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<h3 class="section">3.2 Function-like Macros</h3>
<p><a name="index-function_002dlike-macros-45"></a>
You can also define macros whose use looks like a function call. These
are called <dfn>function-like macros</dfn>. To define a function-like macro,
you use the same ‘<samp><span class="samp">#define</span></samp>’ directive, but you put a pair of
parentheses immediately after the macro name. For example,
<pre class="smallexample"> #define lang_init() c_init()
lang_init()
==> c_init()
</pre>
<p>A function-like macro is only expanded if its name appears with a pair
of parentheses after it. If you write just the name, it is left alone.
This can be useful when you have a function and a macro of the same
name, and you wish to use the function sometimes.
<pre class="smallexample"> extern void foo(void);
#define foo() /* <span class="roman">optimized inline version</span> */
...
foo();
funcptr = foo;
</pre>
<p>Here the call to <code>foo()</code> will use the macro, but the function
pointer will get the address of the real function. If the macro were to
be expanded, it would cause a syntax error.
<p>If you put spaces between the macro name and the parentheses in the
macro definition, that does not define a function-like macro, it defines
an object-like macro whose expansion happens to begin with a pair of
parentheses.
<pre class="smallexample"> #define lang_init () c_init()
lang_init()
==> () c_init()()
</pre>
<p>The first two pairs of parentheses in this expansion come from the
macro. The third is the pair that was originally after the macro
invocation. Since <code>lang_init</code> is an object-like macro, it does not
consume those parentheses.
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