1 IO多路复用的基本概念
IO多路复用是指内核一旦发现进程指定的一个或者多个IO条件准备读取，它就通知该进程。IO多路复用适用如下场合：
（1）当客户处理多个描述字时（一般是交互式输入和网络套接口），必须使用I/O复用。
（2）当一个客户同时处理多个套接口时，而这种情况是可能的，但很少出现。
（3）如果一个TCP服务器既要处理监听套接口，又要处理已连接套接口，一般也要用到I/O复用。
（4）如果一个服务器即要处理TCP，又要处理UDP，一般要使用I/O复用。
（5）如果一个服务器要处理多个服务或多个协议，一般要使用I/O复用。
与多进程和多线程技术相比，I/O多路复用技术的最大优势是系统开销小，系统不必创建进程/线程，也不必维护这些进程/线程，从而大大减小了系统的开销。
2 select 函数
/* According to POSIX.1-2001, POSIX.1-2008 */ #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);
参数描述：
nfds: 集合中所有文件描述符的范围，即所有文件描述符的最大值加1，描述字0、1、2...nfds-1均将被测试；
readfds: 指向fd_set结构的指针，这个集合中包括需要监控读事件的文件描述符，若可读则返回；当然也可以置空（NULL），表示并不关心；
writefds, exceptfds: 同readfds;
timeout: select的超时时间，这个参数至关重要，它可以使select处于三种状态:
- 若将NULL以形参传入，就是将select置于阻塞状态，一定等到监视文件描述符集合中某个文件描述符发生变化为止；
- 若将时间值设为0秒0毫秒，就变成一个纯粹的非阻塞函数，不管文件描述符是否有变化，都立刻返回继续执行，文件无变化返回0，有变化返回一个正值；
- timeout的值大于0，这就是等待的超时时间，即select在timeout时间内阻塞，超时时间之内有事件到来就返回了，否则在超时后不管怎样一定返回，返回值同上述。
返回值：
- >0：就绪描述字的正数目；
- -1：出错；
- 0 ： 超时；
fd_set ：
select使用描述字集，典型地是一个整数数组，其中每个整数中的每一位对应一个描述字。假设使用32位整数，那么该数组的第一个元素对应于描述字0~31，第二个元素对应于描述字32~63，依此类推。所有的实现细节都与应用程序无关，它们隐藏在名为fd_set的数据类型和以下四个宏中：
void FD_ZERO (fd_set *fdset); // clear all bits in fdset void FD_SET (int fd,fd_set *fdset); // turn on the bit for fd in fdset void FD_CLR (int fd,fd_set *fdset); // turn off the bit for fd in fdset int FD_ISSET(int fd,fd_set *fdset); // is the bit for fd on in fdset
struct timeval：
timeval结构用于指定这段时间的秒数和微秒数，
struct timeval { long tv_sec; //seconds long tv_usec; //microseconds };
select() may update the timeout argument to indicate how much time was left. pselect() does not change this argument.
3 select函数的典型应用
下面介绍一个给予select的echo server；client直接通过telnet连接server：telnet host port，主要包括以下几个
基本功能：
(1) client 发送quit，则server主动断开与客服端的连接；
(2) clinet发送其他data到server，则server会原样返回；
注意点：
(1) 由于通过telnet与server通信，telnet client发送过来的字符串的结束符为"
" ，所以判断client的断开信号时，需判断是否为"quit
"
(2) 同样打印客服端的数据时不需要再添加换行符，因为data form telnet client自带的就有；
(3) 由于本文中，每次读数据时都是放在一个公共的缓冲区buffer，所以每次接收数据(read)并处理完(send)后，应重置缓冲区，比如重置受影响的区域(大小size)都为' '，memset(buffer, 0, size)；
/* Handle multiple socket connections with select and fd_set on Linux */ #include <stdio.h> #include <string.h> //strlen #include <stdlib.h> #include <errno.h> #include <unistd.h> #include <arpa/inet.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <sys/time.h> //FD_SET, FD_ISSET, FD_ZERO macros #define TRUE 1 #define FALSE 0 #define PORT 8888 #define CLIENT_NUM 2 #define BLOCK_NUM 2 #define BUFFER_SIZE 1024 int main(int argc , char *argv[]) { int opt = TRUE; int master_socket , addrlen , new_socket , client_socket[CLIENT_NUM] , activity, i , valread , sd; int max_sd; struct sockaddr_in address; char buffer[BUFFER_SIZE+1]; //data buffer of 1K struct timeval tv; char quit[7]="quit "; //for data from telnet client, the data end with " " //set of socket descriptors fd_set readfds; //a message retuan to client when it connect to the server. char *message = "ECHO Daemon v1.0 "; //initialise all client_socket[] to 0 so not checked for (i = 0; i < CLIENT_NUM; i++) { client_socket[i] = 0; } printf("size of buffer:%lu ",sizeof(buffer)); memset(buffer,0,sizeof(buffer)*sizeof(buffer[0])); //create a master socket for listening if( (master_socket = socket(AF_INET , SOCK_STREAM , 0)) == 0) { perror("socket failed"); exit(EXIT_FAILURE); } /*一个端口释放后会等待两分钟之后才能再被使用，SO_REUSEADDR是让端口释放后立即就可以被再次使用。*/ if( setsockopt(master_socket, SOL_SOCKET, SO_REUSEADDR, (char *)&opt, sizeof(opt)) < 0 ) { perror("setsockopt"); exit(EXIT_FAILURE); } //type of socket created address.sin_family = AF_INET; address.sin_addr.s_addr = INADDR_ANY; address.sin_port = htons( PORT ); //bind the socket to localhost port 8888 if (bind(master_socket, (struct sockaddr *)&address, sizeof(address))<0) { perror("bind failed"); exit(EXIT_FAILURE); } printf("Listener on port %d ", PORT); //try to specify maximum of BLOCK_NUM pending connections for the master socket if (listen(master_socket, BLOCK_NUM) < 0) { perror("listen"); exit(EXIT_FAILURE); } //accept the incoming connection addrlen = sizeof(address); puts("Waiting for connections ..."); /********************************call select in a infinite loop*******************************/ while(TRUE) { /*每次调用select前都要重新设置文件描述符和时间，因为事件发生后，文件描述符和时间都被内核修改啦*/ FD_ZERO(&readfds); /*添加监听套接字*/ FD_SET(master_socket, &readfds); max_sd = master_socket; tv.tv_sec = 30; tv.tv_usec = 0; /*添加客户端套接字*/ for ( i = 0 ; i < CLIENT_NUM ; i++) { //socket descriptor sd = client_socket[i]; //if valid socket descriptor then add to read list if(sd > 0) FD_SET( sd , &readfds); //highest file descriptor number, need it for the select function if(sd > max_sd) max_sd = sd; } printf(" select begin, timeout left:%lds-%ldms. ",tv.tv_sec,tv.tv_usec); //wait for an activity on one of the sockets, timeout is tv; activity = select( max_sd + 1 , &readfds , NULL , NULL , &tv); //return value < 0, error happend; if ((activity < 0) && (errno!