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120 lines
5.0 KiB
HTML
120 lines
5.0 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled "GNU Free Documentation License".
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Defined (The C Preprocessor)</title>
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<meta name="description" content="Defined (The C Preprocessor)">
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<meta name="keywords" content="Defined (The C Preprocessor)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Conditional-Syntax.html#Conditional-Syntax" rel="up" title="Conditional Syntax">
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<link href="Else.html#Else" rel="next" title="Else">
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<link href="If.html#If" rel="prev" title="If">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Defined"></a>
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<div class="header">
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<p>
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Next: <a href="Else.html#Else" accesskey="n" rel="next">Else</a>, Previous: <a href="If.html#If" accesskey="p" rel="prev">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>
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</div>
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<hr>
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<a name="Defined-1"></a>
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<h4 class="subsection">4.2.3 Defined</h4>
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<a name="index-defined"></a>
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<p>The special operator <code>defined</code> is used in ‘<samp>#if</samp>’ and
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‘<samp>#elif</samp>’ expressions to test whether a certain name is defined as a
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macro. <code>defined <var>name</var></code> and <code>defined (<var>name</var>)</code> are
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both expressions whose value is 1 if <var>name</var> is defined as a macro at
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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>.
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</p>
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<p><code>defined</code> is useful when you wish to test more than one macro for
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existence at once. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if defined (__vax__) || defined (__ns16000__)
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</pre></div>
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<p>would succeed if either of the names <code>__vax__</code> or
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<code>__ns16000__</code> is defined as a macro.
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</p>
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<p>Conditionals written like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if defined BUFSIZE && BUFSIZE >= 1024
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</pre></div>
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<p>can generally be simplified to just <code>#if BUFSIZE >= 1024<!-- /@w --></code>,
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since if <code>BUFSIZE</code> is not defined, it will be interpreted as having
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the value zero.
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</p>
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<p>If the <code>defined</code> operator appears as a result of a macro expansion,
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the C standard says the behavior is undefined. GNU cpp treats it as a
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genuine <code>defined</code> operator and evaluates it normally. It will warn
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wherever your code uses this feature if you use the command-line option
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<samp>-Wpedantic</samp>, since other compilers may handle it differently. The
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warning is also enabled by <samp>-Wextra</samp>, and can also be enabled
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individually with <samp>-Wexpansion-to-defined</samp>.
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</p>
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</body>
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</html>
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