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| <h4 class="subsection">6.54.8 Structure-Packing Pragmas</h4> |
| |
| <p>For compatibility with Microsoft Windows compilers, GCC supports a |
| set of <code>#pragma</code> directives which change the maximum alignment of |
| members of structures (other than zero-width bitfields), 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. |
| |
| <ol type=1 start=1> |
| <li><code>#pragma pack(</code><var>n</var><code>)</code> simply sets the new alignment. |
| <li><code>#pragma pack()</code> sets the alignment to the one that was in |
| effect when compilation started (see also command line option |
| <samp><span class="option">-fpack-struct[=<n>]</span></samp> see <a href="Code-Gen-Options.html#Code-Gen-Options">Code Gen Options</a>). |
| <li><code>#pragma pack(push[,</code><var>n</var><code>])</code> pushes the current alignment |
| setting on an internal stack and then optionally sets the new alignment. |
| <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([</code><var>n</var><code>])</code> does not influence this internal |
| stack; thus it is possible to have <code>#pragma pack(push)</code> followed by |
| multiple <code>#pragma pack(</code><var>n</var><code>)</code> instances and finalized by a single |
| <code>#pragma pack(pop)</code>. |
| </ol> |
| |
| <p>Some targets, e.g. i386 and powerpc, support the <code>ms_struct</code> |
| <code>#pragma</code> which lays out a structure as the documented |
| <code>__attribute__ ((ms_struct))</code>. |
| <ol type=1 start=1> |
| <li><code>#pragma ms_struct on</code> turns on the layout for structures |
| declared. |
| <li><code>#pragma ms_struct off</code> turns off the layout for structures |
| declared. |
| <li><code>#pragma ms_struct reset</code> goes back to the default layout. |
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