Two question about kernel global function declaration and directory layout

[daley <daleydeng@gmail.com>]

hi, everyone, I'm a big fan of the kernel, I was always being attracted 
by its clean and intelligent design. I'm new to the mailing list and 
this is my first post. I want to ask two basic question about kernel 
which bother me a lot, I've googled a lot but nobody could give a 
suitable answer .

The first one is about global function declaration, I was confused about 
the usage of keyword "extern" before a global function which declared in 
the header file. for example,

<include/linux/kernel.h>
extern long (*panic_blink)(int state);
__printf(1, 2)
void panic(const char *fmt, ...)
         __noreturn __cold;
extern void oops_enter(void);
extern void oops_exit(void);
void print_oops_end_marker(void);
extern int oops_may_print(void);
void do_exit(long error_code)
         __noreturn;
void complete_and_exit(struct completion *, long)
         __noreturn;

the same thing also appeared on other header files a lot. To my 
knowledge, there's no difference between whether has the "extern" before 
global function, is this due to the legacy problem or have other meaning?

The second question is why not move the *sound* directory into the 
*drivers* directory or put bigger subsystem out of drivers, since there 
are more bigger subsystems(e.g. media, scsi) than sound?

Can anybody help?
Thanks.

-- 
daleydeng
Beijing, China