THMAP(9)

THMAP(9) | Kernel Developer's Manual | THMAP(9)

# NAME

`thmap` —
concurrent trie-hash map

# SYNOPSIS

`#include <thmap.h>`

thmap_t *
`thmap_create`(uintptr_t baseptr, const thmap_ops_t *ops, unsigned flags);

void
`thmap_destroy`(thmap_t *thmap);

void *
`thmap_get`(thmap_t *thmap, const void *key, size_t len);

void *
`thmap_put`(thmap_t *thmap, const void *key, size_t len, void *val);

void *
`thmap_del`(thmap_t *thmap, const void *key, size_t len);

void *
`thmap_stage_gc`(thmap_t *thmap);

void
`thmap_gc`(thmap_t *thmap, void *ref);

void
`thmap_setroot`(thmap_t *thmap, uintptr_t root_offset);

uintptr_t
`thmap_getroot`(const thmap_t *thmap);

# DESCRIPTION

Concurrent trie-hash map — a general purpose associative array, combining the elements of hashing and radix trie. Highlights:

- Very competitive performance, with logarithmic time complexity on average.
- Lookups are lock-free and inserts/deletes are using fine-grained locking.
- Incremental growth of the data structure (no large resizing/rehashing).
- Optional support for use with shared memory, e.g. memory-mapped file.

Delete operations (the key/data destruction) must be synchronized with the readers using some reclamation mechanism.

# FUNCTIONS

`thmap_create`(baseptr, ops, flags)
  Construct a new trie-hash map. The optional ops parameter can used to set the custom allocate/free operations (see the description of thmap_ops_t below). In such case, the baseptr is the base (start) address of the address space mapping (it must be word-aligned). If ops is set to `NULL`, then malloc(3) and free(3) will be used as the default operations and baseptr should be set to zero. Currently, the supported flags are:
  `THMAP_NOCOPY`
    The keys on insert will not be copied and the given pointers to them will be expected to be valid and the values constant until the key is deleted; by default, the put operation will make a copy of the key.
  `THMAP_SETROOT`
    Indicate that the root of the map will be manually set using the `thmap_setroot`() routine; by default, the map is initialized and the root node is set on `thmap_create`().
`thmap_destroy`(thmap)
  Destroy the map, freeing the memory it uses.
`thmap_get`(thmap, key, len)
  Lookup the key (of a given length) and return the value associated with it. Return `NULL` if the key is not found (see the CAVEATS section).
`thmap_put`(thmap, key, len, val)
  Insert the key with an arbitrary value. If the key is already present, return the already existing associated value without changing it. Otherwise, on a successful insert, return the given value. Just compare the result against val to test whether the insert was successful.
`thmap_del`(thmap, key, len)
  Remove the given key. If the key was present, return the associated value; otherwise return `NULL`. The memory associated with the entry is not released immediately, because in the concurrent environment (e.g., multi-threaded application) the caller may need to ensure it is safe to do so. It is managed using the `thmap_stage_gc`() and `thmap_gc`() routines.
`thmap_stage_gc`(thmap)
  Stage the currently pending entries (the memory not yet released after the deletion) for reclamation (G/C). This operation should be called before the synchronization barrier.

  Returns a reference which must be passed to `thmap_gc`(). Not calling the G/C function for the returned reference would result in a memory leak.

`thmap_gc`(thmap, ref)
  Reclaim (G/C) the staged entries i.e. release any memory associated with the deleted keys. The reference must be the value returned by the call to `thmap_stage_gc`().

  This function must be called after the synchronization barrier which guarantees that there are no active readers referencing the staged entries.

If the map is created using the THMAP_SETROOT flag, then the following functions are applicable:

`thmap_setroot`(thmap, root_offset)
  Set the root node. The address must be relative to the base address, as if allocated by the `thmap_ops_t::alloc`() routine. Return 0 on success and -1 on failure (if already set).
`thmap_getroot`(thmap)
  Get the root node address. The returned address will be relative to the base address.

Members of thmap_ops_t are

            uintptr_t (*alloc)(size_t len);
            void      (*free)(uintptr_t addr, size_t len);

# EXAMPLES

Simple case backed by malloc(3), which could be used in multi-threaded environment:

    	thmap_t *kvmap;
    	struct obj *obj;

    	kvmap = thmap_create(0, NULL);
    	assert(kvmap != NULL);
    	...
    	obj = obj_create();
    	thmap_put(kvmap, "test", sizeof("test") - 1, obj);
    	...
    	obj = thmap_get(kvmap, "test", sizeof("test") - 1);
    	...
    	thmap_destroy(kvmap);

# AUTHORS

Mindaugas Rasiukevicius <rmind@noxt.eu>

# CAVEATS

The implementation uses pointer tagging and atomic operations. This requires the base address and the allocations to provide at least word alignment.

While the `NULL` values may be inserted, `thmap_get`() and `thmap_del`() cannot indicate whether the key was not found or a key with a `NULL` value was found. If the caller needs to indicate an "empty" value, it can use a special pointer value, such as `(void *)(uintptr_t)0x1`.

December 11, 2018 | NetBSD 10.1