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author: gdamore <gdamore@users.noreply.github.com> 2024-10-27 18:55:51 +0000
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+
+</style>
+<h1 id="id-map"><a class="header" href="#id-map">ID Map</a></h1>
+<p>Internally, <em>NNG</em> uses a map of numeric identifiers to data structures.
+This feature is also exposed for application use, as a “supplemental” feature.</p>
+<p>When using these functions, it is necessary to add the <code>#include &lt;nng/supplemental/util/idhash.h&gt;</code>
+include file to list of includes.</p>
+<h2 id="id-map-structure"><a class="header" href="#id-map-structure">ID Map Structure</a></h2>
+<pre><code class="language-c">#include &lt;nng/nng.h&gt;
+#include &lt;nng/supplemental/util/idhash.h&gt;
+
+typedef struct nng_id_map_s nng_id_map;
+</code></pre>
+<p>The ID map structure, <a name="a001"></a><code>nng_id_map</code> provides a table of identifiers mapping
+to user-supplied pointers (which must not be <code>NULL</code>). The identifiers can be
+thought of as indices into the table, with the pointers providing the reference
+for the user supplied data.</p>
+<p>The values of identifiers can be supplied by the user, or can be allocated automatically
+by <code>nng_id_map</code> from a predefined range. The starting point for allocations
+can also be randomly within the range.</p>
+<p>The identifiers are 64-bit unsigned integers and can be sparse; the structure
+will use space efficiently even if identifiers are very far apart.
+<sup><a name="to-footnote-1"><a href="#footnote-1">1</a></a></sup></p>
+<div class="mdbook-alerts mdbook-alerts-important">
+<p class="mdbook-alerts-title">
+  <span class="mdbook-alerts-icon"></span>
+  important
+</p>
+<p>The function available for <code>nng_id_map</code> are <em>not</em> thread-safe.
+Callers should use a <a href="/api/synch.html#mutual-exclusion-lock">mutex</a> or similar approach when thread-safety is needed.</p>
+</div>
+<h2 id="create-id-map"><a class="header" href="#create-id-map">Create ID Map</a></h2>
+<pre><code class="language-c">#define NNG_MAP_RANDOM 1
+
+int nng_id_map_alloc(nng_id_map **map_p, uint64_t lo, uint64_t hi, int flags);
+</code></pre>
+<p>The <a name="a002"></a><code>nng_id_map_alloc</code> function allocates a map without any data in it,
+and returns a pointer to it in <em>map_p</em>. When allocating identifiers dynamically,
+the values will be chosen from the range defined by <em>lo</em> and <em>hi</em>, inclusive.</p>
+<p>The <em>flags</em> argument is a bit mask of flags that can adjust behavior of the map.
+The only flag defined at present
+is <code>NNG_MAP_RANDOM</code>, which causes the first identifier allocation to start at a random
+point within the range.
+This is useful to reduce the odds of different instances of an application using
+the same identifiers at the same time.</p>
+<p>If both <em>lo</em> and <em>hi</em> are zero, then the values <code>0</code> and <code>0xffffffff</code> are substituted
+in their place, giving a full range of 32-bit identifiers.</p>
+<p>This function can return <a href="/api/errors.html#NNG_ENOMEM"><code>NNG_ENOMEM</code></a> if it is unable to allocate resources, otherwise
+it returns zero on success.</p>
+<h2 id="destroy-map"><a class="header" href="#destroy-map">Destroy Map</a></h2>
+<pre><code class="language-c">void nng_id_map_free(nng_id_map *map);
+</code></pre>
+<p>The <a name="a003"></a><code>nng_id_map_free</code> function destroys <em>map</em>, releasing any resources associated
+with it.</p>
+<div class="mdbook-alerts mdbook-alerts-note">
+<p class="mdbook-alerts-title">
+  <span class="mdbook-alerts-icon"></span>
+  note
+</p>
+<p>The <code>nng_id_map_free</code> frees the map itself, but will not free memory associated with
+any strctures contained within it.</p>
+</div>
+<h2 id="store-a-value"><a class="header" href="#store-a-value">Store a Value</a></h2>
+<pre><code class="language-c">int nng_id_set(nng_id_map *map, uint64_t id, void *value);
+</code></pre>
+<p>The <a name="a004"></a><code>nng_id_map_set</code> function is used to store the <em>value</em> in the <em>map</em> at
+index <em>id</em>.</p>
+<p>If another value is already stored at that same location, then it is overwritten with
+<em>value</em>.</p>
