关于本章

本章讨论struct alignment，其实它最终的落脚点是memory access。

gcc 6.58.8 Structure-Packing Pragmas

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. i386 and PowerPC, support the ms_struct #pragma which lays out a structure as the documented __attribute__ ((ms_struct)).

gcc 6.35.1 Common Type Attributes

aligned

aligned (alignment)

struct __attribute__ ((aligned (8))) S { short f[3]; };
typedef int more_aligned_int __attribute__ ((aligned (8)));

TODO

songho Data Alignment

http://www.catb.org/esr/structure-packing/

https://software.intel.com/en-us/articles/coding-for-performance-data-alignment-and-structures