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<body class="article">
<div id="header">
<h1>hashmap API</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>The hashmap API is a generic implementation of hash-based key-value mappings.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_data_structures">Data Structures</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
<code>struct hashmap</code>
</dt>
<dd>
<p>
        The hash table structure.
</p>
<div class="paragraph"><p>The <code>size</code> member keeps track of the total number of entries. The <code>cmpfn</code>
member is a function used to compare two entries for equality. The <code>table</code> and
<code>tablesize</code> members store the hash table and its size, respectively.</p></div>
</dd>
<dt class="hdlist1">
<code>struct hashmap_entry</code>
</dt>
<dd>
<p>
        An opaque structure representing an entry in the hash table, which must
        be used as first member of user data structures. Ideally it should be
        followed by an int-sized member to prevent unused memory on 64-bit
        systems due to alignment.
</p>
<div class="paragraph"><p>The <code>hash</code> member is the entry&#8217;s hash code and the <code>next</code> member points to the
next entry in case of collisions (i.e. if multiple entries map to the same
bucket).</p></div>
</dd>
<dt class="hdlist1">
<code>struct hashmap_iter</code>
</dt>
<dd>
<p>
        An iterator structure, to be used with hashmap_iter_* functions.
</p>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_types">Types</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
<code>int (*hashmap_cmp_fn)(const void *entry, const void *entry_or_key, const void *keydata)</code>
</dt>
<dd>
<p>
        User-supplied function to test two hashmap entries for equality. Shall
        return 0 if the entries are equal.
</p>
<div class="paragraph"><p>This function is always called with non-NULL <code>entry</code> / <code>entry_or_key</code>
parameters that have the same hash code. When looking up an entry, the <code>key</code>
and <code>keydata</code> parameters to hashmap_get and hashmap_remove are always passed
as second and third argument, respectively. Otherwise, <code>keydata</code> is NULL.</p></div>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_functions">Functions</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
<code>unsigned int strhash(const char *buf)</code>
</dt>
<dt class="hdlist1">
<code>unsigned int strihash(const char *buf)</code>
</dt>
<dt class="hdlist1">
<code>unsigned int memhash(const void *buf, size_t len)</code>
</dt>
<dt class="hdlist1">
<code>unsigned int memihash(const void *buf, size_t len)</code>
</dt>
<dd>
<p>
        Ready-to-use hash functions for strings, using the FNV-1 algorithm (see
        <a href="http://www.isthe.com/chongo/tech/comp/fnv">http://www.isthe.com/chongo/tech/comp/fnv</a>).
</p>
<div class="paragraph"><p><code>strhash</code> and <code>strihash</code> take 0-terminated strings, while <code>memhash</code> and
<code>memihash</code> operate on arbitrary-length memory.</p></div>
<div class="paragraph"><p><code>strihash</code> and <code>memihash</code> are case insensitive versions.</p></div>
</dd>
<dt class="hdlist1">
<code>void hashmap_init(struct hashmap *map, hashmap_cmp_fn equals_function, size_t initial_size)</code>
</dt>
<dd>
<p>
        Initializes a hashmap structure.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap to initialize.</p></div>
<div class="paragraph"><p>The <code>equals_function</code> can be specified to compare two entries for equality.
If NULL, entries are considered equal if their hash codes are equal.</p></div>
<div class="paragraph"><p>If the total number of entries is known in advance, the <code>initial_size</code>
parameter may be used to preallocate a sufficiently large table and thus
prevent expensive resizing. If 0, the table is dynamically resized.</p></div>
</dd>
<dt class="hdlist1">
<code>void hashmap_free(struct hashmap *map, int free_entries)</code>
</dt>
<dd>
<p>
        Frees a hashmap structure and allocated memory.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap to free.</p></div>
<div class="paragraph"><p>If <code>free_entries</code> is true, each hashmap_entry in the map is freed as well
(using stdlib&#8217;s free()).</p></div>
</dd>
<dt class="hdlist1">
<code>void hashmap_entry_init(void *entry, int hash)</code>
</dt>
<dd>
<p>
        Initializes a hashmap_entry structure.
</p>
<div class="paragraph"><p><code>entry</code> points to the entry to initialize.</p></div>
<div class="paragraph"><p><code>hash</code> is the hash code of the entry.</p></div>
</dd>
<dt class="hdlist1">
<code>void *hashmap_get(const struct hashmap *map, const void *key, const void *keydata)</code>
