12 All declarations are in :file:`jansson.h`, so it's enough to
20 All constants are prefixed with ``JSON_`` (except for those describing
21 the library version, prefixed with ``JANSSON_``). Other identifiers
22 are prefixed with ``json_``. Type names are suffixed with ``_t`` and
23 ``typedef``\ 'd so that the ``struct`` keyword need not be used.
29 The Jansson version is of the form *A.B.C*, where *A* is the major
30 version, *B* is the minor version and *C* is the micro version. If the
31 micro version is zero, it's omitted from the version string, i.e. the
32 version string is just *A.B*.
34 When a new release only fixes bugs and doesn't add new features or
35 functionality, the micro version is incremented. When new features are
36 added in a backwards compatible way, the minor version is incremented
37 and the micro version is set to zero. When there are backwards
38 incompatible changes, the major version is incremented and others are
41 The following preprocessor constants specify the current version of
44 ``JANSSON_VERSION_MAJOR``, ``JANSSON_VERSION_MINOR``, ``JANSSON_VERSION_MICRO``
45 Integers specifying the major, minor and micro versions,
49 A string representation of the current version, e.g. ``"1.2.1"`` or
52 ``JANSSON_VERSION_HEX``
53 A 3-byte hexadecimal representation of the version, e.g.
54 ``0x010201`` for version 1.2.1 and ``0x010300`` for version 1.3.
55 This is useful in numeric comparisions, e.g.::
57 #if JANSSON_VERSION_HEX >= 0x010300
58 /* Code specific to version 1.3 and above */
65 The JSON specification (:rfc:`4627`) defines the following data types:
66 *object*, *array*, *string*, *number*, *boolean*, and *null*. JSON
67 types are used dynamically; arrays and objects can hold any other data
68 type, including themselves. For this reason, Jansson's type system is
69 also dynamic in nature. There's one C type to represent all JSON
70 values, and this structure knows the type of the JSON value it holds.
74 This data structure is used throughout the library to represent all
75 JSON values. It always contains the type of the JSON value it holds
76 and the value's reference count. The rest depends on the type of the
79 Objects of :type:`json_t` are always used through a pointer. There
80 are APIs for querying the type, manipulating the reference count, and
81 for constructing and manipulating values of different types.
83 Unless noted otherwise, all API functions return an error value if an
84 error occurs. Depending on the function's signature, the error value
85 is either *NULL* or -1. Invalid arguments or invalid input are
86 apparent sources for errors. Memory allocation and I/O operations may
93 The type of a JSON value is queried and tested using the following
96 .. type:: enum json_type
98 The type of a JSON value. The following members are defined:
100 +--------------------+
102 +--------------------+
104 +--------------------+
106 +--------------------+
108 +--------------------+
110 +--------------------+
112 +--------------------+
114 +--------------------+
116 +--------------------+
118 These correspond to JSON object, array, string, number, boolean and
119 null. A number is represented by either a value of the type
120 ``JSON_INTEGER`` or of the type ``JSON_REAL``. A true boolean value
121 is represented by a value of the type ``JSON_TRUE`` and false by a
122 value of the type ``JSON_FALSE``.
124 .. function:: int json_typeof(const json_t *json)
126 Return the type of the JSON value (a :type:`json_type` cast to
127 :type:`int`). *json* MUST NOT be *NULL*. This function is actually
128 implemented as a macro for speed.
130 .. function:: json_is_object(const json_t *json)
131 json_is_array(const json_t *json)
132 json_is_string(const json_t *json)
133 json_is_integer(const json_t *json)
134 json_is_real(const json_t *json)
135 json_is_true(const json_t *json)
136 json_is_false(const json_t *json)
137 json_is_null(const json_t *json)
139 These functions (actually macros) return true (non-zero) for values
140 of the given type, and false (zero) for values of other types and
143 .. function:: json_is_number(const json_t *json)
145 Returns true for values of types ``JSON_INTEGER`` and
146 ``JSON_REAL``, and false for other types and for *NULL*.
148 .. function:: json_is_boolean(const json_t *json)
150 Returns true for types ``JSON_TRUE`` and ``JSON_FALSE``, and false
151 for values of other types and for *NULL*.
