I/O多路复用中select指的是什么

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select是用于监视多个文件描述符状态的变化的。即用来监视文件描述符读/写/异常状态是否就绪。

函数原型：int select(int nfds,fd_set *readfds,fd_set *writefds,fd_set *exceptfds,struct timeval *timeout);

select的几大缺点：

（1）每次调用select，都需要把fd集合从用户态拷贝到内核态，这个开销在fd很多时会很大

（2）同时每次调用select都需要在内核遍历传递进来的所有fd，这个开销在fd很多时也很大

（3）select支持的文件描述符数量太小了

  1 #include<stdio.h>   2 #include<string.h>   3 #include<stdlib.h>   4 #include<unistd.h>   5 #include<sys/select.h>   6    7 int main()   8 {   9     int std_in = 0;  10 //  int std_out = 1;  11     fd_set reads;  12 //  fd_set writes;  13     //int max_nums = std_out;  14     int max_nums = std_in;  15     FD_ZERO(&reads);  16 //  FD_ZERO(&writes);  17     FD_SET(std_in,&reads);  18 //  FD_SET(std_out,&writes);  19     struct timeval _timeout = {5,0};  20     int done = 0;  21     while(!done)  22     {  23         _timeout.tv_sec = 5;   24         _timeout.tv_usec = 0;  25         //switch(select(max_nums+1,&reads,&writes,NULL,&_timeout))  26         switch(select(max_nums+1,&reads,NULL,NULL,&_timeout))  27         {  28             case -1:  29                 perror("select");  30                 break;  31             case 0:  32                 printf("timeout...\n");  33                 break;  34             default://success  35                 {  36                     if(FD_ISSET(std_in,&reads))  37                     {//read  38                         char buf[1024];  39                         memset(buf,'\0',sizeof(buf));  40                         ssize_t size = read(std_in,buf,sizeof(buf)-1);  41                         if(size<0)  42                         {  43                             perror("read");  44                             exit(1);  45                         }  46                         if(strncmp(buf,"quit",4)==0)   47                         {  48                             done =1;  49                             continue;  50                         }  51                         printf("echo: %s",buf);  52   53                     }  54                 //  if(FD_ISSET(std_out,&writes))  55                 //  {//writes  56                 //      char buf[1024];  57                 //      while(1)  58                 //      {  59                 //          memset(buf,'\0',sizeof(buf));  60                 //          strcpy(buf,"hello world");  61                             //write(1,buf,sizeof(buf)-1);  62                 //          printf("%s\n",buf);  63                 //      }  64                 //  }  65                     break;  66                 }  67         }  68     }  69     return 0;  70 }  [fbl@localhost select]$ ./select  hello echo: hello hi echo: hi nihao echo: nihao ahhau echo: ahhau quit [fbl@localhost select]$

