select详解
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select
- select能监听的文件描述符个数受限于FD_SETSIZE,一般为1024，单纯改变进程打开的文件描述符个数并不能改变select监听文件个数
- 解决1024以下客户端时使用select是很合适的，但如果链接客户端过多，select采用的是轮询模型，会大大降低服务器响应效率，不应在select上投入更多精力
#include <sys/select.h>/* According to earlier standards */#include <sys/time.h>#include <sys/types.h>#include <unistd.h>int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout); nfds: 监控的文件描述符集里最大文件描述符加1，因为此参数会告诉内核检测前多少个文件描述符的状态 readfds： 监控有读数据到达文件描述符集合，传入传出参数 writefds： 监控写数据到达文件描述符集合，传入传出参数 exceptfds： 监控异常发生达文件描述符集合,如带外数据到达异常，传入传出参数 timeout： 定时阻塞监控时间，3种情况 1.NULL，永远等下去 2.设置timeval，等待固定时间 3.设置timeval里时间均为0，检查描述字后立即返回，轮询 struct timeval { long tv_sec; /* seconds */ long tv_usec; /* microseconds */ }; void FD_CLR(int fd, fd_set *set); //把文件描述符集合里fd清0 int FD_ISSET(int fd, fd_set *set); //测试文件描述符集合里fd是否置1 void FD_SET(int fd, fd_set *set); //把文件描述符集合里fd位置1 void FD_ZERO(fd_set *set); //把文件描述符集合里所有位清0为了代码的简洁性首先将常用的函数进行了一次封装，分装了出错处理的内容
wrap.h
#ifndef __WRAP_H_#define __WRAP_H_void perr_exit(const char *s);int Accept(int fd, struct sockaddr *sa, socklen_t *salenptr);int Bind(int fd, const struct sockaddr *sa, socklen_t salen);int Connect(int fd, const struct sockaddr *sa, socklen_t salen);int Listen(int fd, int backlog);int Socket(int family, int type, int protocol);ssize_t Read(int fd, void *ptr, size_t nbytes);ssize_t Write(int fd, const void *ptr, size_t nbytes);int Close(int fd);ssize_t Readn(int fd, void *vptr, size_t n);ssize_t Writen(int fd, const void *vptr, size_t n);ssize_t my_read(int fd, char *ptr);ssize_t Readline(int fd, void *vptr, size_t maxlen);#endifwrap.c
#include <stdlib.h>#include <stdio.h>#include <unistd.h>#include <errno.h>#include <sys/socket.h>void perr_exit(const char *s){ perror(s); exit(-1);}int Accept(int fd, struct sockaddr *sa, socklen_t *salenptr){ int n;again: if ((n = accept(fd, sa, salenptr)) < 0) { if ((errno == ECONNABORTED) || (errno == EINTR)) goto again; else perr_exit("accept error"); } return n;}int Bind(int fd, const struct sockaddr *sa, socklen_t salen){ int n; if ((n = bind(fd, sa, salen)) < 0) perr_exit("bind error"); return n;}int Connect(int fd, const struct sockaddr *sa, socklen_t salen){ int n; if ((n = connect(fd, sa, salen)) < 0) perr_exit("connect error"); return n;}int Listen(int fd, int backlog){ int n; if ((n = listen(fd, backlog)) < 0) perr_exit("listen error"); return n;}int Socket(int family, int type, int protocol){ int n; if ((n = socket(family, type, protocol)) < 0) perr_exit("socket error"); return n;}ssize_t Read(int fd, void *ptr, size_t nbytes){ ssize_t n;again: if ( (n = read(fd, ptr, nbytes)) == -1) { if (errno == EINTR) goto again; else return -1; } return n;}ssize_t Write(int fd, const void *ptr, size_t nbytes){ ssize_t n;again: if ( (n = write(fd, ptr, nbytes)) == -1) { if (errno == EINTR) goto again; else return -1; } return n;}int Close(int fd){ int n; if ((n = close(fd)) == -1) perr_exit("close error"); return n;}/*参三: 应该读取的字节数*/ssize_t Readn(int fd, void *vptr, size_t n){ size_t nleft; //usigned int 剩余未读取的字节数 ssize_t nread; //int 实际读到的字节数 char *ptr; ptr = vptr; nleft = n; while (nleft > 0) { if ((nread = read(fd, ptr, nleft)) < 0) { if (errno == EINTR) nread = 0; else return -1; } else if (nread == 0) break; nleft -= nread; ptr += nread; } return n - nleft;}ssize_t Writen(int fd, const void *vptr, size_t n){ size_t nleft; ssize_t nwritten; const char *ptr; ptr = vptr; nleft = n; while (nleft > 0) { if ( (nwritten = write(fd, ptr, nleft)) <= 0) { if (nwritten < 0 && errno == EINTR) nwritten = 0; else return -1; } nleft -= nwritten; ptr += nwritten; } return n;}static