154 .. _apiref-reference-count:
159 The reference count is used to track whether a value is still in use
160 or not. When a value is created, it's reference count is set to 1. If
161 a reference to a value is kept (e.g. a value is stored somewhere for
162 later use), its reference count is incremented, and when the value is
163 no longer needed, the reference count is decremented. When the
164 reference count drops to zero, there are no references left, and the
165 value can be destroyed.
167 The following functions are used to manipulate the reference count.
169 .. function:: json_t *json_incref(json_t *json)
171 Increment the reference count of *json* if it's not non-*NULL*.
174 .. function:: void json_decref(json_t *json)
176 Decrement the reference count of *json*. As soon as a call to
177 :func:`json_decref()` drops the reference count to zero, the value
178 is destroyed and it can no longer be used.
180 Functions creating new JSON values set the reference count to 1. These
181 functions are said to return a **new reference**. Other functions
182 returning (existing) JSON values do not normally increase the
183 reference count. These functions are said to return a **borrowed
184 reference**. So, if the user will hold a reference to a value returned
185 as a borrowed reference, he must call :func:`json_incref`. As soon as
186 the value is no longer needed, :func:`json_decref` should be called
187 to release the reference.
189 Normally, all functions accepting a JSON value as an argument will
190 manage the reference, i.e. increase and decrease the reference count
191 as needed. However, some functions **steal** the reference, i.e. they
192 have the same result as if the user called :func:`json_decref()` on
193 the argument right after calling the function. These functions are
194 suffixed with ``_new`` or have ``_new_`` somewhere in their name.
196 For example, the following code creates a new JSON array and appends
199 json_t *array, *integer;
201 array = json_array();
202 integer = json_integer(42);
204 json_array_append(array, integer);
205 json_decref(integer);
207 Note how the caller has to release the reference to the integer value
208 by calling :func:`json_decref()`. By using a reference stealing
209 function :func:`json_array_append_new()` instead of
210 :func:`json_array_append()`, the code becomes much simpler::
212 json_t *array = json_array();
213 json_array_append_new(array, json_integer(42));
215 In this case, the user doesn't have to explicitly release the
216 reference to the integer value, as :func:`json_array_append_new()`
217 steals the reference when appending the value to the array.
219 In the following sections it is clearly documented whether a function
220 will return a new or borrowed reference or steal a reference to its
227 A circular reference is created when an object or an array is,
228 directly or indirectly, inserted inside itself. The direct case is
231 json_t *obj = json_object();
232 json_object_set(obj, "foo", obj);
234 Jansson will refuse to do this, and :func:`json_object_set()` (and
235 all the other such functions for objects and arrays) will return with
236 an error status. The indirect case is the dangerous one::
238 json_t *arr1 = json_array(), *arr2 = json_array();
239 json_array_append(arr1, arr2);
240 json_array_append(arr2, arr1);
242 In this example, the array ``arr2`` is contained in the array
243 ``arr1``, and vice versa. Jansson cannot check for this kind of
244 indirect circular references without a performance hit, so it's up to
245 the user to avoid them.
247 If a circular reference is created, the memory consumed by the values
248 cannot be freed by :func:`json_decref()`. The reference counts never
249 drops to zero because the values are keeping the references to each
250 other. Moreover, trying to encode the values with any of the encoding
251 functions will fail. The encoder detects circular references and
252 returns an error status.
258 These values are implemented as singletons, so each of these functions
259 returns the same value each time.
261 .. function:: json_t *json_true(void)
265 Returns the JSON true value.
267 .. function:: json_t *json_false(void)
271 Returns the JSON false value.
273 .. function:: json_t *json_null(void)
277 Returns the JSON null value.
283 Jansson uses UTF-8 as the character encoding. All JSON strings must be
284 valid UTF-8 (or ASCII, as it's a subset of UTF-8). Normal null
285 terminated C strings are used, so JSON strings may not contain
286 embedded null characters. All other Unicode codepoints U+0001 through
287 U+10FFFF are allowed.
289 .. function:: json_t *json_string(const char *value)
293 Returns a new JSON string, or *NULL* on error. *value* must be a
294 valid UTF-8 encoded Unicode string.
