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:: json_t *json_string_nocheck(const char *value)
236 Like :cfunc:`json_string`, but doesn't check that *value* is valid
237 UTF-8. Use this function only if you are certain that this really
238 is the case (e.g. you have already checked it by other means).
240 .. versionadded:: 1.2
242 .. cfunction:: const char *json_string_value(const json_t *string)
244 Returns the associated value of *string* as a null terminated UTF-8
245 encoded string, or *NULL* if *string* is not a JSON string.
247 .. cfunction:: int json_string_set(const json_t *string, const char *value)
249 Sets the associated value of *string* to *value*. *value* must be a
250 valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
253 .. versionadded:: 1.1
255 .. cfunction:: int json_string_set_nocheck(const json_t *string, const char *value)
257 Like :cfunc:`json_string_set`, but doesn't check that *value* is
258 valid UTF-8. Use this function only if you are certain that this
259 really is the case (e.g. you have already checked it by other
262 .. versionadded:: 1.2
268 .. cfunction:: json_t *json_integer(int value)
272 Returns a new JSON integer, or *NULL* on error.
274 .. cfunction:: int json_integer_value(const json_t *integer)
276 Returns the associated value of *integer*, or 0 if *json* is not a
279 .. cfunction:: int json_integer_set(const json_t *integer, int value)
281 Sets the associated value of *integer* to *value*. Returns 0 on
282 success and -1 if *integer* is not a JSON integer.
284 .. versionadded:: 1.1
286 .. cfunction:: json_t *json_real(double value)
290 Returns a new JSON real, or *NULL* on error.
292 .. cfunction:: double json_real_value(const json_t *real)
294 Returns the associated value of *real*, or 0.0 if *real* is not a
297 .. cfunction:: int json_real_set(const json_t *real, double value)
299 Sets the associated value of *real* to *value*. Returns 0 on
300 success and -1 if *real* is not a JSON real.
302 .. versionadded:: 1.1
304 In addition to the functions above, there's a common query function
305 for integers and reals:
307 .. cfunction:: double json_number_value(const json_t *json)
309 Returns the associated value of the JSON integer or JSON real
310 *json*, cast to double regardless of the actual type. If *json* is
311 neither JSON real nor JSON integer, 0.0 is returned.
317 A JSON array is an ordered collection of other JSON values.
319 .. cfunction:: json_t *json_array(void)
323 Returns a new JSON array, or *NULL* on error. Initially, the array
326 .. cfunction:: unsigned int json_array_size(const json_t *array)
328 Returns the number of elements in *array*, or 0 if *array* is NULL
331 .. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
333 .. refcounting:: borrow
335 Returns the element in *array* at position *index*. The valid range
336 for *index* is from 0 to the return value of
337 :cfunc:`json_array_size()` minus 1. If *array* is not a JSON array,
338 if *array* is *NULL*, or if *index* is out of range, *NULL* is
341 .. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
343 Replaces the element in *array* at position *index* with *value*.
344 The valid range for *index* is from 0 to the return value of
345 :cfunc:`json_array_size()` minus 1. Returns 0 on success and -1 on
348 .. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
350 Like :cfunc:`json_array_set()` but steals the reference to *value*.
351 This is useful when *value* is newly created and not used after
354 .. versionadded:: 1.1
356 .. cfunction:: int json_array_append(json_t *array, json_t *value)
358 Appends *value* to the end of *array*, growing the size of *array*
359 by 1. Returns 0 on success and -1 on error.
361 .. cfunction:: int json_array_append_new(json_t *array, json_t *value)
363 Like :cfunc:`json_array_append()` but steals the reference to
364 *value*. This is useful when *value* is newly created and not used
367 .. versionadded:: 1.1
369 .. cfunction:: int json_array_insert(json_t *array, unsigned int index, json_t *value)
371 Inserts *value* to *array* at position *index*, shifting the
372 elements at *index* and after it one position towards the end of
373 the array. Returns 0 on success and -1 on error.
