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<h5 class="subsubsection">3.2.4.5 Obstack Functions and Macros</h5>

<p><a name="index-macros-313"></a>
The interfaces for using obstacks may be defined either as functions or
as macros, depending on the compiler.  The obstack facility works with
all C compilers, including both ISO&nbsp;C<!-- /@w --> and traditional C, but there are
precautions you must take if you plan to use compilers other than GNU C.

   <p>If you are using an old-fashioned non-ISO&nbsp;C<!-- /@w --> compiler, all the obstack
&ldquo;functions&rdquo; are actually defined only as macros.  You can call these
macros like functions, but you cannot use them in any other way (for
example, you cannot take their address).

   <p>Calling the macros requires a special precaution: namely, the first
operand (the obstack pointer) may not contain any side effects, because
it may be computed more than once.  For example, if you write this:

<pre class="smallexample">     obstack_alloc (get_obstack (), 4);
</pre>
   <p class="noindent">you will find that <code>get_obstack</code> may be called several times. 
If you use <code>*obstack_list_ptr++</code> as the obstack pointer argument,
you will get very strange results since the incrementation may occur
several times.

   <p>In ISO&nbsp;C<!-- /@w -->, each function has both a macro definition and a function
definition.  The function definition is used if you take the address of the
function without calling it.  An ordinary call uses the macro definition by
default, but you can request the function definition instead by writing the
function name in parentheses, as shown here:

<pre class="smallexample">     char *x;
     void *(*funcp) ();
     /* <span class="roman">Use the macro</span>.  */
     x = (char *) obstack_alloc (obptr, size);
     /* <span class="roman">Call the function</span>.  */
     x = (char *) (obstack_alloc) (obptr, size);
     /* <span class="roman">Take the address of the function</span>.  */
     funcp = obstack_alloc;
</pre>
   <p class="noindent">This is the same situation that exists in ISO&nbsp;C<!-- /@w --> for the standard library
functions.  See <a href="Macro-Definitions.html#Macro-Definitions">Macro Definitions</a>.

   <p><strong>Warning:</strong> When you do use the macros, you must observe the
precaution of avoiding side effects in the first operand, even in ISO&nbsp;C<!-- /@w -->.

   <p>If you use the GNU C compiler, this precaution is not necessary, because
various language extensions in GNU C permit defining the macros so as to
compute each argument only once.

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