https://www.jianshu.com/p/8ea60a8d3abb
基本的socket变成是阻塞/同步的，每个操作除非已经完成，出错，或者超时才会返回，这样对于每一个请求，要使用一个线程或者单独的进程去处理，系统资源没有办法支撑大量的请求。posix定义了可以使用异步的select系统调用，但是因为它采用了轮询的方式来判断某个fd是否变成active，效率不高。于是各系统就分别提出了基于异步的系统调用，例如Linux的epoll，由于在内核层面做了支持，所以可以用O(1)的效率查找到active的fd。基本上，libevent就是对这些高效IO的封装，提供统一的API，简化开发。
libevent默认情况下是单线程的，可以配置成多线程，每个线程有且只有一个event_base，对应一个struct event_base结构体以及附于其上的事件管理器，用来调度托管给它的一系列event，可以和操作系统的进程管理类比。当一个事件发生后，event_base会在合适的时间，不一定是立即去调用绑定在这个事件上的函数，直到这个函数执行完，再去调度其他的事件。
//创建一个event_base
struct event_base *base = event_base_new();
assert(base != NULL);
event_base内部有一个循环，循环阻塞在epoll等系统调用上，直到有一个/一些时间发生，然后去处理这些事件。当然，这些事件要被绑定在这个event_base上，每个事件对应一个struct event，可以是监听一个fd或者信号量之类的，struct event使用event_new来创建和绑定，使用event_add来将event绑定到event_base中。
// 创建并绑定一个event
struct event* listen_event;
//参数：event_base,监听的对象，需要监听的事件，事件发生后的回调函数，传给回调函数的参数
listen_event = event_new(base, listener, EV_READ | EV_PERSIST, callback_func, (void*)base);
//参数：event，超时时间，NULL表示无超时设置
event_add(listen_event, NULL);
注：libevent支持的事件及属性包括（使用bitfield实现）
然后启动event_base的循环，开始处理事件。循环地启动使用event_base_dispatch，循环将一直持续，找到不再有需要关注的事件，或者是遇到event_loopbreak()/event_loopexit()函数。
//启动循环，开始处理事件
event_base_dispatch(base);
接下来再来关注事件发生时的回调函数callback_func，callback_func的原型如下所示
typedef void(* event_callback_fn)(evutil_socket_t sockfd, short event_type, void *arg)
传给callback_func的是一个监听的fd，监听的事件类型，以及event_new中最后一个参数。在上述程序中，是将event_base传给了callback_func，实际中更常用的是构造一个结构体，把需要传给回调函数的参数都放进来，然后传给event_new，event_new再传给回调函数。
所以总结一下，对于一个服务器而言，流程大致如下：
下面的代码实现了一个简单的echo server，server启动后，client端启动并连接，在cmd中输入文字，server端收到后，将文字再返回给client。
server端代码：
/**
You need libevent2 to compile this piece of code
Please see: http://libevent.org/
Or you can simply run this command to install on Mac: brew install libevent
Cmd to compile this piece of code: g++ LibeventQuickStartServer.c -o LibeventQuickStartServer /usr/local/lib/libevent.a
**/
#include<stdio.h>
#include<string.h>
#include<errno.h>
#include<unistd.h>
#include<event.h>
void accept_cb(int fd, short events, void* arg);
void socket_read_cb(int fd, short events, void* arg);
int tcp_server_init(int port, int listen_num);
int main(int argc, char const *argv[])
{
/* code */
int listener = tcp_server_init(9999, 10);
if (listener == -1)
{
perror("tcp_server_init error");
return -1;
}
struct event_base* base = event_base_new();
// 监听客户端请求链接事件
struct event* ev_listen = event_new(base, listener, EV_READ | EV_PERSIST, accept_cb, base);
event_add(ev_listen, NULL);
event_base_dispatch(base);
return 0;
}
void accept_cb(int fd, short events, void* arg)
{
evutil_socket_t sockfd;
struct sockaddr_in client;
socklen_t len = sizeof(client);
sockfd = ::accept(fd, (struct sockaddr*)&client, &len);
evutil_make_socket_nonblocking(sockfd);
printf("accept a client %d\n", sockfd);
struct event_base* base = (event_base*)arg;
//动态创建一个event结构体，并将其作为回调参数传递给
struct event* ev = event_new(NULL, -1, 0, NULL, NULL);
event_assign(ev, base, sockfd, EV_READ | EV_PERSIST, socket_read_cb, (void*)ev);
event_add(ev, NULL);
}
void socket_read_cb(int fd, short events, void* arg)
