Consider:
struct mystruct_A
{
char a;
int b;
char c;
} x;
struct mystruct_B
{
int b;
char a;
} y;
The sizes of the structures are 12 and 8 respectively.
Are these structures padded or packed?
When does padding or packing take place?
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.