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 :type:`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 .. type:: enum json_type
61 The type of a JSON value. The following members are defined:
63 +--------------------+
65 +--------------------+
67 +--------------------+
69 +--------------------+
71 +--------------------+
73 +--------------------+
75 +--------------------+
77 +--------------------+
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 ``JSON_INTEGER`` or of the type ``JSON_REAL``. A true boolean value
84 is represented by a value of the type ``JSON_TRUE`` and false by a
85 value of the type ``JSON_FALSE``.
87 .. function:: int json_typeof(const json_t *json)
89 Return the type of the JSON value (a :type:`json_type` cast to
90 :type:`int`). *json* MUST NOT be *NULL*. This function is actually
91 implemented as a macro for speed.
93 .. function:: json_is_object(const json_t *json)
94 json_is_array(const json_t *json)
95 json_is_string(const json_t *json)
96 json_is_integer(const json_t *json)
97 json_is_real(const json_t *json)
98 json_is_true(const json_t *json)
99 json_is_false(const json_t *json)
100 json_is_null(const json_t *json)
102 These functions (actually macros) return true (non-zero) for values
103 of the given type, and false (zero) for values of other types and
106 .. function:: json_is_number(const json_t *json)
108 Returns true for values of types ``JSON_INTEGER`` and
109 ``JSON_REAL``, and false for other types and for *NULL*.
111 .. function:: json_is_boolean(const json_t *json)
113 Returns true for types ``JSON_TRUE`` and ``JSON_FALSE``, and false
114 for values of other types and for *NULL*.
117 .. _apiref-reference-count:
122 The reference count is used to track whether a value is still in use
123 or not. When a value is created, it's reference count is set to 1. If
124 a reference to a value is kept (e.g. a value is stored somewhere for
125 later use), its reference count is incremented, and when the value is
126 no longer needed, the reference count is decremented. When the
127 reference count drops to zero, there are no references left, and the
128 value can be destroyed.
130 The following functions are used to manipulate the reference count.
132 .. function:: json_t *json_incref(json_t *json)
134 Increment the reference count of *json* if it's not non-*NULL*.
137 .. function:: void json_decref(json_t *json)
139 Decrement the reference count of *json*. As soon as a call to
140 :func:`json_decref()` drops the reference count to zero, the value
141 is destroyed and it can no longer be used.
143 Functions creating new JSON values set the reference count to 1. These
144 functions are said to return a **new reference**. Other functions
145 returning (existing) JSON values do not normally increase the
146 reference count. These functions are said to return a **borrowed
147 reference**. So, if the user will hold a reference to a value returned
148 as a borrowed reference, he must call :func:`json_incref`. As soon as
149 the value is no longer needed, :func:`json_decref` should be called
150 to release the reference.
152 Normally, all functions accepting a JSON value as an argument will
153 manage the reference, i.e. increase and decrease the reference count
154 as needed. However, some functions **steal** the reference, i.e. they
155 have the same result as if the user called :func:`json_decref()` on
156 the argument right after calling the function. These functions are
157 suffixed with ``_new`` or have ``_new_`` somewhere in their name.
159 For example, the following code creates a new JSON array and appends
162 json_t *array, *integer;
164 array = json_array();
165 integer = json_integer(42);
167 json_array_append(array, integer);
168 json_decref(integer);
170 Note how the caller has to release the reference to the integer value
171 by calling :func:`json_decref()`. By using a reference stealing
172 function :func:`json_array_append_new()` instead of
173 :func:`json_array_append()`, the code becomes much simpler::
175 json_t *array = json_array();
176 json_array_append_new(array, json_integer(42));
178 In this case, the user doesn't have to explicitly release the
179 reference to the integer value, as :func:`json_array_append_new()`
180 steals the reference when appending the value to the array.
