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 references to each
214 other. Moreover, trying to encode the values with any of the encoding
215 functions will fail. The encoder detects circular references and
216 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 .. ctype:: json_int_t
292 This is the C type that is used to store JSON integer values. It
293 represents the widest integer type available on your system. In
294 practice it's just a typedef of ``long long`` if your compiler
295 supports it, otherwise ``long``.
297 Usually, you can safely use plain ``int`` in place of
298 ``json_int_t``, and the implicit C integer conversion handles the
299 rest. Only when you know that you need the full 64-bit range, you
300 should use ``json_int_t`` explicitly.
302 ``JSON_INTEGER_IS_LONG_LONG``
304 This is a preprocessor variable that holds the value 1 if
305 :ctype:`json_int_t` is ``long long``, and 0 if it's ``long``. It
306 can be used as follows::
308 #if JSON_INTEGER_IS_LONG_LONG
309 /* Code specific for long long */
311 /* Code specific for long */
314 ``JSON_INTEGER_FORMAT``
316 This is a macro that expands to a :cfunc:`printf()` conversion
317 specifier that corresponds to :ctype:`json_int_t`, without the
318 leading ``%`` sign, i.e. either ``"lld"`` or ``"ld"``. This macro
319 is required because the actual type of :ctype:`json_int_t` can be
320 either ``long`` or ``long long``, and :cfunc:`printf()` reuiqres
321 different length modifiers for the two.
325 json_int_t x = 123123123;
326 printf("x is %" JSON_INTEGER_FORMAT "\n", x);
329 .. cfunction:: json_t *json_integer(json_int_t value)
333 Returns a new JSON integer, or *NULL* on error.
335 .. cfunction:: json_int_t json_integer_value(const json_t *integer)
337 Returns the associated value of *integer*, or 0 if *json* is not a
340 .. cfunction:: int json_integer_set(const json_t *integer, json_int_t value)
342 Sets the associated value of *integer* to *value*. Returns 0 on
343 success and -1 if *integer* is not a JSON integer.
345 .. versionadded:: 1.1
347 .. cfunction:: json_t *json_real(double value)
351 Returns a new JSON real, or *NULL* on error.
353 .. cfunction:: double json_real_value(const json_t *real)
355 Returns the associated value of *real*, or 0.0 if *real* is not a
358 .. cfunction:: int json_real_set(const json_t *real, double value)
360 Sets the associated value of *real* to *value*. Returns 0 on
361 success and -1 if *real* is not a JSON real.
363 .. versionadded:: 1.1
365 In addition to the functions above, there's a common query function
366 for integers and reals:
368 .. cfunction:: double json_number_value(const json_t *json)
370 Returns the associated value of the JSON integer or JSON real
371 *json*, cast to double regardless of the actual type. If *json* is
372 neither JSON real nor JSON integer, 0.0 is returned.
378 A JSON array is an ordered collection of other JSON values.
380 .. cfunction:: json_t *json_array(void)
384 Returns a new JSON array, or *NULL* on error. Initially, the array
387 .. cfunction:: size_t json_array_size(const json_t *array)
389 Returns the number of elements in *array*, or 0 if *array* is NULL
392 .. cfunction:: json_t *json_array_get(const json_t *array, size_t index)
394 .. refcounting:: borrow
396 Returns the element in *array* at position *index*. The valid range
397 for *index* is from 0 to the return value of
398 :cfunc:`json_array_size()` minus 1. If *array* is not a JSON array,
399 if *array* is *NULL*, or if *index* is out of range, *NULL* is
402 .. cfunction:: int json_array_set(json_t *array, size_t index, json_t *value)
404 Replaces the element in *array* at position *index* with *value*.
405 The valid range for *index* is from 0 to the return value of
406 :cfunc:`json_array_size()` minus 1. Returns 0 on success and -1 on
409 .. cfunction:: int json_array_set_new(json_t *array, size_t index, json_t *value)
411 Like :cfunc:`json_array_set()` but steals the reference to *value*.
412 This is useful when *value* is newly created and not used after
415 .. versionadded:: 1.1
417 .. cfunction:: int json_array_append(json_t *array, json_t *value)
419 Appends *value* to the end of *array*, growing the size of *array*
420 by 1. Returns 0 on success and -1 on error.
