***********

Portability

***********

.. _portability-thread-safety:

Thread safety

-------------

Jansson is thread safe and has no mutable global state. The only

exceptions are the hash function seed and memory allocation functions,

see below.

There's no locking performed inside Jansson's code, so a multithreaded

program must perform its own locking if JSON values are shared by

multiple threads. Jansson's reference counting semantics may make this

a bit harder than it seems, as it's possible to have a reference to a

value that's also stored inside a list or object. Modifying the

container (adding or removing values) may trigger concurrent access to

such values, as containers manage the reference count of their

contained values. Bugs involving concurrent incrementing or

decrementing of deference counts may be hard to track.

The encoding functions (:func:`json_dumps()` and friends) track

reference loops by modifying the internal state of objects and arrays.

For this reason, encoding functions must not be run on the same JSON

values in two separate threads at the same time. As already noted

above, be especially careful if two arrays or objects share their

contained values with another array or object.

If you want to make sure that two JSON value hierarchies do not

contain shared values, use :func:`json_deep_copy()` to make copies.

Hash function seed

==================

To prevent an attacker from intentionally causing large JSON objects

with specially crafted keys to perform very slow, the hash function

used by Jansson is randomized using a seed value. The seed is

automatically generated on the first explicit or implicit call to

:func:`json_object()`, if :func:`json_object_seed()` has not been

called beforehand.

The seed is generated by using operating system's entropy sources if

they are available (``/dev/urandom``, ``CryptGenRandom()``). The

initialization is done in as thread safe manner as possible, by using

architecture specific lockless operations if provided by the platform

or the compiler.

If you're using threads, it's recommended to autoseed the hashtable

explicitly before spawning any threads by calling

``json_object_seed(0)`` , especially if you're unsure whether the

initialization is thread safe on your platform.

Memory allocation functions

===========================

Memory allocation functions should be set at most once, and only on

program startup. See :ref:`apiref-custom-memory-allocation`.

Locale

------

Jansson works fine under any locale.

However, if the host program is multithreaded and uses ``setlocale()``

to switch the locale in one thread while Jansson is currently encoding

or decoding JSON data in another thread, the result may be wrong or

the program may even crash.

Jansson uses locale specific functions for certain string conversions

in the encoder and decoder, and then converts the locale specific

values to/from the JSON representation. This fails if the locale

changes between the string conversion and the locale-to-JSON

conversion. This can only happen in multithreaded programs that use

``setlocale()``, because ``setlocale()`` switches the locale for all

running threads, not only the thread that calls ``setlocale()``.

If your program uses ``setlocale()`` as described above, consider

using the thread-safe ``uselocale()`` instead.