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148 lines
7.3 KiB
HTML
148 lines
7.3 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>Overview (The C Preprocessor)</title>
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<meta name="description" content="Overview (The C Preprocessor)">
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<meta name="keywords" content="Overview (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="index.html#Top" rel="up" title="Top">
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<link href="Character-sets.html#Character-sets" rel="next" title="Character sets">
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<link href="index.html#Top" rel="prev" title="Top">
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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="Overview"></a>
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<div class="header">
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<p>
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Next: <a href="Header-Files.html#Header-Files" accesskey="n" rel="next">Header Files</a>, Previous: <a href="index.html#Top" accesskey="p" rel="prev">Top</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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="Overview-1"></a>
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<h2 class="chapter">1 Overview</h2>
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<p>The C preprocessor, often known as <em>cpp</em>, is a <em>macro processor</em>
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that is used automatically by the C compiler to transform your program
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before compilation. It is called a macro processor because it allows
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you to define <em>macros</em>, which are brief abbreviations for longer
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constructs.
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</p>
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<p>The C preprocessor is intended to be used only with C, C++, and
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Objective-C source code. In the past, it has been abused as a general
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text processor. It will choke on input which does not obey C’s lexical
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rules. For example, apostrophes will be interpreted as the beginning of
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character constants, and cause errors. Also, you cannot rely on it
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preserving characteristics of the input which are not significant to
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C-family languages. If a Makefile is preprocessed, all the hard tabs
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will be removed, and the Makefile will not work.
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</p>
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<p>Having said that, you can often get away with using cpp on things which
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are not C. Other Algol-ish programming languages are often safe
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(Pascal, Ada, etc.) So is assembly, with caution. <samp>-traditional-cpp</samp>
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mode preserves more white space, and is otherwise more permissive. Many
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of the problems can be avoided by writing C or C++ style comments
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instead of native language comments, and keeping macros simple.
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</p>
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<p>Wherever possible, you should use a preprocessor geared to the language
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you are writing in. Modern versions of the GNU assembler have macro
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facilities. Most high level programming languages have their own
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conditional compilation and inclusion mechanism. If all else fails,
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try a true general text processor, such as GNU M4.
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</p>
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<p>C preprocessors vary in some details. This manual discusses the GNU C
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preprocessor, which provides a small superset of the features of ISO
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Standard C. In its default mode, the GNU C preprocessor does not do a
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few things required by the standard. These are features which are
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rarely, if ever, used, and may cause surprising changes to the meaning
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of a program which does not expect them. To get strict ISO Standard C,
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you should use the <samp>-std=c90</samp>, <samp>-std=c99</samp>,
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<samp>-std=c11</samp> or <samp>-std=c17</samp> options, depending
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on which version of the standard you want. To get all the mandatory
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diagnostics, you must also use <samp>-pedantic</samp>. See <a href="Invocation.html#Invocation">Invocation</a>.
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</p>
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<p>This manual describes the behavior of the ISO preprocessor. To
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minimize gratuitous differences, where the ISO preprocessor’s
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behavior does not conflict with traditional semantics, the
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traditional preprocessor should behave the same way. The various
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differences that do exist are detailed in the section <a href="Traditional-Mode.html#Traditional-Mode">Traditional Mode</a>.
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</p>
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<p>For clarity, unless noted otherwise, references to ‘<samp>CPP</samp>’ in this
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manual refer to GNU CPP.
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</p>
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<table class="menu" border="0" cellspacing="0">
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<tr><td align="left" valign="top">• <a href="Character-sets.html#Character-sets" accesskey="1">Character sets</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Initial-processing.html#Initial-processing" accesskey="2">Initial processing</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Tokenization.html#Tokenization" accesskey="3">Tokenization</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="The-preprocessing-language.html#The-preprocessing-language" accesskey="4">The preprocessing language</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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</table>
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<hr>
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|
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<div class="header">
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<p>
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Next: <a href="Header-Files.html#Header-Files" accesskey="n" rel="next">Header Files</a>, Previous: <a href="index.html#Top" accesskey="p" rel="prev">Top</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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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