Finding offset of a structure element in c

Question

struct a {     struct b     {         int i;         float j;     }x;     struct c     {         int k;           float l;     }y; }z; 

Can anybody explain me how to find the offset of int k so that we can find the address of int i?

Answer 1

Use offsetof() to find the offset from the start of z or from the start of x.

offsetof() - offset of a structure member

SYNOPSIS

   #include <stddef.h>     size_t offsetof(type, member); 

offsetof() returns the offset of the field member from the start of the structure type.

EXAMPLE

   #include <stddef.h>    #include <stdio.h>    #include <stdlib.h>     int    main(void)    {        struct s {            int i;            char c;            double d;            char a[];        };         /* Output is compiler dependent */         printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\n",                (long) offsetof(struct s, i),                (long) offsetof(struct s, c),                (long) offsetof(struct s, d),                (long) offsetof(struct s, a));        printf("sizeof(struct s)=%ld\n", (long) sizeof(struct s));         exit(EXIT_SUCCESS);    } 

You will get the following output on a Linux, if you compile with GCC:

       offsets: i=0; c=4; d=8 a=16        sizeof(struct s)=16