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 a full 64-bit range, you
300 should use ``json_int_t`` explicitly.
302 ``JSON_INTEGER_FORMAT``
304 This is a macro that expands to a :cfunc:`printf()` conversion
305 specifier that corresponds to :ctype:`json_int_t`, without the
306 leading ``%`` sign, i.e. either ``"lld"`` or ``"ld"``. This macro
307 is required because the actual type of :ctype:`json_int_t` can be
308 either ``long`` or ``long long``, and :cfunc:`printf()` reuiqres
309 different length modifiers for the two.
313 json_int_t x = 123123123;
314 printf("x is %" JSON_INTEGER_FORMAT "\n", x);
317 .. cfunction:: json_t *json_integer(json_int_t value)
321 Returns a new JSON integer, or *NULL* on error.
323 .. cfunction:: json_int_t json_integer_value(const json_t *integer)
325 Returns the associated value of *integer*, or 0 if *json* is not a
328 .. cfunction:: int json_integer_set(const json_t *integer, json_int_t value)
330 Sets the associated value of *integer* to *value*. Returns 0 on
331 success and -1 if *integer* is not a JSON integer.
333 .. versionadded:: 1.1
335 .. cfunction:: json_t *json_real(double value)
339 Returns a new JSON real, or *NULL* on error.
341 .. cfunction:: double json_real_value(const json_t *real)
343 Returns the associated value of *real*, or 0.0 if *real* is not a
346 .. cfunction:: int json_real_set(const json_t *real, double value)
348 Sets the associated value of *real* to *value*. Returns 0 on
349 success and -1 if *real* is not a JSON real.
351 .. versionadded:: 1.1
353 In addition to the functions above, there's a common query function
354 for integers and reals:
356 .. cfunction:: double json_number_value(const json_t *json)
358 Returns the associated value of the JSON integer or JSON real
359 *json*, cast to double regardless of the actual type. If *json* is
360 neither JSON real nor JSON integer, 0.0 is returned.
366 A JSON array is an ordered collection of other JSON values.
368 .. cfunction:: json_t *json_array(void)
372 Returns a new JSON array, or *NULL* on error. Initially, the array
375 .. cfunction:: size_t json_array_size(const json_t *array)
377 Returns the number of elements in *array*, or 0 if *array* is NULL
380 .. cfunction:: json_t *json_array_get(const json_t *array, size_t index)
382 .. refcounting:: borrow
384 Returns the element in *array* at position *index*. The valid range
385 for *index* is from 0 to the return value of
386 :cfunc:`json_array_size()` minus 1. If *array* is not a JSON array,
387 if *array* is *NULL*, or if *index* is out of range, *NULL* is
390 .. cfunction:: int json_array_set(json_t *array, size_t index, json_t *value)
392 Replaces the element in *array* at position *index* with *value*.
393 The valid range for *index* is from 0 to the return value of
394 :cfunc:`json_array_size()` minus 1. Returns 0 on success and -1 on
397 .. cfunction:: int json_array_set_new(json_t *array, size_t index, json_t *value)
399 Like :cfunc:`json_array_set()` but steals the reference to *value*.
400 This is useful when *value* is newly created and not used after
403 .. versionadded:: 1.1
405 .. cfunction:: int json_array_append(json_t *array, json_t *value)
407 Appends *value* to the end of *array*, growing the size of *array*
408 by 1. Returns 0 on success and -1 on error.
410 .. cfunction:: int json_array_append_new(json_t *array, json_t *value)
412 Like :cfunc:`json_array_append()` but steals the reference to
413 *value*. This is useful when *value* is newly created and not used
416 .. versionadded:: 1.1
418 .. cfunction:: int json_array_insert(json_t *array, size_t index, json_t *value)
420 Inserts *value* to *array* at position *index*, shifting the
421 elements at *index* and after it one position towards the end of
422 the array. Returns 0 on success and -1 on error.