{
char msg[4096];
struct event* ev = (struct event*)arg;
int len = read(fd, msg, sizeof(msg) - 1);
if(len <= 0)
{
printf("some error happen when read\n");
event_free(ev);
close(fd);
return;
}
msg[len] = '\0';
printf("recv the client msg : %s\n", msg);
char reply_msg[4096] = "I have received the msg: ";
strcat(reply_msg + strlen(reply_msg), msg);
write(fd, reply_msg, strlen(reply_msg));
}
typedef struct sockaddr SA;
int tcp_server_init(int port, int listen_num)
{
int errno_save;
evutil_socket_t listener;
listener = ::socket(AF_INET, SOCK_STREAM, 0);
if( listener == -1 )
return -1;
//允许多次绑定同一个地址。要用在socket和bind之间
evutil_make_listen_socket_reuseable(listener);
struct sockaddr_in sin;
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = 0;
sin.sin_port = htons(port);
if( ::bind(listener, (SA*)&sin, sizeof(sin)) < 0 )
goto error;
if( ::listen(listener, listen_num) < 0)
goto error;
//跨平台统一接口，将套接字设置为非阻塞状态
evutil_make_socket_nonblocking(listener);
return listener;
error:
errno_save = errno;
evutil_closesocket(listener);
errno = errno_save;
return -1;
}
client端代码：
/**
You need libevent2 to compile this piece of code
Please see: http://libevent.org/
Or you can simply run this command to install on Mac: brew install libevent
Cmd to compile this piece of code: g++ LibeventQuickStartClient.c -o LibeventQuickStartClient /usr/local/lib/libevent.a
**/
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<errno.h>
#include<unistd.h>
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<event.h>
#include<event2/util.h>
int tcp_connect_server(const char* server_ip, int port);
void cmd_msg_cb(int fd, short events, void* arg);
void socket_read_cb(int fd, short events, void *arg);
int main(int argc, char** argv)
{
if( argc < 3 )
{
printf("please input 2 parameter\n");
return -1;
}
//两个参数依次是服务器端的IP地址、端口号
int sockfd = tcp_connect_server(argv[1], atoi(argv[2]));
if( sockfd == -1)
{
perror("tcp_connect error ");
return -1;
}
printf("connect to server successful\n");
struct event_base* base = event_base_new();
struct event *ev_sockfd = event_new(base, sockfd,
EV_READ | EV_PERSIST,
socket_read_cb, NULL);
event_add(ev_sockfd, NULL);
//监听终端输入事件
struct event* ev_cmd = event_new(base, STDIN_FILENO,
EV_READ | EV_PERSIST, cmd_msg_cb,
(void*)&sockfd);
event_add(ev_cmd, NULL);
event_base_dispatch(base);
printf("finished \n");
return 0;
}
void cmd_msg_cb(int fd, short events, void* arg)
{
char msg[1024];
int ret = read(fd, msg, sizeof(msg));
if( ret <= 0 )
{
perror("read fail ");
exit(1);
}
int sockfd = *((int*)arg);
//把终端的消息发送给服务器端
//为了简单起见，不考虑写一半数据的情况
write(sockfd, msg, ret);
}
void socket_read_cb(int fd, short events, void *arg)
{
char msg[1024];
//为了简单起见，不考虑读一半数据的情况
int len = read(fd, msg, sizeof(msg)-1);
if( len <= 0 )
{
perror("read fail ");
exit(1);
}
msg[len] = '\0';
printf("recv %s from server\n", msg);
}
typedef struct sockaddr SA;
int tcp_connect_server(const char* server_ip, int port)
{
int sockfd, status, save_errno;
struct sockaddr_in server_addr;
memset(&server_addr, 0, sizeof(server_addr) );
