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<h3 class="section">6.49 Support for <code>offsetof</code></h3>
<p><a name="index-g_t_005f_005fbuiltin_005foffsetof-3518"></a>
GCC implements for both C and C++ a syntactic extension to implement
the <code>offsetof</code> macro.
<pre class="smallexample"> primary:
"__builtin_offsetof" "(" <code>typename</code> "," offsetof_member_designator ")"
offsetof_member_designator:
<code>identifier</code>
| offsetof_member_designator "." <code>identifier</code>
| offsetof_member_designator "[" <code>expr</code> "]"
</pre>
<p>This extension is sufficient such that
<pre class="smallexample"> #define offsetof(<var>type</var>, <var>member</var>) __builtin_offsetof (<var>type</var>, <var>member</var>)
</pre>
<p class="noindent">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.
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