424 .. versionadded:: 1.1
426 .. cfunction:: int json_array_insert_new(json_t *array, size_t index, json_t *value)
428 Like :cfunc:`json_array_insert()` but steals the reference to
429 *value*. This is useful when *value* is newly created and not used
432 .. versionadded:: 1.1
434 .. cfunction:: int json_array_remove(json_t *array, size_t index)
436 Removes the element in *array* at position *index*, shifting the
437 elements after *index* one position towards the start of the array.
438 Returns 0 on success and -1 on error.
440 .. versionadded:: 1.1
442 .. cfunction:: int json_array_clear(json_t *array)
444 Removes all elements from *array*. Returns 0 on sucess and -1 on
447 .. versionadded:: 1.1
449 .. cfunction:: int json_array_extend(json_t *array, json_t *other_array)
451 Appends all elements in *other_array* to the end of *array*.
452 Returns 0 on success and -1 on error.
454 .. versionadded:: 1.1
460 A JSON object is a dictionary of key-value pairs, where the key is a
461 Unicode string and the value is any JSON value.
463 .. cfunction:: json_t *json_object(void)
467 Returns a new JSON object, or *NULL* on error. Initially, the
470 .. cfunction:: size_t json_object_size(const json_t *object)
472 Returns the number of elements in *object*, or 0 if *object* is not
475 .. versionadded:: 1.1
477 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
479 .. refcounting:: borrow
481 Get a value corresponding to *key* from *object*. Returns *NULL* if
482 *key* is not found and on error.
484 .. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)
486 Set the value of *key* to *value* in *object*. *key* must be a
487 valid null terminated UTF-8 encoded Unicode string. If there
488 already is a value for *key*, it is replaced by the new value.
489 Returns 0 on success and -1 on error.
491 .. cfunction:: int json_object_set_nocheck(json_t *object, const char *key, json_t *value)
493 Like :cfunc:`json_object_set`, but doesn't check that *key* is
494 valid UTF-8. Use this function only if you are certain that this
495 really is the case (e.g. you have already checked it by other
498 .. versionadded:: 1.2
500 .. cfunction:: int json_object_set_new(json_t *object, const char *key, json_t *value)
502 Like :cfunc:`json_object_set()` but steals the reference to
503 *value*. This is useful when *value* is newly created and not used
506 .. versionadded:: 1.1
508 .. cfunction:: int json_object_set_new_nocheck(json_t *object, const char *key, json_t *value)
510 Like :cfunc:`json_object_set_new`, but doesn't check that *key* is
511 valid UTF-8. Use this function only if you are certain that this
512 really is the case (e.g. you have already checked it by other
515 .. versionadded:: 1.2
517 .. cfunction:: int json_object_del(json_t *object, const char *key)
519 Delete *key* from *object* if it exists. Returns 0 on success, or
520 -1 if *key* was not found.
523 .. cfunction:: int json_object_clear(json_t *object)
525 Remove all elements from *object*. Returns 0 on success and -1 if
526 *object* is not a JSON object.
528 .. versionadded:: 1.1
530 .. cfunction:: int json_object_update(json_t *object, json_t *other)
532 Update *object* with the key-value pairs from *other*, overwriting
533 existing keys. Returns 0 on success or -1 on error.
535 .. versionadded:: 1.1
538 The following functions implement an iteration protocol for objects:
540 .. cfunction:: void *json_object_iter(json_t *object)
542 Returns an opaque iterator which can be used to iterate over all
543 key-value pairs in *object*, or *NULL* if *object* is empty.
545 .. cfunction:: void *json_object_iter_at(json_t *object, const char *key)
547 Like :cfunc:`json_object_iter()`, but returns an iterator to the
548 key-value pair in *object* whose key is equal to *key*, or NULL if
549 *key* is not found in *object*. Iterating forward to the end of
550 *object* only yields all key-value pairs of the object if *key*
551 happens to be the first key in the underlying hash table.
