In this link, https://backreference.org/2010/03/26/tuntap-interface-tutorial/, there's a code sample that uses tun/tap interface to create a TCP tunnel as below.
/* 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);
}
}
What's the purpose of "maxfd" in that code excerpt? The exact lines are:
maxfd = (tap_fd > net_fd)?tap_fd:net_fd;
ret = select(maxfd + 1, &rd_set, NULL, NULL, NULL);
It's an artifact of the way the dangerous and obsolete select function works. It requires an argument that is a bound on the size (in bits) of the fd_set objects passed to it, and cannot work with fd numbers larger than an arbitrary limit imposed by FD_SETSIZE. If you fail to meet these requirements, Undefined Behavior results.
Wherever you see select, you should replace it by poll which does not suffer from these limitations, has an easier-to-use interface, and has more features.