select详解

select

1. 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
2. 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0int FD_ISSET(int fd, fd_set *set);  //测试文件描述符集合里fd是否置1void FD_SET(int fd, fd_set *set);   //把文件描述符集合里fd位置1void 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);#endif

wrap.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;elseperr_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;elsereturn -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;elsereturn -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;elsereturn -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;elsereturn -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;} elsereturn -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-1int 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acceptif(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;}elseWrite(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。