109 lines
4.6 KiB
HTML
109 lines
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HTML
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<title>Using the GNU Compiler Collection (GCC): Offsetof</title>
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<a name="Offsetof"></a>
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<div class="header">
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<p>
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Next: <a href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins" accesskey="n" rel="next">__sync Builtins</a>, Previous: <a href="Vector-Extensions.html#Vector-Extensions" accesskey="p" rel="prev">Vector Extensions</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Support-for-offsetof"></a>
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<h3 class="section">6.50 Support for <code>offsetof</code></h3>
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<a name="index-_005f_005fbuiltin_005foffsetof"></a>
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<p>GCC implements for both C and C++ a syntactic extension to implement
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the <code>offsetof</code> macro.
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</p>
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<div class="smallexample">
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<pre class="smallexample">primary:
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"__builtin_offsetof" "(" <code>typename</code> "," offsetof_member_designator ")"
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offsetof_member_designator:
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<code>identifier</code>
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| offsetof_member_designator "." <code>identifier</code>
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| offsetof_member_designator "[" <code>expr</code> "]"
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</pre></div>
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<p>This extension is sufficient such that
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define offsetof(<var>type</var>, <var>member</var>) __builtin_offsetof (<var>type</var>, <var>member</var>)
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</pre></div>
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<p>is a suitable definition of the <code>offsetof</code> macro. In C++, <var>type</var>
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may be dependent. In either case, <var>member</var> may consist of a single
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identifier, or a sequence of member accesses and array references.
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</p>
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