jansson/doc/apiref.rst

*************
API Reference
*************

.. highlight:: c

Preliminaries
=============

All declarations are in :file:`jansson.h`, so it's enough to

::

   #include <jansson.h>

in each source file.

All constants are prefixed ``JSON_`` and other identifiers with
``json_``. Type names are suffixed with ``_t`` and ``typedef``\ 'd so
that the ``struct`` keyword need not be used.


Value Representation
====================

The JSON specification (:rfc:`4627`) defines the following data types:
*object*, *array*, *string*, *number*, *boolean*, and *null*. JSON
types are used dynamically; arrays and objects can hold any other data
type, including themselves. For this reason, Jansson's type system is
also dynamic in nature. There's one C type to represent all JSON
values, and this structure knows the type of the JSON value it holds.

.. ctype:: json_t

  This data structure is used throughout the library to represent all
  JSON values. It always contains the type of the JSON value it holds
  and the value's reference count. The rest depends on the type of the
  value.

Objects of :ctype:`json_t` are always used through a pointer. There
are APIs for querying the type, manipulating the reference count, and
for constructing and manipulating values of different types.


Type
----

The type of a JSON value is queried and tested using the following
functions:

.. ctype:: enum json_type

   The type of a JSON value. The following members are defined:

   +-------------------------+
   | :const:`JSON_OBJECT`    |
   +-------------------------+
   | :const:`JSON_ARRAY`     |
   +-------------------------+
   | :const:`JSON_STRING`    |
   +-------------------------+
   | :const:`JSON_INTEGER`   |
   +-------------------------+
   | :const:`JSON_REAL`      |
   +-------------------------+
   | :const:`JSON_TRUE`      |
   +-------------------------+
   | :const:`JSON_FALSE`     |
   +-------------------------+
   | :const:`JSON_NULL`      |
   +-------------------------+

   These correspond to JSON object, array, string, number, boolean and
   null. A number is represented by either a value of the type
   :const:`JSON_INTEGER` or of the type :const:`JSON_REAL`. A true
   boolean value is represented by a value of the type
   :const:`JSON_TRUE` and false by a value of the type
   :const:`JSON_FALSE`.

.. cfunction:: int json_typeof(const json_t *json)

   Return the type of the JSON value (a :ctype:`json_type` cast to
   :ctype:`int`). This function is actually implemented as a macro for
   speed.

.. cfunction:: json_is_object(const json_t *json)
               json_is_array(const json_t *json)
               json_is_string(const json_t *json)
               json_is_integer(const json_t *json)
               json_is_real(const json_t *json)
               json_is_true(const json_t *json)
               json_is_false(const json_t *json)
               json_is_null(const json_t *json)

   These functions (actually macros) return true (non-zero) for values
   of the given type, and false (zero) for values of other types.

.. cfunction:: json_is_number(const json_t *json)

   Returns true for values of types :const:`JSON_INTEGER` and
   :const:`JSON_REAL`, and false for other types.

.. cfunction:: json_is_boolean(const json_t *json)

   Returns true for types :const:`JSON_TRUE` and :const:`JSON_FALSE`,
   and false for values of other types.


Reference Count
---------------

The reference count is used to track whether a value is still in use
or not. When a value is created, it's reference count is set to 1. If
a reference to a value is kept (e.g. a value is stored somewhere for
later use), its reference count is incremented, and when the value is
no longer needed, the reference count is decremented. When the
reference count drops to zero, there are no references left, and the
value can be destroyed.

The following functions are used to manipulate the reference count.

.. cfunction:: json_t *json_incref(json_t *json)

   Increment the reference count of *json*.

.. cfunction:: void json_decref(json_t *json)

   Decrement the reference count of *json*. As soon as a call to
   :cfunc:`json_decref()` drops the reference count to zero, the value
   is destroyed and it can no longer be used.

Functions creating new JSON values set the reference count to 1. These
functions are said to return a **new reference**. Other functions
returning (existing) JSON values do not normally increase the
reference count. These functions are said to return a **borrowed
reference**. So, if the user will hold a reference to a value returned
as a borrowed reference, he must call :cfunc:`json_incref`. As soon as
the value is no longer needed, :cfunc:`json_decref` should be called
to release the reference.

