请教大神u.k.a,u.k.b这两个值是怎么计算的............

#include <stdio.h>
void main()
{
struct data
{
char a ;
char b ;
};
union
{
int c ;
struct data k;
}u;
u.k.a=0x56;
u.k.b=0x78;
u.c=0x1234;
printf("%d,%ox,%x\n",sizeof(u),u.k.a,u.k.b);
}