IO的多路复用和信号驱动

　　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：

1. 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）
2. 设置fd的属主。F_SETOWN,要接受信号的进程，fcntl().
3. 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

4. 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

 #include <sys/select.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 #include <signal.h>

 void endServ(int sig)
 {
     printf("Server ended!\n");
     exit(-);
 }

 int main()
 {
     // signal
     struct sigaction act;
     sigemptyset(&act.sa_mask);
     act.sa_handler = endServ;
     act.sa_flags = ;
     sigaction(SIGINT,&act,);
     printf("This server is started,enter CTRL+C to end. \n");

     int servfd = socket(AF_INET,SOCK_STREAM,);
     if(servfd == -) {
         printf("something wrong with socket():%m\n");
         exit(-);
     }

     struct sockaddr_in addr;
     addr.sin_family = AF_INET;
     addr.sin_port = htons();
     inet_pton(AF_INET,"127.0.0.1",&addr.sin_addr);

     if (bind(servfd,(struct sockaddr*)&addr,sizeof(addr)) == -) {
         printf("Some thing wrong with bind():%m\n");
         exit(-);
     }
     printf("Bind success!\n");

     listen(servfd,);

     int numbers=;//how many clients has accepted
     fd_set fs;
     FD_ZERO(&fs);
     int client_fd[];
     int i;
     for(i=;i<;++i)
     {
         client_fd[i] = -;
     }

     int maxfd=;
     char buf[];
     for(;;)
     {
         maxfd =;
         FD_ZERO(&fs);
         FD_SET(servfd,&fs);
         maxfd = maxfd>servfd?maxfd:servfd;
         for(i=;i<numbers;++i)
         {
             if(client_fd[i] != -) {
                 FD_SET(client_fd[i],&fs);
                 maxfd = maxfd>client_fd[i]?maxfd:client_fd[i];
             }
         }

         int res = select(maxfd+,&fs,,,);
         if(res == -) {
             printf("Something wrong with select():%m\n");
             exit(-);
         }

         if(FD_ISSET(servfd,&fs) && numbers < ) {
             printf("New client!\n");
             client_fd[numbers] = accept(servfd,,);
             numbers++;
         }

         for(i=;i<numbers;++i)
         {
             bzero(buf,sizeof(buf));
             //judge if client_fd[i] equal -1 is necessary
             //if a client quited,next time the program will
             //have a segment default
             //also it should be in the front.
             if(client_fd[i] != - && FD_ISSET(client_fd[i],&fs))
             {
                 res = recv(client_fd[i],buf,sizeof(buf),);
                 if(res == ) {
                     printf("A client quit\n");
                     close(client_fd[i]);
                     FD_CLR(client_fd[i],&fs);
                     client_fd[i] = -;
                 }
                 else if(res == -) {
                     printf("Something wrong with net.\n");
                     exit(-);
                 }
                 else {
                     int j;
                     for(j=;j<numbers;++j)
                     {
                         if(client_fd[j] != -)
                             send(client_fd[j],buf,sizeof(buf),);
                     }
                 }
             }
         }
     }
 }

client：

 #include <signal.h>
 #include <unistd.h>
 #include <sys/socket.h>
 #include <fcntl.h>
 #include <arpa/inet.h>
 #include <netinet/in.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
 #include <errno.h>

 static int fd;

 void show(int sig)
 {
     char buf[] = {};
     int n = read(fd,buf,sizeof(buf));
     buf[n] = ;
     write(,"MSG:",strlen("MSG:"));
     write(,buf,strlen(buf));
 }

 int main()
 {
     struct sigaction act;
     sigemptyset(&act.sa_mask);
     act.sa_handler = show;
     //This is necessary,in last loop read() counld be interrupt;
     act.sa_flags = SA_RESTART;
     sigaction(SIGIO,&act,);

     fd = socket(AF_INET,SOCK_STREAM,);
     if(fd == -) {
         printf("Cannot get socketfd!:%m\n");
         exit(-);
     }

     struct sockaddr_in addr;
     addr.sin_family = AF_INET;
     addr.sin_port = htons();
     inet_pton(AF_INET,"127.0.0.1",&addr.sin_addr);

     if(connect(fd,(struct sockaddr*)&addr,sizeof(addr)) != -)
         printf("connect success!\n");
     else
         exit(-);

     int flag = fcntl(fd,F_GETFL);
     flag |= O_ASYNC;
     fcntl(fd,F_SETFL,flag);
     fcntl(fd,F_SETOWN,getpid());

     char buffer[]={};
     for(;;)
     {
         int n = read(,buffer,sizeof(buffer));
         if(n==-)
             break;
         send(fd,buffer,n,);
     }

     write(,"Closed.",strlen("Closed."));
 }