375 .. versionadded:: 1.1
377 .. cfunction:: int json_array_insert_new(json_t *array, unsigned int index, json_t *value)
379 Like :cfunc:`json_array_insert()` but steals the reference to
380 *value*. This is useful when *value* is newly created and not used
383 .. versionadded:: 1.1
385 .. cfunction:: int json_array_remove(json_t *array, unsigned int index)
387 Removes the element in *array* at position *index*, shifting the
388 elements after *index* one position towards the start of the array.
389 Returns 0 on success and -1 on error.
391 .. versionadded:: 1.1
393 .. cfunction:: int json_array_clear(json_t *array)
395 Removes all elements from *array*. Returns 0 on sucess and -1 on
398 .. versionadded:: 1.1
400 .. cfunction:: int json_array_extend(json_t *array, json_t *other_array)
402 Appends all elements in *other_array* to the end of *array*.
403 Returns 0 on success and -1 on error.
405 .. versionadded:: 1.1
411 A JSON object is a dictionary of key-value pairs, where the key is a
412 Unicode string and the value is any JSON value.
414 .. cfunction:: json_t *json_object(void)
418 Returns a new JSON object, or *NULL* on error. Initially, the
421 .. cfunction:: unsigned int json_object_size(const json_t *object)
423 Returns the number of elements in *object*, or 0 if *object* is not
426 .. versionadded:: 1.1
428 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
430 .. refcounting:: borrow
432 Get a value corresponding to *key* from *object*. Returns *NULL* if
433 *key* is not found and on error.
435 .. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)
437 Set the value of *key* to *value* in *object*. *key* must be a
438 valid null terminated UTF-8 encoded Unicode string. If there
439 already is a value for *key*, it is replaced by the new value.
440 Returns 0 on success and -1 on error.
442 .. cfunction:: int json_object_set_nocheck(json_t *object, const char *key, json_t *value)
444 Like :cfunc:`json_object_set`, but doesn't check that *key* is
445 valid UTF-8. Use this function only if you are certain that this
446 really is the case (e.g. you have already checked it by other
449 .. versionadded:: 1.2
451 .. cfunction:: int json_object_set_new(json_t *object, const char *key, json_t *value)
453 Like :cfunc:`json_object_set()` but steals the reference to
454 *value*. This is useful when *value* is newly created and not used
457 .. versionadded:: 1.1
459 .. cfunction:: int json_object_set_new_nocheck(json_t *object, const char *key, json_t *value)
461 Like :cfunc:`json_object_set_new`, but doesn't check that *key* is
462 valid UTF-8. Use this function only if you are certain that this
463 really is the case (e.g. you have already checked it by other
466 .. versionadded:: 1.2
468 .. cfunction:: int json_object_del(json_t *object, const char *key)
470 Delete *key* from *object* if it exists. Returns 0 on success, or
471 -1 if *key* was not found.
474 .. cfunction:: int json_object_clear(json_t *object)
476 Remove all elements from *object*. Returns 0 on success and -1 if
477 *object* is not a JSON object.
479 .. versionadded:: 1.1
481 .. cfunction:: int json_object_update(json_t *object, json_t *other)
483 Update *object* with the key-value pairs from *other*, overwriting
484 existing keys. Returns 0 on success or -1 on error.
486 .. versionadded:: 1.1
489 The following functions implement an iteration protocol for objects:
491 .. cfunction:: void *json_object_iter(json_t *object)
493 Returns an opaque iterator which can be used to iterate over all
494 key-value pairs in *object*, or *NULL* if *object* is empty.
496 .. cfunction:: void *json_object_iter_at(json_t *object, const char *key)
498 Like :cfunc:`json_object_iter()`, but returns an iterator to the
499 key-value pair in *object* whose key is equal to *key*, or NULL if
500 *key* is not found in *object*. Iterating forward to the end of
501 *object* only yields all key-value pairs of the object if *key*
502 happens to be the first key in the underlying hash table.
