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 (
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
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
json_object_seed() has not been
The seed is generated by using operating system’s entropy sources if
they are available (
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.
Jansson works fine under any locale.
However, if the host program is multithreaded and uses
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() switches the locale for all
running threads, not only the thread that calls
If your program uses
setlocale() as described above, consider
using the thread-safe