如何让poll函数永久阻塞
函数
int poll(struct pollfd * fds, int numfds, int timeout);
其中timeout是设置超时时间量，如果设置为0，则poll函数不阻塞，
但是如何让他一直阻塞不超时，直到其中的fds满足条件，timeout 应当设置为何值？
timeout 用INFTIM
If timeout is neither zero nor INFTIM (-1), it specifies a maximum inter-
val to wait for any file descriptor to become ready, in milliseconds. If
timeout is INFTIM (-1), the poll blocks indefinitely. If timeout is
zero, then poll() will return without blocking.
