10 All declarations are in :file:`jansson.h`, so it's enough to
18 All constants are prefixed ``JSON_`` and other identifiers with
19 ``json_``. Type names are suffixed with ``_t`` and ``typedef``\ 'd so
20 that the ``struct`` keyword need not be used.
26 The JSON specification (:rfc:`4627`) defines the following data types:
27 *object*, *array*, *string*, *number*, *boolean*, and *null*. JSON
28 types are used dynamically; arrays and objects can hold any other data
29 type, including themselves. For this reason, Jansson's type system is
30 also dynamic in nature. There's one C type to represent all JSON
31 values, and this structure knows the type of the JSON value it holds.
35 This data structure is used throughout the library to represent all
36 JSON values. It always contains the type of the JSON value it holds
37 and the value's reference count. The rest depends on the type of the
40 Objects of :ctype:`json_t` are always used through a pointer. There
41 are APIs for querying the type, manipulating the reference count, and
42 for constructing and manipulating values of different types.
44 Unless noted otherwise, all API functions return an error value if an
45 error occurs. Depending on the function's signature, the error value
46 is either *NULL* or -1. Invalid arguments or invalid input are
47 apparent sources for errors. Memory allocation and I/O operations may
54 The type of a JSON value is queried and tested using the following
57 .. ctype:: enum json_type
59 The type of a JSON value. The following members are defined:
61 +-------------------------+
62 | :const:`JSON_OBJECT` |
63 +-------------------------+
64 | :const:`JSON_ARRAY` |
65 +-------------------------+
66 | :const:`JSON_STRING` |
67 +-------------------------+
68 | :const:`JSON_INTEGER` |
69 +-------------------------+
70 | :const:`JSON_REAL` |
71 +-------------------------+
72 | :const:`JSON_TRUE` |
73 +-------------------------+
74 | :const:`JSON_FALSE` |
75 +-------------------------+
76 | :const:`JSON_NULL` |
77 +-------------------------+
79 These correspond to JSON object, array, string, number, boolean and
80 null. A number is represented by either a value of the type
81 :const:`JSON_INTEGER` or of the type :const:`JSON_REAL`. A true
82 boolean value is represented by a value of the type
83 :const:`JSON_TRUE` and false by a value of the type
86 .. cfunction:: int json_typeof(const json_t *json)
88 Return the type of the JSON value (a :ctype:`json_type` cast to
89 :ctype:`int`). *json* MUST NOT be *NULL*. This function is actually
90 implemented as a macro for speed.
92 .. cfunction:: json_is_object(const json_t *json)
93 json_is_array(const json_t *json)
94 json_is_string(const json_t *json)
95 json_is_integer(const json_t *json)
96 json_is_real(const json_t *json)
97 json_is_true(const json_t *json)
98 json_is_false(const json_t *json)
99 json_is_null(const json_t *json)
101 These functions (actually macros) return true (non-zero) for values
102 of the given type, and false (zero) for values of other types and
105 .. cfunction:: json_is_number(const json_t *json)
107 Returns true for values of types :const:`JSON_INTEGER` and
108 :const:`JSON_REAL`, and false for other types and for *NULL*.
110 .. cfunction:: json_is_boolean(const json_t *json)
112 Returns true for types :const:`JSON_TRUE` and :const:`JSON_FALSE`,
113 and false for values of other types and for *NULL*.
119 The reference count is used to track whether a value is still in use
120 or not. When a value is created, it's reference count is set to 1. If
121 a reference to a value is kept (e.g. a value is stored somewhere for
122 later use), its reference count is incremented, and when the value is
123 no longer needed, the reference count is decremented. When the
124 reference count drops to zero, there are no references left, and the
125 value can be destroyed.
127 The following functions are used to manipulate the reference count.
129 .. cfunction:: json_t *json_incref(json_t *json)
131 Increment the reference count of *json* if it's not non-*NULL*.
134 .. cfunction:: void json_decref(json_t *json)
136 Decrement the reference count of *json*. As soon as a call to
137 :cfunc:`json_decref()` drops the reference count to zero, the value
138 is destroyed and it can no longer be used.
