offsetofを試した

offsetoftry.c

#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct address 
{
   char name[50];
   char street[50];
   int phone;
};
   
int main()
{
   struct address *hoge = malloc(sizeof(struct address));
   strcpy(hoge->name, "taro");
   strcpy(hoge->street, "hokkaido");
   hoge->phone = 123456;
   printf("name        = %s\n", hoge->name);
   printf("street      = %s\n", hoge->street);
   printf("phone       = %d\n", hoge->phone);

   printf("-------\n");
   
   printf("hoge addr    = %p\n", hoge);
   printf("name addr    = %p\n", hoge->name);
   printf("street addr  = %p\n", hoge->street);
   printf("phone adrr   = %p\n", &hoge->phone);

   printf("-------\n");
   size_t offset = offsetof(struct address, phone);
   printf("phone offset = %d\n", offset);
   printf("phone offset = 0x%x\n", offset);

   printf("\n\n");
   int *phone_addr = &hoge->phone;
   struct address *t = (struct address *)((char *)phone_addr - offset);
   printf("struct address *t = (struct address *)((char *)phone_addr - offset);\n");
   printf("result:    t = %p\n", t);
   printf("t->name      = %s\n", t->name);
   
   return(0);
}