1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 // ignore-lexer-test FIXME #15883
15 use hash::{Hash, Hasher};
16 use iter::{Iterator, count};
17 use kinds::{Sized, marker};
18 use mem::{min_align_of, size_of};
20 use num::{CheckedAdd, CheckedMul, is_power_of_two};
21 use ops::{Deref, DerefMut, Drop};
22 use option::{Some, None, Option};
23 use ptr::{RawPtr, copy_nonoverlapping_memory, zero_memory};
25 use rt::heap::{allocate, deallocate};
27 const EMPTY_BUCKET: u64 = 0u64;
29 /// The raw hashtable, providing safe-ish access to the unzipped and highly
30 /// optimized arrays of hashes, keys, and values.
32 /// This design uses less memory and is a lot faster than the naive
33 /// `Vec<Option<u64, K, V>>`, because we don't pay for the overhead of an
34 /// option on every element, and we get a generally more cache-aware design.
36 /// Essential invariants of this structure:
38 /// - if t.hashes[i] == EMPTY_BUCKET, then `Bucket::at_index(&t, i).raw`
39 /// points to 'undefined' contents. Don't read from it. This invariant is
40 /// enforced outside this module with the `EmptyBucket`, `FullBucket`,
41 /// and `SafeHash` types.
43 /// - An `EmptyBucket` is only constructed at an index with
44 /// a hash of EMPTY_BUCKET.
46 /// - A `FullBucket` is only constructed at an index with a
47 /// non-EMPTY_BUCKET hash.
49 /// - A `SafeHash` is only constructed for non-`EMPTY_BUCKET` hash. We get
50 /// around hashes of zero by changing them to 0x8000_0000_0000_0000,
51 /// which will likely map to the same bucket, while not being confused
54 /// - All three "arrays represented by pointers" are the same length:
55 /// `capacity`. This is set at creation and never changes. The arrays
56 /// are unzipped to save space (we don't have to pay for the padding
57 /// between odd sized elements, such as in a map from u64 to u8), and
58 /// be more cache aware (scanning through 8 hashes brings in at most
59 /// 2 cache lines, since they're all right beside each other).
61 /// You can kind of think of this module/data structure as a safe wrapper
62 /// around just the "table" part of the hashtable. It enforces some
63 /// invariants at the type level and employs some performance trickery,
64 /// but in general is just a tricked out `Vec<Option<u64, K, V>>`.
65 #[unsafe_no_drop_flag]
66 pub struct RawTable<K, V> {
70 // Because K/V do not appear directly in any of the types in the struct,
71 // inform rustc that in fact instances of K and V are reachable from here.
72 marker: marker::CovariantType<(K,V)>,
75 struct RawBucket<K, V> {
81 pub struct Bucket<K, V, M> {
87 pub struct EmptyBucket<K, V, M> {
93 pub struct FullBucket<K, V, M> {
99 pub type EmptyBucketImm<'table, K, V> = EmptyBucket<K, V, &'table RawTable<K, V>>;
100 pub type FullBucketImm<'table, K, V> = FullBucket<K, V, &'table RawTable<K, V>>;
102 pub type EmptyBucketMut<'table, K, V> = EmptyBucket<K, V, &'table mut RawTable<K, V>>;
103 pub type FullBucketMut<'table, K, V> = FullBucket<K, V, &'table mut RawTable<K, V>>;
105 pub enum BucketState<K, V, M> {
106 Empty(EmptyBucket<K, V, M>),
107 Full(FullBucket<K, V, M>),
110 // A GapThenFull encapsulates the state of two consecutive buckets at once.
111 // The first bucket, called the gap, is known to be empty.
112 // The second bucket is full.
113 struct GapThenFull<K, V, M> {
114 gap: EmptyBucket<K, V, ()>,
115 full: FullBucket<K, V, M>,
118 /// A hash that is not zero, since we use a hash of zero to represent empty
120 #[deriving(PartialEq)]
121 pub struct SafeHash {
126 /// Peek at the hash value, which is guaranteed to be non-zero.
128 pub fn inspect(&self) -> u64 { self.hash }
131 /// We need to remove hashes of 0. That's reserved for empty buckets.