296 .. function:: json_t *json_string_nocheck(const char *value)
300 Like :func:`json_string`, but doesn't check that *value* is valid
301 UTF-8. Use this function only if you are certain that this really
302 is the case (e.g. you have already checked it by other means).
304 .. function:: const char *json_string_value(const json_t *string)
306 Returns the associated value of *string* as a null terminated UTF-8
307 encoded string, or *NULL* if *string* is not a JSON string.
309 .. function:: int json_string_set(const json_t *string, const char *value)
311 Sets the associated value of *string* to *value*. *value* must be a
312 valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
315 .. function:: int json_string_set_nocheck(const json_t *string, const char *value)
317 Like :func:`json_string_set`, but doesn't check that *value* is
318 valid UTF-8. Use this function only if you are certain that this
319 really is the case (e.g. you have already checked it by other
326 The JSON specification only contains one numeric type, "number". The C
327 programming language has distinct types for integer and floating-point
328 numbers, so for practical reasons Jansson also has distinct types for
329 the two. They are called "integer" and "real", respectively. For more
330 information, see :ref:`rfc-conformance`.
334 This is the C type that is used to store JSON integer values. It
335 represents the widest integer type available on your system. In
336 practice it's just a typedef of ``long long`` if your compiler
337 supports it, otherwise ``long``.
339 Usually, you can safely use plain ``int`` in place of
340 ``json_int_t``, and the implicit C integer conversion handles the
341 rest. Only when you know that you need the full 64-bit range, you
342 should use ``json_int_t`` explicitly.
344 ``JSON_INTEGER_IS_LONG_LONG``
346 This is a preprocessor variable that holds the value 1 if
347 :type:`json_int_t` is ``long long``, and 0 if it's ``long``. It
348 can be used as follows::
350 #if JSON_INTEGER_IS_LONG_LONG
351 /* Code specific for long long */
353 /* Code specific for long */
356 ``JSON_INTEGER_FORMAT``
358 This is a macro that expands to a :func:`printf()` conversion
359 specifier that corresponds to :type:`json_int_t`, without the
360 leading ``%`` sign, i.e. either ``"lld"`` or ``"ld"``. This macro
361 is required because the actual type of :type:`json_int_t` can be
362 either ``long`` or ``long long``, and :func:`printf()` reuiqres
363 different length modifiers for the two.
367 json_int_t x = 123123123;
368 printf("x is %" JSON_INTEGER_FORMAT "\n", x);
371 .. function:: json_t *json_integer(json_int_t value)
375 Returns a new JSON integer, or *NULL* on error.
377 .. function:: json_int_t json_integer_value(const json_t *integer)
379 Returns the associated value of *integer*, or 0 if *json* is not a
382 .. function:: int json_integer_set(const json_t *integer, json_int_t value)
384 Sets the associated value of *integer* to *value*. Returns 0 on
385 success and -1 if *integer* is not a JSON integer.
387 .. function:: json_t *json_real(double value)
391 Returns a new JSON real, or *NULL* on error.
393 .. function:: double json_real_value(const json_t *real)
395 Returns the associated value of *real*, or 0.0 if *real* is not a
398 .. function:: int json_real_set(const json_t *real, double value)
400 Sets the associated value of *real* to *value*. Returns 0 on
401 success and -1 if *real* is not a JSON real.
403 In addition to the functions above, there's a common query function
404 for integers and reals:
406 .. function:: double json_number_value(const json_t *json)
408 Returns the associated value of the JSON integer or JSON real
409 *json*, cast to double regardless of the actual type. If *json* is
410 neither JSON real nor JSON integer, 0.0 is returned.
