Structure offset

[puneetsardana88]

Hi

I want to find offset of structure elements

struct
{
  char a;
  int b;
  char c;
}example;

Code (which i found on books and internet) to find offset is

unsigned int offset;
offset = (unsigned int)(&(((example *)(0))->b));

I am not able to understand the above code. What is significance of 0 here? The above code is also not portable also. I do not wish to use offsetof right now. Please help me in solution and explanation as well.

[Banfa]

In this case 0 is being used as a pointer value ((example *)(0)) casts 0 to a pointer to example and then uses that to calculate the offset of b with-in example.

In what way do you mean this code isn't portable?

[puneetsardana88]

I have got the above from Internet. The above is also giving error. Can you elaborate more on this with detailed explanation?

[donbock]

Refer to offsetof in stddef.h. This is the portable way to find the offset of a structure member.

The code you quoted in the original post is a common way that offsetof is implemented.