125 lines
5.2 KiB
HTML
125 lines
5.2 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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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: Directives Within Macro Arguments</title>
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<meta name="description" content="The C Preprocessor: Directives Within Macro Arguments">
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<meta name="keywords" content="The C Preprocessor: Directives Within Macro Arguments">
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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="Macros.html#Macros" rel="up" title="Macros">
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<link href="Macro-Pitfalls.html#Macro-Pitfalls" rel="next" title="Macro Pitfalls">
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<link href="Undefining-and-Redefining-Macros.html#Undefining-and-Redefining-Macros" rel="prev" title="Undefining and Redefining Macros">
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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="Directives-Within-Macro-Arguments"></a>
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<div class="header">
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<p>
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Next: <a href="Macro-Pitfalls.html#Macro-Pitfalls" accesskey="n" rel="next">Macro Pitfalls</a>, Previous: <a href="Undefining-and-Redefining-Macros.html#Undefining-and-Redefining-Macros" accesskey="p" rel="prev">Undefining and Redefining Macros</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</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="Directives-Within-Macro-Arguments-1"></a>
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<h3 class="section">3.9 Directives Within Macro Arguments</h3>
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<a name="index-macro-arguments-and-directives"></a>
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<p>Occasionally it is convenient to use preprocessor directives within
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the arguments of a macro. The C and C++ standards declare that
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behavior in these cases is undefined.
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</p>
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<p>Versions of CPP prior to 3.2 would reject such constructs with an
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error message. This was the only syntactic difference between normal
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functions and function-like macros, so it seemed attractive to remove
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this limitation, and people would often be surprised that they could
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not use macros in this way. Moreover, sometimes people would use
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conditional compilation in the argument list to a normal library
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function like ‘<samp>printf</samp>’, only to find that after a library upgrade
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‘<samp>printf</samp>’ had changed to be a function-like macro, and their code
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would no longer compile. So from version 3.2 we changed CPP to
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successfully process arbitrary directives within macro arguments in
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exactly the same way as it would have processed the directive were the
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function-like macro invocation not present.
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</p>
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<p>If, within a macro invocation, that macro is redefined, then the new
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definition takes effect in time for argument pre-expansion, but the
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original definition is still used for argument replacement. Here is a
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pathological example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define f(x) x x
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f (1
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#undef f
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#define f 2
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f)
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</pre></div>
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<p>which expands to
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</p>
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<div class="smallexample">
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<pre class="smallexample">1 2 1 2
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</pre></div>
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<p>with the semantics described above.
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</p>
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</body>
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</html>
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