c - 如何在结构上使用 offsetof()？

Question

我想要 mystruct1 中的 offsetof() 参数行。我试过了

offsetof(struct mystruct1, rec.structPtr1.u_line.line) 

还有

offsetof(struct mystruct1, line)  

但两者都不起作用。

union {
    struct mystruct1 structPtr1;
    struct mystruct2 structPtr2;
} rec;

typedef struct mystruct1 {
    union {
        struct {
            short len;
            char buf[2];
        } line;

        struct {
            short len;
        } logo;

    } u_line;
};

Answer 1

offsetof()宏有两个参数。 C99 标准说(在 §7.17 <stddef.h> ):

offsetof(type, member-designator)

which expands to an integer constant expression that has type size_t, the value of which is the offset in bytes, to the structure member (designated by member-designator), from the beginning of its structure (designated by type). The type and member designator shall be such that given

static type t;

then the expression &(t.member-designator) evaluates to an address constant.

所以，你需要写:

offsetof(struct mystruct1, u_line.line);

但是，我们可以观察到答案将为零，因为 mystruct1包含 union作为第一个成员(也是唯一一个)，以及 line它的一部分是并集的一个元素，因此它将位于偏移量 0 处。