hoge.c

#include <stdio.h>

struct Hoge {
  int num;
};
struct Aaaaa {
  struct Hoge hoge;
  int n;
};

int main() {
  struct Hoge h;
  h.num = 10;
  struct Aaaaa a;
  a.hoge = h;
  struct Aaaaa* ap = &a;
  struct Hoge* hop = (struct Hoge*)ap;
  printf("num=%d\n", hop->num);
  return 0;
}