Null function
Encyclopedia
In computer science
, a null function (or null operator) is subroutine
that returns no data values and leaves the program state
unchanged. When it is part of the instruction set
of a processor
, it is called a NOP
or NOOP (No OPeration).
Mathematically, a (computer) function f is null if and only if its execution leaves the program state s unchanged. That is, a null function is an identity function
whose domain
and codomain
are both the state space
S of the program, and for which:
Less rigorous definitions may also be encountered. For example, a function may take a single operand, transform
it into a new data type
, and return the result. While such usages bear a strong visual resemblance to identity functions, they create or alter a binary data value and thus change the program state. From a software maintainability perspective it is better to identify such "minor" alternations of state explicitly, since calling them null functions provides future maintainers of the code with no insights on their actual purposes.
During software development
, null functions with the same names and type signature
s as a planned functions are often used as stub
s—that is, as non-functional placeholders that allow the incomplete body of code to be compiled and tested prior to completion of all planned features.
Null functions, particularly the NOP variety, are also used to provide delays of indeterminate length within wait loop
s. This is a common strategy in dedicated device controllers that must wait for an external input and have no other tasks to perform while they are waiting. Such wait loops are also used in software applications on larger multiprocessing
computer systems. However, for multiprocessing systems a better approach is to use operating system
functions that let other processes use the CPU during the waiting period.
A third use of null functions is as the definition of a program feature that, if created inadvertently, is almost always deleterious. Unintended null functions can arise during the development of complex programs, and like dead code
, such occurrences indicate serious flaws in program structures.
A null function or method is often used as the default behavior of a revectorable function or overrideable method in an object framework.
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
, a null function (or null operator) is subroutine
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
that returns no data values and leaves the program state
Program state
One of the key concepts in computer programming is the idea of state, essentially a snapshot of the measure of various conditions in the system. Most programming languages require a considerable amount of state information in order to operate properly - information which is generally hidden from...
unchanged. When it is part of the instruction set
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
of a processor
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
, it is called a NOP
NOP
In computer science, NOP or NOOP is an assembly language instruction, sequence of programming language statements, or computer protocol command that effectively does nothing at all....
or NOOP (No OPeration).
Mathematically, a (computer) function f is null if and only if its execution leaves the program state s unchanged. That is, a null function is an identity function
Identity function
In mathematics, an identity function, also called identity map or identity transformation, is a function that always returns the same value that was used as its argument...
whose domain
Domain (mathematics)
In mathematics, the domain of definition or simply the domain of a function is the set of "input" or argument values for which the function is defined...
and codomain
Codomain
In mathematics, the codomain or target set of a function is the set into which all of the output of the function is constrained to fall. It is the set in the notation...
are both the state space
State space
In the theory of discrete dynamical systems, a state space is a directed graph where each possible state of a dynamical system is represented by a vertex, and there is a directed edge from a to b if and only if ƒ = b where the function f defines the dynamical system.State spaces are...
S of the program, and for which:
- f(s) = s for all elements s in S.
Less rigorous definitions may also be encountered. For example, a function may take a single operand, transform
Type conversion
In computer science, type conversion, typecasting, and coercion are different ways of, implicitly or explicitly, changing an entity of one data type into another. This is done to take advantage of certain features of type hierarchies or type representations...
it into a new data type
Data type
In computer programming, a data type is a classification identifying one of various types of data, such as floating-point, integer, or Boolean, that determines the possible values for that type; the operations that can be done on values of that type; the meaning of the data; and the way values of...
, and return the result. While such usages bear a strong visual resemblance to identity functions, they create or alter a binary data value and thus change the program state. From a software maintainability perspective it is better to identify such "minor" alternations of state explicitly, since calling them null functions provides future maintainers of the code with no insights on their actual purposes.
Uses
Null functions have several uses.During software development
Software development
Software development is the development of a software product...
, null functions with the same names and type signature
Type signature
In computer science, a type signature or type annotation defines the inputs and outputs for a function, subroutine or method. A type signature includes at least the function name and the number of its arguments...
s as a planned functions are often used as stub
Method stub
A method stub or simply stub in software development is a piece of code used to stand in for some other programming functionality. A stub may simulate the behavior of existing code or be a temporary substitute for yet-to-be-developed code...
s—that is, as non-functional placeholders that allow the incomplete body of code to be compiled and tested prior to completion of all planned features.
Null functions, particularly the NOP variety, are also used to provide delays of indeterminate length within wait loop
Infinite loop
An infinite loop is a sequence of instructions in a computer program which loops endlessly, either due to the loop having no terminating condition, having one that can never be met, or one that causes the loop to start over...
s. This is a common strategy in dedicated device controllers that must wait for an external input and have no other tasks to perform while they are waiting. Such wait loops are also used in software applications on larger multiprocessing
Multiprocessing
Multiprocessing is the use of two or more central processing units within a single computer system. The term also refers to the ability of a system to support more than one processor and/or the ability to allocate tasks between them...
computer systems. However, for multiprocessing systems a better approach is to use operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
functions that let other processes use the CPU during the waiting period.
A third use of null functions is as the definition of a program feature that, if created inadvertently, is almost always deleterious. Unintended null functions can arise during the development of complex programs, and like dead code
Dead code
Dead code is a computer programming term for code in the source code of a program which is executed but whose result is never used in any other computation...
, such occurrences indicate serious flaws in program structures.
A null function or method is often used as the default behavior of a revectorable function or overrideable method in an object framework.