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| 22 | <title>Self-Referential Macros (GNU C Language Manual)</title>
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| 24 | <meta name="description" content="Self-Referential Macros (GNU C Language Manual)">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Macro-Pitfalls.html" rel="up" title="Macro Pitfalls">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Self_002dReferential-Macros"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Argument-Prescan.html" accesskey="n" rel="next">Argument Prescan</a>, Previous: <a href="Macros-and-Auto-Type.html" accesskey="p" rel="prev">Macros and Auto Type</a>, Up: <a href="Macro-Pitfalls.html" accesskey="u" rel="up">Macro Pitfalls</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Self_002dReferential-Macros-1"></span><h4 class="subsubsection">26.5.10.6 Self-Referential Macros</h4>
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| 64 | <span id="index-self_002dreference"></span>
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| 65 |
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| 66 | <p>A <em>self-referential</em> macro is one whose name appears in its
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| 67 | definition. Recall that all macro definitions are rescanned for more
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| 68 | macros to replace. If the self-reference were considered a use of the
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| 69 | macro, it would produce an infinitely large expansion. To prevent
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| 70 | this, the self-reference is not considered a macro call: preprocessing
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| 71 | leaves it unchanged. Consider an example:
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| 72 | </p>
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| 73 | <div class="example">
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| 74 | <pre class="example">#define foo (4 + foo)
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| 75 | </pre></div>
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| 76 |
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| 77 | <p>where <code>foo</code> is also a variable in your program.
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| 78 | </p>
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| 79 | <p>Following the ordinary rules, each reference to <code>foo</code> will expand
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| 80 | into <code>(4 + foo)</code>; then this will be rescanned and will expand into
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| 81 | <code>(4 + (4 + foo))</code>; and so on until the computer runs out of memory.
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| 82 | </p>
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| 83 | <p>The self-reference rule cuts this process short after one step, at
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| 84 | <code>(4 + foo)</code>. Therefore, this macro definition has the possibly
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| 85 | useful effect of causing the program to add 4 to the value of <code>foo</code>
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| 86 | wherever <code>foo</code> is referred to.
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| 87 | </p>
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| 88 | <p>In most cases, it is a bad idea to take advantage of this feature. A
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| 89 | person reading the program who sees that <code>foo</code> is a variable will
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| 90 | not expect that it is a macro as well. The reader will come across the
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| 91 | identifier <code>foo</code> in the program and think its value should be that
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| 92 | of the variable <code>foo</code>, whereas in fact the value is four greater.
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| 93 | </p>
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| 94 | <p>It is useful to make a macro definition that expands to the macro
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| 95 | name itself. If you write
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| 96 | </p>
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| 97 | <div class="example">
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| 98 | <pre class="example">#define EPERM EPERM
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| 99 | </pre></div>
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| 100 |
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| 101 | <p>then the macro <code>EPERM</code> expands to <code>EPERM</code>. Effectively,
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| 102 | preprocessing leaves it unchanged in the source code. You can tell
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| 103 | that it’s a macro with <code>#ifdef</code>. You might do this if you want
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| 104 | to define numeric constants with an <code>enum</code>, but have
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| 105 | <code>#ifdef</code> be true for each constant.
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| 106 | </p>
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| 107 | <p>If a macro <code>x</code> expands to use a macro <code>y</code>, and the expansion of
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| 108 | <code>y</code> refers to the macro <code>x</code>, that is an <em>indirect
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| 109 | self-reference</em> of <code>x</code>. <code>x</code> is not expanded in this case
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| 110 | either. Thus, if we have
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| 111 | </p>
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| 112 | <div class="example">
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| 113 | <pre class="example">#define x (4 + y)
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| 114 | #define y (2 * x)
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| 115 | </pre></div>
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| 116 |
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| 117 | <p>then <code>x</code> and <code>y</code> expand as follows:
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| 118 | </p>
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| 119 | <div class="example">
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| 120 | <pre class="example">x → (4 + y)
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| 121 | → (4 + (2 * x))
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| 122 |
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| 123 | y → (2 * x)
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| 124 | → (2 * (4 + y))
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| 125 | </pre></div>
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| 126 |
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| 127 | <p>Each macro is expanded when it appears in the definition of the other
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| 128 | macro, but not when it indirectly appears in its own definition.
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| 129 | </p>
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| 130 | <hr>
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| 131 | <div class="header">
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| 132 | <p>
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| 133 | Next: <a href="Argument-Prescan.html" accesskey="n" rel="next">Argument Prescan</a>, Previous: <a href="Macros-and-Auto-Type.html" accesskey="p" rel="prev">Macros and Auto Type</a>, Up: <a href="Macro-Pitfalls.html" accesskey="u" rel="up">Macro Pitfalls</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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