Man page 9 : __copy_to_user

__copy_to_user - Copy a block of data into user space, with less checking.

unsigned long __copy_to_user(void __user *  to , const void *  from , unsigned long  n );

to
    Destination address, in user space.

from
    Source address, in kernel space.

n
    Number of bytes to copy.

User context only. This function may sleep.

Copy data from kernel space to user space. Caller must check the specified block with access_ok before calling this function.

Returns number of bytes that could not be copied. On success, this will be zero.