Null (computer programming)

The word Null has several meanings in computer programming, but one way or another all of them refer to operations on data streams or data values, or attributes of parades of (array) the data of type "characters" such as length.

Template:Left66In order to aid understanding, it should be understood that many data types are abstractions, and the actual implementation depends on machine dependent, characteristics such as binary word lengths. So to do the various national or international standards for exchanging or representing text related data such as unicode or ASCII as are referenced below, save those standards are written with explicit instructions on code values for a particular length of computer word—hence data values, when discussed at all below are given as abstractions, not values. In an particular physical (non-abstract) implementation, the two are married, giving users a working computing system that can talk and be understood via those same information interchange standards with other computers.

Null pointer or null reference

• 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 CPUs (although some architectures use a signed address space and use the most negative value). Many operating systems 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 and Ruby use nil, while Visual Basic uses Nothing. Fortran does not consider it a constant, but a property that can be set by the NULLIFY directive and tested by the ASSOCIATED function.

Null value

• In many disciplines, the concept of null allows a three-valued logic, with null indicating "no value" or an "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 character

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

Null string

• A string in all computer languages is an array of characters, normally implemented as the shortest discrete data type the machine can address. A null string, also known as an empty string, is a string (array) of zero contents, which is to say of zero length or to phrase it differently, one that holds zero characters. In the C, language, a zero length string is expressly referred to as a Null string by Kernighan and Ritchie, but in context, C uses pointers to index into a string, so the null string of C is not a Template:Here (for that is defined), but a string data location, the first character of an array, containing the Template:Here '\x00' (or '\0', a zero value—). A properly initialized pointer, pointing to a null character value in C is using two memory locations of two different data types, and the fact that the lvalue of that location is zero is C's definition and test for a null string (See Template:Here below, and the next paragraph about languages which store string lengths).
  
  Depending upon the computing language, current code environment, and other contextual factors a null string may or may not take up significant memory space. A dynamically allocated string (local within a function or sub-program is usually allocated space on the stack frame, its maximum (allocated) length making up part of the overall length of the stack frame. A static variable, with no contents is however allocated N-bytes of memory at the time of compiling and linking, which is part of the binary space the computer addresses when that program is loaded. Both have zero meaningful contents until and unless they are initialized by valid data, and most compilers are written to initialize dynamically allocated character arrays as null strings until they are initialized. In C, that means the first byte or word of the ASCIZ string would be set to zero. Other languages, notably various BASIC programming implementations use the first byte or machine data word of the character array to hold the length of the string. Testing for nullness there means testing for a zero value meaning the string has zero length. (Testing on a "random number", would be the result if this were not the case, which could then cause a program branch to the wrong code handling the results of the hypothetical test.)

Null device

• 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 from and discards all output when written to. The NUL device has similar functionality in DOS and Windows. On AmigaOS it is known as the NIL: device.

• In Windows XP, null device is a file in system32/drivers, named Null.sys, the size is 2944 bytes. It can be seen in device manager, non-plug-and-play group.

• 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 string ends with a "Zero", or null character.
• When electromechanical teleprinters were used as computer output devices, one or more null characters were sent at the end of each printed line to allow time for the mechanism to return to the first printing position on the next line.

Null variant

• Some programming languages (such as LISP; Ruby; and Pascal, Delphi, and other languages based on Pascal) use nil for the null variant, which can sometimes lead to confusing errors if null is accidentally used instead of nil.

Null script / null subroutine

• A command script or program subroutine which does nothing but return a constant value. Also known as a stub function or "stubroutine", which are used during program development, where the functional implementation of routines is delayed while other routines are developed. This is also one of the techniques used by the software pirating community to bypass callbacks and license-checking code: the target program is disassembled and the appropriate code is substituted for a null subroutine that just returns the value expected by the caller.

Null Account

• On certain forums and other internet sites, user database corruption might cause user accounts to suddenly show up as "null" and have all their settings reset. Generally this will remove the user's ability to add new content to the site.