422 .. cfunction:: int json_array_append_new(json_t *array, json_t *value)
424 Like :cfunc:`json_array_append()` but steals the reference to
425 *value*. This is useful when *value* is newly created and not used
428 .. versionadded:: 1.1
430 .. cfunction:: int json_array_insert(json_t *array, size_t index, json_t *value)
432 Inserts *value* to *array* at position *index*, shifting the
433 elements at *index* and after it one position towards the end of
434 the array. Returns 0 on success and -1 on error.
436 .. versionadded:: 1.1
438 .. cfunction:: int json_array_insert_new(json_t *array, size_t index, json_t *value)
440 Like :cfunc:`json_array_insert()` but steals the reference to
441 *value*. This is useful when *value* is newly created and not used
444 .. versionadded:: 1.1
446 .. cfunction:: int json_array_remove(json_t *array, size_t index)
448 Removes the element in *array* at position *index*, shifting the
449 elements after *index* one position towards the start of the array.
450 Returns 0 on success and -1 on error.
452 .. versionadded:: 1.1
454 .. cfunction:: int json_array_clear(json_t *array)
456 Removes all elements from *array*. Returns 0 on sucess and -1 on
459 .. versionadded:: 1.1
461 .. cfunction:: int json_array_extend(json_t *array, json_t *other_array)
463 Appends all elements in *other_array* to the end of *array*.
464 Returns 0 on success and -1 on error.
466 .. versionadded:: 1.1
472 A JSON object is a dictionary of key-value pairs, where the key is a
473 Unicode string and the value is any JSON value.
475 .. cfunction:: json_t *json_object(void)
479 Returns a new JSON object, or *NULL* on error. Initially, the
482 .. cfunction:: size_t json_object_size(const json_t *object)
484 Returns the number of elements in *object*, or 0 if *object* is not
487 .. versionadded:: 1.1
489 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
491 .. refcounting:: borrow
493 Get a value corresponding to *key* from *object*. Returns *NULL* if
494 *key* is not found and on error.
496 .. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)
498 Set the value of *key* to *value* in *object*. *key* must be a
499 valid null terminated UTF-8 encoded Unicode string. If there
500 already is a value for *key*, it is replaced by the new value.
501 Returns 0 on success and -1 on error.
503 .. cfunction:: int json_object_set_nocheck(json_t *object, const char *key, json_t *value)
505 Like :cfunc:`json_object_set`, but doesn't check that *key* is
506 valid UTF-8. Use this function only if you are certain that this
507 really is the case (e.g. you have already checked it by other
510 .. versionadded:: 1.2
512 .. cfunction:: int json_object_set_new(json_t *object, const char *key, json_t *value)
514 Like :cfunc:`json_object_set()` but steals the reference to
515 *value*. This is useful when *value* is newly created and not used
518 .. versionadded:: 1.1
520 .. cfunction:: int json_object_set_new_nocheck(json_t *object, const char *key, json_t *value)
522 Like :cfunc:`json_object_set_new`, but doesn't check that *key* is
523 valid UTF-8. Use this function only if you are certain that this
524 really is the case (e.g. you have already checked it by other
527 .. versionadded:: 1.2
529 .. cfunction:: int json_object_del(json_t *object, const char *key)
531 Delete *key* from *object* if it exists. Returns 0 on success, or
532 -1 if *key* was not found.
535 .. cfunction:: int json_object_clear(json_t *object)
537 Remove all elements from *object*. Returns 0 on success and -1 if
538 *object* is not a JSON object.
540 .. versionadded:: 1.1
542 .. cfunction:: int json_object_update(json_t *object, json_t *other)
544 Update *object* with the key-value pairs from *other*, overwriting
545 existing keys. Returns 0 on success or -1 on error.
547 .. versionadded:: 1.1
550 The following functions implement an iteration protocol for objects:
552 .. cfunction:: void *json_object_iter(json_t *object)
554 Returns an opaque iterator which can be used to iterate over all
555 key-value pairs in *object*, or *NULL* if *object* is empty.