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(port);
status = inet_aton(server_ip, &server_addr.sin_addr);
if( status == 0 ) //the server_ip is not valid value
{
errno = EINVAL;
return -1;
}
sockfd = ::socket(PF_INET, SOCK_STREAM, 0);
if( sockfd == -1 )
return sockfd;
status = ::connect(sockfd, (SA*)&server_addr, sizeof(server_addr) );
if( status == -1 )
{
save_errno = errno;
::close(sockfd);
errno = save_errno; //the close may be error
return -1;
}
evutil_make_socket_nonblocking(sockfd);
return sockfd;
}
程序运行截图
客户端
Paste_Image.png
服务端
Paste_Image.png
在上面的代码中，client的cmd中有信息输入时，client直接将数据写入到fd中，server中收到信息后，也是直接将信息写入到fd中，因为fd是非阻塞的，所以不能保证正确。那么需要一个自己管理的缓存来管理自己的数据。那么步骤将稍微有些变化，如下所示：
代码如下所示：
使用bufferevent的server端代码
#include<stdio.h>
#include<string.h>
#include<errno.h>
#include<event.h>
#include<event2/bufferevent.h>
void accept_cb(int fd, short events, void* arg);
void socket_read_cb(bufferevent* bev, void* arg);
void event_cb(struct bufferevent *bev, short event, void *arg);
int tcp_server_init(int port, int listen_num);
int main(int argc, char** argv)
{
int listener = tcp_server_init(9999, 10);
if( listener == -1 )
{
perror(" tcp_server_init error ");
return -1;
}
struct event_base* base = event_base_new();
//添加监听客户端请求连接事件
struct event* ev_listen = event_new(base, listener, EV_READ | EV_PERSIST,
accept_cb, base);
event_add(ev_listen, NULL);
event_base_dispatch(base);
event_base_free(base);
return 0;
}
void accept_cb(int fd, short events, void* arg)
{
evutil_socket_t sockfd;
struct sockaddr_in client;
socklen_t len = sizeof(client);
sockfd = ::accept(fd, (struct sockaddr*)&client, &len );
evutil_make_socket_nonblocking(sockfd);
printf("accept a client %d\n", sockfd);
struct event_base* base = (event_base*)arg;
bufferevent* bev = bufferevent_socket_new(base, sockfd, BEV_OPT_CLOSE_ON_FREE);
bufferevent_setcb(bev, socket_read_cb, NULL, event_cb, arg);
bufferevent_enable(bev, EV_READ | EV_PERSIST);
}
void socket_read_cb(bufferevent* bev, void* arg)
{
char msg[4096];
size_t len = bufferevent_read(bev, msg, sizeof(msg));
msg[len] = '\0';
printf("recv the client msg: %s", msg);
char reply_msg[4096] = "I have recvieced the msg: ";
strcat(reply_msg + strlen(reply_msg), msg);
bufferevent_write(bev, reply_msg, strlen(reply_msg));
}
void event_cb(struct bufferevent *bev, short event, void *arg)
{
if (event & BEV_EVENT_EOF)
printf("connection closed\n");
else if (event & BEV_EVENT_ERROR)
printf("some other error\n");
//这将自动close套接字和free读写缓冲区
bufferevent_free(bev);
}
typedef struct sockaddr SA;
int tcp_server_init(int port, int listen_num)
{
int errno_save;
evutil_socket_t listener;
listener = ::socket(AF_INET, SOCK_STREAM, 0);
if( listener == -1 )
return -1;
//允许多次绑定同一个地址。要用在socket和bind之间
evutil_make_listen_socket_reuseable(listener);
struct sockaddr_in sin;
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = 0;
sin.sin_port = htons(port);
if( ::bind(listener, (SA*)&sin, sizeof(sin)) < 0 )
goto error;
if( ::listen(listener, listen_num) < 0)
goto error;
//跨平台统一接口，将套接字设置为非阻塞状态
evutil_make_socket_nonblocking(listener);