416 A JSON array is an ordered collection of other JSON values.
418 .. function:: json_t *json_array(void)
422 Returns a new JSON array, or *NULL* on error. Initially, the array
425 .. function:: size_t json_array_size(const json_t *array)
427 Returns the number of elements in *array*, or 0 if *array* is NULL
430 .. function:: json_t *json_array_get(const json_t *array, size_t index)
432 .. refcounting:: borrow
434 Returns the element in *array* at position *index*. The valid range
435 for *index* is from 0 to the return value of
436 :func:`json_array_size()` minus 1. If *array* is not a JSON array,
437 if *array* is *NULL*, or if *index* is out of range, *NULL* is
440 .. function:: int json_array_set(json_t *array, size_t index, json_t *value)
442 Replaces the element in *array* at position *index* with *value*.
443 The valid range for *index* is from 0 to the return value of
444 :func:`json_array_size()` minus 1. Returns 0 on success and -1 on
447 .. function:: int json_array_set_new(json_t *array, size_t index, json_t *value)
449 Like :func:`json_array_set()` but steals the reference to *value*.
450 This is useful when *value* is newly created and not used after
453 .. function:: int json_array_append(json_t *array, json_t *value)
455 Appends *value* to the end of *array*, growing the size of *array*
456 by 1. Returns 0 on success and -1 on error.
458 .. function:: int json_array_append_new(json_t *array, json_t *value)
460 Like :func:`json_array_append()` but steals the reference to
461 *value*. This is useful when *value* is newly created and not used
464 .. function:: int json_array_insert(json_t *array, size_t index, json_t *value)
466 Inserts *value* to *array* at position *index*, shifting the
467 elements at *index* and after it one position towards the end of
468 the array. Returns 0 on success and -1 on error.
470 .. function:: int json_array_insert_new(json_t *array, size_t index, json_t *value)
472 Like :func:`json_array_insert()` but steals the reference to
473 *value*. This is useful when *value* is newly created and not used
476 .. function:: int json_array_remove(json_t *array, size_t index)
478 Removes the element in *array* at position *index*, shifting the
479 elements after *index* one position towards the start of the array.
480 Returns 0 on success and -1 on error.
482 .. function:: int json_array_clear(json_t *array)
484 Removes all elements from *array*. Returns 0 on sucess and -1 on
487 .. function:: int json_array_extend(json_t *array, json_t *other_array)
489 Appends all elements in *other_array* to the end of *array*.
490 Returns 0 on success and -1 on error.
496 A JSON object is a dictionary of key-value pairs, where the key is a
497 Unicode string and the value is any JSON value.
499 .. function:: json_t *json_object(void)
503 Returns a new JSON object, or *NULL* on error. Initially, the
506 .. function:: size_t json_object_size(const json_t *object)
508 Returns the number of elements in *object*, or 0 if *object* is not
511 .. function:: json_t *json_object_get(const json_t *object, const char *key)
513 .. refcounting:: borrow
515 Get a value corresponding to *key* from *object*. Returns *NULL* if
516 *key* is not found and on error.
518 .. function:: int json_object_set(json_t *object, const char *key, json_t *value)
520 Set the value of *key* to *value* in *object*. *key* must be a
521 valid null terminated UTF-8 encoded Unicode string. If there
522 already is a value for *key*, it is replaced by the new value.
523 Returns 0 on success and -1 on error.
525 .. function:: int json_object_set_nocheck(json_t *object, const char *key, json_t *value)
527 Like :func:`json_object_set`, but doesn't check that *key* is
528 valid UTF-8. Use this function only if you are certain that this
529 really is the case (e.g. you have already checked it by other
532 .. function:: int json_object_set_new(json_t *object, const char *key, json_t *value)
534 Like :func:`json_object_set()` but steals the reference to
535 *value*. This is useful when *value* is newly created and not used
538 .. function:: int json_object_set_new_nocheck(json_t *object, const char *key, json_t *value)
540 Like :func:`json_object_set_new`, but doesn't check that *key* is
541 valid UTF-8. Use this function only if you are certain that this
542 really is the case (e.g. you have already checked it by other
545 .. function:: int json_object_del(json_t *object, const char *key)
547 Delete *key* from *object* if it exists. Returns 0 on success, or
548 -1 if *key* was not found.
551 .. function:: int json_object_clear(json_t *object)
553 Remove all elements from *object*. Returns 0 on success and -1 if
554 *object* is not a JSON object.
