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 *hmap);

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

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

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

void *
thmap_stage_gc(thmap_t *hmap);

void
thmap_gc(thmap_t *hmap, 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()

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()

Destroy the map, freeing the memory it uses.

thmap_get()

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()

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()

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()

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()

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()

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()

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);

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.

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>