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167 lines
7.7 KiB
HTML
167 lines
7.7 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>Computed Includes (The C Preprocessor)</title>
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<meta name="description" content="Computed Includes (The C Preprocessor)">
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<meta name="keywords" content="Computed Includes (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="Header-Files.html#Header-Files" rel="up" title="Header Files">
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<link href="Wrapper-Headers.html#Wrapper-Headers" rel="next" title="Wrapper Headers">
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<link href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef" rel="prev" title="Alternatives to Wrapper #ifndef">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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div.display {margin-left: 3.2em}
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div.example {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="Computed-Includes"></a>
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<div class="header">
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<p>
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Next: <a href="Wrapper-Headers.html#Wrapper-Headers" accesskey="n" rel="next">Wrapper Headers</a>, Previous: <a href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef" accesskey="p" rel="prev">Alternatives to Wrapper #ifndef</a>, Up: <a href="Header-Files.html#Header-Files" accesskey="u" rel="up">Header Files</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="Computed-Includes-1"></a>
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<h3 class="section">2.6 Computed Includes</h3>
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<a name="index-computed-includes"></a>
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<a name="index-macros-in-include"></a>
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<p>Sometimes it is necessary to select one of several different header
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files to be included into your program. They might specify
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configuration parameters to be used on different sorts of operating
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systems, for instance. You could do this with a series of conditionals,
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if SYSTEM_1
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# include "system_1.h"
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#elif SYSTEM_2
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# include "system_2.h"
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#elif SYSTEM_3
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…
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#endif
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</pre></div>
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<p>That rapidly becomes tedious. Instead, the preprocessor offers the
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ability to use a macro for the header name. This is called a
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<em>computed include</em>. Instead of writing a header name as the direct
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argument of ‘<samp>#include</samp>’, you simply put a macro name there instead:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define SYSTEM_H "system_1.h"
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…
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#include SYSTEM_H
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</pre></div>
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<p><code>SYSTEM_H</code> will be expanded, and the preprocessor will look for
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<samp>system_1.h</samp> as if the ‘<samp>#include</samp>’ had been written that way
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originally. <code>SYSTEM_H</code> could be defined by your Makefile with a
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<samp>-D</samp> option.
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</p>
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<p>You must be careful when you define the macro. ‘<samp>#define</samp>’ saves
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tokens, not text. The preprocessor has no way of knowing that the macro
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will be used as the argument of ‘<samp>#include</samp>’, so it generates
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ordinary tokens, not a header name. This is unlikely to cause problems
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if you use double-quote includes, which are close enough to string
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constants. If you use angle brackets, however, you may have trouble.
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</p>
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<p>The syntax of a computed include is actually a bit more general than the
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above. If the first non-whitespace character after ‘<samp>#include</samp>’ is
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not ‘<samp>"</samp>’ or ‘<samp><</samp>’, then the entire line is macro-expanded
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like running text would be.
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</p>
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<p>If the line expands to a single string constant, the contents of that
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string constant are the file to be included. CPP does not re-examine the
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string for embedded quotes, but neither does it process backslash
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escapes in the string. Therefore
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define HEADER "a\"b"
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#include HEADER
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</pre></div>
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<p>looks for a file named <samp>a\"b</samp>. CPP searches for the file according
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to the rules for double-quoted includes.
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</p>
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<p>If the line expands to a token stream beginning with a ‘<samp><</samp>’ token
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and including a ‘<samp>></samp>’ token, then the tokens between the ‘<samp><</samp>’ and
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the first ‘<samp>></samp>’ are combined to form the filename to be included.
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Any whitespace between tokens is reduced to a single space; then any
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space after the initial ‘<samp><</samp>’ is retained, but a trailing space
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before the closing ‘<samp>></samp>’ is ignored. CPP searches for the file
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according to the rules for angle-bracket includes.
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</p>
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<p>In either case, if there are any tokens on the line after the file name,
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an error occurs and the directive is not processed. It is also an error
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if the result of expansion does not match either of the two expected
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forms.
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</p>
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<p>These rules are implementation-defined behavior according to the C
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standard. To minimize the risk of different compilers interpreting your
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computed includes differently, we recommend you use only a single
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object-like macro which expands to a string constant. This will also
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minimize confusion for people reading your program.
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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="Wrapper-Headers.html#Wrapper-Headers" accesskey="n" rel="next">Wrapper Headers</a>, Previous: <a href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef" accesskey="p" rel="prev">Alternatives to Wrapper #ifndef</a>, Up: <a href="Header-Files.html#Header-Files" accesskey="u" rel="up">Header Files</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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