12 All declarations are in :file:`jansson.h`, so it's enough to
20 All constants are prefixed ``JSON_`` and other identifiers with
21 ``json_``. Type names are suffixed with ``_t`` and ``typedef``\ 'd so
22 that the ``struct`` keyword need not be used.
28 The JSON specification (:rfc:`4627`) defines the following data types:
29 *object*, *array*, *string*, *number*, *boolean*, and *null*. JSON
30 types are used dynamically; arrays and objects can hold any other data
31 type, including themselves. For this reason, Jansson's type system is
32 also dynamic in nature. There's one C type to represent all JSON
33 values, and this structure knows the type of the JSON value it holds.
37 This data structure is used throughout the library to represent all
38 JSON values. It always contains the type of the JSON value it holds
39 and the value's reference count. The rest depends on the type of the
42 Objects of :ctype:`json_t` are always used through a pointer. There
43 are APIs for querying the type, manipulating the reference count, and
44 for constructing and manipulating values of different types.
46 Unless noted otherwise, all API functions return an error value if an
47 error occurs. Depending on the function's signature, the error value
48 is either *NULL* or -1. Invalid arguments or invalid input are
49 apparent sources for errors. Memory allocation and I/O operations may
56 The type of a JSON value is queried and tested using the following
59 .. ctype:: enum json_type
61 The type of a JSON value. The following members are defined:
63 +-------------------------+
64 | :const:`JSON_OBJECT` |
65 +-------------------------+
66 | :const:`JSON_ARRAY` |
67 +-------------------------+
68 | :const:`JSON_STRING` |
69 +-------------------------+
70 | :const:`JSON_INTEGER` |
71 +-------------------------+
72 | :const:`JSON_REAL` |
73 +-------------------------+
74 | :const:`JSON_TRUE` |
75 +-------------------------+
76 | :const:`JSON_FALSE` |
77 +-------------------------+
78 | :const:`JSON_NULL` |
79 +-------------------------+
81 These correspond to JSON object, array, string, number, boolean and
82 null. A number is represented by either a value of the type
83 :const:`JSON_INTEGER` or of the type :const:`JSON_REAL`. A true
84 boolean value is represented by a value of the type
85 :const:`JSON_TRUE` and false by a value of the type
88 .. cfunction:: int json_typeof(const json_t *json)
90 Return the type of the JSON value (a :ctype:`json_type` cast to
91 :ctype:`int`). *json* MUST NOT be *NULL*. This function is actually
92 implemented as a macro for speed.
94 .. cfunction:: json_is_object(const json_t *json)
95 json_is_array(const json_t *json)
96 json_is_string(const json_t *json)
97 json_is_integer(const json_t *json)
98 json_is_real(const json_t *json)
99 json_is_true(const json_t *json)
100 json_is_false(const json_t *json)
101 json_is_null(const json_t *json)
103 These functions (actually macros) return true (non-zero) for values
104 of the given type, and false (zero) for values of other types and
107 .. cfunction:: json_is_number(const json_t *json)
109 Returns true for values of types :const:`JSON_INTEGER` and
110 :const:`JSON_REAL`, and false for other types and for *NULL*.
112 .. cfunction:: json_is_boolean(const json_t *json)
114 Returns true for types :const:`JSON_TRUE` and :const:`JSON_FALSE`,
115 and false for values of other types and for *NULL*.
118 .. _apiref-reference-count:
123 The reference count is used to track whether a value is still in use
124 or not. When a value is created, it's reference count is set to 1. If
125 a reference to a value is kept (e.g. a value is stored somewhere for
126 later use), its reference count is incremented, and when the value is
127 no longer needed, the reference count is decremented. When the
128 reference count drops to zero, there are no references left, and the
129 value can be destroyed.
131 The following functions are used to manipulate the reference count.
133 .. cfunction:: json_t *json_incref(json_t *json)
135 Increment the reference count of *json* if it's not non-*NULL*.
138 .. cfunction:: void json_decref(json_t *json)
140 Decrement the reference count of *json*. As soon as a call to
141 :cfunc:`json_decref()` drops the reference count to zero, the value
142 is destroyed and it can no longer be used.
