Question

在此链接https://backreference.org/2010/03/26/tuntap-interface-tutorial/中，有一个代码示例，该示例使用tun / tap接口创建TCP隧道，如下所示。

  /* net_fd is the network file descriptor (to the peer), tap_fd is the
     descriptor connected to the tun/tap interface */

  /* use select() to handle two descriptors at once */
  maxfd = (tap_fd > net_fd)?tap_fd:net_fd;

  while(1) {
    int ret;
    fd_set rd_set;

    FD_ZERO(&rd_set);
    FD_SET(tap_fd, &rd_set); FD_SET(net_fd, &rd_set);

    ret = select(maxfd + 1, &rd_set, NULL, NULL, NULL);

    if (ret < 0 && errno == EINTR) {
      continue;
    }

    if (ret < 0) {
      perror("select()");
      exit(1);
    }

    if(FD_ISSET(tap_fd, &rd_set)) {
      /* data from tun/tap: just read it and write it to the network */

      nread = cread(tap_fd, buffer, BUFSIZE);

      /* write length + packet */
      plength = htons(nread);
      nwrite = cwrite(net_fd, (char *)&plength, sizeof(plength));
      nwrite = cwrite(net_fd, buffer, nread);
    }

    if(FD_ISSET(net_fd, &rd_set)) {
      /* data from the network: read it, and write it to the tun/tap interface.
       * We need to read the length first, and then the packet */

      /* Read length */
      nread = read_n(net_fd, (char *)&plength, sizeof(plength));

      /* read packet */
      nread = read_n(net_fd, buffer, ntohs(plength));

      /* now buffer[] contains a full packet or frame, write it into the tun/tap interface */
      nwrite = cwrite(tap_fd, buffer, nread);
    }
  }

该代码摘录中“ maxfd”的目的是什么？确切的行是：

maxfd = (tap_fd > net_fd)?tap_fd:net_fd;

ret = select(maxfd + 1, &rd_set, NULL, NULL, NULL);

Answer 1

这是危险且过时的select函数工作方式的产物。它需要一个参数，该参数受传递给它的fd_set对象的大小（以位为单位）的限制，并且不能使用大于FD_SETSIZE施加的任意限制的fd数字。如果您不能满足这些要求，则会导致未定义行为。

无论您何时看到select，都应将其替换为poll，因为不受这些限制，它具有易于使用的界面并且具有更多功能。

创建隧道时对“ maxfd”有什么要求？

1 个答案: