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131 lines
6.4 KiB
HTML
131 lines
6.4 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Computed Includes - The C Preprocessor</title>
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<meta http-equiv="Content-Type" content="text/html">
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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="Header-Files.html#Header-Files" title="Header Files">
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<link rel="prev" href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef" title="Alternatives to Wrapper #ifndef">
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<link rel="next" href="Wrapper-Headers.html#Wrapper-Headers" title="Wrapper Headers">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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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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funds for GNU development.
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<style type="text/css"><!--
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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="Computed-Includes"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Wrapper-Headers.html#Wrapper-Headers">Wrapper Headers</a>,
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Previous: <a rel="previous" accesskey="p" href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef">Alternatives to Wrapper #ifndef</a>,
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Up: <a rel="up" accesskey="u" href="Header-Files.html#Header-Files">Header Files</a>
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<hr>
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</div>
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<h3 class="section">2.6 Computed Includes</h3>
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<p><a name="index-computed-includes-34"></a><a name="index-macros-in-include-35"></a>
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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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<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>
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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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<dfn>computed include</dfn>. Instead of writing a header name as the direct
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argument of ‘<samp><span class="samp">#include</span></samp>’, you simply put a macro name there instead:
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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>
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<p class="noindent"><code>SYSTEM_H</code> will be expanded, and the preprocessor will look for
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<samp><span class="file">system_1.h</span></samp> as if the ‘<samp><span class="samp">#include</span></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><span class="option">-D</span></samp> option.
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<p>You must be careful when you define the macro. ‘<samp><span class="samp">#define</span></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><span class="samp">#include</span></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>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><span class="samp">#include</span></samp>’ is
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not ‘<samp><span class="samp">"</span></samp>’ or ‘<samp><span class="samp"><</span></samp>’, then the entire line is macro-expanded
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like running text would be.
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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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<pre class="smallexample"> #define HEADER "a\"b"
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#include HEADER
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</pre>
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<p class="noindent">looks for a file named <samp><span class="file">a\"b</span></samp>. CPP searches for the file according
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to the rules for double-quoted includes.
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<p>If the line expands to a token stream beginning with a ‘<samp><span class="samp"><</span></samp>’ token
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and including a ‘<samp><span class="samp">></span></samp>’ token, then the tokens between the ‘<samp><span class="samp"><</span></samp>’ and
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the first ‘<samp><span class="samp">></span></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><span class="samp"><</span></samp>’ is retained, but a trailing space
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before the closing ‘<samp><span class="samp">></span></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>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>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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</body></html>
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