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!
");
    exit(-1);
}

int main()
{
    // signal
    struct sigaction act;
    sigemptyset(&act.sa_mask);
    act.sa_handler = endServ;
    act.sa_flags = 0;
    sigaction(SIGINT,&act,0);
    printf("This server is started,enter CTRL+C to end. 
");

    int servfd = socket(AF_INET,SOCK_STREAM,0);
    if(servfd == -1) {
        printf("something wrong with socket():%m
");
        exit(-1);
    }

    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(9999);
    inet_pton(AF_INET,"127.0.0.1",&addr.sin_addr);

    if (bind(servfd,(struct sockaddr*)&addr,sizeof(addr)) == -1) {
        printf("Some thing wrong with bind():%m
");
        exit(-1);
    }
    printf("Bind success!
");

    listen(servfd,20);
    
    int numbers=0;//how many clients has accepted
    fd_set fs;
    FD_ZERO(&fs);
    int client_fd[100];
    int i;
    for(i=0;i<100;++i)
    {
        client_fd[i] = -1;
    }

    int maxfd=0;
    char buf[1024];
    for(;;)
    {
        maxfd =0;
        FD_ZERO(&fs);
        FD_SET(servfd,&fs);
        maxfd = maxfd>servfd?maxfd:servfd;
        for(i=0;i<numbers;++i)
        {
            if(client_fd[i] != -1) {
                FD_SET(client_fd[i],&fs);
                maxfd = maxfd>client_fd[i]?maxfd:client_fd[i];
            }
        }

        int res = select(maxfd+1,&fs,0,0,0);
        if(res == -1) {
            printf("Something wrong with select():%m
");
            exit(-1);
        }

        if(FD_ISSET(servfd,&fs) && numbers < 100) {
            printf("New client!
");
            client_fd[numbers] = accept(servfd,0,0);
            numbers++;
        }
        
        for(i=0;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] != -1 && FD_ISSET(client_fd[i],&fs)) 
            {
                res = recv(client_fd[i],buf,sizeof(buf),0);
                if(res == 0) {
                    printf("A client quit
");
                    close(client_fd[i]);
                    FD_CLR(client_fd[i],&fs);
                    client_fd[i] = -1;
                }
                else if(res == -1) {
                    printf("Something wrong with net.
");
                    exit(-1);
                }
                else {
                    int j;
                    for(j=0;j<numbers;++j)
                    {
                        if(client_fd[j] != -1)
                            send(client_fd[j],buf,sizeof(buf),0);
                    }
                }
            }
        }
    }
}

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[1024] = {0};
    int n = read(fd,buf,sizeof(buf));
    buf[n] = 0;
    write(1,"MSG:",strlen("MSG:"));
    write(1,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,0);

    fd = socket(AF_INET,SOCK_STREAM,0);
    if(fd == -1) {
        printf("Cannot get socketfd!:%m
");
        exit(-1);
    }

    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(9999);
    inet_pton(AF_INET,"127.0.0.1",&addr.sin_addr);

    if(connect(fd,(struct sockaddr*)&addr,sizeof(addr)) != -1)
        printf("connect success!
");
    else
        exit(-1);

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

    char buffer[1024]={0};
    for(;;)
    {
        int n = read(0,buffer,sizeof(buffer));
        if(n==-1)
            break;
        send(fd,buffer,n,0);
    }

    write(1,"Closed.",strlen("Closed."));
}