144 Functions creating new JSON values set the reference count to 1. These
145 functions are said to return a **new reference**. Other functions
146 returning (existing) JSON values do not normally increase the
147 reference count. These functions are said to return a **borrowed
148 reference**. So, if the user will hold a reference to a value returned
149 as a borrowed reference, he must call :cfunc:`json_incref`. As soon as
150 the value is no longer needed, :cfunc:`json_decref` should be called
151 to release the reference.
153 Normally, all functions accepting a JSON value as an argument will
154 manage the reference, i.e. increase and decrease the reference count
155 as needed. However, some functions **steal** the reference, i.e. they
156 have the same result as if the user called :cfunc:`json_decref()` on
157 the argument right after calling the function. These are usually
158 convenience functions for adding new references to containers and not
159 to worry about the reference count.
161 In the following sections it is clearly documented whether a function
162 will return a new or borrowed reference or steal a reference to its
169 A circular reference is created when an object or an array is,
170 directly or indirectly, inserted inside itself. The direct case is
173 json_t *obj = json_object();
174 json_object_set(obj, "foo", obj);
176 Jansson will refuse to do this, and :cfunc:`json_object_set()` (and
177 all the other such functions for objects and arrays) will return with
178 an error status. The indirect case is the dangerous one::
180 json_t *arr1 = json_array(), *arr2 = json_array();
181 json_array_append(arr1, arr2);
182 json_array_append(arr2, arr1);
184 In this example, the array ``arr2`` is contained in the array
185 ``arr1``, and vice versa. Jansson cannot check for this kind of
186 indirect circular references without a performance hit, so it's up to
187 the user to avoid them.
189 If a circular reference is created, the memory consumed by the values
190 cannot be freed by :cfunc:`json_decref()`. The reference counts never
191 drops to zero because the values are keeping the circular reference to
192 themselves. Moreover, trying to encode the values with any of the
193 encoding functions will fail. The encoder detects circular references
194 and returns an error status.
200 These values are implemented as singletons, so each of these functions
201 returns the same value each time.
203 .. cfunction:: json_t *json_true(void)
207 Returns the JSON true value.
209 .. cfunction:: json_t *json_false(void)
213 Returns the JSON false value.
215 .. cfunction:: json_t *json_null(void)
219 Returns the JSON null value.
225 .. cfunction:: json_t *json_string(const char *value)
229 Returns a new JSON string, or *NULL* on error. *value* must be a
230 valid UTF-8 encoded Unicode string.
232 .. cfunction:: const char *json_string_value(const json_t *string)
234 Returns the associated value of *string* as a null terminated UTF-8
235 encoded string, or *NULL* if *string* is not a JSON string.
237 .. cfunction:: int json_string_set(const json_t *string, const char *value)
239 Sets the associated value of *string* to *value*. *value* must be a
240 valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
243 .. versionadded:: 1.1
249 .. cfunction:: json_t *json_integer(int value)
253 Returns a new JSON integer, or *NULL* on error.
255 .. cfunction:: int json_integer_value(const json_t *integer)
257 Returns the associated value of *integer*, or 0 if *json* is not a
260 .. cfunction:: int json_integer_set(const json_t *integer, int value)
262 Sets the associated value of *integer* to *value*. Returns 0 on
263 success and -1 if *integer* is not a JSON integer.
265 .. versionadded:: 1.1
267 .. cfunction:: json_t *json_real(double value)
271 Returns a new JSON real, or *NULL* on error.
273 .. cfunction:: double json_real_value(const json_t *real)
275 Returns the associated value of *real*, or 0.0 if *real* is not a
278 .. cfunction:: int json_real_set(const json_t *real, double value)
280 Sets the associated value of *real* to *value*. Returns 0 on
281 success and -1 if *real* is not a JSON real.
283 .. versionadded:: 1.1
285 In addition to the functions above, there's a common query function
286 for integers and reals:
288 .. cfunction:: double json_number_value(const json_t *json)
290 Returns the associated value of the JSON integer or JSON real
291 *json*, cast to double regardless of the actual type. If *json* is
292 neither JSON real nor JSON integer, 0.0 is returned.
