网络编程中I/O复用select的用法

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/qq_40712616/article/details/82945502

注：本文对select函数、相关参数及结构体不做解释

select使用流程图

注：没有写socket流程

在网络编程中需要添加的代码行以及意义

1.定义select相关变量

	int listenfd,connfd,i;
	/*套接字描述符/链接描述符（录入环节）/变量*/	

	int nread,maxfd,maxi,sockfd;
	/*就绪数/最大链接描述符/用于记录链接描述符数组当前最大使用的下标，减少无用的遍历/
		链接描述符（区别在于此处用于I/O复用的遍历环节）*/
	
	int client[FD_SETSIZE];
	/*存放录入的链接描述符*/
	
	fd_set allest,rset;
	/*文件集合描述符:所有集合/就绪可读集合*/

2.select相关变量的相关数据初始化

	maxfd = listenfd; 
	/*初始化最大链接描述符，*/
	maxi = -1;
	for (i = 0; i < FD_SETSIZE; i++) client[i] = -1;
	FD_ZERO(&allest);
	/*清空allset描述符集*/
	FD_SET(listenfd, &allest);
	 /*将监听描述符加到allset中*/

3.select 监视可读事件

	rset = allest;
	nread = select(maxfd+1, &rset, NULL, NULL, NULL);
		/*调用select 监视可读事件*/

4.新的链接描述符放入数组和描述符集

	if (FD_ISSET(listenfd, &rset))
	/*判断服务器端套接字中是否有变化（是否有新的链接）*/
	{
		cliaddr_lin = sizeof(cliaddr);
		connfd = accept(listenfd, (struct sockaddr *)&cliaddr, &cliaddr_lin);
		for (i = 0; i < FD_SETSIZE; i++)
		{
			if (client[i] < 0)
			{
				client[i] = connfd;
/				/*新的链接描述符放入数组*/
				break;
			}
		}
		FD_SET(connfd, &allest);
		 /* 将来自客户的连接connfd加入描述符集 */
		if (connfd > maxfd) maxfd = connfd;
		/*新的链接描述符*/
		if (i > maxi) maxi = i;
		if (--nread == 0) continue;
	}

5.批处理可读链接描述符

for (i = 0; i <= maxi; i++)
{
	if ((sockfd = client[i]) < 0) continue;
	if (FD_ISSET(sockfd, &rset))
	{
		if ((n = read (sockfd, buf, MAXLIN)) == 0)
		{
			close(sockfd);
			FD_CLR(sockfd, &allest);
			client[i] = -1;
		}else
		{
			if (n < 0) perror("read:");
			if (n > 0) write(sockfd, buf, n);
		}
	if (--nread == 0) break;
	}
}

例程

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

#define LINK_NUMBER 20
#define SERV_PORT 8000
#define MAXLIN 100

int main(int argc, char const *argv[])
{
	int listenfd,connfd,i;
	int nread,maxfd,maxi,sockfd;
	int client[FD_SETSIZE];
	ssize_t n;
	fd_set allest,rset;
	socklen_t cliaddr_lin;
	struct sockaddr_in servaddr,cliaddr;
	char buf[MAXLIN],str[INET_ADDRSTRLEN];

	listenfd = socket(AF_INET,SOCK_STREAM,0);

	bzero(&servaddr, sizeof(servaddr));
	servaddr.sin_family = AF_INET;
	servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
	servaddr.sin_port = htons(SERV_PORT);

	bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr));

	listen(listenfd,LINK_NUMBER);

	maxfd = listenfd;
	maxi = -1;
	for (i = 0; i < FD_SETSIZE; i++) client[i] = -1;
	FD_ZERO(&allest);
	FD_SET(listenfd, &allest);

	while(1)
	{
		rset = allest;
		nread = select(maxfd+1, &rset, NULL, NULL, NULL);
		if (nread < 0)
		{
			perror("select error:");
			break;
		}

		if (FD_ISSET(listenfd, &rset))
		{
			cliaddr_lin = sizeof(cliaddr);
			connfd = accept(listenfd, (struct sockaddr *)&cliaddr, &cliaddr_lin);

			printf("received from %s at PORT %d\n",
			       inet_ntop(AF_INET, &cliaddr.sin_addr, str, sizeof(str)),
			       ntohs(cliaddr.sin_port));

			for (i = 0; i < FD_SETSIZE; i++)
			{
				if (client[i] < 0)
				{
					client[i] = connfd;
					break;
				}
			}
			if (i == FD_SETSIZE)
			{
				perror("too many clients");
				exit(1);
			}
			FD_SET(connfd, &allest);
			if (connfd > maxfd) maxfd = connfd;
			if (i > maxi) maxi = i;
			if (--nread == 0) continue;	
		}

		for (i = 0; i <= maxi; i++)
		{
			if ((sockfd = client[i]) < 0) continue;

			if (FD_ISSET(sockfd, &rset))
			{
				if ((n = read (sockfd, buf, MAXLIN)) == 0)
				{
					close(sockfd);
					FD_CLR(sockfd, &allest);
					client[i] = -1;
				}else
				{
					if (n < 0)
					{
						perror("read:");
					}
					if (n > 0)
					{
						write(sockfd, buf, n);
					}
	
				}
			
				if (--nread == 0) break;
			}
		}
		/*socket*/
		/*while(1)
		{
			if((n = read(connfd,buf,MAXLIN)) <= 0) 
			{
				printf("err" );
				break;
			}

			printf("received from %s at PORT %d::%s\n",
			       inet_ntop(AF_INET, &cliaddr.sin_addr, str, sizeof(str)),
			       ntohs(cliaddr.sin_port),buf);

			write(connfd,buf,n);
		}
		close(connfd);*/
	}

	return 0;
}

参考文献及博客

Linux聊天室项目 – ChatRome（select实现）
Linux I/O复用之select函数详解
unix网络编程第二版