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143 lines
6.5 KiB
HTML
143 lines
6.5 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Traditional macros - 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="Traditional-Mode.html#Traditional-Mode" title="Traditional Mode">
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<link rel="prev" href="Traditional-lexical-analysis.html#Traditional-lexical-analysis" title="Traditional lexical analysis">
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<link rel="next" href="Traditional-miscellany.html#Traditional-miscellany" title="Traditional miscellany">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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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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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Traditional-macros"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Traditional-miscellany.html#Traditional-miscellany">Traditional miscellany</a>,
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Previous: <a rel="previous" accesskey="p" href="Traditional-lexical-analysis.html#Traditional-lexical-analysis">Traditional lexical analysis</a>,
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Up: <a rel="up" accesskey="u" href="Traditional-Mode.html#Traditional-Mode">Traditional Mode</a>
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<hr>
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</div>
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<h3 class="section">10.2 Traditional macros</h3>
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<p>The major difference between traditional and ISO macros is that the
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former expand to text rather than to a token sequence. CPP removes
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all leading and trailing horizontal whitespace from a macro's
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replacement text before storing it, but preserves the form of internal
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whitespace.
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<p>One consequence is that it is legitimate for the replacement text to
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contain an unmatched quote (see <a href="Traditional-lexical-analysis.html#Traditional-lexical-analysis">Traditional lexical analysis</a>). An
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unclosed string or character constant continues into the text
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following the macro call. Similarly, the text at the end of a macro's
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expansion can run together with the text after the macro invocation to
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produce a single token.
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<p>Normally comments are removed from the replacement text after the
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macro is expanded, but if the <samp><span class="option">-CC</span></samp> option is passed on the
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command-line comments are preserved. (In fact, the current
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implementation removes comments even before saving the macro
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replacement text, but it careful to do it in such a way that the
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observed effect is identical even in the function-like macro case.)
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<p>The ISO stringification operator ‘<samp><span class="samp">#</span></samp>’ and token paste operator
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‘<samp><span class="samp">##</span></samp>’ have no special meaning. As explained later, an effect
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similar to these operators can be obtained in a different way. Macro
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names that are embedded in quotes, either from the main file or after
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macro replacement, do not expand.
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<p>CPP replaces an unquoted object-like macro name with its replacement
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text, and then rescans it for further macros to replace. Unlike
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standard macro expansion, traditional macro expansion has no provision
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to prevent recursion. If an object-like macro appears unquoted in its
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replacement text, it will be replaced again during the rescan pass,
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and so on <em>ad infinitum</em>. GCC detects when it is expanding
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recursive macros, emits an error message, and continues after the
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offending macro invocation.
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<pre class="smallexample"> #define PLUS +
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#define INC(x) PLUS+x
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INC(foo);
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==> ++foo;
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</pre>
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<p>Function-like macros are similar in form but quite different in
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behavior to their ISO counterparts. Their arguments are contained
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within parentheses, are comma-separated, and can cross physical lines.
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Commas within nested parentheses are not treated as argument
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separators. Similarly, a quote in an argument cannot be left
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unclosed; a following comma or parenthesis that comes before the
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closing quote is treated like any other character. There is no
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facility for handling variadic macros.
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<p>This implementation removes all comments from macro arguments, unless
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the <samp><span class="option">-C</span></samp> option is given. The form of all other horizontal
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whitespace in arguments is preserved, including leading and trailing
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whitespace. In particular
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<pre class="smallexample"> f( )
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</pre>
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<p class="noindent">is treated as an invocation of the macro ‘<samp><span class="samp">f</span></samp>’ with a single
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argument consisting of a single space. If you want to invoke a
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function-like macro that takes no arguments, you must not leave any
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whitespace between the parentheses.
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<p>If a macro argument crosses a new line, the new line is replaced with
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a space when forming the argument. If the previous line contained an
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unterminated quote, the following line inherits the quoted state.
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<p>Traditional preprocessors replace parameters in the replacement text
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with their arguments regardless of whether the parameters are within
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quotes or not. This provides a way to stringize arguments. For
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example
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<pre class="smallexample"> #define str(x) "x"
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str(/* <span class="roman">A comment</span> */some text )
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==> "some text "
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</pre>
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<p class="noindent">Note that the comment is removed, but that the trailing space is
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preserved. Here is an example of using a comment to effect token
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pasting.
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<pre class="smallexample"> #define suffix(x) foo_/**/x
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suffix(bar)
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==> foo_bar
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</pre>
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</body></html>
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