Memory alignment in C-structs

At least on most machines, a type is only ever aligned to a boundary as large as the type itself [Edit: you can't really demand any "more" alignment than that, because you have to be able to create arrays, and you can't insert padding into an array]. On your implementation, short is apparently 2 bytes, and int 4 bytes.

That means your first struct is aligned to a 2-byte boundary. Since all the members are 2 bytes apiece, no padding is inserted between them.

The second contains a 4-byte item, which gets aligned to a 4-byte boundary. Since it's preceded by 6 bytes, 2 bytes of padding is inserted between v3 and i, giving 6 bytes of data in the shorts, two bytes of padding, and 4 more bytes of data in the int for a total of 12.

Forget about having different members, even if you write two structs whose members are exactly same, with a difference is that the order in which they're declared is different, then size of each struct can be (and often is) different.

For example, see this,

#include <iostream>
using namespace std;
struct A
{
   char c;
   char d;
   int i; 
};
struct B
{
   char c;
   int i;   //note the order is different!
   char d;
};
int main() {
        cout << sizeof(A) << endl;
        cout << sizeof(B) << endl;
}

Compile it with gcc-4.3.4, and you get this output:

8
12

That is, sizes are different even though both structs has same members!

Code at Ideone : http://ideone.com/HGGVl

The bottomline is that the Standard doesn't talk about how padding should be done, and so the compilers are free to make any decision and you cannot assume all compilers make the same decision.