imported>Ira Leviton: differring→differing - typos.toolforge.org

2025-06-19T07:31:16Z

differring→differing - typos.toolforge.org

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{{short description|When a compiler produces a program which can change based on given parameters}}

In [[computer programming]], '''conditional compilation''' is a [[Compiler|compilation]] technique which results in differing [[executable program]]s depending on [[Parameter (computer programming)|parameters]] specified. This technique is commonly used when these differences in the program are needed to run it on [[Cross-platform|different platforms]], or with different versions of required [[Library (computing)|libraries]] or [[Computer hardware|hardware]].

Many [[programming language]]s support conditional compilation. Typically [[compiler directives]] define or "undefine" certain [[Variable (computer science)|variable]]s; 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:


<syntaxhighlight lang="pascal">
(* System generic code *)
if mac != Null then
(* macOS specific code *)
else if pc != Null
(* Windows specific code *)
</syntaxhighlight>

In [[C (programming language)|C]] and some languages with a similar syntax, this is done using an [[C preprocessor#Conditional compilation|'#ifdef' directive]].

A similar procedure, using the name "conditional comment", is used by [[Microsoft Internet Explorer]] from version 5 to 9 to interpret [[HTML]] code. There is also a similar proprietary mechanism for adding conditional comments within [[JScript]], known as conditional compilation.<ref name="MS2">{{cite web
| url = http://msdn2.microsoft.com/en-us/library/ahx1z4fs(VS.80).aspx
| title = Conditional Compilation
| publisher = Microsoft Corporation
| access-date = 2011-11-27
| archive-url = https://web.archive.org/web/20080906144358/http://msdn2.microsoft.com/en-us/library/ahx1z4fs(VS.80).aspx
| archive-date = 2008-09-06
| url-status = dead
}}</ref>

[[C Sharp (programming language)|C#]] have preprocessor directives for conditional compilation.<ref>{{cite web |title=Preprocessor directives - C# reference |url=https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/preprocessor-directives#conditional-compilation |website=learn.microsoft.com |access-date=31 May 2025 |language=en-us}}</ref>
<syntaxhighlight lang="csharp">
#if DEBUG
Console.WriteLine("Debug version");
#endif
</syntaxhighlight>

[[Rust (programming language)|Rust]] have conditional compilation.<ref>{{cite web |title=Conditional compilation - The Rust Reference |url=https://doc.rust-lang.org/stable/reference/conditional-compilation.html |website=doc.rust-lang.org |access-date=31 May 2025}}</ref>
<syntaxhighlight lang="rust">
#[cfg_attr(target_os = "linux", path = "linux.rs")]
#[cfg_attr(windows, path = "windows.rs")]
mod os;
</syntaxhighlight>

==Criticism==

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.<ref name="icse19">{{cite conference
| url= https://www.paulgazzillo.com/papers/icse19nier.pdf
| title= Conditional Compilation is Dead, Long Live Conditional Compilation!
| last1= Gazzillo
| first1= Paul
| last2= Wei
| first2= Shiyi
| date= 2019-05-27
| conference= 2019 IEEE/ACM 41st International Conference on Software Engineering: New Ideas and Emerging Results (ICSE-NIER)
| conference-url= https://2019.icse-conferences.org/
| book-title= ICSE-NIER '19: Proceedings of the 41st International Conference on Software Engineering: New Ideas and Emerging Results
| publisher= IEEE Press
| archive-url= https://web.archive.org/web/20221107174330/https://www.paulgazzillo.com/papers/icse19nier.pdf
| archive-date= 2022-11-07
| url-status= live
| location= Montreal, QC, Canada
| pages= 105–108
| isbn= 978-1-7281-1758-4
| doi= 10.1109/ICSE-NIER.2019.00035
| access-date= 2023-01-21
| language= en}}</ref><ref name="meinicke">{{cite book |last1=Meinicke |first1=Jens |title=Quality Assurance for Conditional Compilation |date=2017 |url=https://doi.org/10.1007/978-3-319-61443-4_12 |work=Mastering Software Variability with FeatureIDE |pages=131–139 |editor-last=Meinicke |editor-first=Jens |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-61443-4_12 |isbn=978-3-319-61443-4 |access-date=2023-01-21 |last2=Thüm |first2=Thomas |last3=Schröter |first3=Reimar |last4=Benduhn |first4=Fabian |last5=Leich |first5=Thomas |last6=Saake |first6=Gunter |editor2-last=Thüm |editor2-first=Thomas |editor3-last=Schröter |editor3-first=Reimar |editor4-last=Benduhn |editor4-first=Fabian}}</ref><ref name="se-cc-quality">{{Cite web |title=compiler - How does conditional compilation impact product quality, security and code complexity? |url=https://softwareengineering.stackexchange.com/questions/405980/how-does-conditional-compilation-impact-product-quality-security-and-code-compl |access-date=2023-01-21 |website=Software Engineering Stack Exchange |language=en}}</ref> When conditional compilation is done via a [[preprocessor]] that does not guarantee syntactically correct output in the source language, such as the [[C preprocessor]], this may lead to hard-to-debug compilation errors,<ref name="ifdef-harmful">{{Cite book |last1=Le |first1=Duc |last2=Walkingshaw |first2=Eric |last3=Erwig |first3=Martin |title=2011 IEEE Symposium on Visual Languages and Human-Centric Computing (VL/HCC) |chapter=#ifdef confirmed harmful: Promoting understandable software variation |date=2011-09-18 |url=https://ieeexplore.ieee.org/document/6070391 |pages=143–150 |doi=10.1109/VLHCC.2011.6070391|isbn=978-1-4577-1246-3 }}</ref><ref name="so-avoid-ifdef">{{Cite web |title=conditional compilation - Why should #ifdef be avoided in .c files? |url=https://stackoverflow.com/questions/1851181/why-should-ifdef-be-avoided-in-c-files |access-date=2023-01-21 |website=Stack Overflow |language=en}}</ref><ref name="do-dont-cc">{{Cite web |title=c++ - Dos and Don'ts of Conditional Compile |url=https://stackoverflow.com/questions/4469645/dos-and-donts-of-conditional-compile |access-date=2023-01-21 |website=Stack Overflow |language=en}}</ref> which is sometimes called "#ifdef hell."<ref name="preschern">{{Cite conference |last=Preschern |first=Christopher |title=Proceedings of the 24th European Conference on Pattern Languages of Programs |chapter=Patterns to escape the #ifdef hell |date=2019-07-03 |url=http://preschern.azurewebsites.net/2019-os-specific-code.pdf |location=New York, NY, USA |publisher=Association for Computing Machinery |pages=1–12 |doi=10.1145/3361149.3361151 |isbn=978-1-4503-6206-1 |archive-url=https://web.archive.org/web/20221221151021/http://preschern.azurewebsites.net/2019-os-specific-code.pdf| archive-date=2022-12-21|url-status=live}}</ref><ref name="ifdef-hell">{{Cite web |title=Living in the #ifdef Hell |url=http://www.cqse.eu/en/news/blog/living-in-the-ifdef-hell/ |access-date=2023-01-21 |website=www.cqse.eu |date=28 October 2015 |language=en-GB| archive-url=https://web.archive.org/web/20221128024445/https://www.cqse.eu/en/news/blog/living-in-the-ifdef-hell/ |archive-date=2022-11-28 |url-status=live}}</ref>

==References==
<references/>

[[Category:Programming language implementation]]

Conditional compilation - Revision history

imported>Ira Leviton: differring→differing - typos.toolforge.org