=EINTR)) { printf("select error"); } //return value = 0, timeout; else if(activity == 0){ //no event happend in specific time, timeout; printf("select return, activity:%d, timeout left:%lds-%ldms. ", activity, tv.tv_sec, tv.tv_usec); continue; } //If something happened on the master socket , then its an incoming connection if (FD_ISSET(master_socket, &readfds)) { printf("select return, timeout left:%lds-%ldms. ",tv.tv_sec,tv.tv_usec); if ((new_socket = accept(master_socket, (struct sockaddr *)&address, (socklen_t*)&addrlen))<0) { perror("accept"); exit(EXIT_FAILURE); } //inform user of socket number - used in send and receive commands printf("New connection , socket fd is %d , ip is : %s , port : %d " , new_socket , inet_ntoa(address.sin_addr) , ntohs(address.sin_port)); //send new connection greeting message if( send(new_socket, message, strlen(message), 0) != strlen(message) ) { perror("send"); } puts("Welcome message sent successfully"); //add new socket to array of sockets for (i = 0; i < CLIENT_NUM; i++) { //if position is empty if( client_socket[i] == 0 ) { client_socket[i] = new_socket; printf("Adding to list of sockets as %d " , i); break; } } } //else there is some IO operation on some other socket :) for (i = 0; i < CLIENT_NUM; i++) { sd = client_socket[i]; printf("check client--%d:fd--%d ",i,sd); if (FD_ISSET( sd , &readfds)) { printf("select return, timeout left:%lds-%ldms. ",tv.tv_sec,tv.tv_usec); //Check if it was for closing , and also read the incoming message valread = read( sd , buffer, 1024); printf("read form client,size:%d ",valread); if (valread <= 0) { //Somebody disconnected , get his details and print getpeername(sd , (struct sockaddr*)&address , (socklen_t*)&addrlen); printf("Host disconnected , ip %s , port %d " , inet_ntoa(address.sin_addr) , ntohs(address.sin_port)); //Close the socket and mark as 0 in list for reuse close( sd ); client_socket[i] = 0; } //get message "quit", close the connection; else if(valread == strlen(quit) && memcmp(buffer,quit,strlen(quit))==0) { printf("client of fd:%d will be closed. ", sd); client_socket[i] = 0; close(sd); } //Echo back the message that came in else { printf("data come from client:%s",buffer); send(sd , buffer , strlen(buffer) , 0 ); memset(buffer,0,valread); //printf("sleep 5s "); //sleep(5); } } } } return 0; }
运行结果：
[root@localhost LibEvent_practice]# ./echo-server-select size of buffer:1025 Listener on port 8888 Waiting for connections ... select begin, timeout left:30s-0ms. select return, timeout left:25s-213990ms.
//clinet1 connect to server, select return, the time left to timeout is "timeout left"; New connection , socket fd is 4 , ip is : 10.204.214.232 , port : 45042 //detail info of client Welcome message sent successfully Adding to list of sockets as 0 check client--0:fd--4 //client fd info saved in client_socket[CLIENT_NUM] check client--1:fd--0 select begin, timeout left:30s-0ms. select return, timeout left:20s-751072ms. //client2 connect to server New connection , socket fd is 5 , ip is : 10.204.214.248 , port : 48117 Welcome message sent successfully Adding to list of sockets as 1 check client--0:fd--4 check client--1:fd--5 select begin, timeout left:30s-0ms. check client--0:fd--4 select return, timeout left:25s-940254ms.
//client1 fd可读，读取数据并回传给client read form client,size:21 data come from client:aaaaaaaaaaaaaaaaaaa check client--1:fd--5 select begin, timeout left:30s-0ms. check client--0:fd--4 check client--1:fd--5 select return, timeout left:23s-593254ms.
//client2 fd可读，读取数据并回传，可以看到dataSize=7,可以看到的数据为bbbbb,实际来自client的数据为"bbbbb ",打印出"bbbbb"然后换行； read form client,size:7 data come from client:bbbbb select begin, timeout left:30s-0ms. check client--0:fd--4 select return, timeout left:19s-478711ms. read form client,size:6 client of fd:4 will be closed.
//读取到来自client的"quit "字符串，主动关闭client check client--1:fd--5 select begin, timeout left:30s-0ms. select return, activity:0, timeout left:0s-0ms. select begin, timeout left:30s-0ms. ^C
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