132 /// This function wraps up `hash_keyed` to be the only way outside this
133 /// module to generate a SafeHash.
134 pub fn make_hash<Sized? T: Hash<S>, S, H: Hasher<S>>(hasher: &H, t: &T) -> SafeHash {
135 match hasher.hash(t) {
136 // This constant is exceedingly likely to hash to the same
137 // bucket, but it won't be counted as empty! Just so we can maintain
138 // our precious uniform distribution of initial indexes.
139 EMPTY_BUCKET => SafeHash { hash: 0x8000_0000_0000_0000 },
140 h => SafeHash { hash: h },
144 // `replace` casts a `*u64` to a `*SafeHash`. Since we statically
145 // ensure that a `FullBucket` points to an index with a non-zero hash,
146 // and a `SafeHash` is just a `u64` with a different name, this is
149 // This test ensures that a `SafeHash` really IS the same size as a
150 // `u64`. If you need to change the size of `SafeHash` (and
151 // consequently made this test fail), `replace` needs to be
152 // modified to no longer assume this.
154 fn can_alias_safehash_as_u64() {
155 assert_eq!(size_of::<SafeHash>(), size_of::<u64>())
158 impl<K, V> RawBucket<K, V> {
159 unsafe fn offset(self, count: int) -> RawBucket<K, V> {
161 hash: self.hash.offset(count),
162 key: self.key.offset(count),
163 val: self.val.offset(count),
168 // For parameterizing over mutability.
171 impl<'t, K, V> Deref<RawTable<K, V>> for &'t RawTable<K, V> {
172 fn deref(&self) -> &RawTable<K, V> {
178 impl<'t, K, V> Deref<RawTable<K, V>> for &'t mut RawTable<K, V> {
179 fn deref(&self) -> &RawTable<K,V> {
185 impl<'t, K, V> DerefMut<RawTable<K, V>> for &'t mut RawTable<K, V> {
186 fn deref_mut(&mut self) -> &mut RawTable<K,V> {
191 // Buckets hold references to the table.
192 impl<K, V, M> FullBucket<K, V, M> {
193 /// Borrow a reference to the table.
194 pub fn table(&self) -> &M {
197 /// Move out the reference to the table.
198 pub fn into_table(self) -> M {
201 /// Get the raw index.
202 pub fn index(&self) -> uint {
207 impl<K, V, M> EmptyBucket<K, V, M> {
208 /// Borrow a reference to the table.
209 pub fn table(&self) -> &M {
212 /// Move out the reference to the table.
213 pub fn into_table(self) -> M {
218 impl<K, V, M> Bucket<K, V, M> {
219 /// Move out the reference to the table.
220 pub fn into_table(self) -> M {
223 /// Get the raw index.
224 pub fn index(&self) -> uint {
229 impl<K, V, M: Deref<RawTable<K, V>>> Bucket<K, V, M> {
230 pub fn new(table: M, hash: &SafeHash) -> Bucket<K, V, M> {
231 Bucket::at_index(table, hash.inspect() as uint)
234 pub fn at_index(table: M, ib_index: uint) -> Bucket<K, V, M> {
235 let ib_index = ib_index & (table.capacity() - 1);
238 table.first_bucket_raw().offset(ib_index as int)
245 pub fn first(table: M) -> Bucket<K, V, M> {
247 raw: table.first_bucket_raw(),
253 /// Reads a bucket at a given index, returning an enum indicating whether
254 /// it's initialized or not. You need to match on this enum to get
255 /// the appropriate types to call most of the other functions in
257 pub fn peek(self) -> BucketState<K, V, M> {
258 match unsafe { *self.raw.hash } {
274 /// Modifies the bucket pointer in place to make it point to the next slot.
275 pub fn next(&mut self) {
276 // Branchless bucket iteration step.
277 // As we reach the end of the table...
278 // We take the current idx: 0111111b
279 // Xor it by its increment: ^ 1000000b
282 // Then AND with the capacity: & 1000000b
284 // to get the backwards offset: 1000000b
285 // ... and it's zero at all other times.
286 let maybe_wraparound_dist = (self.idx ^ (self.idx + 1)) & self.table.capacity();
287 // Finally, we obtain the offset 1 or the offset -cap + 1.
