fgets的用法：
    char *fgets(char *s, int size, FILE *stream);
       fgets()  reads  in  at most one less than size characters from stream and stores them
       into the buffer pointed to by s.  Reading stops after an EOF or a newline.  If a new-
       line  is read, it is stored into the buffer.  A ‘\0‘ is stored after the last charac-
       ter in the buffer.
最多从流中读取size-1个字符，并且将他们存储在s所指向的缓冲区中。当遇到EOF终止符或新行时，停止读取。‘\0‘存储在读取的最后一个字符之后。

fgets的用法