5. feof()
功 能: 检测流上的文件结束符
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 functionclearerr().
如果文件结束，则返回非0值，否则返回0，文件结束符只能被clearerr()清除
用法： int feof(FILE *stream);