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

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 

   size_t offsetof(type, member);

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

EXAMPLE

   #include 
   #include 
   #include 

   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