C - 释放 'inherited' 结构

Question

在下面的代码中，释放Dairy也会释放Yogurt吗？
据我所知，两者都指向同一个地址。

此外，这种编码风格是否是一种不好的做法？假设我只保留指向 Dairy 的指针并间接释放 Yogurt 和 Cheese？

#include <stdlib.h>

typedef struct {
    int calcium;
    int protein;
} Dairy;

typedef struct {
    Dairy dairy;
    int sugar;
    int color;
} Yogurt;

int main () {
    Yogurt* yogurt = malloc(sizeof(Yogurt));

    Dairy* dairy = &yogurt->dairy;

    free(dairy); // Will this free yogurt?
}

Answer 1

是的，行为定义明确。

根据 C 标准，您需要将完全相同的地址传递给释放函数 free正如 malloc 返回的那样或其他分配函数。

引用:

7.22.3.3 The free function

Synopsis:

§1 #include <stdlib.h> void free(void *ptr);

Description:

§2 The free function causes the space pointed to by ptr to be deallocated, that is, madeavailable for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if the argument does not match a pointer earlier returned by a memory managementfunction, or if the space has been deallocated by a call to free or realloc, thebehavior is undefined.

因此，在这种情况下，当且仅当指向结构的指针与指向它的第一个成员的指针相同时，才能保证行为被明确定义。
这由以下标准保证:

6.7.2.1 结构和 union 说明符
§15

Within a structure object, the non-bit-field members and the units in which bit-fieldsreside 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 unnamedpadding within a structure object, but not at its beginning.