557 .. cfunction:: void *json_object_iter_at(json_t *object, const char *key)
559 Like :cfunc:`json_object_iter()`, but returns an iterator to the
560 key-value pair in *object* whose key is equal to *key*, or NULL if
561 *key* is not found in *object*. Iterating forward to the end of
562 *object* only yields all key-value pairs of the object if *key*
563 happens to be the first key in the underlying hash table.
565 .. versionadded:: 1.3
567 .. cfunction:: void *json_object_iter_next(json_t *object, void *iter)
569 Returns an iterator pointing to the next key-value pair in *object*
570 after *iter*, or *NULL* if the whole object has been iterated
573 .. cfunction:: const char *json_object_iter_key(void *iter)
575 Extract the associated key from *iter*.
577 .. cfunction:: json_t *json_object_iter_value(void *iter)
579 .. refcounting:: borrow
581 Extract the associated value from *iter*.
583 .. cfunction:: int json_object_iter_set(json_t *object, void *iter, json_t *value)
585 Set the value of the key-value pair in *object*, that is pointed to
586 by *iter*, to *value*.
588 .. versionadded:: 1.3
590 .. cfunction:: int json_object_iter_set_new(json_t *object, void *iter, json_t *value)
592 Like :cfunc:`json_object_iter_set()`, but steals the reference to
593 *value*. This is useful when *value* is newly created and not used
596 .. versionadded:: 1.3
598 The iteration protocol can be used for example as follows::
600 /* obj is a JSON object */
603 void *iter = json_object_iter(obj);
606 key = json_object_iter_key(iter);
607 value = json_object_iter_value(iter);
608 /* use key and value ... */
609 iter = json_object_iter_next(obj, iter);
616 This sections describes the functions that can be used to encode
617 values to JSON. Only objects and arrays can be encoded, since they are
618 the only valid "root" values of a JSON text.
620 By default, the output has no newlines, and spaces are used between
621 array and object elements for a readable output. This behavior can be
622 altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
623 described below. A newline is never appended to the end of the encoded
626 Each function takes a *flags* parameter that controls some aspects of
627 how the data is encoded. Its default value is 0. The following macros
628 can be ORed together to obtain *flags*.
631 Pretty-print the result, using newlines between array and object
632 items, and indenting with *n* spaces. The valid range for *n* is
633 between 0 and 32, other values result in an undefined output. If
634 ``JSON_INDENT`` is not used or *n* is 0, no newlines are inserted
635 between array and object items.
638 This flag enables a compact representation, i.e. sets the separator
639 between array and object items to ``","`` and between object keys
640 and values to ``":"``. Without this flag, the corresponding
641 separators are ``", "`` and ``": "`` for more readable output.
643 .. versionadded:: 1.2
645 ``JSON_ENSURE_ASCII``
646 If this flag is used, the output is guaranteed to consist only of
647 ASCII characters. This is achived by escaping all Unicode
648 characters outside the ASCII range.
650 .. versionadded:: 1.2
653 If this flag is used, all the objects in output are sorted by key.
654 This is useful e.g. if two JSON texts are diffed or visually
657 .. versionadded:: 1.2
659 ``JSON_PRESERVE_ORDER``
660 If this flag is used, object keys in the output are sorted into the
661 same order in which they were first inserted to the object. For
662 example, decoding a JSON text and then encoding with this flag
663 preserves the order of object keys.
665 .. versionadded:: 1.3
667 The following functions perform the actual JSON encoding. The result
670 .. cfunction:: char *json_dumps(const json_t *root, size_t flags)
672 Returns the JSON representation of *root* as a string, or *NULL* on
673 error. *flags* is described above. The return value must be freed
674 by the caller using :cfunc:`free()`.
676 .. cfunction:: int json_dumpf(const json_t *root, FILE *output, size_t flags)
678 Write the JSON representation of *root* to the stream *output*.
679 *flags* is described above. Returns 0 on success and -1 on error.
680 If an error occurs, something may have already been written to
681 *output*. In this case, the output is undefined and most likely not
684 .. cfunction:: int json_dump_file(const json_t *json, const char *path, size_t flags)
686 Write the JSON representation of *root* to the file *path*. If
687 *path* already exists, it is overwritten. *flags* is described
688 above. Returns 0 on success and -1 on error.
