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Conditional compilation

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In <a href="Computer_programming" title="Computer programming">computer programming</a>, conditional compilation is a <a href="Compiler" title="Compiler">compilation</a> technique which results in differing <a href="Executable_program" class="mw-redirect" title="Executable program">executable programs</a> depending on <a href="Parameter_(computer_programming)" title="Parameter (computer programming)">parameters</a> specified. This technique is commonly used when these differences in the program are needed to run it on <a href="Cross-platform" class="mw-redirect" title="Cross-platform">different platforms</a>, or with different versions of required <a href="Library_(computing)" title="Library (computing)">libraries</a> or <a href="Computer_hardware" title="Computer hardware">hardware</a>.

Many <a href="Programming_language" title="Programming language">programming languages</a> support conditional compilation. Typically <a href="Compiler_directives" class="mw-redirect" title="Compiler directives">compiler directives</a> define or "undefine" certain <a href="Variable_(computer_science)" title="Variable (computer science)">variables</a>; other directives test these variables and modify compilation accordingly. For example, not using an actual language, the compiler may be set to define "Macintosh" and undefine "PC", and the code may contain:

<div class="mw-highlight mw-highlight-lang-pascal mw-content-ltr" dir="ltr"><pre>(* System generic code *)

if mac != Null then

(* macOS specific code *)

else if pc != Null

(* Windows specific code *)

</pre></div>

In <a href="C_(programming_language)" title="C (programming language)">C</a> and some languages with a similar syntax, this is done using an <a href="C_preprocessor#Conditional_compilation" title="C preprocessor">'#ifdef' directive</a>.

A similar procedure, using the name "conditional comment", is used by <a href="Microsoft_Internet_Explorer" class="mw-redirect" title="Microsoft Internet Explorer">Microsoft Internet Explorer</a> from version 5 to 9 to interpret <a href="HTML" title="HTML">HTML</a> code. There is also a similar proprietary mechanism for adding conditional comments within <a href="JScript" title="JScript">JScript</a>, known as conditional compilation.<a href="#cite_note-MS2-1">[1]</a>

<a href="C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a> have preprocessor directives for conditional compilation.<a href="#cite_note-2">[2]</a>

<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>#if DEBUG

Console.WriteLine("Debug version");

#endif

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<a href="Rust_(programming_language)" title="Rust (programming language)">Rust</a> have conditional compilation.<a href="#cite_note-3">[3]</a>

<div class="mw-highlight mw-highlight-lang-rust mw-content-ltr" dir="ltr"><pre>#[cfg_attr(target_os = "linux", path = "linux.rs")]

#[cfg_attr(windows, path = "windows.rs")]

mod os;

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

When conditional compilation depends on too many variables, it can make the code harder to reason about as the number of possible combinations of configuration increases exponentially.<a href="#cite_note-icse19-4">[4]</a><a href="#cite_note-meinicke-5">[5]</a><a href="#cite_note-se-cc-quality-6">[6]</a> When conditional compilation is done via a <a href="Preprocessor" title="Preprocessor">preprocessor</a> that does not guarantee syntactically correct output in the source language, such as the <a href="C_preprocessor" title="C preprocessor">C preprocessor</a>, this may lead to hard-to-debug compilation errors,<a href="#cite_note-ifdef-harmful-7">[7]</a><a href="#cite_note-so-avoid-ifdef-8">[8]</a><a href="#cite_note-do-dont-cc-9">[9]</a> which is sometimes called "#ifdef hell."<a href="#cite_note-preschern-10">[10]</a><a href="#cite_note-ifdef-hell-11">[11]</a>

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

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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20080906144358/http://msdn2.microsoft.com/en-us/library/ahx1z4fs(VS.80).aspx">"Conditional Compilation"</a>. Microsoft Corporation. Archived from <a rel="nofollow" class="external text" href="http://msdn2.microsoft.com/en-us/library/ahx1z4fs(VS.80).aspx">the original</a> on 2008-09-06. Retrieved 2011-11-27.</cite>

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<li id="cite_note-2"><a href="#cite_ref-2">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/preprocessor-directives#conditional-compilation">"Preprocessor directives - C# reference"</a>. learn.microsoft.com. Retrieved 31 May 2025.</cite>

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<li id="cite_note-3"><a href="#cite_ref-3">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://doc.rust-lang.org/stable/reference/conditional-compilation.html">"Conditional compilation - The Rust Reference"</a>. doc.rust-lang.org. Retrieved 31 May 2025.</cite>

