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156 lines
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<head>
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<title>The C Preprocessor: Self-Referential Macros</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="Argument-Prescan.html#Argument-Prescan" rel="next" title="Argument Prescan">
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<a name="Self_002dReferential-Macros"></a>
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<div class="header">
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<p>
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Next: <a href="Argument-Prescan.html#Argument-Prescan" accesskey="n" rel="next">Argument Prescan</a>, Previous: <a href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects" accesskey="p" rel="prev">Duplication of Side Effects</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="Self_002dReferential-Macros-1"></a>
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<h4 class="subsection">3.10.5 Self-Referential Macros</h4>
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<a name="index-self_002dreference"></a>
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<p>A <em>self-referential</em> macro is one whose name appears in its
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definition. Recall that all macro definitions are rescanned for more
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macros to replace. If the self-reference were considered a use of the
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macro, it would produce an infinitely large expansion. To prevent this,
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the self-reference is not considered a macro call. It is passed into
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the preprocessor output unchanged. Consider an example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define foo (4 + foo)
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</pre></div>
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<p>where <code>foo</code> is also a variable in your program.
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</p>
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<p>Following the ordinary rules, each reference to <code>foo</code> will expand
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into <code>(4 + foo)</code>; then this will be rescanned and will expand into
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<code>(4 + (4 + foo))</code>; and so on until the computer runs out of memory.
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</p>
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<p>The self-reference rule cuts this process short after one step, at
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<code>(4 + foo)</code>. Therefore, this macro definition has the possibly
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useful effect of causing the program to add 4 to the value of <code>foo</code>
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wherever <code>foo</code> is referred to.
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</p>
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<p>In most cases, it is a bad idea to take advantage of this feature. A
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person reading the program who sees that <code>foo</code> is a variable will
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not expect that it is a macro as well. The reader will come across the
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identifier <code>foo</code> in the program and think its value should be that
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of the variable <code>foo</code>, whereas in fact the value is four greater.
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</p>
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<p>One common, useful use of self-reference is to create a macro which
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expands to itself. If you write
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define EPERM EPERM
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</pre></div>
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<p>then the macro <code>EPERM</code> expands to <code>EPERM</code>. Effectively, it is
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left alone by the preprocessor whenever it’s used in running text. You
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can tell that it’s a macro with ‘<samp>#ifdef</samp>’. You might do this if you
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want to define numeric constants with an <code>enum</code>, but have
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‘<samp>#ifdef</samp>’ be true for each constant.
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</p>
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<p>If a macro <code>x</code> expands to use a macro <code>y</code>, and the expansion of
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<code>y</code> refers to the macro <code>x</code>, that is an <em>indirect
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self-reference</em> of <code>x</code>. <code>x</code> is not expanded in this case
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either. Thus, if we have
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define x (4 + y)
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#define y (2 * x)
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</pre></div>
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<p>then <code>x</code> and <code>y</code> expand as follows:
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</p>
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<div class="smallexample">
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<pre class="smallexample">x → (4 + y)
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→ (4 + (2 * x))
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y → (2 * x)
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→ (2 * (4 + y))
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</pre></div>
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<p>Each macro is expanded when it appears in the definition of the other
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macro, but not when it indirectly appears in its own definition.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Argument-Prescan.html#Argument-Prescan" accesskey="n" rel="next">Argument Prescan</a>, Previous: <a href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects" accesskey="p" rel="prev">Duplication of Side Effects</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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