ssize_t my_read(int fd, char *ptr){ static int read_cnt; static char *read_ptr; static char read_buf[100]; if (read_cnt <= 0) {again: if ( (read_cnt = read(fd, read_buf, sizeof(read_buf))) < 0) { if (errno == EINTR) goto again; return -1; } else if (read_cnt == 0) return 0; read_ptr = read_buf; } read_cnt--; *ptr = *read_ptr++; return 1;}ssize_t Readline(int fd, void *vptr, size_t maxlen){ ssize_t n, rc; char c, *ptr; ptr = vptr; for (n = 1; n < maxlen; n++) { if ( (rc = my_read(fd, &c)) == 1) { *ptr++ = c; if (c == '\n') break; } else if (rc == 0) { *ptr = 0; return n - 1; } else return -1; } *ptr = 0; return n;}server.c
#include<string.h>#include<stdio.h>#include<stdlib.h>#include<sys/types.h>#include<sys/socket.h>#include<sys/select.h>#include<unistd.h>#include<sys/time.h>#include<netinet/in.h>#include<arpa/inet.h>#include<assert.h>#define SIZE 128#include"wrap.h"int startup(char *ip,int port){ assert(ip); int sock=Socket(AF_INET,SOCK_STREAM,0); int opt=1; setsockopt(sock,SOL_SOCKET,SO_REUSEADDR,&opt,sizeof(opt)); struct sockaddr_in local; local.sin_family=AF_INET; local.sin_port=htons(port); local.sin_addr.s_addr=inet_addr(ip); Bind(sock,(struct sockaddr*)&local,sizeof(local)); Listen(sock,5); return sock;}int main(int argc,char* argv[]){ if(argc!=3) { printf("usae: %s [IP] [PORT]\n",argv[0]); return 0; } int lis_sock=startup(argv[1],atoi(argv[2])); //自定义数组gfds用于保存事件集合 int gfds[SIZE]; memset(gfds,-1,SIZE*4); //读事件集合 fd_set rfds; FD_ZERO(&rfds); while(1) { struct timeval timeout={5,0}; //一开始lis_sock的文件描述符集合就已经就绪了 gfds[0]=lis_sock; //现将最大的文件描述符设为-1 int max_fd=-1; int i=0; for(;i<SIZE;i++) { if(max_fd<gfds[i]) { max_fd=gfds[i]; } //将就绪的事件设置进文件描述集合（位图） if(gfds[i]>=0) { FD_SET(gfds[i],&rfds); } } int ret=select(max_fd+1,&rfds,NULL,NULL,NULL); switch(ret) { case 0: printf("timeout...\n"); break; case -1: printf("error"); break; default: //如果有主机连上服务器，也就是listen有了结果，则accept if(FD_ISSET(gfds[0],&rfds)) { struct sockaddr_in peer; socklen_t len=sizeof(peer); //listen 就绪，则accept; int connfd=accept(lis_sock,(struct sockaddr *)&peer,&len); if(connfd<0) { perror("accept"); } else { printf("client: %s:%d fd(%d)\n",inet_ntoa(peer.sin_addr),\ ntohs(peer.sin_port),connfd); int k=0; //将就绪的文件描述符加入集合 for(;k<SIZE;k++) { if(gfds[k]==-1) { gfds[k]=connfd; break; } } if(k>=SIZE) { Close(connfd); gfds[k]=-1; } } } int j=1; for(;j<SIZE;j++) { if(FD_ISSET(gfds[j],&rfds)) { char buf[SIZE]; //从已经就绪的文件描述符里读数据到buf里 ssize_t s=read(gfds[j],buf,sizeof(buf)); if(s<0) { perror("read"); continue; } else if(s==0) { printf("client is quit!\n"); Close(gfds[j]); gfds[j]=-1; } else { //逻辑处理模块 //将读到的数据打印到标准输出上 buf[s]=0; printf("client# %s\n",buf); } } } break; } } return 0;}client.c
/* client.c */#include <stdio.h>#include <string.h>#include <unistd.h>#include <arpa/inet.h>#include <netinet/in.h>#include "wrap.h"#define MAXLINE 80int main(int argc, char *argv[]){ struct sockaddr_in servaddr; char buf[MAXLINE]; int sockfd, n; if (argc != 3) printf("./client IP\n"); sockfd = Socket(AF_INET, SOCK_STREAM, 0); bzero(&servaddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; inet_pton(AF_INET, argv[1], &servaddr.sin_addr); servaddr.sin_port = htons(atoi(argv[2])); Connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)); printf("------------connect ok----------------\n"); while (1) { fgets(buf, sizeof(buf), stdin); Write(sockfd, buf, strlen(buf)); n = Read(sockfd, buf, sizeof(buf)); if (n == 0) { printf("the other side has been closed.\n"); break; } else Write(STDOUT_FILENO, buf, n); } Close(sockfd); return 0;}使用 nc 127.0.0.1 8080命令链接服务器，或者使用客户端链接服务器。
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