MVC 中奇怪的 sizeof 结果（对齐？）

Question

下面的代码给出了结果 sizeof(t1)=16 sizeof(t2)=16 我本来期望 sizeof(t2)=12 因为 sizeof(fpos_t)=8 和 sizeof(int)=4。有人可以解释一下吗？

int main()
{
    typedef struct {
        fpos_t fpos;
        char* s;
        int a;
    } t1;
    typedef struct {
        fpos_t fpos;
        int a;
    } t2;
    t1 it1;
    t2 it2;
    printf("sizeof(t1)=%d sizeof(t2)=%d ", sizeof(t1), sizeof(t2));
    return 0;
}

Answer 1

对于alignment reasons，编译器可以自由插入填充。这意味着结构的大小不一定等于各个成员的大小之和。这是 C 标准明确允许的。

结构开头唯一不允许填充的地方，即第一个成员之前。

来自C11 draft, 6.7.2.1：

Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.

（强调我的）。

Answer 2

现在大多数计算机一次只允许分配 "words"（又名 8 字节，又名 64 位）。这叫做填充。

把它想象成酒店房间。无论您是 1 人还是 2 人 (chars) 入住同一个房间 (memory location)，房间的大小都是一样的。

MVC 中奇怪的 sizeof 结果（对齐？）

strange sizeof result in MVC (alignment?)

c

sizeof

alignment