553 .. versionadded:: 1.3
555 .. cfunction:: void *json_object_iter_next(json_t *object, void *iter)
557 Returns an iterator pointing to the next key-value pair in *object*
558 after *iter*, or *NULL* if the whole object has been iterated
561 .. cfunction:: const char *json_object_iter_key(void *iter)
563 Extract the associated key from *iter*.
565 .. cfunction:: json_t *json_object_iter_value(void *iter)
567 .. refcounting:: borrow
569 Extract the associated value from *iter*.
571 .. cfunction:: int json_object_iter_set(json_t *object, void *iter, json_t *value)
573 Set the value of the key-value pair in *object*, that is pointed to
574 by *iter*, to *value*.
576 .. versionadded:: 1.3
578 .. cfunction:: int json_object_iter_set_new(json_t *object, void *iter, json_t *value)
580 Like :cfunc:`json_object_iter_set()`, but steals the reference to
581 *value*. This is useful when *value* is newly created and not used
584 .. versionadded:: 1.3
586 The iteration protocol can be used for example as follows::
588 /* obj is a JSON object */
591 void *iter = json_object_iter(obj);
594 key = json_object_iter_key(iter);
595 value = json_object_iter_value(iter);
596 /* use key and value ... */
597 iter = json_object_iter_next(obj, iter);
604 This sections describes the functions that can be used to encode
605 values to JSON. Only objects and arrays can be encoded, since they are
606 the only valid "root" values of a JSON text.
608 By default, the output has no newlines, and spaces are used between
609 array and object elements for a readable output. This behavior can be
610 altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
611 described below. A newline is never appended to the end of the encoded
614 Each function takes a *flags* parameter that controls some aspects of
615 how the data is encoded. Its default value is 0. The following macros
616 can be ORed together to obtain *flags*.
619 Pretty-print the result, using newlines between array and object
620 items, and indenting with *n* spaces. The valid range for *n* is
621 between 0 and 32, other values result in an undefined output. If
622 ``JSON_INDENT`` is not used or *n* is 0, no newlines are inserted
623 between array and object items.
626 This flag enables a compact representation, i.e. sets the separator
627 between array and object items to ``","`` and between object keys
628 and values to ``":"``. Without this flag, the corresponding
629 separators are ``", "`` and ``": "`` for more readable output.
631 .. versionadded:: 1.2
633 ``JSON_ENSURE_ASCII``
634 If this flag is used, the output is guaranteed to consist only of
635 ASCII characters. This is achived by escaping all Unicode
636 characters outside the ASCII range.
638 .. versionadded:: 1.2
641 If this flag is used, all the objects in output are sorted by key.
642 This is useful e.g. if two JSON texts are diffed or visually
645 .. versionadded:: 1.2
647 ``JSON_PRESERVE_ORDER``
648 If this flag is used, object keys in the output are sorted into the
649 same order in which they were first inserted to the object. For
650 example, decoding a JSON text and then encoding with this flag
651 preserves the order of object keys.
653 .. versionadded:: 1.3
655 The following functions perform the actual JSON encoding. The result
658 .. cfunction:: char *json_dumps(const json_t *root, size_t flags)
660 Returns the JSON representation of *root* as a string, or *NULL* on
661 error. *flags* is described above. The return value must be freed
662 by the caller using :cfunc:`free()`.
664 .. cfunction:: int json_dumpf(const json_t *root, FILE *output, size_t flags)
666 Write the JSON representation of *root* to the stream *output*.
667 *flags* is described above. Returns 0 on success and -1 on error.
668 If an error occurs, something may have already been written to
669 *output*. In this case, the output is undefined and most likely not
672 .. cfunction:: int json_dump_file(const json_t *json, const char *path, size_t flags)
674 Write the JSON representation of *root* to the file *path*. If
675 *path* already exists, it is overwritten. *flags* is described
676 above. Returns 0 on success and -1 on error.
