DIR结构体类似于FILE，是一个内部结构

1. struct __dirstream
2.    {
3. void *__fd;
4. char *__data;
5. int __entry_data;
6. char *__ptr;
7. int __entry_ptr;
8. size_t __allocation;
9. size_t __size;
10.     __libc_lock_define (, __lock)
11.    };
13. typedef struct __dirstream DIR;

    struct dirent{

    ino_t     d_ino;                                    /*inode number*/

    off_t      d_off;                                    /*offset to the next dirent*/

    unsigned short d_reclen;                /*length of this record*/

    unsigned char d_type ;                    //type of file;not supported by all the file system types;

    char d_name[256];                           //filename

    }