初探libevent网络库

libevent官网:http://libevent.org/

libevent是基于消息事件的网络库。当指定一个文件描述符(可以是socket fd)可读事件或可写事件的时候,libevent API提供执行回调函数的机制。再者,libevent也提供信号或超时的回调。libevent用事件来驱动网络服务。一个应用只需要调用event_dispathc()函数并且在不修改事件驱动的情况下动态添加或者删除事件。libevent支持很多系统,诸如熟悉的Linux,Windows。

但libevent不提供线程安全保障,每一个现场都必须初始化一个event_base。当然,可以对libevent进行拓展,通过每一个现场都初始化一个event_base,又或者通过加锁来访问全局共享的event_base。

一个简单的基于tcp协议的服务器,比如接下来给出的echo服务器,通过libevent可以很容易实现单线程,非阻塞IO模型。

  • 声明并定义套接字可读/可写/出错回调函数,在符合条件的情况,会触发调用相应的函数。

  • 准备监听套接字绑定并开始监听。

  • 设置接受连接(accept)事件并加入时间驱动中。

下面给出示例程序。

#include <event.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

#define SERVER_PORT 8080
int debug = 0;

struct client
{
	int fd;
	struct bufferevent * buf_ev;
};

int setnonblock(int fd)
{
	int flags;

	flags = fcntl(fd,F_GETFL);
	flags |= O_NONBLOCK;
	fcntl(fd,F_SETFL,flags);
}

/*read callback function .*/
void buf_read_callback(struct bufferevent *incoming,
					   void *arg)
{
	printf("this is called in buf_read_callback\n");
	struct evbuffer * evreturn;
	char *req;

	req = evbuffer_readline(incoming->input);

	printf("req is %s\n",req);
	if(req == NULL)
		return;

	evreturn = evbuffer_new();
	evbuffer_add_printf(evreturn,"YOU SAID %s\n",req);
	bufferevent_write_buffer(incoming,evreturn);
	evbuffer_free(evreturn);
	free(req);
}

/*write callback function.*/
void buf_write_callback(struct bufferevent *bev,
						void *arg)
{
	printf("this is called in buf_write_callback\n");
}

/*error callback function.*/
void buf_error_callback(struct bufferevent *bev,    
						short what,void *arg)
{
	struct client *client = (struct client*)arg;
	bufferevent_free(client->buf_ev);
	close(client->fd);
	free(client);
}

/*accept callback function.*/
void accept_callback(int fd,
					 short ev,void *arg)
{
	int client_fd;
	struct sockaddr_in client_addr;
	socklen_t client_len = sizeof(client_addr);
	struct client *client;

	client_fd = accept(fd,
		(struct sockaddr*)&client_addr,
		&client_len);

	if(client_fd < 0)
	{
		printf("Client:accept() faild\n");
		return;
	}

	/*set socket fd to none block.*/
	setnonblock(client_fd);

	/*new a struct client.*/
	client = calloc(1,sizeof(*client));
	if(client == NULL)
	{
		printf("calloc error\n");
		return;
	}// if
	client->fd = client_fd;

	client->buf_ev = bufferevent_new(client_fd,
		buf_read_callback,
		buf_write_callback,
		buf_error_callback,
		client);

	bufferevent_enable(client->buf_ev,EV_READ);
}

int main(int argc,
		 char **argv)
{
	int socketlisten;
	struct sockaddr_in addresslisten;
	struct event accept_event;
	int reuse = 1;

	/*initialize the event_base in this thread and if this is single-thread application,there is no need for saving the event_base.*/
	event_init();

	socketlisten = socket(AF_INET, SOCK_STREAM, 0);

	if (socketlisten < 0)
	{
		fprintf(stderr,"Failed to create listen socket");
		return 1;
	}

	memset(&addresslisten, 0, sizeof(addresslisten));

	addresslisten.sin_family = AF_INET;
	addresslisten.sin_addr.s_addr = INADDR_ANY;
	addresslisten.sin_port = htons(SERVER_PORT);

	if (bind(socketlisten,
		(struct sockaddr *)&addresslisten,
		sizeof(addresslisten)) < 0)
	{
		fprintf(stderr,"Failed to bind");
		return 1;
	}

	if (listen(socketlisten, 5) < 0)
	{
		fprintf(stderr,"Failed to listen to socket");
		return 1;
	}

	setsockopt(socketlisten,
		SOL_SOCKET,
		SO_REUSEADDR,
		&reuse,
		sizeof(reuse));

	setnonblock(socketlisten);

	/*set accept callback function including read/write/error callback function.
	  EV_READ:Wait for a socket or FD to become readable  
	  EV_WRITE:Wait for a socket or FD to become writeable 
	*/
	event_set(&accept_event,
		socketlisten,
		EV_READ|EV_PERSIST,
		accept_callback,
		NULL);

	/*add an event to the set of pending events.*/
	event_add(&accept_event,
		NULL);

	/*Loop to process events.*/
	event_dispatch();

	close(socketlisten);
	return 0;
}

用telnet测试echo服务器。在使用telnet的时候指明IP地址和端口号便可以连接相应的服务程序。

服务端:

daoluan@daoluan:~$ telnet 192.168.128.99 8080

客户端:

root@daoluan:~# telnet 192.168.128.99 8080
Trying 192.168.128.99...
Connected to 192.168.128.99.
Escape character is '^]'.
hello world.
YOU SAID hello world.

libevent的事件驱动机制是在是太妙了,应该是借助了系统的信号处理功能,当检测可读或者可写的时候,通知并调用事先注册好的回调函数。当然libevent没有那么简单!YY下,既然每个线程都必须有唯一的event_base,那么多线程就需要多个event_base,这又存在很多线程间通信的问题,数据同步等等。但如果说一个工作线程服务一个客户连接的话,即可能不存在线程间的数据同步等问题,那么用libevent实现多线程思路也不会太复杂。

而上述的echo是重复型服务器,而非并发型服务器。所以当多个连接请求时,只能“先到先服务”。

下面是有用的链接:

本文完 2012-09-25

捣乱小子 http://www.daoluan.net/

25 September 2012

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