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<p><b>New page</b></p><div>{{Short description|Type of subroutine in computer science}}<br />
{{distinguish|identity function|empty function|zero function}}<br />
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In [[computer science]], a '''null function''' (or '''null operator''') is a [[subroutine]] that leaves the [[program state]] unchanged. When it is part of the [[instruction set]] of a [[central processing unit|processor]], it is called a [[NOP (code)|NOP]] or NOOP (No OPeration).<br />
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Mathematically, a (computer) function <math>f</math> is null if and only if its execution leaves the program state <math>s</math> unchanged. That is, a null function is an [[identity function]] whose [[domain of a function|domain]] and [[codomain]] are both the [[state space]] <math>S</math> of the program, and for which:<br />
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<math display="block">f(s)=s</math><br />
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for all elements <math>s \in S</math>.<br />
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Less rigorous definitions may also be encountered. For example, a function may take a single operand, [[type conversion|transform]] it into a new [[data type]], and return the result.<ref>Example of a [[type conversion]] function labeled as a "null function": [http://www.thinkage.ca/english/gcos/expl/nsc/lib/0nullf.html _NULLF - null function.]</ref> 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.<br />
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== Uses ==<br />
Null functions have several uses.<br />
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During [[software development]], null functions with the same names and [[type signature]]s as a planned functions are often used as [[method stub|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.<br />
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Null functions, particularly the NOP variety, are also used to provide delays of indeterminate length within [[infinite loop|wait loop]]s. This is a common strategy in dedicated [[embedded system overview|device controller]]s 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.<br />
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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.<br />
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A null function or method is often used as the default behavior of a revectorable function or overrideable method in an object framework.<br />
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==See also==<br />
*[[IEFBR14]]<br />
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==References==<br />
{{Reflist}}<br />
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==External links==<br />
*{{cite IETF|rfc=2410|title=The NULL Encryption Algorithm and Its Use With IPsec|date=November 1998|publisher=[[Internet Engineering Task Force|IETF]]}} Makes humorous statements about the NULL encryption algorithm.<br />
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{{nulls}}<br />
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{{DEFAULTSORT:Null Function}}<br />
[[Category:Subroutines]]</div>imported>Remsense