[U-Boot] Why cmd_tbl_t is referenced as extern in defining __u_boot_cmd_start?

[legendbb]

line 64 and 65 in include/command.h

extern cmd_tbl_t  __u_boot_cmd_start;
extern cmd_tbl_t  __u_boot_cmd_end;

definition of cmd_tbl_t is just one line above these. Why "extern" is used?

Thanks,

struct cmd_tbl_s {
	char		*name;		/* Command Name			*/
	int		maxargs;	/* maximum number of arguments	*/
	int		repeatable;	/* autorepeat allowed?		*/
					/* Implementation function	*/
	int		(*cmd)(struct cmd_tbl_s *, int, int, char *[]);
	char		*usage;		/* Usage message	(short)	*/
#ifdef	CONFIG_SYS_LONGHELP
	char		*help;		/* Help  message	(long)	*/
#endif
#ifdef CONFIG_AUTO_COMPLETE
	/* do auto completion on the arguments */
	int		(*complete)(int argc, char *argv[], char last_char, int maxv, char
*cmdv[]);
#endif
};

typedef struct cmd_tbl_s	cmd_tbl_t;

extern cmd_tbl_t  __u_boot_cmd_start;
extern cmd_tbl_t  __u_boot_cmd_end;
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