298 A JSON array is an ordered collection of other JSON values.
300 .. cfunction:: json_t *json_array(void)
304 Returns a new JSON array, or *NULL* on error. Initially, the array
307 .. cfunction:: unsigned int json_array_size(const json_t *array)
309 Returns the number of elements in *array*, or 0 if *array* is NULL
312 .. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
314 .. refcounting:: borrow
316 Returns the element in *array* at position *index*. The valid range
317 for *index* is from 0 to the return value of
318 :cfunc:`json_array_size()` minus 1. If *array* is not a JSON array,
319 if *array* is *NULL*, or if *index* is out of range, *NULL* is
322 .. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
324 Replaces the element in *array* at position *index* with *value*.
325 The valid range for *index* is from 0 to the return value of
326 :cfunc:`json_array_size()` minus 1. Returns 0 on success and -1 on
329 .. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
331 Like :cfunc:`json_array_set()` but steals the reference to *value*.
332 This is useful when *value* is newly created and not used after
335 .. versionadded:: 1.1
337 .. cfunction:: int json_array_append(json_t *array, json_t *value)
339 Appends *value* to the end of *array*, growing the size of *array*
340 by 1. Returns 0 on success and -1 on error.
342 .. cfunction:: int json_array_append_new(json_t *array, json_t *value)
344 Like :cfunc:`json_array_append()` but steals the reference to
345 *value*. This is useful when *value* is newly created and not used
348 .. versionadded:: 1.1
350 .. cfunction:: int json_array_insert(json_t *array, unsigned int index, json_t *value)
352 Inserts *value* to *array* at position *index*, shifting the
353 elements at *index* and after it one position towards the end of
354 the array. Returns 0 on success and -1 on error.
356 .. versionadded:: 1.1
358 .. cfunction:: int json_array_insert_new(json_t *array, unsigned int index, json_t *value)
360 Like :cfunc:`json_array_insert()` but steals the reference to
361 *value*. This is useful when *value* is newly created and not used
364 .. versionadded:: 1.1
366 .. cfunction:: int json_array_remove(json_t *array, unsigned int index)
368 Removes the element in *array* at position *index*, shifting the
369 elements after *index* one position towards the start of the array.
370 Returns 0 on success and -1 on error.
372 .. versionadded:: 1.1
374 .. cfunction:: int json_array_clear(json_t *array)
376 Removes all elements from *array*. Returns 0 on sucess and -1 on
379 .. versionadded:: 1.1
381 .. cfunction:: int json_array_extend(json_t *array, json_t *other_array)
383 Appends all elements in *other_array* to the end of *array*.
384 Returns 0 on success and -1 on error.
386 .. versionadded:: 1.1
392 A JSON object is a dictionary of key-value pairs, where the key is a
393 Unicode string and the value is any JSON value.
395 .. cfunction:: json_t *json_object(void)
399 Returns a new JSON object, or *NULL* on error. Initially, the
402 .. cfunction:: unsigned int json_object_size(const json_t *object)
404 Returns the number of elements in *object*, or 0 if *object* is not
407 .. versionadded:: 1.1
409 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
411 .. refcounting:: borrow
413 Get a value corresponding to *key* from *object*. Returns *NULL* if
414 *key* is not found and on error.
416 .. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)
418 Set the value of *key* to *value* in *object*. *key* must be a
419 valid null terminated UTF-8 encoded Unicode string. If there
420 already is a value for *key*, it is replaced by the new value.
421 Returns 0 on success and -1 on error.
423 .. cfunction:: int json_object_set_new(json_t *object, const char *key, json_t *value)
425 Like :cfunc:`json_object_set()` but steals the reference to
426 *value*. This is useful when *value* is newly created and not used
429 .. versionadded:: 1.1
431 .. cfunction:: int json_object_del(json_t *object, const char *key)
433 Delete *key* from *object* if it exists. Returns 0 on success, or
434 -1 if *key* was not found.
437 .. cfunction:: int json_object_clear(json_t *object)
439 Remove all elements from *object*. Returns 0 on success and -1 if
440 *object* is not a JSON object.
442 .. versionadded:: 1.1
444 .. cfunction:: int json_object_update(json_t *object, json_t *other)
446 Update *object* with the key-value pairs from *other*, overwriting
447 existing keys. Returns 0 on success or -1 on error.
449 .. versionadded:: 1.1
452 The following functions implement an iteration protocol for objects:
454 .. cfunction:: void *json_object_iter(json_t *object)
456 Returns an opaque iterator which can be used to iterate over all
457 key-value pairs in *object*, or *NULL* if *object* is empty.