288 let dist = 1i - (maybe_wraparound_dist as int);
293 self.raw = self.raw.offset(dist);
298 impl<K, V, M: Deref<RawTable<K, V>>> EmptyBucket<K, V, M> {
300 pub fn next(self) -> Bucket<K, V, M> {
301 let mut bucket = self.into_bucket();
307 pub fn into_bucket(self) -> Bucket<K, V, M> {
315 pub fn gap_peek(self) -> Option<GapThenFull<K, V, M>> {
316 let gap = EmptyBucket {
322 match self.next().peek() {
334 impl<K, V, M: DerefMut<RawTable<K, V>>> EmptyBucket<K, V, M> {
335 /// Puts given key and value pair, along with the key's hash,
336 /// into this bucket in the hashtable. Note how `self` is 'moved' into
337 /// this function, because this slot will no longer be empty when
338 /// we return! A `FullBucket` is returned for later use, pointing to
339 /// the newly-filled slot in the hashtable.
341 /// Use `make_hash` to construct a `SafeHash` to pass to this function.
342 pub fn put(mut self, hash: SafeHash, key: K, value: V)
343 -> FullBucket<K, V, M> {
345 *self.raw.hash = hash.inspect();
346 ptr::write(self.raw.key, key);
347 ptr::write(self.raw.val, value);
350 self.table.size += 1;
352 FullBucket { raw: self.raw, idx: self.idx, table: self.table }
356 impl<K, V, M: Deref<RawTable<K, V>>> FullBucket<K, V, M> {
358 pub fn next(self) -> Bucket<K, V, M> {
359 let mut bucket = self.into_bucket();
365 pub fn into_bucket(self) -> Bucket<K, V, M> {
373 /// Get the distance between this bucket and the 'ideal' location
374 /// as determined by the key's hash stored in it.
376 /// In the cited blog posts above, this is called the "distance to
377 /// initial bucket", or DIB. Also known as "probe count".
378 pub fn distance(&self) -> uint {
379 // Calculates the distance one has to travel when going from
380 // `hash mod capacity` onwards to `idx mod capacity`, wrapping around
381 // if the destination is not reached before the end of the table.
382 (self.idx - self.hash().inspect() as uint) & (self.table.capacity() - 1)
386 pub fn hash(&self) -> SafeHash {
394 /// Gets references to the key and value at a given index.
395 pub fn read(&self) -> (&K, &V) {
403 impl<K, V, M: DerefMut<RawTable<K, V>>> FullBucket<K, V, M> {
404 /// Removes this bucket's key and value from the hashtable.
406 /// This works similarly to `put`, building an `EmptyBucket` out of the
408 pub fn take(mut self) -> (EmptyBucket<K, V, M>, K, V) {
409 let key = self.raw.key as *const K;
410 let val = self.raw.val as *const V;
412 self.table.size -= 1;
415 *self.raw.hash = EMPTY_BUCKET;
428 pub fn replace(&mut self, h: SafeHash, k: K, v: V) -> (SafeHash, K, V) {
430 let old_hash = ptr::replace(self.raw.hash as *mut SafeHash, h);
431 let old_key = ptr::replace(self.raw.key, k);
432 let old_val = ptr::replace(self.raw.val, v);
434 (old_hash, old_key, old_val)
438 /// Gets mutable references to the key and value at a given index.
439 pub fn read_mut(&mut self) -> (&mut K, &mut V) {
447 impl<'t, K, V, M: Deref<RawTable<K, V>> + 't> FullBucket<K, V, M> {
448 /// Exchange a bucket state for immutable references into the table.
449 /// Because the underlying reference to the table is also consumed,
450 /// no further changes to the structure of the table are possible;
451 /// in exchange for this, the returned references have a longer lifetime
452 /// than the references returned by `read()`.
453 pub fn into_refs(self) -> (&'t K, &'t V) {
461 impl<'t, K, V, M: DerefMut<RawTable<K, V>> + 't> FullBucket<K, V, M> {
462 /// This works similarly to `into_refs`, exchanging a bucket state
463 /// for mutable references into the table.