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<li id="cite_note-icse19-4"><a href="#cite_ref-icse19_4-0">^</a> <cite id="CITEREFGazzilloWei2019" class="citation conference cs1">Gazzillo, Paul; Wei, Shiyi (2019-05-27). <a rel="nofollow" class="external text" href="https://www.paulgazzillo.com/papers/icse19nier.pdf">"Conditional Compilation is Dead, Long Live Conditional Compilation!"</a> (PDF). ICSE-NIER '19: Proceedings of the 41st International Conference on Software Engineering: New Ideas and Emerging Results. <a rel="nofollow" class="external text" href="https://2019.icse-conferences.org/">2019 IEEE/ACM 41st International Conference on Software Engineering: New Ideas and Emerging Results (ICSE-NIER)</a>. Montreal, QC, Canada: IEEE Press. pp. 105–108. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1109%2FICSE-NIER.2019.00035">10.1109/ICSE-NIER.2019.00035</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-1-7281-1758-4</bdi>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20221107174330/https://www.paulgazzillo.com/papers/icse19nier.pdf">Archived</a> (PDF) from the original on 2022-11-07. Retrieved 2023-01-21.</cite>

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<li id="cite_note-meinicke-5"><a href="#cite_ref-meinicke_5-0">^</a> <cite id="CITEREFMeinickeThümSchröterBenduhn2017" class="citation book cs1">Meinicke, Jens; Thüm, Thomas; Schröter, Reimar; Benduhn, Fabian; Leich, Thomas; Saake, Gunter (2017). Meinicke, Jens; Thüm, Thomas; Schröter, Reimar; Benduhn, Fabian (eds.). <a rel="nofollow" class="external text" href="https://doi.org/10.1007/978-3-319-61443-4_12">Quality Assurance for Conditional Compilation</a>. Cham: Springer International Publishing. pp. 131–139. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1007%2F978-3-319-61443-4_12">10.1007/978-3-319-61443-4_12</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-3-319-61443-4</bdi>. Retrieved 2023-01-21.</cite> <code class="cs1-code">{{cite book}}</code>: <code class="cs1-code">|work=</code> ignored (help)

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<li id="cite_note-se-cc-quality-6"><a href="#cite_ref-se-cc-quality_6-0">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://softwareengineering.stackexchange.com/questions/405980/how-does-conditional-compilation-impact-product-quality-security-and-code-compl">"compiler - How does conditional compilation impact product quality, security and code complexity?"</a>. Software Engineering Stack Exchange. Retrieved 2023-01-21.</cite>

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<li id="cite_note-ifdef-harmful-7"><a href="#cite_ref-ifdef-harmful_7-0">^</a> <cite id="CITEREFLeWalkingshawErwig2011" class="citation book cs1">Le, Duc; Walkingshaw, Eric; Erwig, Martin (2011-09-18). "#ifdef confirmed harmful: Promoting understandable software variation". <a rel="nofollow" class="external text" href="https://ieeexplore.ieee.org/document/6070391">2011 IEEE Symposium on Visual Languages and Human-Centric Computing (VL/HCC)</a>. pp. 143–150. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1109%2FVLHCC.2011.6070391">10.1109/VLHCC.2011.6070391</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-1-4577-1246-3</bdi>.</cite>

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<li id="cite_note-so-avoid-ifdef-8"><a href="#cite_ref-so-avoid-ifdef_8-0">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://stackoverflow.com/questions/1851181/why-should-ifdef-be-avoided-in-c-files">"conditional compilation - Why should #ifdef be avoided in .c files?"</a>. Stack Overflow. Retrieved 2023-01-21.</cite>

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<li id="cite_note-do-dont-cc-9"><a href="#cite_ref-do-dont-cc_9-0">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://stackoverflow.com/questions/4469645/dos-and-donts-of-conditional-compile">"c++ - Dos and Don'ts of Conditional Compile"</a>. Stack Overflow. Retrieved 2023-01-21.</cite>

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<li id="cite_note-preschern-10"><a href="#cite_ref-preschern_10-0">^</a> <cite id="CITEREFPreschern2019" class="citation conference cs1">Preschern, Christopher (2019-07-03). "Patterns to escape the #ifdef hell". <a rel="nofollow" class="external text" href="http://preschern.azurewebsites.net/2019-os-specific-code.pdf">Proceedings of the 24th European Conference on Pattern Languages of Programs</a> (PDF). New York, NY, USA: Association for Computing Machinery. pp. 1–12. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1145%2F3361149.3361151">10.1145/3361149.3361151</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-1-4503-6206-1</bdi>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20221221151021/http://preschern.azurewebsites.net/2019-os-specific-code.pdf">Archived</a> (PDF) from the original on 2022-12-21.</cite>

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<li id="cite_note-ifdef-hell-11"><a href="#cite_ref-ifdef-hell_11-0">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.cqse.eu/en/news/blog/living-in-the-ifdef-hell/">"Living in the #ifdef Hell"</a>. www.cqse.eu. 28 October 2015. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20221128024445/https://www.cqse.eu/en/news/blog/living-in-the-ifdef-hell/">Archived</a> from the original on 2022-11-28. Retrieved 2023-01-21.</cite>

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