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<div class="section" id="portability">
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<h1>Portability<a class="headerlink" href="#portability" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="thread-safety">
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<span id="portability-thread-safety"></span><h2>Thread safety<a class="headerlink" href="#thread-safety" title="Permalink to this headline">¶</a></h2>
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<p>Jansson is thread safe and has no mutable global state. The only
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exceptions are the hash function seed and memory allocation functions,
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see below.</p>
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<p>There’s no locking performed inside Jansson’s code, so a multithreaded
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program must perform its own locking if JSON values are shared by
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multiple threads. Jansson’s reference counting semantics may make this
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a bit harder than it seems, as it’s possible to have a reference to a
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value that’s also stored inside a list or object. Modifying the
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container (adding or removing values) may trigger concurrent access to
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such values, as containers manage the reference count of their
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contained values. Bugs involving concurrent incrementing or
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decrementing of deference counts may be hard to track.</p>
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<p>The encoding functions (<a class="reference internal" href="apiref.html#c.json_dumps" title="json_dumps"><tt class="xref c c-func docutils literal"><span class="pre">json_dumps()</span></tt></a> and friends) track
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reference loops by modifying the internal state of objects and arrays.
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For this reason, encoding functions must not be run on the same JSON
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values in two separate threads at the same time. As already noted
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above, be especially careful if two arrays or objects share their
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contained values with another array or object.</p>
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<p>If you want to make sure that two JSON value hierarchies do not
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contain shared values, use <a class="reference internal" href="apiref.html#c.json_deep_copy" title="json_deep_copy"><tt class="xref c c-func docutils literal"><span class="pre">json_deep_copy()</span></tt></a> to make copies.</p>
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<div class="section" id="hash-function-seed">
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<h3>Hash function seed<a class="headerlink" href="#hash-function-seed" title="Permalink to this headline">¶</a></h3>
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<p>To prevent an attacker from intentionally causing large JSON objects
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with specially crafted keys to perform very slow, the hash function
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used by Jansson is randomized using a seed value. The seed is
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automatically generated on the first explicit or implicit call to
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<a class="reference internal" href="apiref.html#c.json_object" title="json_object"><tt class="xref c c-func docutils literal"><span class="pre">json_object()</span></tt></a>, if <a class="reference internal" href="apiref.html#c.json_object_seed" title="json_object_seed"><tt class="xref c c-func docutils literal"><span class="pre">json_object_seed()</span></tt></a> has not been
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called beforehand.</p>
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<p>The seed is generated by using operating system’s entropy sources if
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they are available (<tt class="docutils literal"><span class="pre">/dev/urandom</span></tt>, <tt class="docutils literal"><span class="pre">CryptGenRandom()</span></tt>). The
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initialization is done in as thread safe manner as possible, by using
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architecture specific lockless operations if provided by the platform
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or the compiler.</p>
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<p>If you’re using threads, it’s recommended to autoseed the hashtable
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explicitly before spawning any threads by calling
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<tt class="docutils literal"><span class="pre">json_object_seed(0)</span></tt> , especially if you’re unsure whether the
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initialization is thread safe on your platform.</p>
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</div>
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<div class="section" id="memory-allocation-functions">
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<h3>Memory allocation functions<a class="headerlink" href="#memory-allocation-functions" title="Permalink to this headline">¶</a></h3>
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<p>Memory allocation functions should be set at most once, and only on
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program startup. See <a class="reference internal" href="apiref.html#apiref-custom-memory-allocation"><em>Custom Memory Allocation</em></a>.</p>
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</div>
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</div>
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<div class="section" id="locale">
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<h2>Locale<a class="headerlink" href="#locale" title="Permalink to this headline">¶</a></h2>
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<p>Jansson works fine under any locale.</p>
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<p>However, if the host program is multithreaded and uses <tt class="docutils literal"><span class="pre">setlocale()</span></tt>
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to switch the locale in one thread while Jansson is currently encoding
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or decoding JSON data in another thread, the result may be wrong or
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the program may even crash.</p>
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<p>Jansson uses locale specific functions for certain string conversions
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in the encoder and decoder, and then converts the locale specific
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values to/from the JSON representation. This fails if the locale
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changes between the string conversion and the locale-to-JSON
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conversion. This can only happen in multithreaded programs that use
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<tt class="docutils literal"><span class="pre">setlocale()</span></tt>, because <tt class="docutils literal"><span class="pre">setlocale()</span></tt> switches the locale for all
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running threads, not only the thread that calls <tt class="docutils literal"><span class="pre">setlocale()</span></tt>.</p>
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<p>If your program uses <tt class="docutils literal"><span class="pre">setlocale()</span></tt> as described above, consider
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using the thread-safe <tt class="docutils literal"><span class="pre">uselocale()</span></tt> instead.</p>
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<h3><a href="index.html">Table Of Contents</a></h3>
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<ul>
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<li><a class="reference internal" href="#">Portability</a><ul>
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<li><a class="reference internal" href="#thread-safety">Thread safety</a><ul>
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<li><a class="reference internal" href="#hash-function-seed">Hash function seed</a></li>
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<li><a class="reference internal" href="#memory-allocation-functions">Memory allocation functions</a></li>
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<li><a class="reference internal" href="#locale">Locale</a></li>
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