#include <stdlib.h>

typedef struct event {
        size_t * contact_ids;
} event;

static size_t contact_ids_len(size_t * ids)
{
        size_t * itr = ids;
        while (itr && *itr) {
                itr++;
        }
        return itr - ids;
}

static void load_contact_id_from_row(event * e)
{
        size_t val = 4;
        size_t len = contact_ids_len(e->contact_ids);
        e->contact_ids = realloc(e->contact_ids, (len + 2) * sizeof(size_t));
        e->contact_ids[len] = val;
        e->contact_ids[len + 1] = 0;
}

int main()
{
        event * e = malloc(sizeof(event));
        e->contact_ids = NULL;
        load_contact_id_from_row(e);
        return 0;
}