504 .. versionadded:: 1.3
506 .. cfunction:: void *json_object_iter_next(json_t *object, void *iter)
508 Returns an iterator pointing to the next key-value pair in *object*
509 after *iter*, or *NULL* if the whole object has been iterated
512 .. cfunction:: const char *json_object_iter_key(void *iter)
514 Extract the associated key from *iter*.
516 .. cfunction:: json_t *json_object_iter_value(void *iter)
518 .. refcounting:: borrow
520 Extract the associated value from *iter*.
522 .. cfunction:: int json_object_iter_set(json_t *object, void *iter, json_t *value)
524 Set the value of the key-value pair in *object*, that is pointed to
525 by *iter*, to *value*.
527 .. versionadded:: 1.3
529 .. cfunction:: int json_object_iter_set_new(json_t *object, void *iter, json_t *value)
531 Like :cfunc:`json_object_iter_set()`, but steals the reference to
532 *value*. This is useful when *value* is newly created and not used
535 .. versionadded:: 1.3
537 The iteration protocol can be used for example as follows::
539 /* obj is a JSON object */
542 void *iter = json_object_iter(obj);
545 key = json_object_iter_key(iter);
546 value = json_object_iter_value(iter);
547 /* use key and value ... */
548 iter = json_object_iter_next(obj, iter);
555 This sections describes the functions that can be used to encode
556 values to JSON. Only objects and arrays can be encoded, since they are
557 the only valid "root" values of a JSON text.
559 By default, the output has no newlines, and spaces are used between
560 array and object elements for a readable output. This behavior can be
561 altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
562 described below. A newline is never appended to the end of the encoded
565 Each function takes a *flags* parameter that controls some aspects of
566 how the data is encoded. Its default value is 0. The following macros
567 can be ORed together to obtain *flags*.
570 Pretty-print the result, using newlines between array and object
571 items, and indenting with *n* spaces. The valid range for *n* is
572 between 0 and 255, other values result in an undefined output. If
573 ``JSON_INDENT`` is not used or *n* is 0, no newlines are inserted
574 between array and object items.
577 This flag enables a compact representation, i.e. sets the separator
578 between array and object items to ``","`` and between object keys
579 and values to ``":"``. Without this flag, the corresponding
580 separators are ``", "`` and ``": "`` for more readable output.
582 .. versionadded:: 1.2
584 ``JSON_ENSURE_ASCII``
585 If this flag is used, the output is guaranteed to consist only of
586 ASCII characters. This is achived by escaping all Unicode
587 characters outside the ASCII range.
589 .. versionadded:: 1.2
592 If this flag is used, all the objects in output are sorted by key.
593 This is useful e.g. if two JSON texts are diffed or visually
596 .. versionadded:: 1.2
598 The following functions perform the actual JSON encoding. The result
601 .. cfunction:: char *json_dumps(const json_t *root, unsigned long flags)
603 Returns the JSON representation of *root* as a string, or *NULL* on
604 error. *flags* is described above. The return value must be freed
605 by the caller using :cfunc:`free()`.
607 .. cfunction:: int json_dumpf(const json_t *root, FILE *output, unsigned long flags)
609 Write the JSON representation of *root* to the stream *output*.
610 *flags* is described above. Returns 0 on success and -1 on error.
611 If an error occurs, something may have already been written to
612 *output*. In this case, the output is undefined and most likely not
615 .. cfunction:: int json_dump_file(const json_t *json, const char *path, unsigned long flags)
617 Write the JSON representation of *root* to the file *path*. If
618 *path* already exists, it is overwritten. *flags* is described
619 above. Returns 0 on success and -1 on error.
