Structure padding and packing

Question

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?

Answer 1

Structure packing is only done when you tell your compiler explicitly to pack the structure. Padding is what you're seeing. Your 32-bit system is padding each field to word alignment. If you had told your compiler to pack the structures, they'd be 6 and 5 bytes, respectively. Don't do that though. It's not portable and makes compilers generate much slower (and sometimes even buggy) code.