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diff --git a/share/doc/cpp/Computed-Includes.html b/share/doc/cpp/Computed-Includes.html new file mode 100644 index 0000000..970169d --- /dev/null +++ b/share/doc/cpp/Computed-Includes.html @@ -0,0 +1,167 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1987-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation. A copy of +the license is included in the +section entitled "GNU Free Documentation License". + +This manual contains no Invariant Sections. The Front-Cover Texts are +(a) (see below), and the Back-Cover Texts are (b) (see below). + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>The C Preprocessor: Computed Includes</title> + +<meta name="description" content="The C Preprocessor: Computed Includes"> +<meta name="keywords" content="The C Preprocessor: Computed Includes"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Header-Files.html#Header-Files" rel="up" title="Header Files"> +<link href="Wrapper-Headers.html#Wrapper-Headers" rel="next" title="Wrapper Headers"> +<link href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef" rel="previous" title="Alternatives to Wrapper #ifndef"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Computed-Includes"></a> +<div class="header"> +<p> +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="previous">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> +</div> +<hr> +<a name="Computed-Includes-1"></a> +<h3 class="section">2.6 Computed Includes</h3> +<a name="index-computed-includes"></a> +<a name="index-macros-in-include"></a> + +<p>Sometimes it is necessary to select one of several different header +files to be included into your program. They might specify +configuration parameters to be used on different sorts of operating +systems, for instance. You could do this with a series of conditionals, +</p> +<div class="smallexample"> +<pre class="smallexample">#if SYSTEM_1 +# include "system_1.h" +#elif SYSTEM_2 +# include "system_2.h" +#elif SYSTEM_3 +… +#endif +</pre></div> + +<p>That rapidly becomes tedious. Instead, the preprocessor offers the +ability to use a macro for the header name. This is called a +<em>computed include</em>. Instead of writing a header name as the direct +argument of ‘<samp>#include</samp>’, you simply put a macro name there instead: +</p> +<div class="smallexample"> +<pre class="smallexample">#define SYSTEM_H "system_1.h" +… +#include SYSTEM_H +</pre></div> + +<p><code>SYSTEM_H</code> will be expanded, and the preprocessor will look for +<samp>system_1.h</samp> as if the ‘<samp>#include</samp>’ had been written that way +originally. <code>SYSTEM_H</code> could be defined by your Makefile with a +<samp>-D</samp> option. +</p> +<p>You must be careful when you define the macro. ‘<samp>#define</samp>’ saves +tokens, not text. The preprocessor has no way of knowing that the macro +will be used as the argument of ‘<samp>#include</samp>’, so it generates +ordinary tokens, not a header name. This is unlikely to cause problems +if you use double-quote includes, which are close enough to string +constants. If you use angle brackets, however, you may have trouble. +</p> +<p>The syntax of a computed include is actually a bit more general than the +above. If the first non-whitespace character after ‘<samp>#include</samp>’ is +not ‘<samp>"</samp>’ or ‘<samp><</samp>’, then the entire line is macro-expanded +like running text would be. +</p> +<p>If the line expands to a single string constant, the contents of that +string constant are the file to be included. CPP does not re-examine the +string for embedded quotes, but neither does it process backslash +escapes in the string. Therefore +</p> +<div class="smallexample"> +<pre class="smallexample">#define HEADER "a\"b" +#include HEADER +</pre></div> + +<p>looks for a file named <samp>a\"b</samp>. CPP searches for the file according +to the rules for double-quoted includes. +</p> +<p>If the line expands to a token stream beginning with a ‘<samp><</samp>’ token +and including a ‘<samp>></samp>’ token, then the tokens between the ‘<samp><</samp>’ and +the first ‘<samp>></samp>’ are combined to form the filename to be included. +Any whitespace between tokens is reduced to a single space; then any +space after the initial ‘<samp><</samp>’ is retained, but a trailing space +before the closing ‘<samp>></samp>’ is ignored. CPP searches for the file +according to the rules for angle-bracket includes. +</p> +<p>In either case, if there are any tokens on the line after the file name, +an error occurs and the directive is not processed. It is also an error +if the result of expansion does not match either of the two expected +forms. +</p> +<p>These rules are implementation-defined behavior according to the C +standard. To minimize the risk of different compilers interpreting your +computed includes differently, we recommend you use only a single +object-like macro which expands to a string constant. This will also +minimize confusion for people reading your program. +</p> +<hr> +<div class="header"> +<p> +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="previous">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> +</div> + + + +</body> +</html> |