使用select处理任意数目个客户的单进程程序

改写http://www.cnblogs.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;
            }
        }
    }
}