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<title>Null function</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Null function</span></span>

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</style><div role="note" class="hatnote navigation-not-searchable">Not to be confused with <a href="Identity_function" title="Identity function">identity function</a>, <a href="Empty_function" class="mw-redirect" title="Empty function">empty function</a>, or <a href="Zero_function" class="mw-redirect" title="Zero function">zero function</a>.</div>

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<p>In <a href="Computer_science" title="Computer science">computer science</a>, a <b>null function</b> (or <b>null operator</b>) is a <a href="Subroutine" class="mw-redirect" title="Subroutine">subroutine</a> that leaves the <a href="Program_state" class="mw-redirect" title="Program state">program state</a> unchanged. When it is part of the <a href="Instruction_set" class="mw-redirect" title="Instruction set">instruction set</a> of a <a href="Central_processing_unit" title="Central processing unit">processor</a>, it is called a <a href="NOP_(code)" title="NOP (code)">NOP</a> or NOOP (No OPeration).

</p><p>Mathematically, a (computer) function <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle f}">

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<annotation encoding="application/x-tex">{\displaystyle f}</annotation>

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</math></span><img src="./132e57acb643253e7810ee9702d9581f159a1c61.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:1.279ex; height:2.509ex;" alt="{\displaystyle f}" loading="lazy"></span> is null if and only if its execution leaves the program state <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle s}">

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<annotation encoding="application/x-tex">{\displaystyle s}</annotation>

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</math></span><img src="./01d131dfd7673938b947072a13a9744fe997e632.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.09ex; height:1.676ex;" alt="{\displaystyle s}" loading="lazy"></span> unchanged. That is, a null function is an <a href="Identity_function" title="Identity function">identity function</a> whose <a href="Domain_of_a_function" title="Domain of a function">domain</a> and <a href="Codomain" title="Codomain">codomain</a> are both the <a href="State_space" class="mw-redirect" title="State space">state space</a> <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle S}">

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<annotation encoding="application/x-tex">{\displaystyle S}</annotation>

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</math></span><img src="./4611d85173cd3b508e67077d4a1252c9c05abca2.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.499ex; height:2.176ex;" alt="{\displaystyle S}" loading="lazy"></span> of the program, and for which:

</p><p><span class="mwe-math-element mwe-math-element-block"><span class="mwe-math-mathml-display mwe-math-mathml-a11y" style="display: none;"><math display="block" xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle f(s)=s}">

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<annotation encoding="application/x-tex">{\displaystyle f(s)=s}</annotation>

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</p><p>for all elements <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle s\in S}">

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<annotation encoding="application/x-tex">{\displaystyle s\in S}</annotation>

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</math></span><img src="./acce52dffd84d073a24f4606a175da60148fd0c6.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:5.43ex; height:2.176ex;" alt="{\displaystyle s\in S}" loading="lazy"></span>.

</p><p>Less rigorous definitions may also be encountered. For example, a function may take a single operand, <a href="Type_conversion" title="Type conversion">transform</a> it into a new <a href="Data_type" title="Data type">data type</a>, and return the result.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> 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.

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<p>Null functions have several uses.

</p><p>During <a href="Software_development" title="Software development">software development</a>, null functions with the same names and <a href="Type_signature" title="Type signature">type signatures</a> as a planned functions are often used as <a href="Method_stub" title="Method stub">stubs</a>—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.

</p><p>Null functions, particularly the NOP variety, are also used to provide delays of indeterminate length within <a href="Infinite_loop" title="Infinite loop">wait loops</a>. This is a common strategy in dedicated <a href="Embedded_system_overview" class="mw-redirect" title="Embedded system overview">device controllers</a> 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 <a href="Software_applications" class="mw-redirect" title="Software applications">software applications</a> on larger <a href="Multiprocessing" title="Multiprocessing">multiprocessing</a> computer systems. However, for multiprocessing systems a better approach is to use <a href="Operating_system" title="Operating system">operating system</a> functions that let other processes use the <a href="CPU" class="mw-redirect" title="CPU">CPU</a> during the waiting period.

</p><p>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 <a href="Dead_code" title="Dead code">dead code</a>, such occurrences indicate serious flaws in program structures.

</p><p>A null function or method is often used as the default behavior of a revectorable function or overrideable method in an object framework.

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<ul><li><a href="IEFBR14" title="IEFBR14">IEFBR14</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text">Example of a <a href="Type_conversion" title="Type conversion">type conversion</a> function labeled as a "null function": <a rel="nofollow" class="external text" href="http://www.thinkage.ca/english/gcos/expl/nsc/lib/0nullf.html">_NULLF - null function.</a></span>

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</style><cite class="citation cs1"><a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc2410"><i>The NULL Encryption Algorithm and Its Use With IPsec</i></a>. <a href="Internet_Engineering_Task_Force" title="Internet Engineering Task Force">IETF</a>. November 1998. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.17487%2FRFC2410">10.17487/RFC2410</a></span>. <a href="Request_for_Comments" title="Request for Comments">RFC</a> <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc2410">2410</a>.</cite> Makes humorous statements about the NULL encryption algorithm.</li></ul>

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</style><div id="Nulls_in_computing108" style="font-size:114%;margin:0 4em">Nulls in <a href="Computing" title="Computing">computing</a></div></th></tr><tr><td colspan="2" class="navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Null_character" title="Null character">Null character</a></li>

<li><a href="Null_device" title="Null device">Null device</a></li>

<li><a href="Null_modem" title="Null modem">Null modem</a></li>

<li><a href="Null_object_pattern" title="Null object pattern">Null object pattern</a></li>

<li><a href="Null_pointer" title="Null pointer">Null pointer</a></li>

<li><a href="Empty_string" title="Empty string">Null string</a></li>

<li><a href="Null_coalescing_operator" title="Null coalescing operator">Null coalescing operator</a></li></ul>

</div></td></tr><tr><td class="navbox-abovebelow hlist" colspan="2"><div>

<ul><li><i>See also:</i> <a href="Null_coalescing_operator" title="Null coalescing operator">Null coalescing operator</a></li>

<li><a href="Nullable_type" title="Nullable type">Nullable type</a></li>

<li><a href="Undefined_value" title="Undefined value">Undefined value</a></li></ul>

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This article is issued from <a class="external text" title="Last edited on 2025-06-06" href="https://en.wikipedia.org/wiki/?title=Null_function&oldid=1294186597">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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