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<div class="subsection-level-extent" id="Structure-Layout-Pragmas"> <div class="nav-panel"> <p> Next: <a href="weak-pragmas" accesskey="n" rel="next">Weak Pragmas</a>, Previous: <a href="symbol-renaming-pragmas" accesskey="p" rel="prev">Symbol-Renaming Pragmas</a>, Up: <a href="pragmas" accesskey="u" rel="up">Pragmas Accepted by GCC</a> [<a href="index#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="indices" title="Index" rel="index">Index</a>]</p> </div>  <h1 class="subsection" id="Structure-Layout-Pragmas-1"><span>6.62.10 Structure-Layout Pragmas<a class="copiable-link" href="#Structure-Layout-Pragmas-1"> ¶</a></span></h1> <p>For compatibility with Microsoft Windows compilers, GCC supports a set of <code class="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 class="var">n</var> value below always is required to be a small power of two and specifies the new alignment in bytes. </p> <ol class="enumerate"> <li> <code class="code">#pragma pack(<var class="var">n</var>)</code> simply sets the new alignment. </li>
<li> <code class="code">#pragma pack()</code> sets the alignment to the one that was in effect when compilation started (see also command-line option <samp class="option">-fpack-struct[=<var class="var">n</var>]</samp> see <a class="pxref" href="code-gen-options">Options for Code Generation Conventions</a>). </li>
<li> <code class="code">#pragma pack(push[,<var class="var">n</var>])</code> pushes the current alignment setting on an internal stack and then optionally sets the new alignment. </li>
<li> <code class="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 class="code">#pragma pack([<var class="var">n</var>])</code> does not influence this internal stack; thus it is possible to have <code class="code">#pragma pack(push)</code> followed by multiple <code class="code">#pragma pack(<var class="var">n</var>)</code> instances and finalized by a single <code class="code">#pragma pack(pop)</code>. </li>
</ol> <p>Some targets, e.g. x86 and PowerPC, support the <code class="code">#pragma ms_struct</code> directive which lays out structures and unions subsequently defined as the documented <code class="code">__attribute__ ((ms_struct))</code>. </p> <ol class="enumerate"> <li> <code class="code">#pragma ms_struct on</code> turns on the Microsoft layout. </li>
<li> <code class="code">#pragma ms_struct off</code> turns off the Microsoft layout. </li>
<li> <code class="code">#pragma ms_struct reset</code> goes back to the default layout. </li>
</ol> <p>Most targets also support the <code class="code">#pragma scalar_storage_order</code> directive which lays out structures and unions subsequently defined as the documented <code class="code">__attribute__ ((scalar_storage_order))</code>. </p> <ol class="enumerate"> <li> <code class="code">#pragma scalar_storage_order big-endian</code> sets the storage order of the scalar fields to big-endian. </li>
<li> <code class="code">#pragma scalar_storage_order little-endian</code> sets the storage order of the scalar fields to little-endian. </li>
<li> <code class="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 class="option">-fsso-struct=<var class="var">endianness</var></samp> see <a class="pxref" href="c-dialect-options">Options Controlling C Dialect</a>). </li>
</ol> </div>  <div class="nav-panel"> <p> Next: <a href="weak-pragmas">Weak Pragmas</a>, Previous: <a href="symbol-renaming-pragmas">Symbol-Renaming Pragmas</a>, Up: <a href="pragmas">Pragmas Accepted by GCC</a> [<a href="index#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="indices" title="Index" rel="index">Index</a>]</p> </div><div class="_attribution">
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