182 In the following sections it is clearly documented whether a function
183 will return a new or borrowed reference or steal a reference to its
190 A circular reference is created when an object or an array is,
191 directly or indirectly, inserted inside itself. The direct case is
194 json_t *obj = json_object();
195 json_object_set(obj, "foo", obj);
197 Jansson will refuse to do this, and :func:`json_object_set()` (and
198 all the other such functions for objects and arrays) will return with
199 an error status. The indirect case is the dangerous one::
201 json_t *arr1 = json_array(), *arr2 = json_array();
202 json_array_append(arr1, arr2);
203 json_array_append(arr2, arr1);
205 In this example, the array ``arr2`` is contained in the array
206 ``arr1``, and vice versa. Jansson cannot check for this kind of
207 indirect circular references without a performance hit, so it's up to
208 the user to avoid them.
210 If a circular reference is created, the memory consumed by the values
211 cannot be freed by :func:`json_decref()`. The reference counts never
212 drops to zero because the values are keeping the references to each
213 other. Moreover, trying to encode the values with any of the encoding
214 functions will fail. The encoder detects circular references and
215 returns an error status.
221 These values are implemented as singletons, so each of these functions
222 returns the same value each time.
224 .. function:: json_t *json_true(void)
228 Returns the JSON true value.
230 .. function:: json_t *json_false(void)
234 Returns the JSON false value.
236 .. function:: json_t *json_null(void)
240 Returns the JSON null value.
246 Jansson uses UTF-8 as the character encoding. All JSON strings must be
247 valid UTF-8 (or ASCII, as it's a subset of UTF-8). Normal null
248 terminated C strings are used, so JSON strings may not contain
249 embedded null characters. All other Unicode codepoints U+0001 through
250 U+10FFFF are allowed.
252 .. function:: json_t *json_string(const char *value)
256 Returns a new JSON string, or *NULL* on error. *value* must be a
257 valid UTF-8 encoded Unicode string.
259 .. function:: json_t *json_string_nocheck(const char *value)
263 Like :func:`json_string`, but doesn't check that *value* is valid
264 UTF-8. Use this function only if you are certain that this really
265 is the case (e.g. you have already checked it by other means).
267 .. versionadded:: 1.2
269 .. function:: const char *json_string_value(const json_t *string)
271 Returns the associated value of *string* as a null terminated UTF-8
272 encoded string, or *NULL* if *string* is not a JSON string.
274 .. function:: int json_string_set(const json_t *string, const char *value)
276 Sets the associated value of *string* to *value*. *value* must be a
277 valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
280 .. versionadded:: 1.1
282 .. function:: int json_string_set_nocheck(const json_t *string, const char *value)
284 Like :func:`json_string_set`, but doesn't check that *value* is
285 valid UTF-8. Use this function only if you are certain that this
286 really is the case (e.g. you have already checked it by other
289 .. versionadded:: 1.2
295 The JSON specification only contains one numeric type, "number". The C
296 programming language has distinct types for integer and floating-point
297 numbers, so for practical reasons Jansson also has distinct types for
298 the two. They are called "integer" and "real", respectively. For more
299 information, see :ref:`rfc-conformance`.
303 This is the C type that is used to store JSON integer values. It
304 represents the widest integer type available on your system. In
305 practice it's just a typedef of ``long long`` if your compiler
306 supports it, otherwise ``long``.
308 Usually, you can safely use plain ``int`` in place of
309 ``json_int_t``, and the implicit C integer conversion handles the
310 rest. Only when you know that you need the full 64-bit range, you
311 should use ``json_int_t`` explicitly.
313 ``JSON_INTEGER_IS_LONG_LONG``
315 This is a preprocessor variable that holds the value 1 if
316 :type:`json_int_t` is ``long long``, and 0 if it's ``long``. It
317 can be used as follows::
319 #if JSON_INTEGER_IS_LONG_LONG
320 /* Code specific for long long */
322 /* Code specific for long */
325 ``JSON_INTEGER_FORMAT``
327 This is a macro that expands to a :func:`printf()` conversion
328 specifier that corresponds to :type:`json_int_t`, without the
329 leading ``%`` sign, i.e. either ``"lld"`` or ``"ld"``. This macro
330 is required because the actual type of :type:`json_int_t` can be
331 either ``long`` or ``long long``, and :func:`printf()` reuiqres
332 different length modifiers for the two.
