Man page 9 : __clear_user

__clear_user - Zero a block of memory in user space, with less checking.

unsigned long __clear_user(void __user *  to , unsigned long  n );

to
    Destination address, in user space.

n
    Number of bytes to zero.

Zero a block of memory in user space. Caller must check the specified block with access_ok before calling this function.

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