+<div class="mdbook-alerts mdbook-alerts-note">
+<p class="mdbook-alerts-title">
+  <span class="mdbook-alerts-icon"></span>
+  note
+</p>
+<p>The <em>value</em> must not be <code>NULL</code>.</p>
+</div>
+<p>If the table has to grow to accommodate this value, it may fail if insufficient
+memory is available, returning <a href="/api/errors.html#NNG_ENOMEM"><code>NNG_ENOMEM</code></a>. Otherwise it returns zero.</p>
+<h2 id="lookup-a-value"><a class="header" href="#lookup-a-value">Lookup a Value</a></h2>
+<pre><code class="language-c">void *nng_id_get(nng_id_map *map, uint64_t id);
+</code></pre>
+<p>The <a name="a005"></a><code>nng_id_get</code> function looks up the entry for <em>id</em> in <em>map</em>, returning the
+associated value if present, or <code>NULL</code> if no such entry exists.</p>
+<h2 id="allocate-an-id"><a class="header" href="#allocate-an-id">Allocate an ID</a></h2>
+<pre><code class="language-c">int nng_id_alloc(nng_id_map *map, uint64_t *id_p, void *value);
+</code></pre>
+<p>The <a name="a006"></a><code>nng_id_alloc</code> stores the <em>value</em> in the <em>map</em>, at a newly allocated index,
+and returns the index in <em>id_p</em>.</p>
+<p>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.<sup><a name="to-footnote-2"><a href="#footnote-2">2</a></a></sup></p>
+<p>As with <a href="/api/id_map.html#store-a-value"><code>nng_id_set</code></a>, this may need to allocate memory and can thus
+fail with <a href="/api/errors.html#NNG_ENOMEM"><code>NNG_ENOMEM</code></a>.</p>
+<p>Additionally, if there are no more free identifiers within the range specified
+when <em>map</em> was created, then it will return <a href="/api/errors.html#NNG_ENOSPC"><code>NNG_ENOSPC</code></a>.</p>
+<p>Otherwise it returns zero, indicating success.</p>
+<h2 id="remove-an-id"><a class="header" href="#remove-an-id">Remove an ID</a></h2>
+<pre><code class="language-c">int nng_id_remove(nng_id_map *map, uint64_t id);
+</code></pre>
+<p>The <a name="a007"></a><code>nng_id_remove</code> removes the entry at index <em>id</em> from <em>map</em>.</p>
+<p>If no such entry exist, it will return <a href="/api/errors.html#NNG_ENOENT"><code>NNG_ENOENT</code></a>. Otherwise it returns zero.</p>
+<h2 id="iterating-ids"><a class="header" href="#iterating-ids">Iterating IDs</a></h2>
+<pre><code class="language-c">bool nng_id_visit(nng_id_map *map, uint64_t *id_p, void **value_p, uint32_t *cursor);
+</code></pre>
+<p>The <a name="a008"></a><code>nng_id_visit</code> function is used to iterate over all items in the table.
+The caller starts the iteration by setting the <em>cursor</em> to 0 before calling it.
+For each call, the associated key and value of the next item will be returned in <em>id_p</em>,
+and <em>value_p</em> and the <em>cursor</em> will be updated.
+When all items have been iterated, the function returns <code>false</code>.
+The order of items returned is not guaranteed to be sequential.
+The caller must not attempt to derive any value of the <em>cursor</em> as it refers to internal table indices.</p>
+<p>Entries may be safely removed from <em>map</em> while iterating.</p>
+<p>However, if new entries are added to the table while iterating, the result of
+iteration is undefined; entries may be repeated or omitted during such an iteration.</p>
+<p>The caller must not attempt to derive any value of the <em>cursor</em> as it refers to internal
+table indices.</p>
+<!-- Symbol cross reference -->
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+<!-- Concept index -->
+<p><hr/>
+<p><a name="footnote-1"><a href="#to-footnote-1">1</a></a>: The ID map is capable of storing at most 2<sup>32</sup> identifiers, even though the identifers may
+themselves be much larger than this.</p>
+<p><a name="footnote-2"><a href="#to-footnote-2">2</a></a>: 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.</p>
+
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author	gdamore <gdamore@users.noreply.github.com>	2024-10-27 18:55:51 +0000
committer	gdamore <gdamore@users.noreply.github.com>	2024-10-27 18:55:51 +0000
commit	45ac4fa56b6e5c31a28fd08eaad14a09bf3934f6 (patch)
tree	e0ed7b8907d4f6ac5651fb6c9d38556ec46b231a /ref/api/id_map.html
parent	b83fdb3ff1327e4376acd0ca24479b16b24f7e0b (diff)
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