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| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
| commit | abdaadbcae30fe0c9a66c7516798279fdfd97750 (patch) | |
| tree | 00a54a6e25601e43876d03c1a4a12a749d4a914c /share/doc/cpp/Ifdef.html | |
https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads
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diff --git a/share/doc/cpp/Ifdef.html b/share/doc/cpp/Ifdef.html new file mode 100644 index 0000000..41b2b5c --- /dev/null +++ b/share/doc/cpp/Ifdef.html @@ -0,0 +1,163 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1987-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation. A copy of +the license is included in the +section entitled "GNU Free Documentation License". + +This manual contains no Invariant Sections. The Front-Cover Texts are +(a) (see below), and the Back-Cover Texts are (b) (see below). + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>The C Preprocessor: Ifdef</title> + +<meta name="description" content="The C Preprocessor: Ifdef"> +<meta name="keywords" content="The C Preprocessor: Ifdef"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Conditional-Syntax.html#Conditional-Syntax" rel="up" title="Conditional Syntax"> +<link href="If.html#If" rel="next" title="If"> +<link href="Conditional-Syntax.html#Conditional-Syntax" rel="previous" title="Conditional Syntax"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Ifdef"></a> +<div class="header"> +<p> +Next: <a href="If.html#If" accesskey="n" rel="next">If</a>, Up: <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="u" rel="up">Conditional Syntax</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Ifdef-1"></a> +<h4 class="subsection">4.2.1 Ifdef</h4> +<a name="index-_0023ifdef"></a> +<a name="index-_0023endif"></a> + +<p>The simplest sort of conditional is +</p> +<div class="smallexample"> +<pre class="smallexample">#ifdef <var>MACRO</var> + +<var>controlled text</var> + +#endif /* <var>MACRO</var> */ +</pre></div> + +<a name="index-conditional-group"></a> +<p>This block is called a <em>conditional group</em>. <var>controlled text</var> +will be included in the output of the preprocessor if and only if +<var>MACRO</var> is defined. We say that the conditional <em>succeeds</em> if +<var>MACRO</var> is defined, <em>fails</em> if it is not. +</p> +<p>The <var>controlled text</var> inside of a conditional can include +preprocessing directives. They are executed only if the conditional +succeeds. You can nest conditional groups inside other conditional +groups, but they must be completely nested. In other words, +‘<samp>#endif</samp>’ always matches the nearest ‘<samp>#ifdef</samp>’ (or +‘<samp>#ifndef</samp>’, or ‘<samp>#if</samp>’). Also, you cannot start a conditional +group in one file and end it in another. +</p> +<p>Even if a conditional fails, the <var>controlled text</var> inside it is +still run through initial transformations and tokenization. Therefore, +it must all be lexically valid C. Normally the only way this matters is +that all comments and string literals inside a failing conditional group +must still be properly ended. +</p> +<p>The comment following the ‘<samp>#endif</samp>’ is not required, but it is a +good practice if there is a lot of <var>controlled text</var>, because it +helps people match the ‘<samp>#endif</samp>’ to the corresponding ‘<samp>#ifdef</samp>’. +Older programs sometimes put <var>MACRO</var> directly after the +‘<samp>#endif</samp>’ without enclosing it in a comment. This is invalid code +according to the C standard. CPP accepts it with a warning. It +never affects which ‘<samp>#ifndef</samp>’ the ‘<samp>#endif</samp>’ matches. +</p> +<a name="index-_0023ifndef"></a> +<p>Sometimes you wish to use some code if a macro is <em>not</em> defined. +You can do this by writing ‘<samp>#ifndef</samp>’ instead of ‘<samp>#ifdef</samp>’. +One common use of ‘<samp>#ifndef</samp>’ is to include code only the first +time a header file is included. See <a href="Once_002dOnly-Headers.html#Once_002dOnly-Headers">Once-Only Headers</a>. +</p> +<p>Macro definitions can vary between compilations for several reasons. +Here are some samples. +</p> +<ul> +<li> Some macros are predefined on each kind of machine +(see <a href="System_002dspecific-Predefined-Macros.html#System_002dspecific-Predefined-Macros">System-specific Predefined Macros</a>). This allows you to provide +code specially tuned for a particular machine. + +</li><li> System header files define more macros, associated with the features +they implement. You can test these macros with conditionals to avoid +using a system feature on a machine where it is not implemented. + +</li><li> Macros can be defined or undefined with the <samp>-D</samp> and <samp>-U</samp> +command-line options when you compile the program. You can arrange to +compile the same source file into two different programs by choosing a +macro name to specify which program you want, writing conditionals to +test whether or how this macro is defined, and then controlling the +state of the macro with command-line options, perhaps set in the +Makefile. See <a href="Invocation.html#Invocation">Invocation</a>. + +</li><li> Your program might have a special header file (often called +<samp>config.h</samp>) that is adjusted when the program is compiled. It can +define or not define macros depending on the features of the system and +the desired capabilities of the program. The adjustment can be +automated by a tool such as <code>autoconf</code>, or done by hand. +</li></ul> + +<hr> +<div class="header"> +<p> +Next: <a href="If.html#If" accesskey="n" rel="next">If</a>, Up: <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="u" rel="up">Conditional Syntax</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |