socket网络编程客户服务器程序范式（三）IO复用select
一：select函数：
/* According to POSIX.1-2001 */
#include
/* According to earlier standards */
#include
#include
#include
int select(int nfds, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);
void FD_CLR(int fd, fd_set *set);
int FD_ISSET(int fd, fd_set *set);
void FD_SET(int fd, fd_set *set);
void FD_ZERO(fd_set *set);
select函数的返回值：若有就绪描述符则为其数目，若超时，函数返回0，否则函数出错返回-1.
先介绍timeout,它告知内核等待所指定描述符中任意一个就绪的最长时间，这个参数有以下3种可能
1.timeout=NULL,永远等待，select阻塞直到有任意一个描述符就绪。
2.*timeout=0,也就是时间的值为0，轮询，select检查是否有描述符就绪，若没有，不阻塞，直接返回。
2.等待一段固定时间，在等待的这段时间内select阻塞，若有任意一个描述符就绪，则select返回；若没有，则超时函数返回0。
struct timeval {
long tv_sec; /* seconds */
long tv_usec; /* microseconds */
};
描述符集的用法：
fd_set rset;
FD_ZERO(&rset);
FD_SET(fileno(fp), &rset);//fileno(fp)为用来读的文件描述符
FD_SET(sockfd, &rset);//sockfd为用来读的套接字描述符描述符就绪条件：
当下列条件发生时，描述符可读：
1. 当套接字接收缓冲区中有数据时；2.套接字的一端发送FIN时(发送端关闭)；3.监听套接字连接数大于0时；4.套接字本身出错，有错误待处理时，描述符都变为可读.
2.当套接字发送缓冲区中有数据时；2.套接字接收端关闭时；3.从connect返回后；4.套接字本身出错，有错误待处理时，描述符都变为可写.
eg:
#include "unp.h"
void str_cli(FILE *fp, int sockfd)
{
int maxfdp1, stdineof;
fd_set rset;
char buf[MAXLINE];
int n;
stdineof = 0;
FD_ZERO(&rset);
for ( ; ; ) {
if (stdineof == 0)
FD_SET(fileno(fp), &rset);
FD_SET(sockfd, &rset);
maxfdp1 = max(fileno(fp), sockfd) + 1;
Select(maxfdp1, &rset, NULL, NULL, NULL);
if (FD_ISSET(sockfd, &rset)) { /* socket is readable */
if ( (n = Read(sockfd, buf, MAXLINE)) == 0) {
if (stdineof == 1)
return; /* normal termination */
else
err_quit("str_cli: server terminated prematurely");
}
Write(fileno(stdout), buf, n);
}
if (FD_ISSET(fileno(fp), &rset)) { /* input is readable */
if ( (n = Read(fileno(fp), buf, MAXLINE)) == 0) {
stdineof = 1;
Shutdown(sockfd, SHUT_WR); /* send FIN */
FD_CLR(fileno(fp), &rset);
continue;
}
Writen(sockfd, buf, n);
}
}
}
二：poll函数
#include
int poll(struct pollfd *fds, nfds_t nfds, int timeout);
struct pollfd {
int fd; /* file descriptor */
short events; /* requested events */
short revents; /* returned events */
};
eg:
/* include fig01 */
#include "unp.h"
#include /* for OPEN_MAX */
int
main(int argc, char **argv)
{
int i, maxi, listenfd, connfd, sockfd;
int nready;
ssize_t n;
char buf[MAXLINE];
socklen_t clilen;
struct pollfd client[OPEN_MAX];
struct sockaddr_in cliaddr, servaddr;
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, (SA *) &servaddr, sizeof(servaddr));
Listen(listenfd, LISTENQ);
client[0].fd = listenfd;
client[0].events = POLLRDNORM;
for (i = 1; i < OPEN_MAX; i++)
client[i].fd = -1; /* -1 indicates available entry */
maxi = 0; /* max index into client[] array */
/* end fig01 */
/* include fig02 */
for ( ; ; ) {
nready = Poll(client, maxi+1, INFTIM);
if (client[0].revents & POLLRDNORM) { /* new client connection */
clilen = sizeof(cliaddr);
connfd = Accept(listenfd, (SA *) &cliaddr, &clilen);
#ifdef NOTDEF
printf("new client: %s\n", Sock_ntop((SA *) &cliaddr, clilen));
#endif
for (i = 1; i < OPEN_MAX; i++)
if (client[i].fd < 0) {
client[i].fd = connfd; /* save descriptor */
break;
}
if (i == OPEN_MAX)
err_quit("too many clients");
client[i].events = POLLRDNORM;
if (i > maxi)
maxi = i; /* max index in client[] array */
if (--nready <= 0)
continue; /* no more readable descriptors */
}
for (i = 1; i <= maxi; i++) { /* check all clients for data */
if ( (sockfd = client[i].fd) < 0)
continue;
if (client[i].revents & (POLLRDNORM | POLLERR)) {
if ( (n = read(sockfd, buf, MAXLINE)) < 0) {
if (errno == ECONNRESET) {
/*4connection reset by client */
#ifdef NOTDEF
printf("client[%d] aborted connection\n", i);
#endif
Close(sockfd);
client[i].fd = -1;
} else
err_sys("read error");
} else if (n == 0) {
/*4connection closed by client */
#ifdef NOTDEF
printf("client[%d] closed connection\n", i);
#endif
Close(sockfd);
client[i].fd = -1;
} else
Writen(sockfd, buf, n);
if (--nready <= 0)
break; /* no more readable descriptors */
}
}
}
}
/* end fig02 */