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 Jansson uses UTF-8 as the character encoding. All JSON strings must be
248 valid UTF-8 (or ASCII, as it's a subset of UTF-8). Normal null
249 terminated C strings are used, so JSON strings may not contain
250 embedded null characters. All other Unicode codepoints U+0001 through
251 U+10FFFF are allowed.
253 .. cfunction:: json_t *json_string(const char *value)
257 Returns a new JSON string, or *NULL* on error. *value* must be a
258 valid UTF-8 encoded Unicode string.
260 .. cfunction:: json_t *json_string_nocheck(const char *value)
264 Like :cfunc:`json_string`, but doesn't check that *value* is valid
265 UTF-8. Use this function only if you are certain that this really
266 is the case (e.g. you have already checked it by other means).
268 .. versionadded:: 1.2
270 .. cfunction:: const char *json_string_value(const json_t *string)
272 Returns the associated value of *string* as a null terminated UTF-8
273 encoded string, or *NULL* if *string* is not a JSON string.
275 .. cfunction:: int json_string_set(const json_t *string, const char *value)
277 Sets the associated value of *string* to *value*. *value* must be a
278 valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
281 .. versionadded:: 1.1
283 .. cfunction:: int json_string_set_nocheck(const json_t *string, const char *value)
285 Like :cfunc:`json_string_set`, but doesn't check that *value* is
286 valid UTF-8. Use this function only if you are certain that this
287 really is the case (e.g. you have already checked it by other
290 .. versionadded:: 1.2
296 The JSON specification only contains one numeric type, "number". The C
297 programming language has distinct types for integer and floating-point
298 numbers, so for practical reasons Jansson also has distinct types for
299 the two. They are called "integer" and "real", respectively. For more
300 information, see :ref:`rfc-conformance`.
302 .. cfunction:: json_t *json_integer(int value)
306 Returns a new JSON integer, or *NULL* on error.
308 .. cfunction:: int json_integer_value(const json_t *integer)
310 Returns the associated value of *integer*, or 0 if *json* is not a
313 .. cfunction:: int json_integer_set(const json_t *integer, int value)
315 Sets the associated value of *integer* to *value*. Returns 0 on
316 success and -1 if *integer* is not a JSON integer.
318 .. versionadded:: 1.1
320 .. cfunction:: json_t *json_real(double value)
324 Returns a new JSON real, or *NULL* on error.
326 .. cfunction:: double json_real_value(const json_t *real)
328 Returns the associated value of *real*, or 0.0 if *real* is not a
331 .. cfunction:: int json_real_set(const json_t *real, double value)
333 Sets the associated value of *real* to *value*. Returns 0 on
334 success and -1 if *real* is not a JSON real.
336 .. versionadded:: 1.1
338 In addition to the functions above, there's a common query function
339 for integers and reals:
341 .. cfunction:: double json_number_value(const json_t *json)
343 Returns the associated value of the JSON integer or JSON real
344 *json*, cast to double regardless of the actual type. If *json* is
345 neither JSON real nor JSON integer, 0.0 is returned.
351 A JSON array is an ordered collection of other JSON values.
353 .. cfunction:: json_t *json_array(void)
357 Returns a new JSON array, or *NULL* on error. Initially, the array
360 .. cfunction:: unsigned int json_array_size(const json_t *array)
362 Returns the number of elements in *array*, or 0 if *array* is NULL
365 .. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
367 .. refcounting:: borrow
369 Returns the element in *array* at position *index*. The valid range
370 for *index* is from 0 to the return value of
371 :cfunc:`json_array_size()` minus 1. If *array* is not a JSON array,
372 if *array* is *NULL*, or if *index* is out of range, *NULL* is
375 .. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
377 Replaces the element in *array* at position *index* with *value*.
378 The valid range for *index* is from 0 to the return value of
379 :cfunc:`json_array_size()` minus 1. Returns 0 on success and -1 on
382 .. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
384 Like :cfunc:`json_array_set()` but steals the reference to *value*.
385 This is useful when *value* is newly created and not used after
388 .. versionadded:: 1.1
390 .. cfunction:: int json_array_append(json_t *array, json_t *value)
392 Appends *value* to the end of *array*, growing the size of *array*
393 by 1. Returns 0 on success and -1 on error.
