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## 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);
```
### Description
These functions provide support for managing tables of data based on 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.
It also 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.]
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 _lo_ and _hi_ are both zero, then a full range of 32-bit identifiers is assumed.footnote:[Consequently, if identifiers larger than or equal to 2^32^ are required, then both _lo_ and _hi_ must be specified with the exact values desired.]
The _flags_ argument is a bit mask of flags for the table, can contain the following value:
((`NNG_MAP_RANDOM`)):: 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 concurrently.
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.
NOTE: These functions are limited to storing at most 2^32^-1 identifiers, even though the identifers may themselves be larger than 2^32^.
IMPORTANT: These functions are *not* thread-safe.
Callers should use a xref:../thr/nng_mtx_lock.adoc[mutex] or similar approach when thread-safety is needed.
### 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
[horizontal]
`NNG_ENOENT`:: The _id_ does not exist in the table.
`NNG_ENOMEM`:: Insufficient memory is available, or the table is full.
### See Also
xref:../thr/index.adoc[Threads & Synchronization]
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