Using the GNU Compiler Collection (GCC): Structure-Layout Pragmas

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6.62.10 Structure-Layout Pragmas

For compatibility with Microsoft Windows compilers, GCC supports a set of #pragma directives that change the maximum alignment of members of structures (other than zero-width bit-fields), unions, and classes subsequently defined. The n value below always is required to be a small power of two and specifies the new alignment in bytes.

1. #pragma pack(n) simply sets the new alignment.
2. #pragma pack() sets the alignment to the one that was in effect when compilation started (see also command-line option -fpack-struct[=n] see Code Gen Options).
3. #pragma pack(push[,n]) pushes the current alignment setting on an internal stack and then optionally sets the new alignment.
4. #pragma pack(pop) restores the alignment setting to the one saved at the top of the internal stack (and removes that stack entry). Note that #pragma pack([n]) does not influence this internal stack; thus it is possible to have #pragma pack(push) followed by multiple #pragma pack(n) instances and finalized by a single #pragma pack(pop).

Some targets, e.g. x86 and PowerPC, support the #pragma ms_struct directive which lays out structures and unions subsequently defined as the documented __attribute__ ((ms_struct)).

1. #pragma ms_struct on turns on the Microsoft layout.
2. #pragma ms_struct off turns off the Microsoft layout.
3. #pragma ms_struct reset goes back to the default layout.

Most targets also support the #pragma scalar_storage_order directive which lays out structures and unions subsequently defined as the documented __attribute__ ((scalar_storage_order)).

1. #pragma scalar_storage_order big-endian sets the storage order of the scalar fields to big-endian.
2. #pragma scalar_storage_order little-endian sets the storage order of the scalar fields to little-endian.
3. #pragma scalar_storage_order default goes back to the endianness that was in effect when compilation started (see also command-line option -fsso-struct=endianness see C Dialect Options).

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