395 .. cfunction:: int json_array_append_new(json_t *array, json_t *value)
397 Like :cfunc:`json_array_append()` but steals the reference to
398 *value*. This is useful when *value* is newly created and not used
401 .. versionadded:: 1.1
403 .. cfunction:: int json_array_insert(json_t *array, unsigned int index, json_t *value)
405 Inserts *value* to *array* at position *index*, shifting the
406 elements at *index* and after it one position towards the end of
407 the array. Returns 0 on success and -1 on error.
409 .. versionadded:: 1.1
411 .. cfunction:: int json_array_insert_new(json_t *array, unsigned int index, json_t *value)
413 Like :cfunc:`json_array_insert()` but steals the reference to
414 *value*. This is useful when *value* is newly created and not used
417 .. versionadded:: 1.1
419 .. cfunction:: int json_array_remove(json_t *array, unsigned int index)
421 Removes the element in *array* at position *index*, shifting the
422 elements after *index* one position towards the start of the array.
423 Returns 0 on success and -1 on error.
425 .. versionadded:: 1.1
427 .. cfunction:: int json_array_clear(json_t *array)
429 Removes all elements from *array*. Returns 0 on sucess and -1 on
432 .. versionadded:: 1.1
434 .. cfunction:: int json_array_extend(json_t *array, json_t *other_array)
436 Appends all elements in *other_array* to the end of *array*.
437 Returns 0 on success and -1 on error.
439 .. versionadded:: 1.1
445 A JSON object is a dictionary of key-value pairs, where the key is a
446 Unicode string and the value is any JSON value.
448 .. cfunction:: json_t *json_object(void)
452 Returns a new JSON object, or *NULL* on error. Initially, the
455 .. cfunction:: unsigned int json_object_size(const json_t *object)
457 Returns the number of elements in *object*, or 0 if *object* is not
460 .. versionadded:: 1.1
462 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
464 .. refcounting:: borrow
466 Get a value corresponding to *key* from *object*. Returns *NULL* if
467 *key* is not found and on error.
469 .. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)
471 Set the value of *key* to *value* in *object*. *key* must be a
472 valid null terminated UTF-8 encoded Unicode string. If there
473 already is a value for *key*, it is replaced by the new value.
474 Returns 0 on success and -1 on error.
476 .. cfunction:: int json_object_set_nocheck(json_t *object, const char *key, json_t *value)
478 Like :cfunc:`json_object_set`, but doesn't check that *key* is
479 valid UTF-8. Use this function only if you are certain that this
480 really is the case (e.g. you have already checked it by other
483 .. versionadded:: 1.2
485 .. cfunction:: int json_object_set_new(json_t *object, const char *key, json_t *value)
487 Like :cfunc:`json_object_set()` but steals the reference to
488 *value*. This is useful when *value* is newly created and not used
491 .. versionadded:: 1.1
493 .. cfunction:: int json_object_set_new_nocheck(json_t *object, const char *key, json_t *value)
495 Like :cfunc:`json_object_set_new`, but doesn't check that *key* is
496 valid UTF-8. Use this function only if you are certain that this
497 really is the case (e.g. you have already checked it by other
500 .. versionadded:: 1.2
502 .. cfunction:: int json_object_del(json_t *object, const char *key)
504 Delete *key* from *object* if it exists. Returns 0 on success, or
505 -1 if *key* was not found.
508 .. cfunction:: int json_object_clear(json_t *object)
510 Remove all elements from *object*. Returns 0 on success and -1 if
511 *object* is not a JSON object.
513 .. versionadded:: 1.1
515 .. cfunction:: int json_object_update(json_t *object, json_t *other)
517 Update *object* with the key-value pairs from *other*, overwriting
518 existing keys. Returns 0 on success or -1 on error.
520 .. versionadded:: 1.1
523 The following functions implement an iteration protocol for objects:
525 .. cfunction:: void *json_object_iter(json_t *object)
527 Returns an opaque iterator which can be used to iterate over all
528 key-value pairs in *object*, or *NULL* if *object* is empty.
