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<a name="Structure_002dLayout-Pragmas-1"></a>
<h4 class="subsection">6.62.11 Structure-Layout Pragmas</h4>

<p>For compatibility with Microsoft Windows compilers, GCC supports a
set of <code>#pragma</code> directives that change the maximum alignment of
members of structures (other than zero-width bit-fields), unions, and
classes subsequently defined. The <var>n</var> value below always is required
to be a small power of two and specifies the new alignment in bytes.
</p>
<ol>
<li> <code>#pragma pack(<var>n</var>)</code> simply sets the new alignment.
</li><li> <code>#pragma pack()</code> sets the alignment to the one that was in
effect when compilation started (see also command-line option
<samp>-fpack-struct[=<var>n</var>]</samp> see <a href="Code-Gen-Options.html#Code-Gen-Options">Code Gen Options</a>).
</li><li> <code>#pragma pack(push[,<var>n</var>])</code> pushes the current alignment
setting on an internal stack and then optionally sets the new alignment.
</li><li> <code>#pragma pack(pop)</code> restores the alignment setting to the one
saved at the top of the internal stack (and removes that stack entry).
Note that <code>#pragma pack([<var>n</var>])</code> does not influence this internal
stack; thus it is possible to have <code>#pragma pack(push)</code> followed by
multiple <code>#pragma pack(<var>n</var>)</code> instances and finalized by a single
<code>#pragma pack(pop)</code>.
</li></ol>

<p>Some targets, e.g. x86 and PowerPC, support the <code>#pragma ms_struct</code>
directive which lays out structures and unions subsequently defined as the
documented <code>__attribute__ ((ms_struct))</code>.
</p>
<ol>
<li> <code>#pragma ms_struct on</code> turns on the Microsoft layout.
</li><li> <code>#pragma ms_struct off</code> turns off the Microsoft layout.
</li><li> <code>#pragma ms_struct reset</code> goes back to the default layout.
</li></ol>

<p>Most targets also support the <code>#pragma scalar_storage_order</code> directive
which lays out structures and unions subsequently defined as the documented
<code>__attribute__ ((scalar_storage_order))</code>.
</p>
<ol>
<li> <code>#pragma scalar_storage_order big-endian</code> sets the storage order
of the scalar fields to big-endian.
</li><li> <code>#pragma scalar_storage_order little-endian</code> sets the storage order
of the scalar fields to little-endian.
</li><li> <code>#pragma scalar_storage_order default</code> goes back to the endianness
that was in effect when compilation started (see also command-line option
<samp>-fsso-struct=<var>endianness</var></samp> see <a href="C-Dialect-Options.html#C-Dialect-Options">C Dialect Options</a>).
</li></ol>

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