5.49.7 Structure-Packing Pragmas

For compatibility with Win32, GCC supports a set of #pragma directives which change the maximum alignment of members of structures (other than zero-width bitfields), 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).