147 lines
6.4 KiB
HTML
147 lines
6.4 KiB
HTML
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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) 1987-2016 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 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The C Preprocessor: If</title>
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<meta name="description" content="The C Preprocessor: If">
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<meta name="keywords" content="The C Preprocessor: If">
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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="Defined.html#Defined" rel="next" title="Defined">
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<link href="Ifdef.html#Ifdef" rel="prev" title="Ifdef">
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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.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.indentedblock {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.smallindentedblock {margin-left: 3.2em; font-size: smaller}
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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.nocodebreak {white-space:nowrap}
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span.nolinebreak {white-space:nowrap}
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span.roman {font-family:serif; 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" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="If"></a>
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<div class="header">
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<p>
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Next: <a href="Defined.html#Defined" accesskey="n" rel="next">Defined</a>, Previous: <a href="Ifdef.html#Ifdef" accesskey="p" rel="prev">Ifdef</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="If-1"></a>
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<h4 class="subsection">4.2.2 If</h4>
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<p>The ‘<samp>#if</samp>’ directive allows you to test the value of an arithmetic
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expression, rather than the mere existence of one macro. Its syntax is
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if <var>expression</var>
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<var>controlled text</var>
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#endif /* <var>expression</var> */
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</pre></div>
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<p><var>expression</var> is a C expression of integer type, subject to stringent
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restrictions. It may contain
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</p>
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<ul>
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<li> Integer constants.
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</li><li> Character constants, which are interpreted as they would be in normal
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code.
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</li><li> Arithmetic operators for addition, subtraction, multiplication,
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division, bitwise operations, shifts, comparisons, and logical
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operations (<code>&&</code> and <code>||</code>). The latter two obey the usual
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short-circuiting rules of standard C.
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</li><li> Macros. All macros in the expression are expanded before actual
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computation of the expression’s value begins.
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</li><li> Uses of the <code>defined</code> operator, which lets you check whether macros
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are defined in the middle of an ‘<samp>#if</samp>’.
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</li><li> Identifiers that are not macros, which are all considered to be the
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number zero. This allows you to write <code>#if MACRO<!-- /@w --></code> instead of
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<code>#ifdef MACRO<!-- /@w --></code>, if you know that MACRO, when defined, will
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always have a nonzero value. Function-like macros used without their
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function call parentheses are also treated as zero.
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<p>In some contexts this shortcut is undesirable. The <samp>-Wundef</samp>
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option causes GCC to warn whenever it encounters an identifier which is
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not a macro in an ‘<samp>#if</samp>’.
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</p></li></ul>
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<p>The preprocessor does not know anything about types in the language.
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Therefore, <code>sizeof</code> operators are not recognized in ‘<samp>#if</samp>’, and
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neither are <code>enum</code> constants. They will be taken as identifiers
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which are not macros, and replaced by zero. In the case of
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<code>sizeof</code>, this is likely to cause the expression to be invalid.
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</p>
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<p>The preprocessor calculates the value of <var>expression</var>. It carries
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out all calculations in the widest integer type known to the compiler;
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on most machines supported by GCC this is 64 bits. This is not the same
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rule as the compiler uses to calculate the value of a constant
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expression, and may give different results in some cases. If the value
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comes out to be nonzero, the ‘<samp>#if</samp>’ succeeds and the <var>controlled
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text</var> is included; otherwise it is skipped.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Defined.html#Defined" accesskey="n" rel="next">Defined</a>, Previous: <a href="Ifdef.html#Ifdef" accesskey="p" rel="prev">Ifdef</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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</body>
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</html>
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