如何计算C中两个不同结构成员之间的偏移量？

Question

Consider the following C code:

struct Foo
{
  short a;
  long b;
  char c;
  void* d;
};

I know it is possible to know the size of the entire structure using sizeof. Is it possible to know the size of a subset of this structure, or (stated differently) know the distance between two members? I realize could nest structures based on the offsets I want (then it is possible to use sizeof), but say I wanted to know the number of bytes between a and d? Also, I'd like to do this without instantiating an object (something like sizeof). (I realize if I have a object, then I could look at the address differences between foo.d and foo.a). Is this possible?

Answer 1

offsetof(Foo, d) - offsetof(Foo, a)

offsetof gives you the offset from the beginning of Foo to the member specified.

You'll need #include <stddef.h>