Null is a special Pointer value (or other kind of object Reference ) used to signify that a pointer intentionally does not point to (or refer to) an object. Such a pointer is called a Null Pointer . Many implementations use a value of 0 (all bits zero) to represent the null pointer, as this is at the bottom of the address space of most CPU s (e.g., the INMOS Transputer has a signed address space and uses the most negative value). Many Operating System s generate an exception when an attempt is made to access this memory address. Some languages use other nomenclature for such a pointer, e.g., Pascal uses nil.
In many disciplines, the concept of Null allows a Three-valued Logic , with null indicating "unknown value". The SQL database query language uses Null in this way, as do Visual Basic and its derivatives. In this model, an expression that depends on the value of a Null operand will evaluate to Null (VB) or "unknown" (SQL). So, for any A, the expressions "A = Null" and "A <> Null" are neither true nor false. However, the boolean operation "A and False" produces false, and similarly "A or True" is true, even when A is Null, because these expressions do not depend on the value of A. (Some SQL implementations may consider A = Null to be "true" if A is Null; see Null (SQL) ).
;Null device
In (also referred to as the Black Hole or Bit Bucket ) is a Special File that delivers no input when read from and discards all output when written to. The NUL device has similar functionality in DOS and Windows.