return listener;
error:
errno_save = errno;
evutil_closesocket(listener);
errno = errno_save;
return -1;
}
使用bufferevent的client端代码
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<errno.h>
#include<unistd.h>
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<event.h>
#include<event2/bufferevent.h>
#include<event2/buffer.h>
#include<event2/util.h>
int tcp_connect_server(const char* server_ip, int port);
void cmd_msg_cb(int fd, short events, void* arg);
void server_msg_cb(struct bufferevent* bev, void* arg);
void event_cb(struct bufferevent *bev, short event, void *arg);
int main(int argc, char** argv)
{
if( argc < 3 )
{
printf("please input 2 parameter\n");
return -1;
}
//两个参数依次是服务器端的IP地址、端口号
int sockfd = tcp_connect_server(argv[1], atoi(argv[2]));
if( sockfd == -1)
{
perror("tcp_connect error ");
return -1;
}
printf("connect to server successful\n");
struct event_base* base = event_base_new();
struct bufferevent* bev = bufferevent_socket_new(base, sockfd,
BEV_OPT_CLOSE_ON_FREE);
//监听终端输入事件
struct event* ev_cmd = event_new(base, STDIN_FILENO,
EV_READ | EV_PERSIST, cmd_msg_cb,
(void*)bev);
event_add(ev_cmd, NULL);
//当socket关闭时会用到回调参数
bufferevent_setcb(bev, server_msg_cb, NULL, event_cb, (void*)ev_cmd);
bufferevent_enable(bev, EV_READ | EV_PERSIST);
event_base_dispatch(base);
printf("finished \n");
return 0;
}
void cmd_msg_cb(int fd, short events, void* arg)
{
char msg[1024];
int ret = read(fd, msg, sizeof(msg));
if( ret < 0 )
{
perror("read fail ");
exit(1);
}
struct bufferevent* bev = (struct bufferevent*)arg;
//把终端的消息发送给服务器端
bufferevent_write(bev, msg, ret);
}
void server_msg_cb(struct bufferevent* bev, void* arg)
{
char msg[1024];
size_t len = bufferevent_read(bev, msg, sizeof(msg));
msg[len] = '\0';
printf("recv %s from server\n", msg);
}
void event_cb(struct bufferevent *bev, short event, void *arg)
{
if (event & BEV_EVENT_EOF)
printf("connection closed\n");
else if (event & BEV_EVENT_ERROR)
printf("some other error\n");
//这将自动close套接字和free读写缓冲区
bufferevent_free(bev);
struct event *ev = (struct event*)arg;
//因为socket已经没有，所以这个event也没有存在的必要了
event_free(ev);
}
typedef struct sockaddr SA;
int tcp_connect_server(const char* server_ip, int port)
{
int sockfd, status, save_errno;
struct sockaddr_in server_addr;
memset(&server_addr, 0, sizeof(server_addr) );
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(port);
status = inet_aton(server_ip, &server_addr.sin_addr);
if( status == 0 ) //the server_ip is not valid value
{
errno = EINVAL;
return -1;
}
sockfd = ::socket(PF_INET, SOCK_STREAM, 0);
if( sockfd == -1 )
return sockfd;
status = ::connect(sockfd, (SA*)&server_addr, sizeof(server_addr) );
if( status == -1 )
{
save_errno = errno;
::close(sockfd);
errno = save_errno; //the close may be error
return -1;
}
evutil_make_socket_nonblocking(sockfd);
return sockfd;
}
欢迎留言交流学习
参考链接：
https://www.felix021.com/blog/read.php?2068
http://blog.csdn.net/luotuo44/article/details/39670221
作者：shareLin
链接：https://www.jianshu.com/p/8ea60a8d3abb
来源：简书
简书著作权归作者所有，任何形式的转载都请联系作者获得授权并注明出处。