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.. _apiref:
*************
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.
Unless noted otherwise, all API functions return an error value if an
error occurs. Depending on the function's signature, the error value
is either *NULL* or -1. Invalid arguments or invalid input are
apparent sources for errors. Memory allocation and I/O operations may
also cause errors.
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`). *json* MUST NOT be *NULL*. 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 and
for *NULL*.
.. 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 and for *NULL*.
.. 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 and for *NULL*.
.. _apiref-reference-count:
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* if it's not non-*NULL*.
Returns *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.
Circular References
-------------------
A circular reference is created when an object or an array is,
directly or indirectly, inserted inside itself. The direct case is
simple::
json_t *obj = json_object();
json_object_set(obj, "foo", obj);
Jansson will refuse to do this, and :cfunc:`json_object_set()` (and
all the other such functions for objects and arrays) will return with
an error status. The indirect case is the dangerous one::
json_t *arr1 = json_array(), *arr2 = json_array();
json_array_append(arr1, arr2);
json_array_append(arr2, arr1);
In this example, the array ``arr2`` is contained in the array
``arr1``, and vice versa. Jansson cannot check for this kind of
indirect circular references without a performance hit, so it's up to
the user to avoid them.
If a circular reference is created, the memory consumed by the values
cannot be freed by :cfunc:`json_decref()`. The reference counts never
drops to zero because the values are keeping the circular reference to
themselves. Moreover, trying to encode the values with any of the
encoding functions will fail. The encoder detects circular references
and returns an error status.
True, False and Null
====================
These values are implemented as singletons, so each of these functions
returns the same value each time.
.. cfunction:: json_t *json_true(void)
.. refcounting:: new
Returns the JSON true value.
.. cfunction:: json_t *json_false(void)
.. refcounting:: new
Returns the JSON false value.
.. cfunction:: json_t *json_null(void)
.. refcounting:: new
Returns the JSON null value.
String
======
.. cfunction:: json_t *json_string(const char *value)
.. refcounting:: new
Returns a new 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 *string)
Returns the associated value of *string* as a null terminated UTF-8
encoded string, or *NULL* if *string* is not a JSON string.
.. cfunction:: int json_string_set(const json_t *string, const char *value)
Sets the associated value of *string* to *value*. *value* must be a
valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
error.
.. versionadded:: 1.1
Number
======
.. cfunction:: json_t *json_integer(int value)
.. refcounting:: new
Returns a new JSON integer, or *NULL* on error.
.. cfunction:: int json_integer_value(const json_t *integer)
Returns the associated value of *integer*, or 0 if *json* is not a
JSON integer.
.. cfunction:: int json_integer_set(const json_t *integer, int value)
Sets the associated value of *integer* to *value*. Returns 0 on
success and -1 if *integer* is not a JSON integer.
.. versionadded:: 1.1
.. cfunction:: json_t *json_real(double value)
.. refcounting:: new
Returns a new JSON real, or *NULL* on error.
.. cfunction:: double json_real_value(const json_t *real)
Returns the associated value of *real*, or 0.0 if *real* is not a
JSON real.
.. cfunction:: int json_real_set(const json_t *real, double value)
Sets the associated value of *real* to *value*. Returns 0 on
success and -1 if *real* is not a JSON real.
.. versionadded:: 1.1
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 associated value of the JSON integer or JSON real
*json*, cast to double regardless of the actual type. If *json* is
neither JSON real nor JSON integer, 0.0 is returned.
Array
=====
A JSON array is an ordered collection of other JSON values.
.. cfunction:: json_t *json_array(void)
.. refcounting:: new
Returns a new 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*, or 0 if *array* is NULL
or not a JSON array.
.. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
.. refcounting:: borrow
Returns the element in *array* at position *index*. The valid range
for *index* is from 0 to the return value of
:cfunc:`json_array_size()` minus 1. If *array* is not a JSON array,
if *array* is *NULL*, or if *index* is out of range, *NULL* is
returned.
.. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
Replaces the element in *array* at position *index* with *value*.