464 pub fn into_mut_refs(self) -> (&'t mut K, &'t mut V) {
472 impl<K, V, M> BucketState<K, V, M> {
474 pub fn expect_full(self) -> FullBucket<K, V, M> {
477 Empty(..) => panic!("Expected full bucket")
482 impl<K, V, M: Deref<RawTable<K, V>>> GapThenFull<K, V, M> {
484 pub fn full(&self) -> &FullBucket<K, V, M> {
488 pub fn shift(mut self) -> Option<GapThenFull<K, V, M>> {
490 *self.gap.raw.hash = mem::replace(&mut *self.full.raw.hash, EMPTY_BUCKET);
491 copy_nonoverlapping_memory(self.gap.raw.key, self.full.raw.key as *const K, 1);
492 copy_nonoverlapping_memory(self.gap.raw.val, self.full.raw.val as *const V, 1);
495 let FullBucket { raw: prev_raw, idx: prev_idx, .. } = self.full;
497 match self.full.next().peek() {
499 self.gap.raw = prev_raw;
500 self.gap.idx = prev_idx;
512 /// Rounds up to a multiple of a power of two. Returns the closest multiple
513 /// of `target_alignment` that is higher or equal to `unrounded`.
517 /// Fails if `target_alignment` is not a power of two.
518 fn round_up_to_next(unrounded: uint, target_alignment: uint) -> uint {
519 assert!(is_power_of_two(target_alignment));
520 (unrounded + target_alignment - 1) & !(target_alignment - 1)
525 assert_eq!(round_up_to_next(0, 4), 0);
526 assert_eq!(round_up_to_next(1, 4), 4);
527 assert_eq!(round_up_to_next(2, 4), 4);
528 assert_eq!(round_up_to_next(3, 4), 4);
529 assert_eq!(round_up_to_next(4, 4), 4);
530 assert_eq!(round_up_to_next(5, 4), 8);
533 // Returns a tuple of (key_offset, val_offset),
534 // from the start of a mallocated array.
535 fn calculate_offsets(hashes_size: uint,
536 keys_size: uint, keys_align: uint,
539 let keys_offset = round_up_to_next(hashes_size, keys_align);
540 let end_of_keys = keys_offset + keys_size;
542 let vals_offset = round_up_to_next(end_of_keys, vals_align);
544 (keys_offset, vals_offset)
547 // Returns a tuple of (minimum required malloc alignment, hash_offset,
548 // array_size), from the start of a mallocated array.
549 fn calculate_allocation(hash_size: uint, hash_align: uint,
550 keys_size: uint, keys_align: uint,
551 vals_size: uint, vals_align: uint)
552 -> (uint, uint, uint) {
554 let (_, vals_offset) = calculate_offsets(hash_size,
555 keys_size, keys_align,
557 let end_of_vals = vals_offset + vals_size;
559 let min_align = cmp::max(hash_align, cmp::max(keys_align, vals_align));
561 (min_align, hash_offset, end_of_vals)
565 fn test_offset_calculation() {
566 assert_eq!(calculate_allocation(128, 8, 15, 1, 4, 4), (8, 0, 148));
567 assert_eq!(calculate_allocation(3, 1, 2, 1, 1, 1), (1, 0, 6));
568 assert_eq!(calculate_allocation(6, 2, 12, 4, 24, 8), (8, 0, 48));
569 assert_eq!(calculate_offsets(128, 15, 1, 4), (128, 144));
570 assert_eq!(calculate_offsets(3, 2, 1, 1), (3, 5));
571 assert_eq!(calculate_offsets(6, 12, 4, 8), (8, 24));
574 impl<K, V> RawTable<K, V> {
575 /// Does not initialize the buckets. The caller should ensure they,
576 /// at the very least, set every hash to EMPTY_BUCKET.
577 unsafe fn new_uninitialized(capacity: uint) -> RawTable<K, V> {
582 hashes: 0 as *mut u64,
583 marker: marker::CovariantType,
586 // No need for `checked_mul` before a more restrictive check performed
587 // later in this method.
588 let hashes_size = capacity * size_of::<u64>();
589 let keys_size = capacity * size_of::< K >();
590 let vals_size = capacity * size_of::< V >();
592 // Allocating hashmaps is a little tricky. We need to allocate three
593 // arrays, but since we know their sizes and alignments up front,
594 // we just allocate a single array, and then have the subarrays
597 // This is great in theory, but in practice getting the alignment
598 // right is a little subtle. Therefore, calculating offsets has been
599 // factored out into a different function.
