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