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 functions are
158 suffixed with ``_new`` or have ``_new_`` somewhere in their name.
160 For example, the following code creates a new JSON array and appends
163 json_t *array, *integer;
165 array = json_array();
166 integer = json_integer(42);
168 json_array_append(array, integer);
169 json_decref(integer);
171 Note how the caller has to release the reference to the integer value
172 by calling :cfunc:`json_decref()`. By using a reference stealing
173 function :cfunc:`json_array_append_new()` instead of
174 :cfunc:`json_array_append()`, the code becomes much simpler::
176 json_t *array = json_array();
177 json_array_append_new(array, json_integer(42));
179 In this case, the user doesn't have to explicitly release the
180 reference to the integer value, as :cfunc:`json_array_append_new()`
181 steals the reference when appending the value to the array.
183 In the following sections it is clearly documented whether a function
184 will return a new or borrowed reference or steal a reference to its
191 A circular reference is created when an object or an array is,
192 directly or indirectly, inserted inside itself. The direct case is
195 json_t *obj = json_object();
196 json_object_set(obj, "foo", obj);
198 Jansson will refuse to do this, and :cfunc:`json_object_set()` (and
199 all the other such functions for objects and arrays) will return with
200 an error status. The indirect case is the dangerous one::
202 json_t *arr1 = json_array(), *arr2 = json_array();
203 json_array_append(arr1, arr2);
204 json_array_append(arr2, arr1);
206 In this example, the array ``arr2`` is contained in the array
207 ``arr1``, and vice versa. Jansson cannot check for this kind of
208 indirect circular references without a performance hit, so it's up to
209 the user to avoid them.
211 If a circular reference is created, the memory consumed by the values
212 cannot be freed by :cfunc:`json_decref()`. The reference counts never
213 drops to zero because the values are keeping the circular reference to
214 themselves. Moreover, trying to encode the values with any of the
215 encoding functions will fail. The encoder detects circular references
216 and returns an error status.
222 These values are implemented as singletons, so each of these functions
223 returns the same value each time.
225 .. cfunction:: json_t *json_true(void)
229 Returns the JSON true value.
231 .. cfunction:: json_t *json_false(void)
235 Returns the JSON false value.
237 .. cfunction:: json_t *json_null(void)
241 Returns the JSON null value.
247 .. cfunction:: json_t *json_string(const char *value)
251 Returns a new JSON string, or *NULL* on error. *value* must be a
252 valid UTF-8 encoded Unicode string.
254 .. cfunction:: json_t *json_string_nocheck(const char *value)
258 Like :cfunc:`json_string`, but doesn't check that *value* is valid
259 UTF-8. Use this function only if you are certain that this really
260 is the case (e.g. you have already checked it by other means).
262 .. versionadded:: 1.2
264 .. cfunction:: const char *json_string_value(const json_t *string)
266 Returns the associated value of *string* as a null terminated UTF-8
267 encoded string, or *NULL* if *string* is not a JSON string.
269 .. cfunction:: int json_string_set(const json_t *string, const char *value)
271 Sets the associated value of *string* to *value*. *value* must be a
272 valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
275 .. versionadded:: 1.1
277 .. cfunction:: int json_string_set_nocheck(const json_t *string, const char *value)
279 Like :cfunc:`json_string_set`, but doesn't check that *value* is
280 valid UTF-8. Use this function only if you are certain that this
281 really is the case (e.g. you have already checked it by other
284 .. versionadded:: 1.2
290 .. cfunction:: json_t *json_integer(int value)
294 Returns a new JSON integer, or *NULL* on error.
296 .. cfunction:: int json_integer_value(const json_t *integer)
298 Returns the associated value of *integer*, or 0 if *json* is not a
301 .. cfunction:: int json_integer_set(const json_t *integer, int value)
303 Sets the associated value of *integer* to *value*. Returns 0 on
304 success and -1 if *integer* is not a JSON integer.
306 .. versionadded:: 1.1
308 .. cfunction:: json_t *json_real(double value)
312 Returns a new JSON real, or *NULL* on error.
314 .. cfunction:: double json_real_value(const json_t *real)
316 Returns the associated value of *real*, or 0.0 if *real* is not a
319 .. cfunction:: int json_real_set(const json_t *real, double value)
321 Sets the associated value of *real* to *value*. Returns 0 on
322 success and -1 if *real* is not a JSON real.
324 .. versionadded:: 1.1
326 In addition to the functions above, there's a common query function
327 for integers and reals:
329 .. cfunction:: double json_number_value(const json_t *json)
331 Returns the associated value of the JSON integer or JSON real
332 *json*, cast to double regardless of the actual type. If *json* is
333 neither JSON real nor JSON integer, 0.0 is returned.
