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<h5 class="subsubsection">3.2.4.9 Alignment of Data in Obstacks</h5>

<p><a name="index-alignment-_0028in-obstacks_0029-336"></a>
Each obstack has an <dfn>alignment boundary</dfn>; each object allocated in
the obstack automatically starts on an address that is a multiple of the
specified boundary.  By default, this boundary is aligned so that
the object can hold any type of data.

   <p>To access an obstack's alignment boundary, use the macro
<code>obstack_alignment_mask</code>, whose function prototype looks like
this:

<!-- obstack.h -->
<!-- GNU -->
<div class="defun">
&mdash; Macro: int <b>obstack_alignment_mask</b> (<var>struct obstack *obstack-ptr</var>)<var><a name="index-obstack_005falignment_005fmask-337"></a></var><br>
<blockquote><p>The value is a bit mask; a bit that is 1 indicates that the corresponding
bit in the address of an object should be 0.  The mask value should be one
less than a power of 2; the effect is that all object addresses are
multiples of that power of 2.  The default value of the mask is a value
that allows aligned objects to hold any type of data: for example, if
its value is 3, any type of data can be stored at locations whose
addresses are multiples of 4.  A mask value of 0 means an object can start
on any multiple of 1 (that is, no alignment is required).

        <p>The expansion of the macro <code>obstack_alignment_mask</code> is an lvalue,
so you can alter the mask by assignment.  For example, this statement:

     <pre class="smallexample">          obstack_alignment_mask (obstack_ptr) = 0;
</pre>
        <p class="noindent">has the effect of turning off alignment processing in the specified obstack. 
</p></blockquote></div>

   <p>Note that a change in alignment mask does not take effect until
<em>after</em> the next time an object is allocated or finished in the
obstack.  If you are not growing an object, you can make the new
alignment mask take effect immediately by calling <code>obstack_finish</code>. 
This will finish a zero-length object and then do proper alignment for
the next object.

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