For compatibility with Win32, GCC supports a set of #pragma
directives which change the maximum alignment of members of structures,
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.
#pragma pack(n) simply sets the new alignment.
#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).
#pragma pack(push[,n]) pushes the current alignment
setting on an internal stack and then optionally sets the new alignment.
#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).