339 A JSON array is an ordered collection of other JSON values.
341 .. cfunction:: json_t *json_array(void)
345 Returns a new JSON array, or *NULL* on error. Initially, the array
348 .. cfunction:: unsigned int json_array_size(const json_t *array)
350 Returns the number of elements in *array*, or 0 if *array* is NULL
353 .. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
355 .. refcounting:: borrow
357 Returns the element in *array* at position *index*. The valid range
358 for *index* is from 0 to the return value of
359 :cfunc:`json_array_size()` minus 1. If *array* is not a JSON array,
360 if *array* is *NULL*, or if *index* is out of range, *NULL* is
363 .. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
365 Replaces the element in *array* at position *index* with *value*.
366 The valid range for *index* is from 0 to the return value of
367 :cfunc:`json_array_size()` minus 1. Returns 0 on success and -1 on
370 .. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
372 Like :cfunc:`json_array_set()` but steals the reference to *value*.
373 This is useful when *value* is newly created and not used after
376 .. versionadded:: 1.1
378 .. cfunction:: int json_array_append(json_t *array, json_t *value)
380 Appends *value* to the end of *array*, growing the size of *array*
381 by 1. Returns 0 on success and -1 on error.
383 .. cfunction:: int json_array_append_new(json_t *array, json_t *value)
385 Like :cfunc:`json_array_append()` but steals the reference to
386 *value*. This is useful when *value* is newly created and not used
389 .. versionadded:: 1.1
391 .. cfunction:: int json_array_insert(json_t *array, unsigned int index, json_t *value)
393 Inserts *value* to *array* at position *index*, shifting the
394 elements at *index* and after it one position towards the end of
395 the array. Returns 0 on success and -1 on error.
397 .. versionadded:: 1.1
399 .. cfunction:: int json_array_insert_new(json_t *array, unsigned int index, json_t *value)
401 Like :cfunc:`json_array_insert()` but steals the reference to
402 *value*. This is useful when *value* is newly created and not used
405 .. versionadded:: 1.1
407 .. cfunction:: int json_array_remove(json_t *array, unsigned int index)
409 Removes the element in *array* at position *index*, shifting the
410 elements after *index* one position towards the start of the array.
411 Returns 0 on success and -1 on error.
413 .. versionadded:: 1.1
415 .. cfunction:: int json_array_clear(json_t *array)
417 Removes all elements from *array*. Returns 0 on sucess and -1 on
420 .. versionadded:: 1.1
422 .. cfunction:: int json_array_extend(json_t *array, json_t *other_array)
424 Appends all elements in *other_array* to the end of *array*.
425 Returns 0 on success and -1 on error.
427 .. versionadded:: 1.1
433 A JSON object is a dictionary of key-value pairs, where the key is a
434 Unicode string and the value is any JSON value.
436 .. cfunction:: json_t *json_object(void)
440 Returns a new JSON object, or *NULL* on error. Initially, the
443 .. cfunction:: unsigned int json_object_size(const json_t *object)
445 Returns the number of elements in *object*, or 0 if *object* is not
448 .. versionadded:: 1.1
450 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
452 .. refcounting:: borrow
454 Get a value corresponding to *key* from *object*. Returns *NULL* if
455 *key* is not found and on error.
457 .. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)
459 Set the value of *key* to *value* in *object*. *key* must be a
460 valid null terminated UTF-8 encoded Unicode string. If there
461 already is a value for *key*, it is replaced by the new value.
462 Returns 0 on success and -1 on error.
464 .. cfunction:: int json_object_set_nocheck(json_t *object, const char *key, json_t *value)
466 Like :cfunc:`json_object_set`, but doesn't check that *key* is
467 valid UTF-8. Use this function only if you are certain that this
468 really is the case (e.g. you have already checked it by other
471 .. versionadded:: 1.2
473 .. cfunction:: int json_object_set_new(json_t *object, const char *key, json_t *value)
475 Like :cfunc:`json_object_set()` but steals the reference to
476 *value*. This is useful when *value* is newly created and not used
479 .. versionadded:: 1.1
481 .. cfunction:: int json_object_set_new_nocheck(json_t *object, const char *key, json_t *value)
483 Like :cfunc:`json_object_set_new`, but doesn't check that *key* is
484 valid UTF-8. Use this function only if you are certain that this
485 really is the case (e.g. you have already checked it by other
488 .. versionadded:: 1.2
490 .. cfunction:: int json_object_del(json_t *object, const char *key)
492 Delete *key* from *object* if it exists. Returns 0 on success, or
493 -1 if *key* was not found.