140 Functions creating new JSON values set the reference count to 1. These
141 functions are said to return a **new reference**. Other functions
142 returning (existing) JSON values do not normally increase the
143 reference count. These functions are said to return a **borrowed
144 reference**. So, if the user will hold a reference to a value returned
145 as a borrowed reference, he must call :cfunc:`json_incref`. As soon as
146 the value is no longer needed, :cfunc:`json_decref` should be called
147 to release the reference.
149 Normally, all functions accepting a JSON value as an argument will
150 manage the reference, i.e. increase and decrease the reference count
151 as needed. However, some functions **steal** the reference, i.e. they
152 have the same result as if the user called :cfunc:`json_decref()` on
153 the argument right after calling the function. These are usually
154 convenience functions for adding new references to containers and not
155 to worry about the reference count.
157 In the following sections it is clearly documented whether a function
158 will return a new or borrowed reference or steal a reference to its
165 A circular reference is created when an object or an array is,
166 directly or indirectly, inserted inside itself. The direct case is
169 json_t *obj = json_object();
170 json_object_set(obj, "foo", obj);
172 Jansson will refuse to do this, and :cfunc:`json_object_set()` (and
173 all the other such functions for objects and arrays) will return with
174 an error status. The indirect case is the dangerous one::
176 json_t *arr1 = json_array(), *arr2 = json_array();
177 json_array_append(arr1, arr2);
178 json_array_append(arr2, arr1);
180 In this example, the array ``arr2`` is contained in the array
181 ``arr1``, and vice versa. Jansson cannot check for this kind of
182 indirect circular references without a performance hit, so it's up to
183 the user to avoid them.
185 If a circular reference is created, the memory consumed by the values
186 cannot be freed by :cfunc:`json_decref()`. The reference counts never
187 drops to zero because the values are keeping the circular reference to
188 themselves. Moreover, trying to encode the values with any of the
189 encoding functions will fail. The encoder detects circular references
190 and returns an error status.
196 These values are implemented as singletons, so each of these functions
197 returns the same value each time.
199 .. cfunction:: json_t *json_true(void)
203 Returns the JSON true value.
205 .. cfunction:: json_t *json_false(void)
209 Returns the JSON false value.
211 .. cfunction:: json_t *json_null(void)
215 Returns the JSON null value.
221 .. cfunction:: json_t *json_string(const char *value)
225 Returns a new JSON string, or *NULL* on error. *value* must be a
226 valid UTF-8 encoded Unicode string.
228 .. cfunction:: const char *json_string_value(const json_t *string)
230 Returns the associated value of *string* as a null terminated UTF-8
231 encoded string, or *NULL* if *string* is not a JSON string.
233 .. cfunction:: int json_string_set(const json_t *string, const char *value)
235 Sets the associated value of *string* to *value*. *value* must be a
236 valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
239 .. versionadded:: 1.1
245 .. cfunction:: json_t *json_integer(int value)
249 Returns a new JSON integer, or *NULL* on error.
251 .. cfunction:: int json_integer_value(const json_t *integer)
253 Returns the associated value of *integer*, or 0 if *json* is not a
256 .. cfunction:: int json_integer_set(const json_t *integer, int value)
258 Sets the associated value of *integer* to *value*. Returns 0 on
259 success and -1 if *integer* is not a JSON integer.
261 .. versionadded:: 1.1
263 .. cfunction:: json_t *json_real(double value)
267 Returns a new JSON real, or *NULL* on error.
269 .. cfunction:: double json_real_value(const json_t *real)
271 Returns the associated value of *real*, or 0.0 if *real* is not a
274 .. cfunction:: int json_real_set(const json_t *real, double value)
276 Sets the associated value of *real* to *value*. Returns 0 on
277 success and -1 if *real* is not a JSON real.
279 .. versionadded:: 1.1
281 In addition to the functions above, there's a common query function
282 for integers and reals:
284 .. cfunction:: double json_number_value(const json_t *json)
286 Returns the associated value of the JSON integer or JSON real
287 *json*, cast to double regardless of the actual type. If *json* is
288 neither JSON real nor JSON integer, 0.0 is returned.