Normally, all functions accepting a JSON value as an argument will
manage the reference, i.e. increase and decrease the reference count
as needed. However, some functions **steal** the reference, i.e. they
have the same result as if the user called :cfunc:`json_decref()` on
the argument right after calling the function. These are usually
convenience functions for adding new references to containers and not
to worry about the reference count.

In the following sections it is clearly documented whether a function
will return a new or borrowed reference or steal a reference to its
argument.


True, False and Null
====================

.. cfunction:: json_t *json_true(void)

   .. refcounting:: new

   Returns a value of the type :const:`JSON_TRUE`, or *NULL* on
   error.

.. cfunction:: json_t *json_false(void)

   .. refcounting:: new

   Returns a value of the type :const:`JSON_FALSE`, or *NULL* on
   error.

.. cfunction:: json_t *json_null(void)

   .. refcounting:: new

   Returns a value of the type :const:`JSON_NULL`, or *NULL* on
   error.


String
======

.. cfunction:: json_t *json_string(const char *value)

   .. refcounting:: new

   Returns a new value of the type :const:`JSON_STRING`, or *NULL* on
   error. *value* must be a valid UTF-8 encoded Unicode string.

.. cfunction:: const char *json_string_value(const json_t *json)

   Returns the associated value of a :const:`JSON_STRING` value as a
   null terminated UTF-8 encoded string.


Number
======

.. cfunction:: json_t *json_integer(int value)

   .. refcounting:: new

   Returns a new value of the type :const:`JSON_INTEGER`, or *NULL* on
   error.

.. cfunction:: int json_integer_value(const json_t *json)

   Returns the associated integer value of values of the type
   :const:`JSON_INTEGER`, or 0 for values of other types.

.. cfunction:: json_t *json_real(double value)

   .. refcounting:: new

   Returns a new value of the type :const:`JSON_REAL`, or *NULL* on
   error.

.. cfunction:: double json_real_value(const json_t *json)

   Returns the associated real value of values of the type
   :const:`JSON_INTEGER`, or 0 for values of other types.

In addition to the functions above, there's a common query function
for integers and reals:

.. cfunction:: double json_number_value(const json_t *json)

   Returns the value of either ``JSON_INTEGER`` or ``JSON_REAL``, cast
   to double regardless of the actual type.


Array
=====

A JSON array is an ordered collection of other JSON values.

.. cfunction:: json_t *json_array(void)

   .. refcounting:: new

   Returns a new value of the type :const:`JSON_ARRAY`, or *NULL* on
   error. Initially, the array is empty.

.. cfunction:: unsigned int json_array_size(const json_t *array)

   Returns the number of elements in *array*.

.. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)

   .. refcounting:: borrow

   Returns the element in *array* at position *index*, or *NULL* if
   *index* is out of range. The valid range for *index* is from 0 to
   the return value of :cfunc:`json_array_size()` minus 1.

.. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)

   Replaces the element in *array* at position *index* with *value*.
   Returns 0 on success, or -1 if *index* is out of range. The valid
   range for *index* is from 0 to the return value of
   :cfunc:`json_array_size()` minus 1.

.. cfunction:: int json_array_append(json_t *array, json_t *value)

   Appends *value* to the end of *array*, growing the size of *array*
   by 1. Returns 0 on success and -1 on error.


Object
======

A JSON object is a dictionary of key-value pairs, where the key is a
Unicode string and the value is any JSON value.

.. cfunction:: json_t *json_object(void)

   .. refcounting:: new

   Returns a new value of the type :const:`JSON_OBJECT`, or *NULL* on
   error. Initially, the object is empty.

.. cfunction:: json_t *json_object_get(const json_t *object, const char *key)

   .. refcounting:: borrow

   Get a value corresponding to *key* from *object*. Returns *NULL* if
   *key* is not found and on error.

.. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)

   Set the value of *key* to *value* in *object*. *key* must be a
   valid terminated UTF-8 encoded Unicode string. If there already is
   a value for *key*, it is replaced by the new value. Returns 0 on
   success and -1 on error.

.. cfunction:: int json_object_del(json_t *object, const char *key)

   Delete *key* from *object* if it exists. Returns 0 on success, or
   -1 if *key* was not found.