336 json_int_t x = 123123123;
337 printf("x is %" JSON_INTEGER_FORMAT "\n", x);
340 .. function:: json_t *json_integer(json_int_t value)
344 Returns a new JSON integer, or *NULL* on error.
346 .. function:: json_int_t json_integer_value(const json_t *integer)
348 Returns the associated value of *integer*, or 0 if *json* is not a
351 .. function:: int json_integer_set(const json_t *integer, json_int_t value)
353 Sets the associated value of *integer* to *value*. Returns 0 on
354 success and -1 if *integer* is not a JSON integer.
356 .. versionadded:: 1.1
358 .. function:: json_t *json_real(double value)
362 Returns a new JSON real, or *NULL* on error.
364 .. function:: double json_real_value(const json_t *real)
366 Returns the associated value of *real*, or 0.0 if *real* is not a
369 .. function:: int json_real_set(const json_t *real, double value)
371 Sets the associated value of *real* to *value*. Returns 0 on
372 success and -1 if *real* is not a JSON real.
374 .. versionadded:: 1.1
376 In addition to the functions above, there's a common query function
377 for integers and reals:
379 .. function:: double json_number_value(const json_t *json)
381 Returns the associated value of the JSON integer or JSON real
382 *json*, cast to double regardless of the actual type. If *json* is
383 neither JSON real nor JSON integer, 0.0 is returned.
389 A JSON array is an ordered collection of other JSON values.
391 .. function:: json_t *json_array(void)
395 Returns a new JSON array, or *NULL* on error. Initially, the array
398 .. function:: size_t json_array_size(const json_t *array)
400 Returns the number of elements in *array*, or 0 if *array* is NULL
403 .. function:: json_t *json_array_get(const json_t *array, size_t index)
405 .. refcounting:: borrow
407 Returns the element in *array* at position *index*. The valid range
408 for *index* is from 0 to the return value of
409 :func:`json_array_size()` minus 1. If *array* is not a JSON array,
410 if *array* is *NULL*, or if *index* is out of range, *NULL* is
413 .. function:: int json_array_set(json_t *array, size_t index, json_t *value)
415 Replaces the element in *array* at position *index* with *value*.
416 The valid range for *index* is from 0 to the return value of
417 :func:`json_array_size()` minus 1. Returns 0 on success and -1 on
420 .. function:: int json_array_set_new(json_t *array, size_t index, json_t *value)
422 Like :func:`json_array_set()` but steals the reference to *value*.
423 This is useful when *value* is newly created and not used after
426 .. versionadded:: 1.1
428 .. function:: int json_array_append(json_t *array, json_t *value)
430 Appends *value* to the end of *array*, growing the size of *array*
431 by 1. Returns 0 on success and -1 on error.
433 .. function:: int json_array_append_new(json_t *array, json_t *value)
435 Like :func:`json_array_append()` but steals the reference to
436 *value*. This is useful when *value* is newly created and not used
439 .. versionadded:: 1.1
441 .. function:: int json_array_insert(json_t *array, size_t index, json_t *value)
443 Inserts *value* to *array* at position *index*, shifting the
444 elements at *index* and after it one position towards the end of
445 the array. Returns 0 on success and -1 on error.
447 .. versionadded:: 1.1
449 .. function:: int json_array_insert_new(json_t *array, size_t index, json_t *value)
451 Like :func:`json_array_insert()` but steals the reference to
452 *value*. This is useful when *value* is newly created and not used
455 .. versionadded:: 1.1
457 .. function:: int json_array_remove(json_t *array, size_t index)
459 Removes the element in *array* at position *index*, shifting the
460 elements after *index* one position towards the start of the array.
