这是返回值的结构体：
typedef struct {
rec_header header; /**< A processed version of the last record read */
int fd; /**< Actual file descriptor for the backingfile */
union {
byte_t padding[4];
} __fd;
stdf_format file_format; /**< Compressed file format */
char *filename; /**< Filename that was given to stdf_open() */
__stdf_fops *fops; /**< Virtual file i/o functions to hide compression details */
int byte_order; /**< Byte order of the file */
uint32_t opts; /**< Misc options to control libstdf behavior */
dtc_U1 ver; /**< Spec version of the file */
byte_t *__data;
byte_t *rec_pos;
byte_t *rec_end;
byte_t *__output;
byte_t *_write_pos;
dtc_U2 _write_chunk_size;
} stdf_file;
stdf_file里面有另外的结构体
