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<title>Using the GNU Compiler Collection (GCC): Structure-Layout Pragmas</title>
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<a name="Structure_002dLayout-Pragmas"></a>
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<p>
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Next: <a href="Weak-Pragmas.html#Weak-Pragmas" accesskey="n" rel="next">Weak Pragmas</a>, Previous: <a href="Symbol_002dRenaming-Pragmas.html#Symbol_002dRenaming-Pragmas" accesskey="p" rel="prev">Symbol-Renaming Pragmas</a>, Up: <a href="Pragmas.html#Pragmas" accesskey="u" rel="up">Pragmas</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="Structure_002dLayout-Pragmas-1"></a>
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<h4 class="subsection">6.61.10 Structure-Layout Pragmas</h4>
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<p>For compatibility with Microsoft Windows compilers, GCC supports a
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set of <code>#pragma</code> directives that change the maximum alignment of
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members of structures (other than zero-width bit-fields), unions, and
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classes subsequently defined. The <var>n</var> value below always is required
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to be a small power of two and specifies the new alignment in bytes.
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</p>
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<ol>
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<li> <code>#pragma pack(<var>n</var>)</code> simply sets the new alignment.
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</li><li> <code>#pragma pack()</code> sets the alignment to the one that was in
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effect when compilation started (see also command-line option
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<samp>-fpack-struct[=<var>n</var>]</samp> see <a href="Code-Gen-Options.html#Code-Gen-Options">Code Gen Options</a>).
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</li><li> <code>#pragma pack(push[,<var>n</var>])</code> pushes the current alignment
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setting on an internal stack and then optionally sets the new alignment.
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</li><li> <code>#pragma pack(pop)</code> restores the alignment setting to the one
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saved at the top of the internal stack (and removes that stack entry).
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Note that <code>#pragma pack([<var>n</var>])</code> does not influence this internal
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stack; thus it is possible to have <code>#pragma pack(push)</code> followed by
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multiple <code>#pragma pack(<var>n</var>)</code> instances and finalized by a single
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<code>#pragma pack(pop)</code>.
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</li></ol>
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<p>Some targets, e.g. x86 and PowerPC, support the <code>#pragma ms_struct</code>
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directive which lays out structures and unions subsequently defined as the
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documented <code>__attribute__ ((ms_struct))</code>.
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</p>
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<ol>
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<li> <code>#pragma ms_struct on</code> turns on the Microsoft layout.
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</li><li> <code>#pragma ms_struct off</code> turns off the Microsoft layout.
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</li><li> <code>#pragma ms_struct reset</code> goes back to the default layout.
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</li></ol>
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<p>Most targets also support the <code>#pragma scalar_storage_order</code> directive
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which lays out structures and unions subsequently defined as the documented
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<code>__attribute__ ((scalar_storage_order))</code>.
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</p>
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<ol>
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<li> <code>#pragma scalar_storage_order big-endian</code> sets the storage order
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of the scalar fields to big-endian.
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</li><li> <code>#pragma scalar_storage_order little-endian</code> sets the storage order
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of the scalar fields to little-endian.
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</li><li> <code>#pragma scalar_storage_order default</code> goes back to the endianness
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that was in effect when compilation started (see also command-line option
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<samp>-fsso-struct=<var>endianness</var></samp> see <a href="C-Dialect-Options.html#C-Dialect-Options">C Dialect Options</a>).
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</li></ol>
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<p>
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Next: <a href="Weak-Pragmas.html#Weak-Pragmas" accesskey="n" rel="next">Weak Pragmas</a>, Previous: <a href="Symbol_002dRenaming-Pragmas.html#Symbol_002dRenaming-Pragmas" accesskey="p" rel="prev">Symbol-Renaming Pragmas</a>, Up: <a href="Pragmas.html#Pragmas" accesskey="u" rel="up">Pragmas</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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