Null (computer)

Null has several meanings in computer programming.

Null is a special value for a pointer (or other kind of object reference) used to signify that the pointer intentionally does not have a target. Such a pointer is called a null pointer. Many implementations use a binary 0 (all bits zero) as the null value, as most operating systems consider it an error to try to access such a low memory address. Some languages use other nomenclature for such a pointer, e.g., nil, undefined, void reference, etc.

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 (termed "Nothing") 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)).

In UNIX systems, /dev/null (also referred to as the black hole or bit bucket) is a special file that delivers no input when read and absorbs all output when written. The NUL device has similar functionality in DOS and Windows.

In the C0 control code set defined in the ISO 646, ASCII, and Unicode character set standards, code value 0 is the null character (NUL). Most programming languages provide syntax for representing this character as a literal value (e.g., Pascal uses chr(0), C and its many derivatives use '\0').

In some programming languages, the null character denotes the end of a character string. Such strings that are encoded with the ASCII character set are sometimes referred to as ASCIIZ or ASCIZ strings - the 'Z' indicating that the terminating 'Zero' or null character is present.

Some programming languages (such as LISP, Pascal, Delphi, and other languages based on Pascal) use nil for the null variant (which can lead to somewhat confusing errors if null is accidentally used instead of nil). In Ruby, the designation is called nil.cs:NULL