Padding aligns structure members to “natural” address boundaries – say, int
members would have offsets, which are mod(4) == 0
on 32-bit platform. Padding is on by default. It inserts the following “gaps” into your first structure:
struct mystruct_A { char a; char gap_0[3]; /* inserted by compiler: for alignment of b */ int b; char c; char gap_1[3]; /* -"-: for alignment of the whole struct in an array */ } x;
Packing, on the other hand prevents compiler from doing padding – this has to be explicitly requested – under GCC it’s __attribute__((__packed__))
, so the following:
struct __attribute__((__packed__)) mystruct_A { char a; int b; char c; };
would produce structure of size 6
on a 32-bit architecture.
A note though – unaligned memory access is slower on architectures that allow it (like x86 and amd64), and is explicitly prohibited on strict alignment architectures like SPARC.