461 Returns 0 on success and -1 on error.
463 .. versionadded:: 1.1
465 .. function:: int json_array_clear(json_t *array)
467 Removes all elements from *array*. Returns 0 on sucess and -1 on
470 .. versionadded:: 1.1
472 .. function:: int json_array_extend(json_t *array, json_t *other_array)
474 Appends all elements in *other_array* to the end of *array*.
475 Returns 0 on success and -1 on error.
477 .. versionadded:: 1.1
483 A JSON object is a dictionary of key-value pairs, where the key is a
484 Unicode string and the value is any JSON value.
486 .. function:: json_t *json_object(void)
490 Returns a new JSON object, or *NULL* on error. Initially, the
493 .. function:: size_t json_object_size(const json_t *object)
495 Returns the number of elements in *object*, or 0 if *object* is not
498 .. versionadded:: 1.1
500 .. function:: json_t *json_object_get(const json_t *object, const char *key)
502 .. refcounting:: borrow
504 Get a value corresponding to *key* from *object*. Returns *NULL* if
505 *key* is not found and on error.
507 .. function:: int json_object_set(json_t *object, const char *key, json_t *value)
509 Set the value of *key* to *value* in *object*. *key* must be a
510 valid null terminated UTF-8 encoded Unicode string. If there
511 already is a value for *key*, it is replaced by the new value.
512 Returns 0 on success and -1 on error.
514 .. function:: int json_object_set_nocheck(json_t *object, const char *key, json_t *value)
516 Like :func:`json_object_set`, but doesn't check that *key* is
517 valid UTF-8. Use this function only if you are certain that this
518 really is the case (e.g. you have already checked it by other
521 .. versionadded:: 1.2
523 .. function:: int json_object_set_new(json_t *object, const char *key, json_t *value)
525 Like :func:`json_object_set()` but steals the reference to
526 *value*. This is useful when *value* is newly created and not used
529 .. versionadded:: 1.1
531 .. function:: int json_object_set_new_nocheck(json_t *object, const char *key, json_t *value)
533 Like :func:`json_object_set_new`, but doesn't check that *key* is
534 valid UTF-8. Use this function only if you are certain that this
535 really is the case (e.g. you have already checked it by other
538 .. versionadded:: 1.2
540 .. function:: int json_object_del(json_t *object, const char *key)
542 Delete *key* from *object* if it exists. Returns 0 on success, or
543 -1 if *key* was not found.
546 .. function:: int json_object_clear(json_t *object)
548 Remove all elements from *object*. Returns 0 on success and -1 if
549 *object* is not a JSON object.
551 .. versionadded:: 1.1
553 .. function:: int json_object_update(json_t *object, json_t *other)
555 Update *object* with the key-value pairs from *other*, overwriting
556 existing keys. Returns 0 on success or -1 on error.
558 .. versionadded:: 1.1
561 The following functions implement an iteration protocol for objects:
563 .. function:: void *json_object_iter(json_t *object)
565 Returns an opaque iterator which can be used to iterate over all
566 key-value pairs in *object*, or *NULL* if *object* is empty.
568 .. function:: void *json_object_iter_at(json_t *object, const char *key)
570 Like :func:`json_object_iter()`, but returns an iterator to the
571 key-value pair in *object* whose key is equal to *key*, or NULL if
572 *key* is not found in *object*. Iterating forward to the end of
573 *object* only yields all key-value pairs of the object if *key*
574 happens to be the first key in the underlying hash table.
576 .. versionadded:: 1.3
578 .. function:: void *json_object_iter_next(json_t *object, void *iter)
580 Returns an iterator pointing to the next key-value pair in *object*
581 after *iter*, or *NULL* if the whole object has been iterated
584 .. function:: const char *json_object_iter_key(void *iter)
586 Extract the associated key from *iter*.
588 .. function:: json_t *json_object_iter_value(void *iter)
590 .. refcounting:: borrow
592 Extract the associated value from *iter*.
594 .. function:: int json_object_iter_set(json_t *object, void *iter, json_t *value)
596 Set the value of the key-value pair in *object*, that is pointed to
597 by *iter*, to *value*.
