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180 lines
8.1 KiB
HTML
180 lines
8.1 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>Argument Prescan (The C Preprocessor)</title>
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<meta name="description" content="Argument Prescan (The C Preprocessor)">
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<meta name="keywords" content="Argument Prescan (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="Macro-Pitfalls.html#Macro-Pitfalls" rel="up" title="Macro Pitfalls">
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<link href="Newlines-in-Arguments.html#Newlines-in-Arguments" rel="next" title="Newlines in Arguments">
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<link href="Self_002dReferential-Macros.html#Self_002dReferential-Macros" rel="prev" title="Self-Referential Macros">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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span.roman {font-family: initial; 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="Argument-Prescan"></a>
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<div class="header">
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<p>
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Next: <a href="Newlines-in-Arguments.html#Newlines-in-Arguments" accesskey="n" rel="next">Newlines in Arguments</a>, Previous: <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros" accesskey="p" rel="prev">Self-Referential Macros</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="Argument-Prescan-1"></a>
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<h4 class="subsection">3.10.6 Argument Prescan</h4>
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<a name="index-expansion-of-arguments"></a>
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<a name="index-macro-argument-expansion"></a>
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<a name="index-prescan-of-macro-arguments"></a>
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<p>Macro arguments are completely macro-expanded before they are
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substituted into a macro body, unless they are stringized or pasted
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with other tokens. After substitution, the entire macro body, including
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the substituted arguments, is scanned again for macros to be expanded.
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The result is that the arguments are scanned <em>twice</em> to expand
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macro calls in them.
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</p>
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<p>Most of the time, this has no effect. If the argument contained any
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macro calls, they are expanded during the first scan. The result
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therefore contains no macro calls, so the second scan does not change
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it. If the argument were substituted as given, with no prescan, the
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single remaining scan would find the same macro calls and produce the
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same results.
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</p>
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<p>You might expect the double scan to change the results when a
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self-referential macro is used in an argument of another macro
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(see <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros">Self-Referential Macros</a>): the self-referential macro would be
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expanded once in the first scan, and a second time in the second scan.
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However, this is not what happens. The self-references that do not
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expand in the first scan are marked so that they will not expand in the
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second scan either.
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</p>
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<p>You might wonder, “Why mention the prescan, if it makes no difference?
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And why not skip it and make the preprocessor faster?” The answer is
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that the prescan does make a difference in three special cases:
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</p>
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<ul>
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<li> Nested calls to a macro.
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<p>We say that <em>nested</em> calls to a macro occur when a macro’s argument
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contains a call to that very macro. For example, if <code>f</code> is a macro
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that expects one argument, <code>f (f (1))</code> is a nested pair of calls to
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<code>f</code>. The desired expansion is made by expanding <code>f (1)</code> and
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substituting that into the definition of <code>f</code>. The prescan causes
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the expected result to happen. Without the prescan, <code>f (1)</code> itself
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would be substituted as an argument, and the inner use of <code>f</code> would
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appear during the main scan as an indirect self-reference and would not
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be expanded.
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</p>
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</li><li> Macros that call other macros that stringize or concatenate.
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<p>If an argument is stringized or concatenated, the prescan does not
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occur. If you <em>want</em> to expand a macro, then stringize or
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concatenate its expansion, you can do that by causing one macro to call
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another macro that does the stringizing or concatenation. For
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instance, if you have
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define AFTERX(x) X_ ## x
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#define XAFTERX(x) AFTERX(x)
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#define TABLESIZE 1024
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#define BUFSIZE TABLESIZE
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</pre></div>
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<p>then <code>AFTERX(BUFSIZE)</code> expands to <code>X_BUFSIZE</code>, and
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<code>XAFTERX(BUFSIZE)</code> expands to <code>X_1024</code>. (Not to
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<code>X_TABLESIZE</code>. Prescan always does a complete expansion.)
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</p>
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</li><li> Macros used in arguments, whose expansions contain unshielded commas.
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<p>This can cause a macro expanded on the second scan to be called with the
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wrong number of arguments. Here is an example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define foo a,b
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#define bar(x) lose(x)
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#define lose(x) (1 + (x))
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</pre></div>
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<p>We would like <code>bar(foo)</code> to turn into <code>(1 + (foo))</code>, which
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would then turn into <code>(1 + (a,b))</code>. Instead, <code>bar(foo)</code>
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expands into <code>lose(a,b)</code>, and you get an error because <code>lose</code>
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requires a single argument. In this case, the problem is easily solved
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by the same parentheses that ought to be used to prevent misnesting of
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arithmetic operations:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define foo (a,b)
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</pre><pre class="smallexample">or
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</pre><pre class="smallexample">#define bar(x) lose((x))
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</pre></div>
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<p>The extra pair of parentheses prevents the comma in <code>foo</code>’s
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definition from being interpreted as an argument separator.
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</p>
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Newlines-in-Arguments.html#Newlines-in-Arguments" accesskey="n" rel="next">Newlines in Arguments</a>, Previous: <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros" accesskey="p" rel="prev">Self-Referential Macros</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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</body>
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</html>
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