</dt>
<dd>
<p>
        Returns the hashmap entry for the specified key, or NULL if not found.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>key</code> is a hashmap_entry structure (or user data structure that starts with
hashmap_entry) that has at least been initialized with the proper hash code
(via <code>hashmap_entry_init</code>).</p></div>
<div class="paragraph"><p>If an entry with matching hash code is found, <code>key</code> and <code>keydata</code> are passed
to <code>hashmap_cmp_fn</code> to decide whether the entry matches the key.</p></div>
</dd>
<dt class="hdlist1">
<code>void *hashmap_get_from_hash(const struct hashmap *map, unsigned int hash, const void *keydata)</code>
</dt>
<dd>
<p>
        Returns the hashmap entry for the specified hash code and key data,
        or NULL if not found.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>hash</code> is the hash code of the entry to look up.</p></div>
<div class="paragraph"><p>If an entry with matching hash code is found, <code>keydata</code> is passed to
<code>hashmap_cmp_fn</code> to decide whether the entry matches the key. The
<code>entry_or_key</code> parameter points to a bogus hashmap_entry structure that
should not be used in the comparison.</p></div>
</dd>
<dt class="hdlist1">
<code>void *hashmap_get_next(const struct hashmap *map, const void *entry)</code>
</dt>
<dd>
<p>
        Returns the next equal hashmap entry, or NULL if not found. This can be
        used to iterate over duplicate entries (see <code>hashmap_add</code>).
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>entry</code> is the hashmap_entry to start the search from, obtained via a previous
call to <code>hashmap_get</code> or <code>hashmap_get_next</code>.</p></div>
</dd>
<dt class="hdlist1">
<code>void hashmap_add(struct hashmap *map, void *entry)</code>
</dt>
<dd>
<p>
        Adds a hashmap entry. This allows to add duplicate entries (i.e.
        separate values with the same key according to hashmap_cmp_fn).
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>entry</code> is the entry to add.</p></div>
</dd>
<dt class="hdlist1">
<code>void *hashmap_put(struct hashmap *map, void *entry)</code>
</dt>
<dd>
<p>
        Adds or replaces a hashmap entry. If the hashmap contains duplicate
        entries equal to the specified entry, only one of them will be replaced.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>entry</code> is the entry to add or replace.</p></div>
<div class="paragraph"><p>Returns the replaced entry, or NULL if not found (i.e. the entry was added).</p></div>
</dd>
<dt class="hdlist1">
<code>void *hashmap_remove(struct hashmap *map, const void *key, const void *keydata)</code>
</dt>
<dd>
<p>
        Removes a hashmap entry matching the specified key. If the hashmap
        contains duplicate entries equal to the specified key, only one of
        them will be removed.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>key</code> is a hashmap_entry structure (or user data structure that starts with
hashmap_entry) that has at least been initialized with the proper hash code
(via <code>hashmap_entry_init</code>).</p></div>
<div class="paragraph"><p>If an entry with matching hash code is found, <code>key</code> and <code>keydata</code> are
passed to <code>hashmap_cmp_fn</code> to decide whether the entry matches the key.</p></div>
<div class="paragraph"><p>Returns the removed entry, or NULL if not found.</p></div>
</dd>
<dt class="hdlist1">
<code>void hashmap_iter_init(struct hashmap *map, struct hashmap_iter *iter)</code>
</dt>
<dt class="hdlist1">
<code>void *hashmap_iter_next(struct hashmap_iter *iter)</code>
</dt>
<dt class="hdlist1">
<code>void *hashmap_iter_first(struct hashmap *map, struct hashmap_iter *iter)</code>
</dt>
<dd>
<p>
        Used to iterate over all entries of a hashmap.
</p>
<div class="paragraph"><p><code>hashmap_iter_init</code> initializes a <code>hashmap_iter</code> structure.</p></div>
<div class="paragraph"><p><code>hashmap_iter_next</code> returns the next hashmap_entry, or NULL if there are no
more entries.</p></div>
<div class="paragraph"><p><code>hashmap_iter_first</code> is a combination of both (i.e. initializes the iterator
and returns the first entry, if any).</p></div>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_usage_example">Usage example</h2>
<div class="sectionbody">
<div class="paragraph"><p>Here&#8217;s a simple usage example that maps long keys to double values.</p></div>
<div class="listingblock">
<div class="content">
<pre><tt><span style="font-weight: bold"><span style="color: #0000FF">struct</span></span> <span style="color: #008080">hashmap</span> map<span style="color: #990000">;</span>