The following functions implement an iteration protocol for objects:

.. cfunction:: void *json_object_iter(json_t *object)

   Returns an opaque iterator which can be used to iterate over all
   key-value pairs in *object*, or *NULL* if *object* is empty.

.. cfunction:: void *json_object_iter_next(json_t *object, void *iter)

   Returns an iterator pointing to the next key-value pair in *object*
   after *iter*, or *NULL* if the whole object has been iterated
   through.

.. cfunction:: const char *json_object_iter_key(void *iter)

   Extract the associated key from *iter*.

.. cfunction:: json_t *json_object_iter_value(void *iter)

   .. refcounting:: borrow

   Extract the associated value from *iter*.


Encoding
========

This sections describes the functions that can be used to encode
values to JSON. Only objects and arrays can be encoded, since they are
the only valid "root" values of a JSON text.

Each function takes a *flags* parameter that controls some aspects of
how the data is encoded. Its default value is 0. The following macros
can be ORed together to obtain *flags*.

``JSON_INDENT(n)``
   Pretty-print the result, indenting arrays and objects by *n*
   spaces. The valid range for *n* is between 0 and 255, other values
   result in an undefined output. If ``JSON_INDENT`` is not used or
   *n* is 0, no pretty-printing is done and the result is a compact
   representation.

The following functions perform the actual JSON encoding. The result
is in UTF-8.

.. cfunction:: char *json_dumps(const json_t *root, uint32_t flags)

   Returns the JSON representation of *root* as a string, or *NULL* on
   error. *flags* is described above. The return value must be freed
   by the caller using :cfunc:`free()`.

.. cfunction:: int json_dumpf(const json_t *root, FILE *output, uint32_t flags)

   Write the JSON representation of *root* to the stream *output*.
   *flags* is described above. Returns 0 on success and -1 on error.

.. cfunction:: int json_dump_file(const json_t *json, const char *path, uint32_t flags)

   Write the JSON representation of *root* to the file *path*. If
   *path* already exists, it is overwritten. *flags* is described
   above. Returns 0 on success and -1 on error.


Decoding
========

This sections describes the functions that can be used to decode JSON
text to the Jansson representation of JSON data. The JSON
specification requires that a JSON text is either a serialized array
or object, and this requirement is also enforced with the following
functions.

The only supported character encoding is UTF-8 (which ASCII is a
subset of).

.. ctype:: json_error_t

   This data structure is used to return information on decoding
   errors from the decoding functions. Its definition is repeated
   here::

      #define JSON_ERROR_TEXT_LENGTH  160

      typedef struct {
          char text[JSON_ERROR_TEXT_LENGTH];
          int line;
      } json_error_t;

   *line* is the line number on which the error occurred, or -1 if
   this information is not available. *text* contains the error
   message (in UTF-8), or an empty string if a message is not
   available.

   The normal usef of :ctype:`json_error_t` is to allocate it normally
   on the stack, and pass a pointer to a decoding function. Example::

      int main() {
          json_t *json;
          json_error_t error;

          json = json_load_file("/path/to/file.json", &error);
          if(!json) {
              /* the error variable contains error information */
          }
          ...
      }

   Also note that if the decoding succeeded (``json != NULL`` in the
   above example), the contents of ``error`` are unspecified.

   All decoding functions also accept *NULL* as the
   :ctype:`json_error_t` pointer, in which case no error information
   is returned to the caller.

The following functions perform the actual JSON decoding.

.. cfunction:: json_t *json_loads(const char *input, json_error_t *error)

   .. refcounting:: new

   Decodes the JSON string *input* and returns the array or object it
   contains, or *NULL* on error, in which case *error* is filled with
   information about the error. See above for discussion on the
   *error* parameter.

.. cfunction:: json_t *json_loadf(FILE *input, json_error_t *error)

   .. refcounting:: new

   Decodes the JSON text in stream *input* and returns the array or
   object it contains, or *NULL* on error, in which case *error* is
   filled with information about the error. See above for discussion
   on the *error* parameter.

.. cfunction:: json_t *json_load_file(const char *path, json_error_t *error)

   .. refcounting:: new

   Decodes the JSON text in file *path* and returns the array or
   object it contains, or *NULL* on error, in which case *error* is
   filled with information about the error. See above for discussion
   on the *error* parameter.