The valid range for *index* is from 0 to the return value of
:cfunc:`json_array_size()` minus 1. Returns 0 on success and -1 on
error.
.. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
Like :cfunc:`json_array_set()` but steals the reference to *value*.
This is useful when *value* is newly created and not used after
the call.
.. versionadded:: 1.1
.. 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.
.. cfunction:: int json_array_append_new(json_t *array, json_t *value)
Like :cfunc:`json_array_append()` but steals the reference to
*value*. This is useful when *value* is newly created and not used
after the call.
.. versionadded:: 1.1
.. cfunction:: int json_array_insert(json_t *array, unsigned int index, json_t *value)
Inserts *value* to *array* at position *index*, shifting the
elements at *index* and after it one position towards the end of
the array. Returns 0 on success and -1 on error.
.. versionadded:: 1.1
.. cfunction:: int json_array_insert_new(json_t *array, unsigned int index, json_t *value)
Like :cfunc:`json_array_insert()` but steals the reference to
*value*. This is useful when *value* is newly created and not used
after the call.
.. versionadded:: 1.1
.. cfunction:: int json_array_remove(json_t *array, unsigned int index)
Removes the element in *array* at position *index*, shifting the
elements after *index* one position towards the start of the array.
Returns 0 on success and -1 on error.
.. versionadded:: 1.1
.. cfunction:: int json_array_clear(json_t *array)
Removes all elements from *array*. Returns 0 on sucess and -1 on
error.
.. versionadded:: 1.1
.. cfunction:: int json_array_extend(json_t *array, json_t *other_array)
Appends all elements in *other_array* to the end of *array*.
Returns 0 on success and -1 on error.
.. versionadded:: 1.1
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 JSON object, or *NULL* on error. Initially, the
object is empty.
.. cfunction:: unsigned int json_object_size(const json_t *object)
Returns the number of elements in *object*, or 0 if *object* is not
a JSON object.
.. versionadded:: 1.1
.. 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 null 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_set_new(json_t *object, const char *key, json_t *value)
Like :cfunc:`json_object_set()` but steals the reference to
*value*. This is useful when *value* is newly created and not used
after the call.
.. versionadded:: 1.1
.. 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.
.. cfunction:: int json_object_clear(json_t *object)
Remove all elements from *object*. Returns 0 on success and -1 if
*object* is not a JSON object.
.. versionadded:: 1.1
.. cfunction:: int json_object_update(json_t *object, json_t *other)
Update *object* with the key-value pairs from *other*, overwriting
existing keys. Returns 0 on success or -1 on error.
.. versionadded:: 1.1
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*.
The iteration protocol can be used for example as follows::
/* obj is a JSON object */
const char *key;
json_t *value;
void *iter = json_object_iter(obj);
while(iter)
{
key = json_object_iter_key(iter);
value = json_object_iter_value(iter);
/* use key and value ... */
iter = json_object_iter_next(obj, 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.
By default, the output has no newlines, and spaces are used between
array and object elements for a readable output. This behavior can be
altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
described below. A newline is never appended to the end of the encoded
JSON data.
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, using newlines between array and object
items, and indenting with *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 newlines are inserted
between array and object items.
``JSON_COMPACT``
This flag enables a compact representation, i.e. sets the separator
between array and object items to ``","`` and between object keys
and values to ``":"``. Without this flag, the corresponding
separators are ``", "`` and ``": "`` for more readable output.
.. versionadded:: 1.2
``JSON_ENSURE_ASCII``
If this flag is used, the output is guaranteed to consist only of
ASCII characters. This is achived by escaping all Unicode
characters outside the ASCII range.
.. versionadded:: 1.2
The following functions perform the actual JSON encoding. The result
is in UTF-8.
.. cfunction:: char *json_dumps(const json_t *root, unsigned long 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, unsigned long flags)
Write the JSON representation of *root* to the stream *output*.
*flags* is described above. Returns 0 on success and -1 on error.
If an error occurs, something may have already been written to
*output*. In this case, the output is undefined and most likely not
valid JSON.
.. cfunction:: int json_dump_file(const json_t *json, const char *path, unsigned long 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.
|