<span style="font-weight: bold"><span style="color: #0000FF">struct</span></span> <span style="color: #008080">long2double</span> <span style="color: #FF0000">{</span>
        <span style="font-weight: bold"><span style="color: #0000FF">struct</span></span> <span style="color: #008080">hashmap_entry</span> ent<span style="color: #990000">;</span> <span style="font-style: italic"><span style="color: #9A1900">/* must be the first member! */</span></span>
        <span style="color: #009900">long</span> key<span style="color: #990000">;</span>
        <span style="color: #009900">double</span> value<span style="color: #990000">;</span>
<span style="color: #FF0000">}</span><span style="color: #990000">;</span>

<span style="font-weight: bold"><span style="color: #0000FF">static</span></span> <span style="color: #009900">int</span> <span style="font-weight: bold"><span style="color: #000000">long2double_cmp</span></span><span style="color: #990000">(</span><span style="font-weight: bold"><span style="color: #0000FF">const</span></span> <span style="font-weight: bold"><span style="color: #0000FF">struct</span></span> <span style="color: #008080">long2double</span> <span style="color: #990000">*</span>e1<span style="color: #990000">,</span> <span style="font-weight: bold"><span style="color: #0000FF">const</span></span> <span style="font-weight: bold"><span style="color: #0000FF">struct</span></span> <span style="color: #008080">long2double</span> <span style="color: #990000">*</span>e2<span style="color: #990000">,</span> <span style="font-weight: bold"><span style="color: #0000FF">const</span></span> <span style="color: #009900">void</span> <span style="color: #990000">*</span>unused<span style="color: #990000">)</span>
<span style="color: #FF0000">{</span>
        <span style="font-weight: bold"><span style="color: #0000FF">return</span></span> <span style="color: #990000">!(</span>e1<span style="color: #990000">-&gt;</span>key <span style="color: #990000">==</span> e2<span style="color: #990000">-&gt;</span>key<span style="color: #990000">);</span>
<span style="color: #FF0000">}</span>

<span style="color: #009900">void</span> <span style="font-weight: bold"><span style="color: #000000">long2double_init</span></span><span style="color: #990000">(</span><span style="color: #009900">void</span><span style="color: #990000">)</span>
<span style="color: #FF0000">{</span>
        <span style="font-weight: bold"><span style="color: #000000">hashmap_init</span></span><span style="color: #990000">(&amp;</span>map<span style="color: #990000">,</span> <span style="color: #990000">(</span>hashmap_cmp_fn<span style="color: #990000">)</span> long2double_cmp<span style="color: #990000">,</span> <span style="color: #993399">0</span><span style="color: #990000">);</span>
<span style="color: #FF0000">}</span>

<span style="color: #009900">void</span> <span style="font-weight: bold"><span style="color: #000000">long2double_free</span></span><span style="color: #990000">(</span><span style="color: #009900">void</span><span style="color: #990000">)</span>
<span style="color: #FF0000">{</span>
        <span style="font-weight: bold"><span style="color: #000000">hashmap_free</span></span><span style="color: #990000">(&amp;</span>map<span style="color: #990000">,</span> <span style="color: #993399">1</span><span style="color: #990000">);</span>
<span style="color: #FF0000">}</span>

<span style="font-weight: bold"><span style="color: #0000FF">static</span></span> <span style="font-weight: bold"><span style="color: #0000FF">struct</span></span> <span style="color: #008080">long2double</span> <span style="color: #990000">*</span><span style="font-weight: bold"><span style="color: #000000">find_entry</span></span><span style="color: #990000">(</span><span style="color: #009900">long</span> key<span style="color: #990000">)</span>
<span style="color: #FF0000">{</span>
        <span style="font-weight: bold"><span style="color: #0000FF">struct</span></span> <span style="color: #008080">long2double</span> k<span style="color: #990000">;</span>
        <span style="font-weight: bold"><span style="color: #000000">hashmap_entry_init</span></span><span style="color: #990000">(&amp;</span>k<span style="color: #990000">,</span> <span style="font-weight: bold"><span style="color: #000000">memhash</span></span><span style="color: #990000">(&amp;</span>key<span style="color: #990000">,</span> <span style="font-weight: bold"><span style="color: #0000FF">sizeof</span></span><span style="color: #990000">(</span><span style="color: #009900">long</span><span style="color: #990000">)));</span>
        k<span style="color: #990000">.</span>key <span style="color: #990000">=</span> key<span style="color: #990000">;</span>
        <span style="font-weight: bold"><span style="color: #0000FF">return</span></span> <span style="font-weight: bold"><span style="color: #000000">hashmap_get</span></span><span style="color: #990000">(&amp;</span>map<span style="color: #990000">,</span> <span style="color: #990000">&amp;</span>k<span style="color: #990000">,</span> NULL<span style="color: #990000">);</span>
<span style="color: #FF0000">}</span>

