Thread: Understanding offsetof(struct ,member);

Code:

#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);
}

Output : offsets: i=0; c=4; d=8 a=16sizeof(struct s)=16
My Hypothesis :
Integer is 4 bytes ..
its first index so i=0;
Character is 1 byte ..
but d=8 it should be 5.Can someone explain me how offset works..