test.c

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

struct Foo {
  int a;
  int arr[];
};

struct Foo*
f(int n) {
  struct Foo *a = calloc(sizeof *a, n * sizeof (int));

  a->a = 42;
  a->arr[n] = 666;

  a[1].arr[n+2] = 420;
  a[n].arr[n] = 420666;

  return a;
}

int
main() {
  struct Foo *test = f(12);
  printf("%d\n", test[12].arr[12]);
  free(test);
  return 0;
}