15.3 Field Offset

To determine the offset of a given field field in a structure type type, use the macro offsetof, which is defined in the file stddef.h. It is used like this:

offsetof (type, field)

Here is an example:

struct foo
{
  int element;
  struct foo *next;
};

offsetof (struct foo, next)
/* On most machines that is 4.  It may be 8.  */