The length given to strnlen() is, as you yourself said, greater than the size of the actual buffer in order to detect an oversize source. So strnlen() has the

The length given to strnlen() is, as you yourself said, greater than the size of the actual buffer in order to detect an oversize source. So strnlen() has the potential to access the character beyond the end of the array, checking it for NUL, incurring page faults and whatnot.