530 .. cfunction:: void *json_object_iter_at(json_t *object, const char *key)
532 Like :cfunc:`json_object_iter()`, but returns an iterator to the
533 key-value pair in *object* whose key is equal to *key*, or NULL if
534 *key* is not found in *object*. Iterating forward to the end of
535 *object* only yields all key-value pairs of the object if *key*
536 happens to be the first key in the underlying hash table.
538 .. versionadded:: 1.3
540 .. cfunction:: void *json_object_iter_next(json_t *object, void *iter)
542 Returns an iterator pointing to the next key-value pair in *object*
543 after *iter*, or *NULL* if the whole object has been iterated
546 .. cfunction:: const char *json_object_iter_key(void *iter)
548 Extract the associated key from *iter*.
550 .. cfunction:: json_t *json_object_iter_value(void *iter)
552 .. refcounting:: borrow
554 Extract the associated value from *iter*.
556 .. cfunction:: int json_object_iter_set(json_t *object, void *iter, json_t *value)
558 Set the value of the key-value pair in *object*, that is pointed to
559 by *iter*, to *value*.
561 .. versionadded:: 1.3
563 .. cfunction:: int json_object_iter_set_new(json_t *object, void *iter, json_t *value)
565 Like :cfunc:`json_object_iter_set()`, but steals the reference to
566 *value*. This is useful when *value* is newly created and not used
569 .. versionadded:: 1.3
571 The iteration protocol can be used for example as follows::
573 /* obj is a JSON object */
576 void *iter = json_object_iter(obj);
579 key = json_object_iter_key(iter);
580 value = json_object_iter_value(iter);
581 /* use key and value ... */
582 iter = json_object_iter_next(obj, iter);
589 This sections describes the functions that can be used to encode
590 values to JSON. Only objects and arrays can be encoded, since they are
591 the only valid "root" values of a JSON text.
593 By default, the output has no newlines, and spaces are used between
594 array and object elements for a readable output. This behavior can be
595 altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
596 described below. A newline is never appended to the end of the encoded
599 Each function takes a *flags* parameter that controls some aspects of
600 how the data is encoded. Its default value is 0. The following macros
601 can be ORed together to obtain *flags*.
604 Pretty-print the result, using newlines between array and object
605 items, and indenting with *n* spaces. The valid range for *n* is
606 between 0 and 255, other values result in an undefined output. If
607 ``JSON_INDENT`` is not used or *n* is 0, no newlines are inserted
608 between array and object items.
611 This flag enables a compact representation, i.e. sets the separator
612 between array and object items to ``","`` and between object keys
613 and values to ``":"``. Without this flag, the corresponding
614 separators are ``", "`` and ``": "`` for more readable output.
616 .. versionadded:: 1.2
618 ``JSON_ENSURE_ASCII``
619 If this flag is used, the output is guaranteed to consist only of
620 ASCII characters. This is achived by escaping all Unicode
621 characters outside the ASCII range.
623 .. versionadded:: 1.2
626 If this flag is used, all the objects in output are sorted by key.
627 This is useful e.g. if two JSON texts are diffed or visually
630 .. versionadded:: 1.2
632 ``JSON_PRESERVE_ORDER``
633 If this flag is used, object keys in the output are sorted into the
634 same order in which they were first inserted to the object. For
635 example, decoding a JSON text and then encoding with this flag
636 preserves the order of object keys.
638 .. versionadded:: 1.3
640 The following functions perform the actual JSON encoding. The result
643 .. cfunction:: char *json_dumps(const json_t *root, unsigned long flags)
645 Returns the JSON representation of *root* as a string, or *NULL* on
646 error. *flags* is described above. The return value must be freed
647 by the caller using :cfunc:`free()`.
649 .. cfunction:: int json_dumpf(const json_t *root, FILE *output, unsigned long flags)
651 Write the JSON representation of *root* to the stream *output*.
652 *flags* is described above. Returns 0 on success and -1 on error.
653 If an error occurs, something may have already been written to
654 *output*. In this case, the output is undefined and most likely not
657 .. cfunction:: int json_dump_file(const json_t *json, const char *path, unsigned long flags)
659 Write the JSON representation of *root* to the file *path*. If
660 *path* already exists, it is overwritten. *flags* is described
661 above. Returns 0 on success and -1 on error.
