如何理解结构填充?

我学习过 The Lost Art of Structure Packing ,但是下面的实验把我弄糊涂了。

这是代码:

#include <stdio.h>

int main(int argc, char **argv)
{
    struct a_t {
        char aa;
    };

    struct b_t {
        int bb;
        int bbb;
    };

    struct c_t {
        struct a_t  parent_a;
        struct b_t  parent_b;
        char cc;
    };

    struct c_t c;
    printf("sizeof(a_t)=%d\n", sizeof(struct a_t));
    printf("sizeof(b_t)=%d\n", sizeof(struct b_t));
    printf("c address=%p\n", &c);
    printf("c's parent_a=%p\n", &(c.parent_a));
    printf("c's parent_b=%p\n", &(c.parent_b));
    printf("c + sizeof(a_t)=%p\n", (struct b_t *)(&c + sizeof(struct a_t)));

    return 0;
}

输出如下:

sizeof(a_t)=1
sizeof(b_t)=8
c address=0x7fff9bca7680
c's parent_a=0x7fff9bca7680
c's parent_b=0x7fff9bca7684
c + sizeof(a_t)=0x7fff9bca7690

输出中的最后3行令人惊讶:

1. c's parent_a=0x7fff9bca7680 ,这是意料之中的事。parent_a地址等于结构变量c的地址。
2. c's parent_b=0x7fff9bca7684 ,这是一个惊喜,parent_b的大小是8字节,为什么它的地址不是8的倍数?
3. c + sizeof(a_t)=0x7fff9bca7690 ,这是一个惊喜,这超出了c的地址16个字节! struct a_t 大小为1字节,我原以为它是0x7fff9bca7681。

你能发光吗?