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79 lines
3.4 KiB
HTML
79 lines
3.4 KiB
HTML
4 years ago
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<title>Offsetof - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Vector-Extensions.html#Vector-Extensions" title="Vector Extensions">
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<link rel="next" href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins" title="__sync Builtins">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Offsetof"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins">__sync Builtins</a>,
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Previous: <a rel="previous" accesskey="p" href="Vector-Extensions.html#Vector-Extensions">Vector Extensions</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.49 Support for <code>offsetof</code></h3>
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<p><a name="index-g_t_005f_005fbuiltin_005foffsetof-3518"></a>
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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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<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>
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<p>This extension is sufficient such that
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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>
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<p class="noindent">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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