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85 lines
3.3 KiB
HTML
85 lines
3.3 KiB
HTML
4 years ago
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<head>
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<title>Misnesting - The C Preprocessor</title>
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<meta name="description" content="The C Preprocessor">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Macro-Pitfalls.html#Macro-Pitfalls" title="Macro Pitfalls">
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<link rel="next" href="Operator-Precedence-Problems.html#Operator-Precedence-Problems" title="Operator Precedence Problems">
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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="Misnesting"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Operator-Precedence-Problems.html#Operator-Precedence-Problems">Operator Precedence Problems</a>,
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Up: <a rel="up" accesskey="u" href="Macro-Pitfalls.html#Macro-Pitfalls">Macro Pitfalls</a>
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<hr>
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</div>
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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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<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>
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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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<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>
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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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</body></html>
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