600 let (malloc_alignment, hash_offset, size) =
601 calculate_allocation(
602 hashes_size, min_align_of::<u64>(),
603 keys_size, min_align_of::< K >(),
604 vals_size, min_align_of::< V >());
606 // One check for overflow that covers calculation and rounding of size.
607 let size_of_bucket = size_of::<u64>().checked_add(&size_of::<K>()).unwrap()
608 .checked_add(&size_of::<V>()).unwrap();
609 assert!(size >= capacity.checked_mul(&size_of_bucket)
610 .expect("capacity overflow"),
611 "capacity overflow");
613 let buffer = allocate(size, malloc_alignment);
614 if buffer.is_null() { ::alloc::oom() }
616 let hashes = buffer.offset(hash_offset as int) as *mut u64;
622 marker: marker::CovariantType,
626 fn first_bucket_raw(&self) -> RawBucket<K, V> {
627 let hashes_size = self.capacity * size_of::<u64>();
628 let keys_size = self.capacity * size_of::<K>();
630 let buffer = self.hashes as *mut u8;
631 let (keys_offset, vals_offset) = calculate_offsets(hashes_size,
632 keys_size, min_align_of::<K>(),
633 min_align_of::<V>());
638 key: buffer.offset(keys_offset as int) as *mut K,
639 val: buffer.offset(vals_offset as int) as *mut V
644 /// Creates a new raw table from a given capacity. All buckets are
646 #[allow(experimental)]
647 pub fn new(capacity: uint) -> RawTable<K, V> {
649 let ret = RawTable::new_uninitialized(capacity);
650 zero_memory(ret.hashes, capacity);
655 /// The hashtable's capacity, similar to a vector's.
656 pub fn capacity(&self) -> uint {
660 /// The number of elements ever `put` in the hashtable, minus the number
661 /// of elements ever `take`n.
662 pub fn size(&self) -> uint {
666 fn raw_buckets(&self) -> RawBuckets<K, V> {
668 raw: self.first_bucket_raw(),
670 self.hashes.offset(self.capacity as int)
672 marker: marker::ContravariantLifetime,
676 pub fn iter(&self) -> Entries<K, V> {
678 iter: self.raw_buckets(),
679 elems_left: self.size(),
683 pub fn iter_mut(&mut self) -> MutEntries<K, V> {
685 iter: self.raw_buckets(),
686 elems_left: self.size(),
690 pub fn into_iter(self) -> MoveEntries<K, V> {
691 let RawBuckets { raw, hashes_end, .. } = self.raw_buckets();
692 // Replace the marker regardless of lifetime bounds on parameters.
696 hashes_end: hashes_end,
697 marker: marker::ContravariantLifetime,
703 /// Returns an iterator that copies out each entry. Used while the table
704 /// is being dropped.
705 unsafe fn rev_move_buckets(&mut self) -> RevMoveBuckets<K, V> {
706 let raw_bucket = self.first_bucket_raw();
708 raw: raw_bucket.offset(self.capacity as int),
709 hashes_end: raw_bucket.hash,
710 elems_left: self.size,
711 marker: marker::ContravariantLifetime,
716 /// A raw iterator. The basis for some other iterators in this module. Although
717 /// this interface is safe, it's not used outside this module.
718 struct RawBuckets<'a, K, V> {
719 raw: RawBucket<K, V>,
720 hashes_end: *mut u64,
721 marker: marker::ContravariantLifetime<'a>,
724 impl<'a, K, V> Iterator<RawBucket<K, V>> for RawBuckets<'a, K, V> {
725 fn next(&mut self) -> Option<RawBucket<K, V>> {
726 while self.raw.hash != self.hashes_end {
728 // We are swapping out the pointer to a bucket and replacing
729 // it with the pointer to the next one.
730 let prev = ptr::replace(&mut self.raw, self.raw.offset(1));
731 if *prev.hash != EMPTY_BUCKET {
741 /// An iterator that moves out buckets in reverse order. It leaves the table
742 /// in an an inconsistent state and should only be used for dropping
743 /// the table's remaining entries. It's used in the implementation of Drop.
