C preserve const for array of pointer to struct

/* preserving const of pointer in array of pointer to struct */

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

struct ds {
    size_t sz;
    char word[10];
};

void *new_ds() {
    void *p = malloc (sizeof (struct ds));
    if (!p) {
        fprintf (stderr, "error: virtual memory exhausted.\n");
        exit (EXIT_FAILURE);
    }
    return p;
}

void prn_ds (const void *a, size_t n)
{
    size_t i;

    struct ds * const *b = (struct ds * const *)a;

    for (i = 0; i < n; i++)
        printf (" size: %zu  word: %s\n", b[i]->sz, b[i]->word);
}

int main (void) {

    struct ds *arr[2];
    arr[0] = new_ds();
    arr[1] = new_ds();

    strncpy (arr[0]->word, "hello", 10);
    strncpy (arr[1]->word, "world", 10);

    arr[0]->sz = strlen (arr[0]->word) + 1;
    arr[1]->sz = strlen (arr[1]->word) + 1;

    prn_ds (arr, 2);

    free (arr[0]);
    free (arr[1]);

    return 0;
}