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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/Defined.html | |
https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads
Change-Id: I7303388733328cd98ab9aa3c30236db67f2e9e9c
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| -rw-r--r-- | share/doc/cpp/Defined.html | 120 |
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diff --git a/share/doc/cpp/Defined.html b/share/doc/cpp/Defined.html new file mode 100644 index 0000000..d79832f --- /dev/null +++ b/share/doc/cpp/Defined.html @@ -0,0 +1,120 @@ +<!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: Defined</title> + +<meta name="description" content="The C Preprocessor: Defined"> +<meta name="keywords" content="The C Preprocessor: Defined"> +<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="Else.html#Else" rel="next" title="Else"> +<link href="If.html#If" rel="previous" title="If"> +<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="Defined"></a> +<div class="header"> +<p> +Next: <a href="Else.html#Else" accesskey="n" rel="next">Else</a>, Previous: <a href="If.html#If" accesskey="p" rel="previous">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="Defined-1"></a> +<h4 class="subsection">4.2.3 Defined</h4> + +<a name="index-defined"></a> +<p>The special operator <code>defined</code> is used in ‘<samp>#if</samp>’ and +‘<samp>#elif</samp>’ expressions to test whether a certain name is defined as a +macro. <code>defined <var>name</var></code> and <code>defined (<var>name</var>)</code> are +both expressions whose value is 1 if <var>name</var> is defined as a macro at +the current point in the program, and 0 otherwise. Thus, <code>#if defined MACRO<!-- /@w --></code> is precisely equivalent to <code>#ifdef MACRO<!-- /@w --></code>. +</p> +<p><code>defined</code> is useful when you wish to test more than one macro for +existence at once. For example, +</p> +<div class="smallexample"> +<pre class="smallexample">#if defined (__vax__) || defined (__ns16000__) +</pre></div> + +<p>would succeed if either of the names <code>__vax__</code> or +<code>__ns16000__</code> is defined as a macro. +</p> +<p>Conditionals written like this: +</p> +<div class="smallexample"> +<pre class="smallexample">#if defined BUFSIZE && BUFSIZE >= 1024 +</pre></div> + +<p>can generally be simplified to just <code>#if BUFSIZE >= 1024<!-- /@w --></code>, +since if <code>BUFSIZE</code> is not defined, it will be interpreted as having +the value zero. +</p> +<p>If the <code>defined</code> operator appears as a result of a macro expansion, +the C standard says the behavior is undefined. GNU cpp treats it as a +genuine <code>defined</code> operator and evaluates it normally. It will warn +wherever your code uses this feature if you use the command-line option +<samp>-Wpedantic</samp>, since other compilers may handle it differently. The +warning is also enabled by <samp>-Wextra</samp>, and can also be enabled +individually with <samp>-Wexpansion-to-defined</samp>. +</p> + + + +</body> +</html> |