struct {
      unsigned char x1 : 2;
      unsigned char x2 : 2;
      unsigned char x3 : 2;
      unsigned char x4 : 2;
} Bunch;

/* sizeof(Bunch) => 1 */

struct {
      unsigned char x1: 2;
      unsigned char x2: 2;
      unsigned char : 0; /* 下一个位域在内存分配边界对齐 */
      unsigned char x4: 2;
} Bunch;
/* sizeof(Bunch) => 2 */