are used to get and put single values (such as an int, char, or
long) from and to userspace. A pointer into userspace should
never be simply dereferenced: data should be copied using these
routines. Both return
-EFAULT or 0.
copy_from_user() are more general: they copy
an arbitrary amount of data to and from userspace.
get_user(), they return the amount of
uncopied data (ie. 0 still means
[Yes, this moronic interface makes me cringe. The flamewar comes up every year or so. --RR.]
The functions may sleep implicitly. This should never be called outside user context (it makes no sense), with interrupts disabled, or a spinlock held.