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+# nng_id_map
+
+## NAME
+
+nng_id_map - identifier based mapping table
+
+## SYNOPSIS
+
+```c
+#include <nng/nng.h>
+#include <nng/supplemental/util/idhash.h>
+
+typedef struct nng_id_map_s nng_id_map;
+
+#define NNG_MAP_RANDOM 1
+
+int   nng_id_map_alloc(nng_id_map **map_p, uint64_t lo, uint64_t hi, int flags);
+void  nng_id_map_free(nng_id_map *map);
+void *nng_id_get(nng_id_map *map, uint64_t id);
+int   nng_id_set(nng_id_map *map, uint64_t, void *value);
+int   nng_id_alloc(nng_id_map *map, uint64_t *id_p, void *value);
+int   nng_id_remove(nng_id_map *map, uint64_t id);
+bool  nng_id_visit(nng_id_map *map, uint64_t *id_p, void **value_p, uint32_t *cursor);
+```
+
+## DESCRIPTION
+
+These functions provide support for managing tables of data based on
+{{i:identifiers}}, ensuring that identifiers are allocated uniquely and within
+specified range limits.
+
+The table stores data pointers (which must not be `NULL`) at a logical numeric index.
+It does so efficiently, even if large gaps exist, and it provides a means to efficiently
+allocate a numeric identifier from a pool of unused identifiers.
+
+Identifiers are allocated in increasing order, without reusing old identifiers until the
+largest possible identifier is allocated. After wrapping, only identifiers that are no longer
+in use will be considered.
+No effort is made to order the availability of identifiers based on
+when they were freed.{{footnote: The concern about possibly reusing a
+recently released identifier comes into consideration after the range has wrapped.
+Given a sufficiently large range, this is unlikely to be a concern.}}
+
+> [!IMPORTANT]
+> These functions are _not_ thread-safe.
+> Callers should use a [mutex][mutex] or similar approach when thread-safety is needed.
+
+### Initialization
+
+An initial table is allocated with `nng_id_map_alloc()`, which takes the lowest legal identifier in _lo_,
+and the largest legal identifier in _hi_.
+The new table is returned in _map_p_, and should be used as the _map_ argument to the rest of these functions.
+
+If these are specified as zero, then a full range of 32-bit identifiers is assumed.
+If identifiers beyond 32-bits are required,
+then both _lo_ and _hi_ must be specified with the exact values desired.
+{{footnote: These functions are limited to storing at most 2<sup>32</sup> identifiers, even though the identifers may
+themselves be larger than this.}}
+
+The _flags_ argument is a bit mask of flags for the table.
+If `NNG_MAP_RANDOM` is specified, then the starting point for allocations is randomized, but subsequent allocations will then be monotonically increasing.
+This is useful to reduce the odds of different instances of an application using the same identifiers at the same time.
+
+### Accessors
+
+The `nng_id_get()` function returns the value previously stored with the given identifier.
+If no value is currently associated with the identifer, it returns `NULL`.
+
+The `nng_id_set()` function sets the value with the associated identifier.
+This can be used to replace a previously allocated identifier.
+If the identifier was not previously allocated, then it is allocated as part of the call.
+This function does not necessarily honor the identifier range limits set for the map when it was allocated.
+
+The `nng_id_alloc()` function allocates a new identifier from the range for the map, and associates it with
+the supplied _value_.
+
+The `nng_id_remove()` function removes the identifier and its associated value from the table.
+
+### Iteration
+
+The `nng_id_visit()` function is used to iterate over all items in the table.
+The caller starts the iteration by setting the _cursor_ to 0 before calling it.
+For each call, the associated key and value of the next item will be returned in _id_p_,
+and _value_p_ and the _cursor_ will be updated.
+When all items have been iterated, the function returns `false`.
+The order of items returned is not guaranteed to be sequential.
+The caller must not attempt to derive any value of the _cursor_ as it refers to internal table indices.
+
+## RETURN VALUES
+
+The `nng_id_map_alloc()`, `nng_id_set()`, `nng_id_alloc()`, and `nng_id_remove()` functions
+return 0 on success, or -1 on failure.
+
+The `nng_id_map_get()` function returns the requested data pointer, or `NULL` if the identifier was not found.
+
+## ERRORS
+
+- `NNG_ENOENT`: The _id_ does not exist in the table.
+- `NNG_ENOMEM`: Insufficient memory is available, or the table is full.
+
+[mutex]: ../thr/mutex.md