使用select处理任意数目个客户的单进程程序

改写http://www.CUOXin.com/nufangrensheng/p/3587962.html中的TCP回射服务器程序，把它重写成使用select来处理任意数目个客户的单进程程序，而不是为每个客户派生一个子进程。

int main(int argc, char **argv){    int                   i, maxi, maxfd, listenfd, connfd, sockfd;    int                   nready, client[FD_SETSIZE];    ssize_t               n;    fd_set                rset, allset;    char                  buf[4096];    socklen_t             clilen;    struct sockaddr_in    cliaddr, servaddr;    listenfd = socket(AF_INET, SOCK_STREAM, 0);        bzero(&servaddr, sizeof(servaddr));    servaddr.sin_family = AF_INET;    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);    servaddr.sin_port = htons(9877);        bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr));        listen(listenfd, 5);    maxfd = listenfd;    maxi = -1;    for(i = 0; i < FD_SETSIZE; i++)        client[i] = -1;        /* -1 indicates available entry */    FD_ZERO(&allset);    FD_SET(listenfd, &allset);    for(;;)    {        rset = allset;        nready = select(maxfd + 1, &rset, NULL, NULL, NULL);        if(FD_ISSET(listenfd, &rset))        {            clilen = sizeof(cliaddr);            connfd = accept(listenfd, (struct sockaddr *)&cliaddr, &clilen);                        for(i = 0; i < FD_SETSIZE; i++)            {                if(client[i] < 0)                {                    client[i] = connfd;    /* save descriptor */                    break;                }            }            if(i == FD_SETSIZE)            {                PRintf("too many clients/n");                exit(1);            }            FD_SET(connfd, &allset);    /* add new descriptor to set */            if(connfd > maxfd)                maxfd = connfd;            if(i > maxi)                maxi = i;            if(--nready <= 0)                continue;    /* no more readable descriptor */        }        for(i = 0; i <= maxi; i++)    /* check all clients for data */        {            if((sockfd = client[i]) < 0)                continue;            if(FD_ISSET(sockfd, &rset))            {                if((n = read(sockfd, buf, 4096)) == 0)    /* connection closed by client */                {                    close(sockfd);                    FD_CLR(sockfd, &rset);                    client[i] = -1;                }                else                {                    writen(sockfd, buf, n);                        }                                if(--nready <= 0)    /* no more readable descriptors */                    break;            }        }    }}