599 .. versionadded:: 1.3
601 .. function:: int json_object_iter_set_new(json_t *object, void *iter, json_t *value)
603 Like :func:`json_object_iter_set()`, but steals the reference to
604 *value*. This is useful when *value* is newly created and not used
607 .. versionadded:: 1.3
609 The iteration protocol can be used for example as follows::
611 /* obj is a JSON object */
614 void *iter = json_object_iter(obj);
617 key = json_object_iter_key(iter);
618 value = json_object_iter_value(iter);
619 /* use key and value ... */
620 iter = json_object_iter_next(obj, iter);
627 This sections describes the functions that can be used to encode
628 values to JSON. Only objects and arrays can be encoded, since they are
629 the only valid "root" values of a JSON text.
631 By default, the output has no newlines, and spaces are used between
632 array and object elements for a readable output. This behavior can be
633 altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
634 described below. A newline is never appended to the end of the encoded
637 Each function takes a *flags* parameter that controls some aspects of
638 how the data is encoded. Its default value is 0. The following macros
639 can be ORed together to obtain *flags*.
642 Pretty-print the result, using newlines between array and object
643 items, and indenting with *n* spaces. The valid range for *n* is
644 between 0 and 32, other values result in an undefined output. If
645 ``JSON_INDENT`` is not used or *n* is 0, no newlines are inserted
646 between array and object items.
649 This flag enables a compact representation, i.e. sets the separator
650 between array and object items to ``","`` and between object keys
651 and values to ``":"``. Without this flag, the corresponding
652 separators are ``", "`` and ``": "`` for more readable output.
654 .. versionadded:: 1.2
656 ``JSON_ENSURE_ASCII``
657 If this flag is used, the output is guaranteed to consist only of
658 ASCII characters. This is achived by escaping all Unicode
659 characters outside the ASCII range.
661 .. versionadded:: 1.2
664 If this flag is used, all the objects in output are sorted by key.
665 This is useful e.g. if two JSON texts are diffed or visually
668 .. versionadded:: 1.2
670 ``JSON_PRESERVE_ORDER``
671 If this flag is used, object keys in the output are sorted into the
672 same order in which they were first inserted to the object. For
673 example, decoding a JSON text and then encoding with this flag
674 preserves the order of object keys.
676 .. versionadded:: 1.3
678 The following functions perform the actual JSON encoding. The result
681 .. function:: char *json_dumps(const json_t *root, size_t flags)
683 Returns the JSON representation of *root* as a string, or *NULL* on
684 error. *flags* is described above. The return value must be freed
685 by the caller using :func:`free()`.
687 .. function:: int json_dumpf(const json_t *root, FILE *output, size_t flags)
689 Write the JSON representation of *root* to the stream *output*.
690 *flags* is described above. Returns 0 on success and -1 on error.
691 If an error occurs, something may have already been written to
692 *output*. In this case, the output is undefined and most likely not
695 .. function:: int json_dump_file(const json_t *json, const char *path, size_t flags)
697 Write the JSON representation of *root* to the file *path*. If
698 *path* already exists, it is overwritten. *flags* is described
699 above. Returns 0 on success and -1 on error.
705 This sections describes the functions that can be used to decode JSON
706 text to the Jansson representation of JSON data. The JSON
707 specification requires that a JSON text is either a serialized array
708 or object, and this requirement is also enforced with the following
709 functions. In other words, the top level value in the JSON text being
710 decoded must be either array or object.
712 See :ref:`rfc-conformance` for a discussion on Jansson's conformance
713 to the JSON specification. It explains many design decisions that
714 affect especially the behavior of the decoder.
