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<a name="Offsetof"></a>
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<a name="Support-for-offsetof"></a>
<h3 class="section">6.53 Support for <code>offsetof</code></h3>
<a name="index-_005f_005fbuiltin_005foffsetof"></a>

<p>GCC implements for both C and C++ a syntactic extension to implement
the <code>offsetof</code> macro.
</p>
<div class="smallexample">
<pre class="smallexample">primary:
        &quot;__builtin_offsetof&quot; &quot;(&quot; <code>typename</code> &quot;,&quot; offsetof_member_designator &quot;)&quot;

offsetof_member_designator:
          <code>identifier</code>
        | offsetof_member_designator &quot;.&quot; <code>identifier</code>
        | offsetof_member_designator &quot;[&quot; <code>expr</code> &quot;]&quot;
</pre></div>

<p>This extension is sufficient such that
</p>
<div class="smallexample">
<pre class="smallexample">#define offsetof(<var>type</var>, <var>member</var>)  __builtin_offsetof (<var>type</var>, <var>member</var>)
</pre></div>

<p>is a suitable definition of the <code>offsetof</code> macro.  In C++, <var>type</var>
may be dependent.  In either case, <var>member</var> may consist of a single
identifier, or a sequence of member accesses and array references.
</p>


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