how to tell 64 vs 32 bit architecture ?

hi,
i was wondering what is the proper way to tell a 64 vs 32 bit architecture.

I see that some code in sys/ uses  ' #ifdef __LP64__ ' but i am not
sure if this is generic enough (ie not gcc or FreeBSD specific),
and also suitable for userland (i.e. works on linux or other platforms
as well).

	cheers
	luigi