294 A JSON array is an ordered collection of other JSON values.
296 .. cfunction:: json_t *json_array(void)
300 Returns a new JSON array, or *NULL* on error. Initially, the array
303 .. cfunction:: unsigned int json_array_size(const json_t *array)
305 Returns the number of elements in *array*, or 0 if *array* is NULL
308 .. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
310 .. refcounting:: borrow
312 Returns the element in *array* at position *index*. The valid range
313 for *index* is from 0 to the return value of
314 :cfunc:`json_array_size()` minus 1. If *array* is not a JSON array,
315 if *array* is *NULL*, or if *index* is out of range, *NULL* is
318 .. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
320 Replaces the element in *array* at position *index* with *value*.
321 The valid range for *index* is from 0 to the return value of
322 :cfunc:`json_array_size()` minus 1. Returns 0 on success and -1 on
325 .. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
327 Like :cfunc:`json_array_set()` but steals the reference to *value*.
328 This is useful when *value* is newly created and not used after
331 .. versionadded:: 1.1
333 .. cfunction:: int json_array_append(json_t *array, json_t *value)
335 Appends *value* to the end of *array*, growing the size of *array*
336 by 1. Returns 0 on success and -1 on error.
338 .. cfunction:: int json_array_append_new(json_t *array, json_t *value)
340 Like :cfunc:`json_array_append()` but steals the reference to
341 *value*. This is useful when *value* is newly created and not used
344 .. versionadded:: 1.1
346 .. cfunction:: int json_array_insert(json_t *array, unsigned int index, json_t *value)
348 Inserts *value* to *array* at position *index*, shifting the
349 elements at *index* and after it one position towards the end of
350 the array. Returns 0 on success and -1 on error.
352 .. versionadded:: 1.1
354 .. cfunction:: int json_array_insert_new(json_t *array, unsigned int index, json_t *value)
356 Like :cfunc:`json_array_insert()` but steals the reference to
357 *value*. This is useful when *value* is newly created and not used
360 .. versionadded:: 1.1
362 .. cfunction:: int json_array_remove(json_t *array, unsigned int index)
364 Removes the element in *array* at position *index*, shifting the
365 elements after *index* one position towards the start of the array.
366 Returns 0 on success and -1 on error.
368 .. versionadded:: 1.1
370 .. cfunction:: int json_array_clear(json_t *array)
372 Removes all elements from *array*. Returns 0 on sucess and -1 on
375 .. versionadded:: 1.1
377 .. cfunction:: int json_array_extend(json_t *array, json_t *other_array)
379 Appends all elements in *other_array* to the end of *array*.
380 Returns 0 on success and -1 on error.
382 .. versionadded:: 1.1
388 A JSON object is a dictionary of key-value pairs, where the key is a
389 Unicode string and the value is any JSON value.
391 .. cfunction:: json_t *json_object(void)
395 Returns a new JSON object, or *NULL* on error. Initially, the
398 .. cfunction:: unsigned int json_object_size(const json_t *object)
400 Returns the number of elements in *object*, or 0 if *object* is not
403 .. versionadded:: 1.1
405 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
407 .. refcounting:: borrow
409 Get a value corresponding to *key* from *object*. Returns *NULL* if
410 *key* is not found and on error.
412 .. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)
414 Set the value of *key* to *value* in *object*. *key* must be a
415 valid null terminated UTF-8 encoded Unicode string. If there
416 already is a value for *key*, it is replaced by the new value.
417 Returns 0 on success and -1 on error.
419 .. cfunction:: int json_object_set_new(json_t *object, const char *key, json_t *value)
421 Like :cfunc:`json_object_set()` but steals the reference to
422 *value*. This is useful when *value* is newly created and not used
425 .. versionadded:: 1.1
427 .. cfunction:: int json_object_del(json_t *object, const char *key)
429 Delete *key* from *object* if it exists. Returns 0 on success, or
430 -1 if *key* was not found.
433 .. cfunction:: int json_object_clear(json_t *object)
435 Remove all elements from *object*. Returns 0 on success and -1 if
436 *object* is not a JSON object.
438 .. versionadded:: 1.1
440 .. cfunction:: int json_object_update(json_t *object, json_t *other)
442 Update *object* with the key-value pairs from *other*, overwriting
443 existing keys. Returns 0 on success or -1 on error.
445 .. versionadded:: 1.1
448 The following functions implement an iteration protocol for objects:
450 .. cfunction:: void *json_object_iter(json_t *object)
452 Returns an opaque iterator which can be used to iterate over all
453 key-value pairs in *object*, or *NULL* if *object* is empty.
