Linux為多路複用IO提供了較多的介面，有select(),pselect(),poll()的方式，繼承自BSD和System V 兩大派系。

　　select模型比較簡單，“輪詢”檢測fd_set的狀態，然後再採取相應的措施。

　　訊號驅動模型有必要仔細研究一下，一般有如下步驟：

1. 設定安裝函式，訊號是驅動訊號是SIGIO（最好使用sigaction的方式，方便設定flag為SA_RESTART，因為client中讀取終端的syscall可能會被中斷，有必要重啟。當然，使用signal()的方式然後再對errno進行判斷是否為ETNTR，自行重啟也是一種方法。但是signal()的可移植性問題，我強烈不建議使用）
2. 設定fd的屬主。F_SETOWN,要接受訊號的程序，fcntl().
3. 設定fd的非同步標誌。小細節，用'|'新增，要把之前的狀態保留，也就是先F_GETFL再F_SETFL。（注意：在開啟檔案open()時設定標識O_ASYNC無實質效果）  

   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

4. sigaction()安裝

　　具體進下文client例程。

　　寫了一個聊天程式的demo，把這兩種技術都使用了。服務端採取多路複用的IO方式，代替多進（線）程的模型，客服端採取的是訊號驅動，如下：

容易產生bug的地方都寫註釋裡邊了。

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 }