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 subjects 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).
Some targets, e.g. i386 and powerpc, support the ms_struct
#pragma which lays out a structure as the documented
__attribute__ ((ms_struct)).
#pragma ms_struct on turns on the layout for structures
declared.
#pragma ms_struct off turns off the layout for structures
declared.
#pragma ms_struct reset goes back to the default layout.