459 .. cfunction:: void *json_object_iter_next(json_t *object, void *iter)
461 Returns an iterator pointing to the next key-value pair in *object*
462 after *iter*, or *NULL* if the whole object has been iterated
465 .. cfunction:: const char *json_object_iter_key(void *iter)
467 Extract the associated key from *iter*.
469 .. cfunction:: json_t *json_object_iter_value(void *iter)
471 .. refcounting:: borrow
473 Extract the associated value from *iter*.
475 The iteration protocol can be used for example as follows::
477 /* obj is a JSON object */
480 void *iter = json_object_iter(obj);
483 key = json_object_iter_key(iter);
484 value = json_object_iter_value(iter);
485 /* use key and value ... */
486 iter = json_object_iter_next(obj, iter);
493 This sections describes the functions that can be used to encode
494 values to JSON. Only objects and arrays can be encoded, since they are
495 the only valid "root" values of a JSON text.
497 Each function takes a *flags* parameter that controls some aspects of
498 how the data is encoded. Its default value is 0. The following macros
499 can be ORed together to obtain *flags*.
502 Pretty-print the result, indenting arrays and objects by *n*
503 spaces. The valid range for *n* is between 0 and 255, other values
504 result in an undefined output. If ``JSON_INDENT`` is not used or
505 *n* is 0, no pretty-printing is done and the result is a compact
508 The following functions perform the actual JSON encoding. The result
511 .. cfunction:: char *json_dumps(const json_t *root, unsigned long flags)
513 Returns the JSON representation of *root* as a string, or *NULL* on
514 error. *flags* is described above. The return value must be freed
515 by the caller using :cfunc:`free()`.
517 .. cfunction:: int json_dumpf(const json_t *root, FILE *output, unsigned long flags)
519 Write the JSON representation of *root* to the stream *output*.
520 *flags* is described above. Returns 0 on success and -1 on error.
521 If an error occurs, something may have already been written to
522 *output*. In this case, the output is undefined and most likely not
525 .. cfunction:: int json_dump_file(const json_t *json, const char *path, unsigned long flags)
527 Write the JSON representation of *root* to the file *path*. If
528 *path* already exists, it is overwritten. *flags* is described
529 above. Returns 0 on success and -1 on error.
535 This sections describes the functions that can be used to decode JSON
536 text to the Jansson representation of JSON data. The JSON
537 specification requires that a JSON text is either a serialized array
538 or object, and this requirement is also enforced with the following
541 The only supported character encoding is UTF-8 (which ASCII is a
544 .. ctype:: json_error_t
546 This data structure is used to return information on decoding
547 errors from the decoding functions. Its definition is repeated
550 #define JSON_ERROR_TEXT_LENGTH 160
553 char text[JSON_ERROR_TEXT_LENGTH];
557 *line* is the line number on which the error occurred, or -1 if
558 this information is not available. *text* contains the error
559 message (in UTF-8), or an empty string if a message is not
562 The normal usef of :ctype:`json_error_t` is to allocate it normally
563 on the stack, and pass a pointer to a decoding function. Example::
569 json = json_load_file("/path/to/file.json", &error);
571 /* the error variable contains error information */
576 Also note that if the decoding succeeded (``json != NULL`` in the
577 above example), the contents of ``error`` are unspecified.
579 All decoding functions also accept *NULL* as the
580 :ctype:`json_error_t` pointer, in which case no error information
581 is returned to the caller.
583 The following functions perform the actual JSON decoding.
585 .. cfunction:: json_t *json_loads(const char *input, json_error_t *error)
589 Decodes the JSON string *input* and returns the array or object it
590 contains, or *NULL* on error, in which case *error* is filled with
591 information about the error. See above for discussion on the
594 .. cfunction:: json_t *json_loadf(FILE *input, json_error_t *error)
598 Decodes the JSON text in stream *input* and returns the array or
599 object it contains, or *NULL* on error, in which case *error* is
600 filled with information about the error. See above for discussion
601 on the *error* parameter.
603 .. cfunction:: json_t *json_load_file(const char *path, json_error_t *error)
607 Decodes the JSON text in file *path* and returns the array or
608 object it contains, or *NULL* on error, in which case *error* is
609 filled with information about the error. See above for discussion
610 on the *error* parameter.