setnonblock(int fd)
 33 {
 34     int flags;
 35     flags = fcntl(fd, F_GETFL);
 36     if (flags < 0)
 37         return flags;
 38     flags |= O_NONBLOCK;
 39     if (fcntl(fd, F_SETFL, flags) < 0)
 40         return -1;
 41     return 0;
 42 }
 43 
 44 //键盘事件
 45 void cmd_msg_cb(int fd, short events, void* arg)
 46 {
 47     printf("cmd_msg_cb
");
 48     char msg[1024];
 49 
 50     int ret = read(fd, msg, sizeof(msg));
 51     if (ret < 0)
 52     {
 53         perror("read fail ");
 54         exit(1);
 55     }
 56     struct bufferevent* bev = (struct bufferevent*)arg;
 57     //把终端的消息发送给服务器端
 58     bufferevent_write(bev, msg, ret);
 59 }
 60 
 61 //读服务端发来的数据
 62 void read_msg_cb(struct bufferevent* bev, void* arg)
 63 {
 64     printf("read_msg_cb
");
 65     char msg[1024];
 66 
 67     size_t len = bufferevent_read(bev, msg, sizeof(msg));
 68     msg[len] = '';
 69     printf("recv %s from server", msg);
 70 }
 71 
 72 //连接断开或者出错回调
 73 void event_error(struct bufferevent *bev, short event, void *arg)
 74 {
 75     printf("event_error
");
 76     if (event & EVBUFFER_EOF)
 77         printf("connection closed
");
 78     else if (event & EVBUFFER_ERROR)
 79         printf("some other error
");
 80     struct event *ev = (struct event*)arg;
 81     //因为socket已经没有，所以这个event也没有存在的必要了  
 82     free(ev);
 83     //当发生错误退出事件循环
 84     event_loopexit(0);
 85     bufferevent_free(bev);
 86 }
 87 
 88 //连接到server
 89 typedef struct sockaddr SA;
 90 int tcp_connect_server(const char* server_ip, int port)
 91 {
 92     int sockfd, status, save_errno;
 93     struct sockaddr_in server_addr;
 94 
 95     memset(&server_addr, 0, sizeof(server_addr));
 96 
 97     server_addr.sin_family = AF_INET;
 98     server_addr.sin_port = htons(port);
 99     status = inet_aton(server_ip, &server_addr.sin_addr);
100 
101     if (status == 0) //the server_ip is not valid value
102     {
103         errno = EINVAL;
104         return -1;
105     }
106 
107     sockfd = socket(AF_INET, SOCK_STREAM, 0);
108     if (sockfd == -1)
109         return sockfd;
110     status = connect(sockfd, (SA*)&server_addr, sizeof(server_addr));
111 
112     if (status == -1)
113     {
114         save_errno = errno;
115         close(sockfd);
116         errno = save_errno; //the close may be error
117         return -1;
118     }
119 
120     setnonblock(sockfd);
121 
122     return sockfd;
123 }
124 
125 
126 int main(int argc, char** argv)
127 {
128 
129     event_init();
130     //测试用直接连接本地server
131     int sockfd = tcp_connect_server("127.0.0.1", SERVER_PORT);
132     if (sockfd == -1)
133     {
134         perror("tcp_connect error ");
135         return -1;
136     }
137     
138     printf("connect to server successful
");
139     serv = calloc(1, sizeof(*serv));
140     if (serv == NULL)
141         err(1, "malloc failed");
142     serv->fd = sockfd;
143     serv->buf_ev = bufferevent_new(sockfd, read_msg_cb,
144         NULL, NULL, (void *)serv);
145 
146     //监听终端输入事件
147     struct event *ev_cmd = calloc(1,sizeof(*ev_cmd));
148     event_set(ev_cmd, STDIN_FILENO,
149         EV_READ | EV_PERSIST, cmd_msg_cb,
150         (void*)serv->buf_ev);
151     event_add(ev_cmd, NULL);
152     //设置下read和发生错误的回调函数。（当socket关闭时会用到回调参数，删除键盘事件）
153     bufferevent_setcb(serv->buf_ev, read_msg_cb, NULL, event_error, (void*)ev_cmd);
154     bufferevent_enable(serv->buf_ev, EV_READ| EV_PERSIST);
155     event_dispatch();
156     printf("finished 
");
157     return 0;
158 }