  1 #include<stdio.h>   2 #include<string.h>   3 #include<stdlib.h>   4 #include<unistd.h>   5 #include<sys/socket.h>   6 #include<sys/select.h>   7 #include<sys/types.h>   8 #include<netinet/in.h>   9 #include<arpa/inet.h>  10 #include<assert.h>  11   12 #define _BACKLOG_ 5  13 int fd[64];  14 void usage(char *_port)  15 {  16     printf("%s,[ip],[port]\n",_port);  17 }  18 int startup(char *ip,int port)  19 {  20     assert(ip);  21     int sock = socket(AF_INET,SOCK_STREAM,0);  22     if(sock<0)  23     {  24         perror("socket");  25         exit(1);  26     }  27     struct sockaddr_in local;  28     local.sin_family = AF_INET;  29     local.sin_port = htons(port);  30     local.sin_addr.s_addr = inet_addr(ip);  31     if(bind(sock,(struct sockaddr*)&local,sizeof(local))<0)  32     {  33         perror("bind");  34         exit(2);  35     }  36     if(listen(sock,_BACKLOG_)<0)  37     {  38         perror("listen");  39         exit(3);  40     }  41     return sock;  42   43   44 }  45 int main(int argc,char *argv[])  46 {  47     if(argc!=3)  48     {  49         usage(argv[0]);  50         return 1;  51     }  52     int port = atoi(argv[2]);  53     char *ip = argv[1];  54     int listen_sock = startup(ip,port);  55     int new_sock = -1;  56     struct sockaddr_in client;  57     socklen_t len = sizeof(client);  58     fd_set reads;  59     fd_set writes;  60     int max_nums;  61     struct timeval _timeout = {5,0};  62     int done = 0;  63     int i =0;  64     int fd_nums = sizeof(fd)/sizeof(fd[0]);  65     for(;i<fd_nums;++i)  66     {  67         fd[i]=-1;  68     }  69     fd[0] = listen_sock;   70     while(!done)  71     {  72         _timeout.tv_sec = 5;  73         _timeout.tv_usec = 0;  74         FD_ZERO(&reads);  75         FD_ZERO(&writes);  76         for(i=0;i<fd_nums;++i)  77         {  78             if(fd[i]>0)  79             {  80                 FD_SET(fd[i],&reads);  81                 if(fd[i]>max_nums)  82                 {  83                     max_nums = fd[i];  84                 }  85   86             }  87         }  88         switch(select(max_nums+1,&reads,&writes,NULL,&_timeout))  89         {  90             case -1:  91                 perror("select");  92                 break;  93             case 0:  94                 printf("timeout...\n");  95                 break;  96             default:  97                 {  98   99                     for(i=0;i<fd_nums;++i) 100                     { 101                         if(fd[i]==listen_sock && FD_ISSET(fd[i],&reads)) 102                         { 103                             new_sock = accept(listen_sock,(struct sockaddr*)    &client,&len); 104                             if(new_sock<0) 105                             { 106                                 perror("accept"); 107                                 continue; 108                             } 109                             for(i=0;i<fd_nums;++i) 110                             { 111                                 if(fd[i]==-1) 112                                 { 113                                     fd[i]=new_sock; 114                                 } 115                             } 116  117                         } 118                         else if(fd[i]>0 && FD_ISSET(fd[i],&reads)) 119                         { 120                             char buf[1024]; 121                             memset(buf,'\0',sizeof(buf)); 122                             ssize_t size = read(new_sock,buf,sizeof(buf)-1); 123                             if(size<0) 124                             { 125                                 perror("read"); 126                                 exit(4); 127                             } 128                             else if(size==0) 129                             { 130                                 printf("client close...\n"); 131                                 close(fd[i]); 132                                 fd[i]=-1; 133                             } 134                             else 135                             { 136                                 buf[size]='\0'; 137                             } 138                             printf("client:%s\n",buf); 139                         } 140                         else 141                         {} 142                     } 143  144                     break; 145                 } 146  147         } 148     } 149     return 0; 150 } 1 #include<stdio.h>   2 #include<string.h>   3 #include<stdlib.h>   4 #include<unistd.h>   5 #include<sys/socket.h>   6 #include<sys/select.h>   7 #include<sys/types.h>   8 #include<netinet/in.h>   9 #include<arpa/inet.h>  10 void usage(char *_port)  11 {  12     printf("%s,[ip],[port]\n",_port);  13 }  14 int main(int argc,char *argv[])  15 {  16     if(argc!=3)  17     {  18         usage(argv[0]);  19         return 1;  20     }  21     int port = atoi(argv[2]);  22     char *ip = argv[1];  23     int sock = socket(AF_INET,SOCK_STREAM,0);  24     if(sock<0)  25     {  26         perror("socket");  27         exit(1);  28     }  29     struct sockaddr_in remote;  30     remote.sin_family = AF_INET;  31     remote.sin_port = htons(port);  32     remote.sin_addr.s_addr = inet_addr(ip);  33     int ret = connect(sock,(struct sockaddr*)&remote,sizeof(remote));  34     if(ret<0)  35     {  36         perror("connect");  37         exit(1);  38     }  39     char buf[1024];  40     while(1)  41     {  42   43         memset(buf,'\0',sizeof(buf));  44         printf("please say:");  45         scanf("%s",buf);  46         ssize_t size = write(sock,buf,sizeof(buf)-1);   47         if(size<0)  48         {  49             perror("write");  50             exit(2);  51         }  52         else if(size ==0)  53         {}  54         else  55         {  56             printf("client : %s\n",buf);  57         }  58     }  59     return 0;  60 } [fbl@localhost select_socket]$ ./server 192.168.1.106 8080 timeout... client:hello client:hi client:huowo client close... client: read: Bad file descriptor [fbl@localhost select_socket]$  [fbl@localhost select_socket]$ ./client 192.168.1.106 8080 please say:hello client : hello please say:hi client : hi please say:huowo client : huowo please say:^C [fbl@localhost select_socket]$

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