496 .. cfunction:: int json_object_clear(json_t *object)
498 Remove all elements from *object*. Returns 0 on success and -1 if
499 *object* is not a JSON object.
501 .. versionadded:: 1.1
503 .. cfunction:: int json_object_update(json_t *object, json_t *other)
505 Update *object* with the key-value pairs from *other*, overwriting
506 existing keys. Returns 0 on success or -1 on error.
508 .. versionadded:: 1.1
511 The following functions implement an iteration protocol for objects:
513 .. cfunction:: void *json_object_iter(json_t *object)
515 Returns an opaque iterator which can be used to iterate over all
516 key-value pairs in *object*, or *NULL* if *object* is empty.
518 .. cfunction:: void *json_object_iter_at(json_t *object, const char *key)
520 Like :cfunc:`json_object_iter()`, but returns an iterator to the
521 key-value pair in *object* whose key is equal to *key*, or NULL if
522 *key* is not found in *object*. Iterating forward to the end of
523 *object* only yields all key-value pairs of the object if *key*
524 happens to be the first key in the underlying hash table.
526 .. versionadded:: 1.3
528 .. cfunction:: void *json_object_iter_next(json_t *object, void *iter)
530 Returns an iterator pointing to the next key-value pair in *object*
531 after *iter*, or *NULL* if the whole object has been iterated
534 .. cfunction:: const char *json_object_iter_key(void *iter)
536 Extract the associated key from *iter*.
538 .. cfunction:: json_t *json_object_iter_value(void *iter)
540 .. refcounting:: borrow
542 Extract the associated value from *iter*.
544 .. cfunction:: int json_object_iter_set(json_t *object, void *iter, json_t *value)
546 Set the value of the key-value pair in *object*, that is pointed to
547 by *iter*, to *value*.
549 .. versionadded:: 1.3
551 .. cfunction:: int json_object_iter_set_new(json_t *object, void *iter, json_t *value)
553 Like :cfunc:`json_object_iter_set()`, but steals the reference to
554 *value*. This is useful when *value* is newly created and not used
557 .. versionadded:: 1.3
559 The iteration protocol can be used for example as follows::
561 /* obj is a JSON object */
564 void *iter = json_object_iter(obj);
567 key = json_object_iter_key(iter);
568 value = json_object_iter_value(iter);
569 /* use key and value ... */
570 iter = json_object_iter_next(obj, iter);
577 This sections describes the functions that can be used to encode
578 values to JSON. Only objects and arrays can be encoded, since they are
579 the only valid "root" values of a JSON text.
581 By default, the output has no newlines, and spaces are used between
582 array and object elements for a readable output. This behavior can be
583 altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
584 described below. A newline is never appended to the end of the encoded
587 Each function takes a *flags* parameter that controls some aspects of
588 how the data is encoded. Its default value is 0. The following macros
589 can be ORed together to obtain *flags*.
592 Pretty-print the result, using newlines between array and object
593 items, and indenting with *n* spaces. The valid range for *n* is
594 between 0 and 255, other values result in an undefined output. If
595 ``JSON_INDENT`` is not used or *n* is 0, no newlines are inserted
596 between array and object items.
599 This flag enables a compact representation, i.e. sets the separator
600 between array and object items to ``","`` and between object keys
601 and values to ``":"``. Without this flag, the corresponding
602 separators are ``", "`` and ``": "`` for more readable output.
604 .. versionadded:: 1.2
606 ``JSON_ENSURE_ASCII``
607 If this flag is used, the output is guaranteed to consist only of
608 ASCII characters. This is achived by escaping all Unicode
609 characters outside the ASCII range.
611 .. versionadded:: 1.2
614 If this flag is used, all the objects in output are sorted by key.
615 This is useful e.g. if two JSON texts are diffed or visually
618 .. versionadded:: 1.2
620 ``JSON_PRESERVE_ORDER``
621 If this flag is used, object keys in the output are sorted into the
622 same order in which they were first inserted to the object. For
623 example, decoding a JSON text and then encoding with this flag
624 preserves the order of object keys.
626 .. versionadded:: 1.3
628 The following functions perform the actual JSON encoding. The result
631 .. cfunction:: char *json_dumps(const json_t *root, unsigned long flags)
633 Returns the JSON representation of *root* as a string, or *NULL* on
634 error. *flags* is described above. The return value must be freed
635 by the caller using :cfunc:`free()`.