744 struct RevMoveBuckets<'a, K, V> {
745 raw: RawBucket<K, V>,
746 hashes_end: *mut u64,
748 marker: marker::ContravariantLifetime<'a>,
751 impl<'a, K, V> Iterator<(K, V)> for RevMoveBuckets<'a, K, V> {
752 fn next(&mut self) -> Option<(K, V)> {
753 if self.elems_left == 0 {
758 debug_assert!(self.raw.hash != self.hashes_end);
761 self.raw = self.raw.offset(-1);
763 if *self.raw.hash != EMPTY_BUCKET {
764 self.elems_left -= 1;
766 ptr::read(self.raw.key as *const K),
767 ptr::read(self.raw.val as *const V)
775 /// Iterator over shared references to entries in a table.
776 pub struct Entries<'a, K: 'a, V: 'a> {
777 iter: RawBuckets<'a, K, V>,
781 /// Iterator over mutable references to entries in a table.
782 pub struct MutEntries<'a, K: 'a, V: 'a> {
783 iter: RawBuckets<'a, K, V>,
787 /// Iterator over the entries in a table, consuming the table.
788 pub struct MoveEntries<K, V> {
789 table: RawTable<K, V>,
790 iter: RawBuckets<'static, K, V>
793 impl<'a, K, V> Iterator<(&'a K, &'a V)> for Entries<'a, K, V> {
794 fn next(&mut self) -> Option<(&'a K, &'a V)> {
795 self.iter.next().map(|bucket| {
796 self.elems_left -= 1;
804 fn size_hint(&self) -> (uint, Option<uint>) {
805 (self.elems_left, Some(self.elems_left))
809 impl<'a, K, V> Iterator<(&'a K, &'a mut V)> for MutEntries<'a, K, V> {
810 fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
811 self.iter.next().map(|bucket| {
812 self.elems_left -= 1;
820 fn size_hint(&self) -> (uint, Option<uint>) {
821 (self.elems_left, Some(self.elems_left))
825 impl<K, V> Iterator<(SafeHash, K, V)> for MoveEntries<K, V> {
826 fn next(&mut self) -> Option<(SafeHash, K, V)> {
827 self.iter.next().map(|bucket| {
828 self.table.size -= 1;
834 ptr::read(bucket.key as *const K),
835 ptr::read(bucket.val as *const V)
841 fn size_hint(&self) -> (uint, Option<uint>) {
842 let size = self.table.size();
847 impl<K: Clone, V: Clone> Clone for RawTable<K, V> {
848 fn clone(&self) -> RawTable<K, V> {
850 let mut new_ht = RawTable::new_uninitialized(self.capacity());
853 let cap = self.capacity();
854 let mut new_buckets = Bucket::first(&mut new_ht);
855 let mut buckets = Bucket::first(self);
856 while buckets.index() != cap {
857 match buckets.peek() {
860 let (k, v) = full.read();
861 (full.hash(), k.clone(), v.clone())
863 *new_buckets.raw.hash = h.inspect();
864 ptr::write(new_buckets.raw.key, k);
865 ptr::write(new_buckets.raw.val, v);
868 *new_buckets.raw.hash = EMPTY_BUCKET;
876 new_ht.size = self.size();
884 impl<K, V> Drop for RawTable<K, V> {
886 if self.hashes.is_null() {
889 // This is done in reverse because we've likely partially taken
890 // some elements out with `.into_iter()` from the front.
891 // Check if the size is 0, so we don't do a useless scan when
892 // dropping empty tables such as on resize.
893 // Also avoid double drop of elements that have been already moved out.
895 for _ in self.rev_move_buckets() {}
898 let hashes_size = self.capacity * size_of::<u64>();
899 let keys_size = self.capacity * size_of::<K>();
900 let vals_size = self.capacity * size_of::<V>();
901 let (align, _, size) = calculate_allocation(hashes_size, min_align_of::<u64>(),
902 keys_size, min_align_of::<K>(),
903 vals_size, min_align_of::<V>());
906 deallocate(self.hashes as *mut u8, size, align);
907 // Remember how everything was allocated out of one buffer
908 // during initialization? We only need one call to free here.