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<h4 class="subsection">3.10.5 Self-Referential Macros</h4>
<p><a name="index-self_002dreference-78"></a>
A <dfn>self-referential</dfn> 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:
<pre class="smallexample"> #define foo (4 + foo)
</pre>
<p class="noindent">where <code>foo</code> is also a variable in your program.
<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>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>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>One common, useful use of self-reference is to create a macro which
expands to itself. If you write
<pre class="smallexample"> #define EPERM EPERM
</pre>
<p class="noindent">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><span class="samp">#ifdef</span></samp>’. You might do this if you
want to define numeric constants with an <code>enum</code>, but have
‘<samp><span class="samp">#ifdef</span></samp>’ be true for each constant.
<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 <dfn>indirect
self-reference</dfn> of <code>x</code>. <code>x</code> is not expanded in this case
either. Thus, if we have
<pre class="smallexample"> #define x (4 + y)
#define y (2 * x)
</pre>
<p class="noindent">then <code>x</code> and <code>y</code> expand as follows:
<pre class="smallexample"> x ==> (4 + y)
==> (4 + (2 * x))
y ==> (2 * x)
==> (2 * (4 + y))
</pre>
<p class="noindent">Each macro is expanded when it appears in the definition of the other
macro, but not when it indirectly appears in its own definition.
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