455 .. cfunction:: void *json_object_iter_next(json_t *object, void *iter)
457 Returns an iterator pointing to the next key-value pair in *object*
458 after *iter*, or *NULL* if the whole object has been iterated
461 .. cfunction:: const char *json_object_iter_key(void *iter)
463 Extract the associated key from *iter*.
465 .. cfunction:: json_t *json_object_iter_value(void *iter)
467 .. refcounting:: borrow
469 Extract the associated value from *iter*.
471 The iteration protocol can be used for example as follows::
473 /* obj is a JSON object */
476 void *iter = json_object_iter(obj);
479 key = json_object_iter_key(iter);
480 value = json_object_iter_value(iter);
481 /* use key and value ... */
482 iter = json_object_iter_next(obj, iter);
489 This sections describes the functions that can be used to encode
490 values to JSON. Only objects and arrays can be encoded, since they are
491 the only valid "root" values of a JSON text.
493 Each function takes a *flags* parameter that controls some aspects of
494 how the data is encoded. Its default value is 0. The following macros
495 can be ORed together to obtain *flags*.
498 Pretty-print the result, indenting arrays and objects by *n*
499 spaces. The valid range for *n* is between 0 and 255, other values
500 result in an undefined output. If ``JSON_INDENT`` is not used or
501 *n* is 0, no pretty-printing is done and the result is a compact
504 The following functions perform the actual JSON encoding. The result
507 .. cfunction:: char *json_dumps(const json_t *root, unsigned long flags)
509 Returns the JSON representation of *root* as a string, or *NULL* on
510 error. *flags* is described above. The return value must be freed
511 by the caller using :cfunc:`free()`.
513 .. cfunction:: int json_dumpf(const json_t *root, FILE *output, unsigned long flags)
515 Write the JSON representation of *root* to the stream *output*.
516 *flags* is described above. Returns 0 on success and -1 on error.
517 If an error occurs, something may have already been written to
518 *output*. In this case, the output is undefined and most likely not
521 .. cfunction:: int json_dump_file(const json_t *json, const char *path, unsigned long flags)
523 Write the JSON representation of *root* to the file *path*. If
524 *path* already exists, it is overwritten. *flags* is described
525 above. Returns 0 on success and -1 on error.
531 This sections describes the functions that can be used to decode JSON
532 text to the Jansson representation of JSON data. The JSON
533 specification requires that a JSON text is either a serialized array
534 or object, and this requirement is also enforced with the following
537 The only supported character encoding is UTF-8 (which ASCII is a
540 .. ctype:: json_error_t
542 This data structure is used to return information on decoding
543 errors from the decoding functions. Its definition is repeated
546 #define JSON_ERROR_TEXT_LENGTH 160
549 char text[JSON_ERROR_TEXT_LENGTH];
553 *line* is the line number on which the error occurred, or -1 if
554 this information is not available. *text* contains the error
555 message (in UTF-8), or an empty string if a message is not
558 The normal usef of :ctype:`json_error_t` is to allocate it normally
559 on the stack, and pass a pointer to a decoding function. Example::
565 json = json_load_file("/path/to/file.json", &error);
567 /* the error variable contains error information */
572 Also note that if the decoding succeeded (``json != NULL`` in the
573 above example), the contents of ``error`` are unspecified.
575 All decoding functions also accept *NULL* as the
576 :ctype:`json_error_t` pointer, in which case no error information
577 is returned to the caller.
579 The following functions perform the actual JSON decoding.
581 .. cfunction:: json_t *json_loads(const char *input, json_error_t *error)
585 Decodes the JSON string *input* and returns the array or object it
586 contains, or *NULL* on error, in which case *error* is filled with
587 information about the error. See above for discussion on the
590 .. cfunction:: json_t *json_loadf(FILE *input, json_error_t *error)
594 Decodes the JSON text in stream *input* and returns the array or
595 object it contains, or *NULL* on error, in which case *error* is
596 filled with information about the error. See above for discussion
597 on the *error* parameter.
599 .. cfunction:: json_t *json_load_file(const char *path, json_error_t *error)
603 Decodes the JSON text in file *path* and returns the array or
604 object it contains, or *NULL* on error, in which case *error* is
605 filled with information about the error. See above for discussion
606 on the *error* parameter.