116 lines
4.6 KiB
HTML
116 lines
4.6 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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<html>
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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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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: Misnesting</title>
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<meta name="description" content="The C Preprocessor: Misnesting">
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<meta name="keywords" content="The C Preprocessor: Misnesting">
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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="Macro-Pitfalls.html#Macro-Pitfalls" rel="up" title="Macro Pitfalls">
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<link href="Operator-Precedence-Problems.html#Operator-Precedence-Problems" rel="next" title="Operator Precedence Problems">
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<link href="Macro-Pitfalls.html#Macro-Pitfalls" rel="prev" title="Macro Pitfalls">
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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="Misnesting"></a>
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<div class="header">
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<p>
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Next: <a href="Operator-Precedence-Problems.html#Operator-Precedence-Problems" accesskey="n" rel="next">Operator Precedence Problems</a>, Up: <a href="Macro-Pitfalls.html#Macro-Pitfalls" accesskey="u" rel="up">Macro Pitfalls</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="Misnesting-1"></a>
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<h4 class="subsection">3.10.1 Misnesting</h4>
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<p>When a macro is called with arguments, the arguments are substituted
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into the macro body and the result is checked, together with the rest of
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the input file, for more macro calls. It is possible to piece together
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a macro call coming partially from the macro body and partially from the
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arguments. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define twice(x) (2*(x))
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#define call_with_1(x) x(1)
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call_with_1 (twice)
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→ twice(1)
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→ (2*(1))
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</pre></div>
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<p>Macro definitions do not have to have balanced parentheses. By writing
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an unbalanced open parenthesis in a macro body, it is possible to create
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a macro call that begins inside the macro body but ends outside of it.
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For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define strange(file) fprintf (file, "%s %d",
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…
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strange(stderr) p, 35)
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→ fprintf (stderr, "%s %d", p, 35)
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</pre></div>
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<p>The ability to piece together a macro call can be useful, but the use of
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unbalanced open parentheses in a macro body is just confusing, and
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should be avoided.
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</p>
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</body>
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</html>
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