rem: return ENOMEM;
add: return FS_ENOMEM;
context: init_hash_table(hash_table *ht){    ht->max   = HT_DEFAULT_MAX;    ht->mask  = ht->max - 1;    ht->num_elements = 0;    ht->table = (hash_ent **)calloc(ht->max, sizeof(hash_ent *));    if (ht->table == NULL)        return ENOMEM;    return 0;}