container.c

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

typedef struct item {
  int id;
} item;

typedef struct container {
  int cur, max;
  item items[0];
} container;

typedef struct bad_container {
  int cur, max;
} bad_container;

container *new_container(int N) {
  container *x = malloc(sizeof(container) + sizeof(item) * N);
  x->cur = 0;
  x->max = N;
  for(int i=0; i<N; i++) {
    x->items[i].id = i;
  }

  return x;
}

bad_container *new_bad_container(int N) {
  bad_container *x = malloc(sizeof(bad_container) + sizeof(item) * N);
  x->cur = 0;
  x->max = N;
  item *items = (item *)(x + 1);
  for(int i=0; i<N; i++) {
    items[i].id = i;
  }

  return x;
}

int main() {
  container *x = new_container(10);
  bad_container *y = new_bad_container(10);
  item *y_items = (item *)(y + 1);

  for(int i=0; i<10; i++) {
    printf("x->items[%d].id = %d\n", i, x->items[i].id);
    printf("y_items[%d].id = %d\n", i, y_items[i].id);
  }
}