#include <stddef.h>

int offsetof (
  structure,      /* structure to use */
  member);        /* member to get offset for */

The offsetof macro calculates the offset of the member structure element from the beginning of the structure. The structure argument must specify the name of a structure. The member argument must specify the name of a member of the structure.

The offsetof macro returns the offset, in bytes, of the member element from the beginning of struct structure.

#include <stddef.h>

struct index_st
  {
  unsigned char type;
  unsigned long num;
  unsigned ing len;
  };

typedef struct index_st index_t;

void main (void) {
  int x, y;

  x = offsetof (struct index_st, len);  /* x = 5 */
  y = offsetof (index_t, num);          /* y = 1 */
}