C: 是否使用转换为 "outer"结构类型的指针访问嵌套结构的初始成员？

Question

<分区>

Answer 1

这是不安全的，因为您正试图访问类型为 struct extended_base 的对象，就好像它是类型为 struct base 的对象一样>.

但是，有些规则允许通过 union 访问两个结构的初始公共(public)序列。来自 C standard 的第 6.5.2.3p6 节:

One special guarantee is made in order to simplify the use of unions: if a union contains several structures that share a common initial sequence (see below), and if the union object currently contains one of these structures, it is permitted to inspect the common initial part of any of them anywhere that a declaration of the completed type of the union is visible. Two structures share a common initial sequence if corresponding members have compatible types (and, for bit-fields, the same widths) for a sequence of one or more initial members

因此，如果您将 struct extended_socket_object 的定义更改为:

struct extended_socket_object {
    union u_base {
        struct base b_attr;
        struct extended_base e_attr;
    };
    int e_id;
    int e_type;
    int e_status;
    int some_other_field;
};

然后 struct extended_socket_object * 可以转换为 union u_base *，后者又可以转换为 struct base *。根据第 6.7.2.1 p15 和 p16 节，这是允许的:

15 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.

16 The size of a union is sufficient to contain the largest of its members. The value of at most one of the members can be stored in a union object at any time. A pointer to a union object, suitably converted, points to each of its members (or if a member is a bit-field, then to the unit in which it resides), and vice versa.

然后允许访问 b_attr->flags，因为它位于 6.5.2.3p6 中的 union 。