155 lines
7.0 KiB
HTML
155 lines
7.0 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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<!-- 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: The preprocessing language</title>
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<meta name="description" content="The C Preprocessor: The preprocessing language">
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<meta name="keywords" content="The C Preprocessor: The preprocessing language">
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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="Overview.html#Overview" rel="up" title="Overview">
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<link href="Header-Files.html#Header-Files" rel="next" title="Header Files">
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<link href="Tokenization.html#Tokenization" rel="prev" title="Tokenization">
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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="The-preprocessing-language"></a>
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<div class="header">
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<p>
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Previous: <a href="Tokenization.html#Tokenization" accesskey="p" rel="prev">Tokenization</a>, Up: <a href="Overview.html#Overview" accesskey="u" rel="up">Overview</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="The-preprocessing-language-1"></a>
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<h3 class="section">1.4 The preprocessing language</h3>
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<a name="index-directives"></a>
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<a name="index-preprocessing-directives"></a>
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<a name="index-directive-line"></a>
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<a name="index-directive-name"></a>
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<p>After tokenization, the stream of tokens may simply be passed straight
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to the compiler’s parser. However, if it contains any operations in the
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<em>preprocessing language</em>, it will be transformed first. This stage
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corresponds roughly to the standard’s “translation phase 4” and is
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what most people think of as the preprocessor’s job.
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</p>
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<p>The preprocessing language consists of <em>directives</em> to be executed
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and <em>macros</em> to be expanded. Its primary capabilities are:
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</p>
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<ul>
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<li> Inclusion of header files. These are files of declarations that can be
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substituted into your program.
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</li><li> Macro expansion. You can define <em>macros</em>, which are abbreviations
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for arbitrary fragments of C code. The preprocessor will replace the
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macros with their definitions throughout the program. Some macros are
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automatically defined for you.
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</li><li> Conditional compilation. You can include or exclude parts of the
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program according to various conditions.
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</li><li> Line control. If you use a program to combine or rearrange source files
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into an intermediate file which is then compiled, you can use line
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control to inform the compiler where each source line originally came
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from.
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</li><li> Diagnostics. You can detect problems at compile time and issue errors
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or warnings.
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</li></ul>
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<p>There are a few more, less useful, features.
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</p>
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<p>Except for expansion of predefined macros, all these operations are
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triggered with <em>preprocessing directives</em>. Preprocessing directives
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are lines in your program that start with ‘<samp>#</samp>’. Whitespace is
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allowed before and after the ‘<samp>#</samp>’. The ‘<samp>#</samp>’ is followed by an
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identifier, the <em>directive name</em>. It specifies the operation to
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perform. Directives are commonly referred to as ‘<samp>#<var>name</var></samp>’
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where <var>name</var> is the directive name. For example, ‘<samp>#define</samp>’ is
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the directive that defines a macro.
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</p>
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<p>The ‘<samp>#</samp>’ which begins a directive cannot come from a macro
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expansion. Also, the directive name is not macro expanded. Thus, if
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<code>foo</code> is defined as a macro expanding to <code>define</code>, that does
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not make ‘<samp>#foo</samp>’ a valid preprocessing directive.
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</p>
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<p>The set of valid directive names is fixed. Programs cannot define new
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preprocessing directives.
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</p>
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<p>Some directives require arguments; these make up the rest of the
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directive line and must be separated from the directive name by
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whitespace. For example, ‘<samp>#define</samp>’ must be followed by a macro
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name and the intended expansion of the macro.
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</p>
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<p>A preprocessing directive cannot cover more than one line. The line
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may, however, be continued with backslash-newline, or by a block comment
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which extends past the end of the line. In either case, when the
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directive is processed, the continuations have already been merged with
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the first line to make one long line.
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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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Previous: <a href="Tokenization.html#Tokenization" accesskey="p" rel="prev">Tokenization</a>, Up: <a href="Overview.html#Overview" accesskey="u" rel="up">Overview</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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