Linux为多路复用IO提供了较多的接口，有select(),pselect(),poll()的方式，继承自BSD和System V 两大派系。
select模型比较简单，“轮询”检测fd_set的状态，然后再采取相应的措施。
信号驱动模型有必要仔细研究一下，一般有如下步骤：
- 设置安装函数，信号是驱动信号是SIGIO（最好使用sigaction的方式，方便设置flag为SA_RESTART，因为client中读取终端的syscall可能会被中断，有必要重启。当然，使用signal()的方式然后再对errno进行判断是否为ETNTR，自行重启也是一种方法。但是signal()的可移植性问题，我强烈不建议使用）
- 设置fd的属主。F_SETOWN,要接受信号的进程，fcntl().
- 设置fd的异步标志。小细节，用'|'添加，要把之前的状态保留，也就是先F_GETFL再F_SETFL。（注意：在打开文件open()时设置标识O_ASYNC无实质效果）
On Linux, specifying the O_ASYNC
flag when calling open() has no effect. To enable signal-driven I/O, we must
instead set this flag using the fcntl() F_SETFL operation (Section 5.3).————《the linux programming interface》 4.3.1 - sigaction()安装
具体进下文client例程。
写了一个聊天程序的demo，把这两种技术都使用了。服务端采取多路复用的IO方式，代替多进（线）程的模型，客服端采取的是信号驱动，如下：
容易产生bug的地方都写注释里边了。
serv.c
1 #include <sys/select.h> 2 #include <sys/socket.h> 3 #include <netinet/in.h> 4 #include <arpa/inet.h> 5 #include <unistd.h> 6 #include <fcntl.h> 7 #include <stdlib.h> 8 #include <string.h> 9 #include <stdio.h> 10 #include <signal.h> 11 12 void endServ(int sig) 13 { 14 printf("Server ended!\n"); 15 exit(-1); 16 } 17 18 int main() 19 { 20 // signal 21 struct sigaction act; 22 sigemptyset(&act.sa_mask); 23 act.sa_handler = endServ; 24 act.sa_flags = 0; 25 sigaction(SIGINT,&act,0); 26 printf("This server is started,enter CTRL+C to end. \n"); 27 28 int servfd = socket(AF_INET,SOCK_STREAM,0); 29 if(servfd == -1) { 30 printf("something wrong with socket():%m\n"); 31 exit(-1); 32 } 33 34 struct sockaddr_in addr; 35 addr.sin_family = AF_INET; 36 addr.sin_port = htons(9999); 37 inet_pton(AF_INET,"127.0.0.1",&addr.sin_addr); 38 39 if (bind(servfd,(struct sockaddr*)&addr,sizeof(addr)) == -1) { 40 printf("Some thing wrong with bind():%m\n"); 41 exit(-1); 42 } 43 printf("Bind success!\n"); 44 45 listen(servfd,20); 46 47 int numbers=0;//how many clients has accepted 48 fd_set fs; 49 FD_ZERO(&fs); 50 int client_fd[100]; 51 int i; 52 for(i=0;i<100;++i) 53 { 54 client_fd[i] = -1; 55 } 56 57 int maxfd=0; 58 char buf[1024]; 59 for(;;) 60 { 61 maxfd =0; 62 FD_ZERO(&fs); 63 FD_SET(servfd,&fs); 64 maxfd = maxfd>servfd?maxfd:servfd; 65 for(i=0;i<numbers;++i) 66 { 67 if(client_fd[i] != -1) { 68 FD_SET(client_fd[i],&fs); 69 maxfd = maxfd>client_fd[i]?maxfd:client_fd[i]; 70 } 71 } 72 73 int res = select(maxfd+1,&fs,0,0,0); 74 if(res == -1) { 75 printf("Something wrong with select():%m\n"); 76 exit(-1); 77 } 78 79 if(FD_ISSET(servfd,&fs) && numbers < 100) { 80 printf("New client!\n"); 81 client_fd[numbers] = accept(servfd,0,0); 82 numbers++; 83 } 84 85 for(i=0;i<numbers;++i) 86 { 87 bzero(buf,sizeof(buf)); 88 //judge if client_fd[i] equal -1 is necessary 89 //if a client quited,next time the program will 90 //have a segment default 91 //also it should be in the front. 92 if(client_fd[i] != -1 && FD_ISSET(client_fd[i],&fs)) 93 { 94 res = recv(client_fd[i],buf,sizeof(buf),0); 95 if(res == 0) { 96 printf("A client quit\n"); 97 close(client_fd[i]); 98 FD_CLR(client_fd[i],&fs); 99 client_fd[i] = -1; 100 } 101 else if(res == -1) { 102 printf("Something wrong with net.\n"); 103 exit(-1); 104 } 105 else { 106 int j; 107 for(j=0;j<numbers;++j) 108 { 109 if(client_fd[j] != -1) 110 send(client_fd[j],buf,sizeof(buf),0); 111 } 112 } 113 } 114 } 115 } 116 }
client：
1 #include <signal.h> 2 #include <unistd.h> 3 #include <sys/socket.h> 4 #include <fcntl.h> 5 #include <arpa/inet.h> 6 #include <netinet/in.h> 7 #include <stdio.h> 8 #include <string.h> 9 #include <stdlib.h> 10 #include <errno.h> 11 12 static int fd; 13 14 void show(int sig) 15 { 16 char buf[1024] = {0}; 17 int n = read(fd,buf,sizeof(buf)); 18 buf[n] = 0; 19 write(1,"MSG:",strlen("MSG:")); 20 write(1,buf,strlen(buf)); 21 } 22 23 int main() 24 { 25 struct sigaction act; 26 sigemptyset(&act.sa_mask); 27 act.sa_handler = show; 28 //This is necessary,in last loop read() counld be interrupt; 29 act.sa_flags = SA_RESTART; 30 sigaction(SIGIO,&act,0); 31 32 fd = socket(AF_INET,SOCK_STREAM,0); 33 if(fd == -1) { 34 printf("Cannot get socketfd!:%m\n"); 35 exit(-1); 36 } 37 38 struct sockaddr_in addr; 39 addr.sin_family = AF_INET; 40 addr.sin_port = htons(9999); 41 inet_pton(AF_INET,"127.0.0.1",&addr.sin_addr); 42 43 if(connect(fd,(struct sockaddr*)&addr,sizeof(addr)) != -1) 44 printf("connect success!\n"); 45 else 46 exit(-1); 47 48 int flag = fcntl(fd,F_GETFL); 49 flag |= O_ASYNC; 50 fcntl(fd,F_SETFL,flag); 51 fcntl(fd,F_SETOWN,getpid()); 52 53 char buffer[1024]={0}; 54 for(;;) 55 { 56 int n = read(0,buffer,sizeof(buffer)); 57 if(n==-1) 58 break; 59 send(fd,buffer,n,0); 60 } 61 62 write(1,"Closed.",strlen("Closed.")); 63 }