556 .. function:: int json_object_update(json_t *object, json_t *other)
558 Update *object* with the key-value pairs from *other*, overwriting
559 existing keys. Returns 0 on success or -1 on error.
562 The following functions implement an iteration protocol for objects:
564 .. function:: void *json_object_iter(json_t *object)
566 Returns an opaque iterator which can be used to iterate over all
567 key-value pairs in *object*, or *NULL* if *object* is empty.
569 .. function:: void *json_object_iter_at(json_t *object, const char *key)
571 Like :func:`json_object_iter()`, but returns an iterator to the
572 key-value pair in *object* whose key is equal to *key*, or NULL if
573 *key* is not found in *object*. Iterating forward to the end of
574 *object* only yields all key-value pairs of the object if *key*
575 happens to be the first key in the underlying hash table.
577 .. function:: void *json_object_iter_next(json_t *object, void *iter)
579 Returns an iterator pointing to the next key-value pair in *object*
580 after *iter*, or *NULL* if the whole object has been iterated
583 .. function:: const char *json_object_iter_key(void *iter)
585 Extract the associated key from *iter*.
587 .. function:: json_t *json_object_iter_value(void *iter)
589 .. refcounting:: borrow
591 Extract the associated value from *iter*.
593 .. function:: int json_object_iter_set(json_t *object, void *iter, json_t *value)
595 Set the value of the key-value pair in *object*, that is pointed to
596 by *iter*, to *value*.
598 .. function:: int json_object_iter_set_new(json_t *object, void *iter, json_t *value)
600 Like :func:`json_object_iter_set()`, but steals the reference to
601 *value*. This is useful when *value* is newly created and not used
604 The iteration protocol can be used for example as follows::
606 /* obj is a JSON object */
609 void *iter = json_object_iter(obj);
612 key = json_object_iter_key(iter);
613 value = json_object_iter_value(iter);
614 /* use key and value ... */
615 iter = json_object_iter_next(obj, iter);
622 This sections describes the functions that can be used to encode
623 values to JSON. Only objects and arrays can be encoded, since they are
624 the only valid "root" values of a JSON text.
626 By default, the output has no newlines, and spaces are used between
627 array and object elements for a readable output. This behavior can be
628 altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
629 described below. A newline is never appended to the end of the encoded
632 Each function takes a *flags* parameter that controls some aspects of
633 how the data is encoded. Its default value is 0. The following macros
634 can be ORed together to obtain *flags*.
637 Pretty-print the result, using newlines between array and object
638 items, and indenting with *n* spaces. The valid range for *n* is
639 between 0 and 32, other values result in an undefined output. If
640 ``JSON_INDENT`` is not used or *n* is 0, no newlines are inserted
641 between array and object items.
644 This flag enables a compact representation, i.e. sets the separator
645 between array and object items to ``","`` and between object keys
646 and values to ``":"``. Without this flag, the corresponding
647 separators are ``", "`` and ``": "`` for more readable output.
649 ``JSON_ENSURE_ASCII``
650 If this flag is used, the output is guaranteed to consist only of
651 ASCII characters. This is achived by escaping all Unicode
652 characters outside the ASCII range.
655 If this flag is used, all the objects in output are sorted by key.
656 This is useful e.g. if two JSON texts are diffed or visually
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 The following functions perform the actual JSON encoding. The result
668 .. function:: char *json_dumps(const json_t *root, size_t flags)
670 Returns the JSON representation of *root* as a string, or *NULL* on
671 error. *flags* is described above. The return value must be freed
672 by the caller using :func:`free()`.
674 .. function:: int json_dumpf(const json_t *root, FILE *output, size_t flags)
676 Write the JSON representation of *root* to the stream *output*.
677 *flags* is described above. Returns 0 on success and -1 on error.
678 If an error occurs, something may have already been written to
679 *output*. In this case, the output is undefined and most likely not
682 .. function:: int json_dump_file(const json_t *json, const char *path, size_t flags)
684 Write the JSON representation of *root* to the file *path*. If
685 *path* already exists, it is overwritten. *flags* is described
686 above. Returns 0 on success and -1 on error.
