Offsetof在Linux中使用 [英] Offsetof used in Linux

问题描述

我正在经历一个特定变量的偏差如何在给定结构中发现。

I was going through how offset of a particular variable is found in a given structure.

我尝试下面的程序。

struct info{
char a;
int b;
char c;
int d;
};

struct info myinfo;

int main(int argc, char **argv)
{
struct info *ptr = &myinfo;
unsigned int offset;

offset = (unsigned int) &((struct info *) 0)->d;
printf("Offset = %d\n",offset);

return 0;
}

我只是想知道如何偏移量=（无符号整数）行及（（结构信息*）0） - D 1和D 的作品。
我因为0提领的困惑

I just wanted to know how the line offset = (unsigned int) &((struct info *) 0)->d works. I am confused because of dereferencing of 0.

推荐答案

它没有的真正的间接引用 0 ，虽然它看起来像它。这确实需要一些成员的地址，如果在地址解引用 0 ，假设

It does not really dereference 0, although it looks like it. It really takes the address of some member if it was dereferenced at address 0, hypothetically.

这是一种肮脏的黑客（另外，一些讨厌的东西宏），但它可以让你什么你感兴趣的（在结构中的偏移量成员）。

This is a kind of dirty hack (plus, some nasty macro stuff), but it gets you what you're interested in (the offset of the member in the struct).

做同样的事情会产生一个有效的对象，以它的地址，并采取成员的地址，然后减去这些的更正确的方式。做同样的一个空指针是不是所有的pretty，但没有创建一个对象，并减去任何工作。

A more "correct" way of doing the same thing would be to generate a valid object, take its address, and take the address of the member, then subtract these. Doing the same with a null pointer is not all pretty, but works without creating an object and subtracting anything.

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