682 This sections describes the functions that can be used to decode JSON
683 text to the Jansson representation of JSON data. The JSON
684 specification requires that a JSON text is either a serialized array
685 or object, and this requirement is also enforced with the following
688 The only supported character encoding is UTF-8 (which ASCII is a
691 .. ctype:: json_error_t
693 This data structure is used to return information on decoding
694 errors from the decoding functions. Its definition is repeated
697 #define JSON_ERROR_TEXT_LENGTH 160
700 char text[JSON_ERROR_TEXT_LENGTH];
704 *line* is the line number on which the error occurred, or -1 if
705 this information is not available. *text* contains the error
706 message (in UTF-8), or an empty string if a message is not
709 The normal usef of :ctype:`json_error_t` is to allocate it normally
710 on the stack, and pass a pointer to a decoding function. Example::
716 json = json_load_file("/path/to/file.json", &error);
718 /* the error variable contains error information */
723 Also note that if the decoding succeeded (``json != NULL`` in the
724 above example), the contents of ``error`` are unspecified.
726 All decoding functions also accept *NULL* as the
727 :ctype:`json_error_t` pointer, in which case no error information
728 is returned to the caller.
730 The following functions perform the actual JSON decoding.
732 .. cfunction:: json_t *json_loads(const char *input, json_error_t *error)
736 Decodes the JSON string *input* and returns the array or object it
737 contains, or *NULL* on error, in which case *error* is filled with
738 information about the error. See above for discussion on the
741 .. cfunction:: json_t *json_loadf(FILE *input, json_error_t *error)
745 Decodes the JSON text in stream *input* and returns the array or
746 object it contains, or *NULL* on error, in which case *error* is
747 filled with information about the error. See above for discussion
748 on the *error* parameter.
750 .. cfunction:: json_t *json_load_file(const char *path, json_error_t *error)
754 Decodes the JSON text in file *path* and returns the array or
755 object it contains, or *NULL* on error, in which case *error* is
756 filled with information about the error. See above for discussion
757 on the *error* parameter.
763 Testing for equality of two JSON values cannot, in general, be
764 achieved using the ``==`` operator. Equality in the terms of the
765 ``==`` operator states that the two :ctype:`json_t` pointers point to
766 exactly the same JSON value. However, two JSON values can be equal not
767 only if they are exactly the same value, but also if they have equal
770 * Two integer or real values are equal if their contained numeric
771 values are equal. An integer value is never equal to a real value,
774 * Two strings are equal if their contained UTF-8 strings are equal.
776 * Two arrays are equal if they have the same number of elements and
777 each element in the first array is equal to the corresponding
778 element in the second array.
780 * Two objects are equal if they have exactly the same keys and the
781 value for each key in the first object is equal to the value of the
782 corresponding key in the second object.
784 * Two true, false or null values have no "contents", so they are equal
785 if their types are equal. (Because these values are singletons,
786 their equality can actually be tested with ``==``.)
788 The following function can be used to test whether two JSON values are
791 .. cfunction:: int json_equal(json_t *value1, json_t *value2)
793 Returns 1 if *value1* and *value2* are equal, as defined above.
794 Returns 0 if they are inequal or one or both of the pointers are
797 .. versionadded:: 1.2
803 Because of reference counting, passing JSON values around doesn't
804 require copying them. But sometimes a fresh copy of a JSON value is
805 needed. For example, if you need to modify an array, but still want to
806 use the original afterwards, you should take a copy of it first.
808 Jansson supports two kinds of copying: shallow and deep. There is a
809 difference between these methods only for arrays and objects. Shallow
810 copying only copies the first level value (array or object) and uses
811 the same child values in the copied value. Deep copying makes a fresh
812 copy of the child values, too. Moreover, all the child values are deep
813 copied in a recursive fashion.
815 .. cfunction:: json_t *json_copy(json_t *value)
819 Returns a shallow copy of *value*, or *NULL* on error.
821 .. versionadded:: 1.2
823 .. cfunction:: json_t *json_deep_copy(json_t *value)
827 Returns a deep copy of *value*, or *NULL* on error.
829 .. versionadded:: 1.2