667 This sections describes the functions that can be used to decode JSON
668 text to the Jansson representation of JSON data. The JSON
669 specification requires that a JSON text is either a serialized array
670 or object, and this requirement is also enforced with the following
671 functions. In other words, the top level value in the JSON text being
672 decoded must be either array or object.
674 See :ref:`rfc-conformance` for a discussion on Jansson's conformance
675 to the JSON specification. It explains many design decisions that
676 affect especially the behavior of the decoder.
678 .. ctype:: json_error_t
680 This data structure is used to return information on decoding
681 errors from the decoding functions. Its definition is repeated
684 #define JSON_ERROR_TEXT_LENGTH 160
687 char text[JSON_ERROR_TEXT_LENGTH];
691 *line* is the line number on which the error occurred, or -1 if
692 this information is not available. *text* contains the error
693 message (in UTF-8), or an empty string if a message is not
696 The normal usef of :ctype:`json_error_t` is to allocate it normally
697 on the stack, and pass a pointer to a decoding function. Example::
703 json = json_load_file("/path/to/file.json", &error);
705 /* the error variable contains error information */
710 Also note that if the decoding succeeded (``json != NULL`` in the
711 above example), the contents of ``error`` are unspecified.
713 All decoding functions also accept *NULL* as the
714 :ctype:`json_error_t` pointer, in which case no error information
715 is returned to the caller.
717 The following functions perform the actual JSON decoding.
719 .. cfunction:: json_t *json_loads(const char *input, json_error_t *error)
723 Decodes the JSON string *input* and returns the array or object it
724 contains, or *NULL* on error, in which case *error* is filled with
725 information about the error. See above for discussion on the
728 .. cfunction:: json_t *json_loadf(FILE *input, json_error_t *error)
732 Decodes the JSON text in stream *input* and returns the array or
733 object it contains, or *NULL* on error, in which case *error* is
734 filled with information about the error. See above for discussion
735 on the *error* parameter.
737 .. cfunction:: json_t *json_load_file(const char *path, json_error_t *error)
741 Decodes the JSON text in file *path* and returns the array or
742 object it contains, or *NULL* on error, in which case *error* is
743 filled with information about the error. See above for discussion
744 on the *error* parameter.
750 Testing for equality of two JSON values cannot, in general, be
751 achieved using the ``==`` operator. Equality in the terms of the
752 ``==`` operator states that the two :ctype:`json_t` pointers point to
753 exactly the same JSON value. However, two JSON values can be equal not
754 only if they are exactly the same value, but also if they have equal
757 * Two integer or real values are equal if their contained numeric
758 values are equal. An integer value is never equal to a real value,
761 * Two strings are equal if their contained UTF-8 strings are equal,
762 byte by byte. Unicode comparison algorithms are not implemented.
764 * Two arrays are equal if they have the same number of elements and
765 each element in the first array is equal to the corresponding
766 element in the second array.
768 * Two objects are equal if they have exactly the same keys and the
769 value for each key in the first object is equal to the value of the
770 corresponding key in the second object.
772 * Two true, false or null values have no "contents", so they are equal
773 if their types are equal. (Because these values are singletons,
774 their equality can actually be tested with ``==``.)
776 The following function can be used to test whether two JSON values are
779 .. cfunction:: int json_equal(json_t *value1, json_t *value2)
781 Returns 1 if *value1* and *value2* are equal, as defined above.
782 Returns 0 if they are inequal or one or both of the pointers are
785 .. versionadded:: 1.2
791 Because of reference counting, passing JSON values around doesn't
792 require copying them. But sometimes a fresh copy of a JSON value is
793 needed. For example, if you need to modify an array, but still want to
794 use the original afterwards, you should take a copy of it first.
796 Jansson supports two kinds of copying: shallow and deep. There is a
797 difference between these methods only for arrays and objects. Shallow
798 copying only copies the first level value (array or object) and uses
799 the same child values in the copied value. Deep copying makes a fresh
800 copy of the child values, too. Moreover, all the child values are deep
801 copied in a recursive fashion.
803 .. cfunction:: json_t *json_copy(json_t *value)
807 Returns a shallow copy of *value*, or *NULL* on error.
809 .. versionadded:: 1.2
811 .. cfunction:: json_t *json_deep_copy(json_t *value)
815 Returns a deep copy of *value*, or *NULL* on error.
817 .. versionadded:: 1.2