hi, Srinivasan

(i)In c offsetof() macro is defined in <stddef.h>. With the help of it you can compute the offset field in structure struct s as offsetof(type,field).

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

e.g.

struct st { int a, b; }; 
offsetof(struct st, b);

(char *) casting arranges that the offset so computed is a byte offset.

(ii)Maintain the offsets field computed by the offsetof() macro.

*(int *)((char *)stp + offsetf) = value;

in the above the value of f can be set indirectly, here stp is the pointer to an instance of the srurcture and the field is an int which has the offset offsetf.