c - 第一个元素偏移量

Question

是否有保证，结构的第一个元素的偏移量为 0？为了更准确，让我们考虑一下

struct foo {
int a;
double b;
};
struct foo *ptr=malloc(sizeof(struct foo));
int *int_ptr = &ptr->a;
free(int_ptr)

是否保证它在任何操作系统或任何其他因素下始终有效？

Answer 1

是的，这是有保证的。会给你一个标准报价，让我查一下。

C99 标准:§6.7.2.1

第 12 段

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.