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115 lines
4.9 KiB
HTML
115 lines
4.9 KiB
HTML
4 years ago
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<title>If - The C Preprocessor</title>
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<meta name="description" content="The C Preprocessor">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Conditional-Syntax.html#Conditional-Syntax" title="Conditional Syntax">
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<link rel="prev" href="Ifdef.html#Ifdef" title="Ifdef">
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<link rel="next" href="Defined.html#Defined" title="Defined">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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Copyright (C) 1987-2015 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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--></style>
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</head>
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<body>
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<div class="node">
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<a name="If"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Defined.html#Defined">Defined</a>,
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Previous: <a rel="previous" accesskey="p" href="Ifdef.html#Ifdef">Ifdef</a>,
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Up: <a rel="up" accesskey="u" href="Conditional-Syntax.html#Conditional-Syntax">Conditional Syntax</a>
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<hr>
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</div>
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<h4 class="subsection">4.2.2 If</h4>
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<p>The ‘<samp><span class="samp">#if</span></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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<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>
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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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<ul>
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<li>Integer constants.
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<li>Character constants, which are interpreted as they would be in normal
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code.
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<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>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>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><span class="samp">#if</span></samp>’.
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<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><span class="option">-Wundef</span></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><span class="samp">#if</span></samp>’.
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</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><span class="samp">#if</span></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>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><span class="samp">#if</span></samp>’ succeeds and the <var>controlled
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text</var> is included; otherwise it is skipped.
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</body></html>
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