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115 lines
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HTML
115 lines
4.5 KiB
HTML
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(a) The FSF's Front-Cover Text is:
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<head>
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<title>Misnesting (The C Preprocessor)</title>
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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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<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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