716 .. type:: json_error_t
718 This data structure is used to return information on decoding
719 errors from the decoding functions. Its definition is repeated
722 #define JSON_ERROR_TEXT_LENGTH 160
725 char text[JSON_ERROR_TEXT_LENGTH];
729 *line* is the line number on which the error occurred, or -1 if
730 this information is not available. *text* contains the error
731 message (in UTF-8), or an empty string if a message is not
734 The normal usef of :type:`json_error_t` is to allocate it normally
735 on the stack, and pass a pointer to a decoding function. Example::
741 json = json_load_file("/path/to/file.json", 0, &error);
743 /* the error variable contains error information */
748 Also note that if the decoding succeeded (``json != NULL`` in the
749 above example), the contents of ``error`` are unspecified.
751 All decoding functions also accept *NULL* as the
752 :type:`json_error_t` pointer, in which case no error information
753 is returned to the caller.
755 The following functions perform the actual JSON decoding.
757 .. function:: json_t *json_loads(const char *input, size_t flags, json_error_t *error)
761 Decodes the JSON string *input* and returns the array or object it
762 contains, or *NULL* on error, in which case *error* is filled with
763 information about the error. See above for discussion on the
764 *error* parameter. *flags* is currently unused, and should be set
767 .. function:: json_t *json_loadf(FILE *input, size_t flags, json_error_t *error)
771 Decodes the JSON text in stream *input* and returns the array or
772 object it contains, or *NULL* on error, in which case *error* is
773 filled with information about the error. See above for discussion
774 on the *error* parameter. *flags* is currently unused, and should
777 .. function:: json_t *json_load_file(const char *path, size_t flags, json_error_t *error)
781 Decodes the JSON text in file *path* and returns the array or
782 object it contains, or *NULL* on error, in which case *error* is
783 filled with information about the error. See above for discussion
784 on the *error* parameter. *flags* is currently unused, and should
791 Testing for equality of two JSON values cannot, in general, be
792 achieved using the ``==`` operator. Equality in the terms of the
793 ``==`` operator states that the two :type:`json_t` pointers point to
794 exactly the same JSON value. However, two JSON values can be equal not
795 only if they are exactly the same value, but also if they have equal
798 * Two integer or real values are equal if their contained numeric
799 values are equal. An integer value is never equal to a real value,
802 * Two strings are equal if their contained UTF-8 strings are equal,
803 byte by byte. Unicode comparison algorithms are not implemented.
805 * Two arrays are equal if they have the same number of elements and
806 each element in the first array is equal to the corresponding
807 element in the second array.
809 * Two objects are equal if they have exactly the same keys and the
810 value for each key in the first object is equal to the value of the
811 corresponding key in the second object.
813 * Two true, false or null values have no "contents", so they are equal
814 if their types are equal. (Because these values are singletons,
815 their equality can actually be tested with ``==``.)
817 The following function can be used to test whether two JSON values are
820 .. function:: int json_equal(json_t *value1, json_t *value2)
822 Returns 1 if *value1* and *value2* are equal, as defined above.
823 Returns 0 if they are inequal or one or both of the pointers are
826 .. versionadded:: 1.2
832 Because of reference counting, passing JSON values around doesn't
833 require copying them. But sometimes a fresh copy of a JSON value is
834 needed. For example, if you need to modify an array, but still want to
835 use the original afterwards, you should take a copy of it first.
837 Jansson supports two kinds of copying: shallow and deep. There is a
838 difference between these methods only for arrays and objects. Shallow
839 copying only copies the first level value (array or object) and uses
840 the same child values in the copied value. Deep copying makes a fresh
841 copy of the child values, too. Moreover, all the child values are deep
842 copied in a recursive fashion.
844 .. function:: json_t *json_copy(json_t *value)
848 Returns a shallow copy of *value*, or *NULL* on error.
850 .. versionadded:: 1.2
852 .. function:: json_t *json_deep_copy(json_t *value)
856 Returns a deep copy of *value*, or *NULL* on error.
858 .. versionadded:: 1.2