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<h5 class="subsubsection">3.2.4.2 Preparing for Using Obstacks</h5>

<p>Each source file in which you plan to use the obstack functions
must include the header file <samp><span class="file">obstack.h</span></samp>, like this:

<pre class="smallexample">     #include &lt;obstack.h&gt;
</pre>
   <p><a name="index-obstack_005fchunk_005falloc-303"></a><a name="index-obstack_005fchunk_005ffree-304"></a>Also, if the source file uses the macro <code>obstack_init</code>, it must
declare or define two functions or macros that will be called by the
obstack library.  One, <code>obstack_chunk_alloc</code>, is used to allocate
the chunks of memory into which objects are packed.  The other,
<code>obstack_chunk_free</code>, is used to return chunks when the objects in
them are freed.  These macros should appear before any use of obstacks
in the source file.

   <p>Usually these are defined to use <code>malloc</code> via the intermediary
<code>xmalloc</code> (see <a href="Unconstrained-Allocation.html#Unconstrained-Allocation">Unconstrained Allocation</a>).  This is done with
the following pair of macro definitions:

<pre class="smallexample">     #define obstack_chunk_alloc xmalloc
     #define obstack_chunk_free free
</pre>
   <p class="noindent">Though the memory you get using obstacks really comes from <code>malloc</code>,
using obstacks is faster because <code>malloc</code> is called less often, for
larger blocks of memory.  See <a href="Obstack-Chunks.html#Obstack-Chunks">Obstack Chunks</a>, for full details.

   <p>At run time, before the program can use a <code>struct obstack</code> object
as an obstack, it must initialize the obstack by calling
<code>obstack_init</code>.

<!-- obstack.h -->
<!-- GNU -->
<div class="defun">
&mdash; Function: int <b>obstack_init</b> (<var>struct obstack *obstack-ptr</var>)<var><a name="index-obstack_005finit-305"></a></var><br>
<blockquote><p>Initialize obstack <var>obstack-ptr</var> for allocation of objects.  This
function calls the obstack's <code>obstack_chunk_alloc</code> function.  If
allocation of memory fails, the function pointed to by
<code>obstack_alloc_failed_handler</code> is called.  The <code>obstack_init</code>
function always returns 1 (Compatibility notice: Former versions of
obstack returned 0 if allocation failed). 
</p></blockquote></div>

   <p>Here are two examples of how to allocate the space for an obstack and
initialize it.  First, an obstack that is a static variable:

<pre class="smallexample">     static struct obstack myobstack;
     ...
     obstack_init (&amp;myobstack);
</pre>
   <p class="noindent">Second, an obstack that is itself dynamically allocated:

<pre class="smallexample">     struct obstack *myobstack_ptr
       = (struct obstack *) xmalloc (sizeof (struct obstack));
     
     obstack_init (myobstack_ptr);
</pre>
   <!-- obstack.h -->
<!-- GNU -->
<div class="defun">
&mdash; Variable: <b>obstack_alloc_failed_handler</b><var><a name="index-obstack_005falloc_005ffailed_005fhandler-306"></a></var><br>
<blockquote><p>The value of this variable is a pointer to a function that
<code>obstack</code> uses when <code>obstack_chunk_alloc</code> fails to allocate
memory.  The default action is to print a message and abort. 
You should supply a function that either calls <code>exit</code>
(see <a href="Program-Termination.html#Program-Termination">Program Termination</a>) or <code>longjmp</code> (see <a href="Non_002dLocal-Exits.html#Non_002dLocal-Exits">Non-Local Exits</a>) and doesn't return.

     <pre class="smallexample">          void my_obstack_alloc_failed (void)
          ...
          obstack_alloc_failed_handler = &amp;my_obstack_alloc_failed;
</pre>
        </blockquote></div>

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