<span style="color: #009900">double</span> <span style="font-weight: bold"><span style="color: #000000">get_value</span></span><span style="color: #990000">(</span><span style="color: #009900">long</span> key<span style="color: #990000">)</span>
<span style="color: #FF0000">{</span>
        <span style="font-weight: bold"><span style="color: #0000FF">struct</span></span> <span style="color: #008080">long2double</span> <span style="color: #990000">*</span>e <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">find_entry</span></span><span style="color: #990000">(</span>key<span style="color: #990000">);</span>
        <span style="font-weight: bold"><span style="color: #0000FF">return</span></span> e <span style="color: #990000">?</span> e<span style="color: #990000">-&gt;</span>value <span style="color: #990000">:</span> <span style="color: #993399">0</span><span style="color: #990000">;</span>
<span style="color: #FF0000">}</span>

<span style="color: #009900">void</span> <span style="font-weight: bold"><span style="color: #000000">set_value</span></span><span style="color: #990000">(</span><span style="color: #009900">long</span> key<span style="color: #990000">,</span> <span style="color: #009900">double</span> value<span style="color: #990000">)</span>
<span style="color: #FF0000">{</span>
        <span style="font-weight: bold"><span style="color: #0000FF">struct</span></span> <span style="color: #008080">long2double</span> <span style="color: #990000">*</span>e <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">find_entry</span></span><span style="color: #990000">(</span>key<span style="color: #990000">);</span>
        <span style="font-weight: bold"><span style="color: #0000FF">if</span></span> <span style="color: #990000">(!</span>e<span style="color: #990000">)</span> <span style="color: #FF0000">{</span>
                e <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">malloc</span></span><span style="color: #990000">(</span><span style="font-weight: bold"><span style="color: #0000FF">sizeof</span></span><span style="color: #990000">(</span><span style="font-weight: bold"><span style="color: #0000FF">struct</span></span> <span style="color: #008080">long2double</span><span style="color: #990000">));</span>
                <span style="font-weight: bold"><span style="color: #000000">hashmap_entry_init</span></span><span style="color: #990000">(</span>e<span style="color: #990000">,</span> <span style="font-weight: bold"><span style="color: #000000">memhash</span></span><span style="color: #990000">(&amp;</span>key<span style="color: #990000">,</span> <span style="font-weight: bold"><span style="color: #0000FF">sizeof</span></span><span style="color: #990000">(</span><span style="color: #009900">long</span><span style="color: #990000">)));</span>
                e<span style="color: #990000">-&gt;</span>key <span style="color: #990000">=</span> key<span style="color: #990000">;</span>
                <span style="font-weight: bold"><span style="color: #000000">hashmap_add</span></span><span style="color: #990000">(&amp;</span>map<span style="color: #990000">,</span> e<span style="color: #990000">);</span>
        <span style="color: #FF0000">}</span>
        e<span style="color: #990000">-&gt;</span>value <span style="color: #990000">=</span> value<span style="color: #990000">;</span>
<span style="color: #FF0000">}</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_using_variable_sized_keys">Using variable-sized keys</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <code>hashmap_entry_get</code> and <code>hashmap_entry_remove</code> functions expect an ordinary
<code>hashmap_entry</code> structure as key to find the correct entry. If the key data is
variable-sized (e.g. a FLEX_ARRAY string) or quite large, it is undesirable
to create a full-fledged entry structure on the heap and copy all the key data
into the structure.</p></div>
<div class="paragraph"><p>In this case, the <code>keydata</code> parameter can be used to pass
variable-sized key data directly to the comparison function, and the <code>key</code>
parameter can be a stripped-down, fixed size entry structure allocated on the
stack.</p></div>
<div class="paragraph"><p>See test-hashmap.c for an example using arbitrary-length strings as keys.</p></div>
</div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Last updated 2026-01-13 07:24:07 UTC
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