GCC 2.96 shows bogus memory allocation when allocating some given structs?

[Claudio Lavecchia]

Hello GCC ppl,

I am trying to compile a really simple test program on my redhat using 
gcc 2.96.

//----------------BEGIN TEST PROGRAM----------------
#include <stdio.h>

struct S1
{
    char        a[6];  
};

struct S2
{
    char        a[6];  
    int            b;
};

struct S3
{
    char        c[6];
    char        d[6];     
};

int main(int argc, char *argv[])
{   
    struct S2 s2;

    printf ("SIZE int = %d\n",sizeof(int));
    printf ("SIZE S1 = %d\n",sizeof(struct S1));
    printf ("SIZE S2 = %d\n",sizeof(struct S2));
    printf ("SIZE S3 = %d\n",sizeof(struct S3));
    printf ("SIZE s2 = %d\n",sizeof(s2));
    printf ("SIZE s2.a = %d\n",sizeof(s2.a));
    printf ("SIZE s2.b = %d\n",sizeof(s2.b));
}
//----------------END TEST PROGRAM----------------

The output is what follows:

SIZE int = 4
SIZE S1 = 6
SIZE S2 = 12
SIZE S3 = 12
SIZE s2 = 12
SIZE s2.a = 6
SIZE s2.b = 4

As you can see, size of structure S2 is just pointless..... please can 
anyone tell me what is going on?

Claudio