c - Typecasting structure pointers -

hie guys. studying structures , pointers using book called pointers in c: hands on approach , on page 107, came across incomplete example of struct type casting. tried make work implementing function receivedata() ,adding headers , making few changes.
complete code:

#include <stdio.h> #include <string.h> #include <stdlib.h>  struct signature {     char  sign;     char version; };  struct id {     char id;     char platform; };  struct data {     struct id idv;     struct signature sig;     char data[100]; };  static void receivedata(struct data *d); static struct signature *extractsignature(struct data *d); static struct id *extractid(struct data *d);  int main(int argc, char *argv[]) { /* actual line in book : *  struct data *img; no memory allocation or assignment null. *  had errors allocated memory on heap before passing value  *  receivedata(); */     struct data *img = malloc(sizeof(struct data));      receivedata(img);  /* actual line in book :  *   struct id *idval = extractid(&img);  *   struct signature *sig = extractsignature(&img);  * erroneous because &img pointer   * pointer (struct data**) not data type   * extract functions' argument */     struct id *idval = extractid(img);     struct signature *sig = extractsignature(img);      printf("for signature:\n");     printf("sign = %c", sig->sign);     printf(" version = %c\n\n", sig->version);      printf("for id:\n");     printf("id = %c", idval->id);     printf(" platform = %c", idval->platform);      printf("\ndata = %s", img->data);      return 0; }  static struct signature *extractsignature(struct data *d) {     struct signature *sig = (struct signature *)d;     return sig; }  static struct id *extractid(struct data *d) {     struct id *idv = (struct id *)d;     return idv; }  static void receivedata(struct data *d) {     struct data *dptr = d;     char *ch = "coding cool!";      dptr->sig.sign = 's';     dptr->sig.version = '1';      dptr->idv.id = 'i';     dptr->idv.platform = 'p';      strncpy(dptr->data, ch, strlen(ch));      return; } 

what understand casting inside function extractid() , extraxtsignature() because seems casting struct data * retrieve individual members. ran program , value of first member of struct data both sig member , id members. here output when struct id comes first in struct data:

for signature: sign = version = p  id: id = platform = p data = coding cool! 

and when struct signature first element:

for signature: sign = s version = 1  id: id = s platform = 1 data = coding cool! 

and finally, when char data[100] first element:

for signature: sign = c version = o  id: id = c platform = o data = coding cool! 

may please explain type of casting, when use , best practices. thank much.

struct data {     struct id idv;     struct signature sig;     char data[100]; }; 

for above definition, extractid function below valid , right results.

static struct id *extractid(struct data *d) {     struct id *idv = (struct id *)d;     return idv; } 

this because, struct id first member of struct data. so, when have struct data *d, d points starting address of struct data , same starting address of first member struct id.

so, if casting first member of struct, right per code have provided above.