625 This sections describes the functions that can be used to decode JSON
626 text to the Jansson representation of JSON data. The JSON
627 specification requires that a JSON text is either a serialized array
628 or object, and this requirement is also enforced with the following
631 The only supported character encoding is UTF-8 (which ASCII is a
634 .. ctype:: json_error_t
636 This data structure is used to return information on decoding
637 errors from the decoding functions. Its definition is repeated
640 #define JSON_ERROR_TEXT_LENGTH 160
643 char text[JSON_ERROR_TEXT_LENGTH];
647 *line* is the line number on which the error occurred, or -1 if
648 this information is not available. *text* contains the error
649 message (in UTF-8), or an empty string if a message is not
652 The normal usef of :ctype:`json_error_t` is to allocate it normally
653 on the stack, and pass a pointer to a decoding function. Example::
659 json = json_load_file("/path/to/file.json", &error);
661 /* the error variable contains error information */
666 Also note that if the decoding succeeded (``json != NULL`` in the
667 above example), the contents of ``error`` are unspecified.
669 All decoding functions also accept *NULL* as the
670 :ctype:`json_error_t` pointer, in which case no error information
671 is returned to the caller.
673 The following functions perform the actual JSON decoding.
675 .. cfunction:: json_t *json_loads(const char *input, json_error_t *error)
679 Decodes the JSON string *input* and returns the array or object it
680 contains, or *NULL* on error, in which case *error* is filled with
681 information about the error. See above for discussion on the
684 .. cfunction:: json_t *json_loadf(FILE *input, json_error_t *error)
688 Decodes the JSON text in stream *input* and returns the array or
689 object it contains, or *NULL* on error, in which case *error* is
690 filled with information about the error. See above for discussion
691 on the *error* parameter.
693 .. cfunction:: json_t *json_load_file(const char *path, json_error_t *error)
697 Decodes the JSON text in file *path* and returns the array or
698 object it contains, or *NULL* on error, in which case *error* is
699 filled with information about the error. See above for discussion
700 on the *error* parameter.
706 Testing for equality of two JSON values cannot, in general, be
707 achieved using the ``==`` operator. Equality in the terms of the
708 ``==`` operator states that the two :ctype:`json_t` pointers point to
709 exactly the same JSON value. However, two JSON values can be equal not
710 only if they are exactly the same value, but also if they have equal
713 * Two integer or real values are equal if their contained numeric
714 values are equal. An integer value is never equal to a real value,
717 * Two strings are equal if their contained UTF-8 strings are equal.
719 * Two arrays are equal if they have the same number of elements and
720 each element in the first array is equal to the corresponding
721 element in the second array.
723 * Two objects are equal if they have exactly the same keys and the
724 value for each key in the first object is equal to the value of the
725 corresponding key in the second object.
727 * Two true, false or null values have no "contents", so they are equal
728 if their types are equal. (Because these values are singletons,
729 their equality can actually be tested with ``==``.)
731 The following function can be used to test whether two JSON values are
734 .. cfunction:: int json_equal(json_t *value1, json_t *value2)
736 Returns 1 if *value1* and *value2* are equal, as defined above.
737 Returns 0 if they are inequal or one or both of the pointers are
740 .. versionadded:: 1.2
746 Because of reference counting, passing JSON values around doesn't
747 require copying them. But sometimes a fresh copy of a JSON value is
748 needed. For example, if you need to modify an array, but still want to
749 use the original afterwards, you should take a copy of it first.
751 Jansson supports two kinds of copying: shallow and deep. There is a
752 difference between these methods only for arrays and objects. Shallow
753 copying only copies the first level value (array or object) and uses
754 the same child values in the copied value. Deep copying makes a fresh
755 copy of the child values, too. Moreover, all the child values are deep
756 copied in a recursive fashion.
758 .. cfunction:: json_t *json_copy(json_t *value)
762 Returns a shallow copy of *value*, or *NULL* on error.
764 .. versionadded:: 1.2
766 .. cfunction:: json_t *json_deep_copy(json_t *value)
770 Returns a deep copy of *value*, or *NULL* on error.
772 .. versionadded:: 1.2