637 .. cfunction:: int json_dumpf(const json_t *root, FILE *output, unsigned long flags)
639 Write the JSON representation of *root* to the stream *output*.
640 *flags* is described above. Returns 0 on success and -1 on error.
641 If an error occurs, something may have already been written to
642 *output*. In this case, the output is undefined and most likely not
645 .. cfunction:: int json_dump_file(const json_t *json, const char *path, unsigned long flags)
647 Write the JSON representation of *root* to the file *path*. If
648 *path* already exists, it is overwritten. *flags* is described
649 above. Returns 0 on success and -1 on error.
655 This sections describes the functions that can be used to decode JSON
656 text to the Jansson representation of JSON data. The JSON
657 specification requires that a JSON text is either a serialized array
658 or object, and this requirement is also enforced with the following
661 The only supported character encoding is UTF-8 (which ASCII is a
664 .. ctype:: json_error_t
666 This data structure is used to return information on decoding
667 errors from the decoding functions. Its definition is repeated
670 #define JSON_ERROR_TEXT_LENGTH 160
673 char text[JSON_ERROR_TEXT_LENGTH];
677 *line* is the line number on which the error occurred, or -1 if
678 this information is not available. *text* contains the error
679 message (in UTF-8), or an empty string if a message is not
682 The normal usef of :ctype:`json_error_t` is to allocate it normally
683 on the stack, and pass a pointer to a decoding function. Example::
689 json = json_load_file("/path/to/file.json", &error);
691 /* the error variable contains error information */
696 Also note that if the decoding succeeded (``json != NULL`` in the
697 above example), the contents of ``error`` are unspecified.
699 All decoding functions also accept *NULL* as the
700 :ctype:`json_error_t` pointer, in which case no error information
701 is returned to the caller.
703 The following functions perform the actual JSON decoding.
705 .. cfunction:: json_t *json_loads(const char *input, json_error_t *error)
709 Decodes the JSON string *input* and returns the array or object it
710 contains, or *NULL* on error, in which case *error* is filled with
711 information about the error. See above for discussion on the
714 .. cfunction:: json_t *json_loadf(FILE *input, json_error_t *error)
718 Decodes the JSON text in stream *input* and returns the array or
719 object it contains, or *NULL* on error, in which case *error* is
720 filled with information about the error. See above for discussion
721 on the *error* parameter.
723 .. cfunction:: json_t *json_load_file(const char *path, json_error_t *error)
727 Decodes the JSON text in file *path* and returns the array or
728 object it contains, or *NULL* on error, in which case *error* is
729 filled with information about the error. See above for discussion
730 on the *error* parameter.
736 Testing for equality of two JSON values cannot, in general, be
737 achieved using the ``==`` operator. Equality in the terms of the
738 ``==`` operator states that the two :ctype:`json_t` pointers point to
739 exactly the same JSON value. However, two JSON values can be equal not
740 only if they are exactly the same value, but also if they have equal
743 * Two integer or real values are equal if their contained numeric
744 values are equal. An integer value is never equal to a real value,
747 * Two strings are equal if their contained UTF-8 strings are equal.
749 * Two arrays are equal if they have the same number of elements and
750 each element in the first array is equal to the corresponding
751 element in the second array.
753 * Two objects are equal if they have exactly the same keys and the
754 value for each key in the first object is equal to the value of the
755 corresponding key in the second object.
757 * Two true, false or null values have no "contents", so they are equal
758 if their types are equal. (Because these values are singletons,
759 their equality can actually be tested with ``==``.)
761 The following function can be used to test whether two JSON values are
764 .. cfunction:: int json_equal(json_t *value1, json_t *value2)
766 Returns 1 if *value1* and *value2* are equal, as defined above.
767 Returns 0 if they are inequal or one or both of the pointers are
770 .. versionadded:: 1.2
776 Because of reference counting, passing JSON values around doesn't
777 require copying them. But sometimes a fresh copy of a JSON value is
778 needed. For example, if you need to modify an array, but still want to
779 use the original afterwards, you should take a copy of it first.
781 Jansson supports two kinds of copying: shallow and deep. There is a
782 difference between these methods only for arrays and objects. Shallow
783 copying only copies the first level value (array or object) and uses
784 the same child values in the copied value. Deep copying makes a fresh
785 copy of the child values, too. Moreover, all the child values are deep
786 copied in a recursive fashion.
788 .. cfunction:: json_t *json_copy(json_t *value)
792 Returns a shallow copy of *value*, or *NULL* on error.
794 .. versionadded:: 1.2
796 .. cfunction:: json_t *json_deep_copy(json_t *value)
800 Returns a deep copy of *value*, or *NULL* on error.
802 .. versionadded:: 1.2