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Conditional compilation
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<p>In <a href="/facts/Computer_programming/QmSsFnu0">computer programming</a>, conditional compilation is a <a href="/facts/Compiler/WNfCDFJe">compilation</a> technique which results in differring <a href="/facts/Executable_program/VSst9eLA">executable programs</a> depending on <a href="/facts/Parameter_(computer_programming)/LcuLbbSq">parameters</a> specified. This technique is commonly used when these differences in the program are needed to run it on <a href="/facts/Cross-platform/16zqdvNR">different platforms</a>, or with different versions of required <a href="/facts/Library_(computing)/xRzL3P7q">libraries</a> or <a href="/facts/Computer_hardware/8jSjsCNv">hardware</a>. </p><p>Many <a href="/facts/Programming_language/cWTYbgWa">programming languages</a> support conditional compilation. Typically <a href="/facts/Compiler_directives/RsozxGG0">compiler directives</a> define or "undefine" certain <a href="/facts/Variable_(computer_science)/VoCKtwml">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: </p> (* System generic code *) if mac != Null then (* macOS specific code *) else if pc != Null (* Windows specific code *) <p>In <a href="/facts/C_(programming_language)/Ky2No763">C</a> and some languages with a similar syntax, this is done using an <a href="/facts/C_preprocessor/KuhuxKYP">'#ifdef' directive</a>. </p><p>A similar procedure, using the name "<a href="/facts/Conditional_comment/tEoYYD7a">conditional comment</a>", is used by <a href="/facts/Microsoft_Internet_Explorer/uL8a4Y5l">Microsoft Internet Explorer</a> from version 5 to 9 to interpret <a href="/facts/HTML/sSc3fT78">HTML</a> code. There is also a similar proprietary mechanism for adding conditional comments within <a href="/facts/JScript/HyuCUU2D">JScript</a>, known as <a href="/facts/Conditional_comment/tEoYYD7a">conditional compilation</a>. </p>