fgets读到文件最后一行的时候（返回不是NULL） ，feof测试为假，再fgets一次(返回为NULL)，再feof测试为真。
feof是通过判断read返回错误时，才判断流已经到达结尾。
在对流操作的时候，应该判断fgets()为NULL
其实正确使用 feof 函数很容易，只要记住一个原则就是了：“先读文件后用 feof 判断”。
foef(stream)针对的是”流“， 用一个indicator(指示）表明文件尾，而feof判断这个指示是否设置，而不是判断当前SEEK数值是否到尾。
seek值到尾了对于“流”不一定是是文件尾，因为“流”是会不断增长的。
对于红字部分确实有些代码确实如此
查看是否有EOF标识符，然后再进行处理。
NAME
clearerr, feof, ferror, fileno - check and reset stream status
SYNOPSIS
#include <stdio.h>
void clearerr(FILE *stream);
int feof(FILE *stream);
int ferror(FILE *stream);
int fileno(FILE *stream);
DESCRIPTION
The function clearerr clears the end-of-file and error indicators for the stream pointed to by stream.
The function feof tests the end-of-file indicator for the stream pointed to by stream, returning non-zero if it is
set. The end-of-file indicator can only be cleared by the function clearerr.
The function ferror tests the error indicator for the stream pointed to by stream, returning non-zero if it is set.
The error indicator can only be reset by the clearerr function.
The function fileno examines the argument stream and returns its integer descriptor.
For non-locking counterparts, see unlocked_stdio(3).
ERRORS
These functions should not fail and do not set the external variable errno. (However, in case fileno detects that
its argument is not a valid stream, it must return -1 and set errno to EBADF.)
CONFORMING TO
The functions clearerr, feof, and ferror conform to X3.159-1989 (鈥樷€楢NSI C鈥欌€?.
SEE ALSO
open(2), unlocked_stdio(3), stdio(3)