694 This sections describes the functions that can be used to decode JSON
695 text to the Jansson representation of JSON data. The JSON
696 specification requires that a JSON text is either a serialized array
697 or object, and this requirement is also enforced with the following
700 The only supported character encoding is UTF-8 (which ASCII is a
703 .. ctype:: json_error_t
705 This data structure is used to return information on decoding
706 errors from the decoding functions. Its definition is repeated
709 #define JSON_ERROR_TEXT_LENGTH 160
712 char text[JSON_ERROR_TEXT_LENGTH];
716 *line* is the line number on which the error occurred, or -1 if
717 this information is not available. *text* contains the error
718 message (in UTF-8), or an empty string if a message is not
721 The normal usef of :ctype:`json_error_t` is to allocate it normally
722 on the stack, and pass a pointer to a decoding function. Example::
728 json = json_load_file("/path/to/file.json", 0, &error);
730 /* the error variable contains error information */
735 Also note that if the decoding succeeded (``json != NULL`` in the
736 above example), the contents of ``error`` are unspecified.
738 All decoding functions also accept *NULL* as the
739 :ctype:`json_error_t` pointer, in which case no error information
740 is returned to the caller.
742 The following functions perform the actual JSON decoding.
744 .. cfunction:: json_t *json_loads(const char *input, size_t flags, json_error_t *error)
748 Decodes the JSON string *input* and returns the array or object it
749 contains, or *NULL* on error, in which case *error* is filled with
750 information about the error. See above for discussion on the
751 *error* parameter. *flags* is currently unused, and should be set
754 .. cfunction:: json_t *json_loadf(FILE *input, size_t flags, json_error_t *error)
758 Decodes the JSON text in stream *input* and returns the array or
759 object it contains, or *NULL* on error, in which case *error* is
760 filled with information about the error. See above for discussion
761 on the *error* parameter. *flags* is currently unused, and should
764 .. cfunction:: json_t *json_load_file(const char *path, size_t flags, json_error_t *error)
768 Decodes the JSON text in file *path* and returns the array or
769 object it contains, or *NULL* on error, in which case *error* is
770 filled with information about the error. See above for discussion
771 on the *error* parameter. *flags* is currently unused, and should
778 Testing for equality of two JSON values cannot, in general, be
779 achieved using the ``==`` operator. Equality in the terms of the
780 ``==`` operator states that the two :ctype:`json_t` pointers point to
781 exactly the same JSON value. However, two JSON values can be equal not
782 only if they are exactly the same value, but also if they have equal
785 * Two integer or real values are equal if their contained numeric
786 values are equal. An integer value is never equal to a real value,
789 * Two strings are equal if their contained UTF-8 strings are equal.
791 * Two arrays are equal if they have the same number of elements and
792 each element in the first array is equal to the corresponding
793 element in the second array.
795 * Two objects are equal if they have exactly the same keys and the
796 value for each key in the first object is equal to the value of the
797 corresponding key in the second object.
799 * Two true, false or null values have no "contents", so they are equal
800 if their types are equal. (Because these values are singletons,
801 their equality can actually be tested with ``==``.)
803 The following function can be used to test whether two JSON values are
806 .. cfunction:: int json_equal(json_t *value1, json_t *value2)
808 Returns 1 if *value1* and *value2* are equal, as defined above.
809 Returns 0 if they are inequal or one or both of the pointers are
812 .. versionadded:: 1.2
818 Because of reference counting, passing JSON values around doesn't
819 require copying them. But sometimes a fresh copy of a JSON value is
820 needed. For example, if you need to modify an array, but still want to
821 use the original afterwards, you should take a copy of it first.
823 Jansson supports two kinds of copying: shallow and deep. There is a
824 difference between these methods only for arrays and objects. Shallow
825 copying only copies the first level value (array or object) and uses
826 the same child values in the copied value. Deep copying makes a fresh
827 copy of the child values, too. Moreover, all the child values are deep
828 copied in a recursive fashion.
830 .. cfunction:: json_t *json_copy(json_t *value)
834 Returns a shallow copy of *value*, or *NULL* on error.
836 .. versionadded:: 1.2
838 .. cfunction:: json_t *json_deep_copy(json_t *value)
842 Returns a deep copy of *value*, or *NULL* on error.
844 .. versionadded:: 1.2