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[MAXLINE];
      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(SERV_PORT);
  
      Bind(listenfd, (SA *) &servaddr, sizeof(servaddr));
  
      Listen(listenfd, LISTENQ);
  
      maxfd = listenfd;            /* initialize */
      maxi = -1;                    /* index into client[] array */
      for (i = 0; i < FD_SETSIZE; i++)
          client[i] = -1;            /* -1 indicates available entry */
      FD_ZERO(&allset);
      FD_SET(listenfd, &allset);
  
      for ( ; ; ) {
          rset = allset;        /* structure assignment */
          nready = Select(maxfd+1, &rset, NULL, NULL, NULL);
  
          if (FD_ISSET(listenfd, &rset)) {    /* new client connection */
              clilen = sizeof(cliaddr);
              connfd = Accept(listenfd, (SA *) &cliaddr, &clilen);
  
              for (i = 0; i < FD_SETSIZE; i++)
                  if (client[i] < 0) {
                      client[i] = connfd;    /* save descriptor */
                      break;
                  }
              if (i == FD_SETSIZE)
                  err_quit("too many clients");
  
              FD_SET(connfd, &allset);    /* add new descriptor to set */
              if (connfd > maxfd)
                  maxfd = connfd;            /* for select */
              if (i > maxi)
                  maxi = i;                /* max index in client[] array */
  
              if (--nready <= 0)
                  continue;                /* no more readable descriptors */
          }
  
          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, MAXLINE)) == 0) {
                        /*4connection closed by client */
                    Close(sockfd);
                    FD_CLR(sockfd, &allset);
                    client[i] = -1;
                } else
                    Writen(sockfd, buf, n);
  
                  if (--nready <= 0)
                      break;                /* no more readable descriptors */
              }
          }
      }
  }

  ssize_t Read(int fd, void *ptr, size_t nbytes) {
      ssize_t        n;
      if ( (n = read(fd, ptr, nbytes)) == -1)
          err_sys("read error");
      return(n);
  }
  
  void Writen(int fd, void *ptr, size_t nbytes) {
      if (writen(fd, ptr, nbytes) != nbytes)
          err_sys("writen error");
  }
  
  void err_sys(const char *fmt, ...) {
      va_list        ap;
      va_start(ap, fmt);
      err_doit(1, LOG_ERR, fmt, ap);
      va_end(ap);
      exit(1);
  }

              if (FD_ISSET(sockfd, &rset)) {
                if ( (n = Read(sockfd, buf, MAXLINE)) <= 0) {
                        /*4connection closed by client */
                      if ( n == 0 ) {
                       puts( "Client hung up!" );
                    } else {
                       puts( "Read error!" );
                    }
                    Close the socket when error or hung up.
                    Close(sockfd);
                    FD_CLR(sockfd, &allset);
                    client[i] = -1;
                } else
                    Writen(sockfd, buf, n);
  
  ssize_t Read(int fd, void *ptr, size_t nbytes) {
       ssize_t        n;
       if ( (n = read(fd, ptr, nbytes)) == -1)
           puts("read error");
       return(n);
   }
   
   void Writen(int fd, void *ptr, size_t nbytes) {
       if (writen(fd, ptr, nbytes) != nbytes)
           puts("writen error");
   }