C offsetof 宏是如何工作的? [英] How does the C offsetof macro work?

问题描述

可能重复:
为什么这个 C 代码可以工作?
如何在结构上使用 offsetof()?

我在网上看到过这个 offsetof 宏，但没有说明它的用途.

I read about this offsetof macro on the Internet, but it doesn't explain what it is used for.

#define offsetof(a,b) ((int)(&(((a*)(0))->b)))

它想做什么，使用它有什么好处?

What is it trying to do and what is the advantage of using it?

推荐答案

它没有优势，不应该使用，因为它会调用未定义的行为(并且使用了错误的类型 - int 而不是 <代码>size_t).

It has no advantages and should not be used, since it invokes undefined behavior (and uses the wrong type - int instead of size_t).

C 标准在 stddef.h 中定义了一个 offsetof 宏，该宏实际上适用于结构中需要元素偏移量的情况，例如:

The C standard defines an offsetof macro in stddef.h which actually works, for cases where you need the offset of an element in a structure, such as:

#include <stddef.h>

struct foo {
    int a;
    int b;
    char *c;
};

struct struct_desc {
    const char *name;
    int type;
    size_t off;
};

static const struct struct_desc foo_desc[] = {
    { "a", INT, offsetof(struct foo, a) },
    { "b", INT, offsetof(struct foo, b) },
    { "c", CHARPTR, offsetof(struct foo, c) },
};

这将让您以编程方式按名称填充 struct foo 的字段，例如读取 JSON 文件时.

which would let you programmatically fill the fields of a struct foo by name, e.g. when reading a JSON file.

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