692 This sections describes the functions that can be used to decode JSON
693 text to the Jansson representation of JSON data. The JSON
694 specification requires that a JSON text is either a serialized array
695 or object, and this requirement is also enforced with the following
696 functions. In other words, the top level value in the JSON text being
697 decoded must be either array or object.
699 See :ref:`rfc-conformance` for a discussion on Jansson's conformance
700 to the JSON specification. It explains many design decisions that
701 affect especially the behavior of the decoder.
703 .. function:: json_t *json_loads(const char *input, size_t flags, json_error_t *error)
707 Decodes the JSON string *input* and returns the array or object it
708 contains, or *NULL* on error, in which case *error* is filled with
709 information about the error. See above for discussion on the
710 *error* parameter. *flags* is currently unused, and should be set
713 .. function:: json_t *json_loadf(FILE *input, size_t flags, json_error_t *error)
717 Decodes the JSON text in stream *input* and returns the array or
718 object it contains, or *NULL* on error, in which case *error* is
719 filled with information about the error. See above for discussion
720 on the *error* parameter. *flags* is currently unused, and should
723 .. function:: json_t *json_load_file(const char *path, size_t flags, json_error_t *error)
727 Decodes the JSON text in file *path* and returns the array or
728 object it contains, or *NULL* on error, in which case *error* is
729 filled with information about the error. See above for discussion
730 on the *error* parameter. *flags* is currently unused, and should
733 .. type:: json_error_t
735 This data structure is used to return information on decoding
736 errors from the decoding functions.
738 .. member:: const char *text
740 The error message (in UTF-8), or an empty string if a message is
745 The line number on which the error occurred, or -1 if this
746 information is not available.
748 .. member:: int column
750 The character column on which the error occurred, or -1 if this
751 information is not available.
753 .. member:: const char *source
755 Source of the error. This is (a part of) the file name when
756 using :func:`json_load_file()`, or a special identifier in angle
757 brackets otherwise (e.g. ``<string>``).
759 The normal use of :type:`json_error_t` is to allocate it on the
760 stack, and pass a pointer to a decoding function. Example::
766 json = json_load_file("/path/to/file.json", 0, &error);
768 /* the error variable contains error information */
773 Also note that if the decoding succeeded (``json != NULL`` in the
774 above example), the contents of ``error`` are unspecified.
776 All decoding functions also accept *NULL* as the
777 :type:`json_error_t` pointer, in which case no error information
778 is returned to the caller.
784 Testing for equality of two JSON values cannot, in general, be
785 achieved using the ``==`` operator. Equality in the terms of the
786 ``==`` operator states that the two :type:`json_t` pointers point to
787 exactly the same JSON value. However, two JSON values can be equal not
788 only if they are exactly the same value, but also if they have equal
791 * Two integer or real values are equal if their contained numeric
792 values are equal. An integer value is never equal to a real value,
795 * Two strings are equal if their contained UTF-8 strings are equal,
796 byte by byte. Unicode comparison algorithms are not implemented.
798 * Two arrays are equal if they have the same number of elements and
799 each element in the first array is equal to the corresponding
800 element in the second array.
802 * Two objects are equal if they have exactly the same keys and the
803 value for each key in the first object is equal to the value of the
804 corresponding key in the second object.
806 * Two true, false or null values have no "contents", so they are equal
807 if their types are equal. (Because these values are singletons,
808 their equality can actually be tested with ``==``.)
810 The following function can be used to test whether two JSON values are
813 .. function:: int json_equal(json_t *value1, json_t *value2)
815 Returns 1 if *value1* and *value2* are equal, as defined above.
816 Returns 0 if they are inequal or one or both of the pointers are
823 Because of reference counting, passing JSON values around doesn't
824 require copying them. But sometimes a fresh copy of a JSON value is
825 needed. For example, if you need to modify an array, but still want to
826 use the original afterwards, you should take a copy of it first.
828 Jansson supports two kinds of copying: shallow and deep. There is a
829 difference between these methods only for arrays and objects. Shallow
830 copying only copies the first level value (array or object) and uses
831 the same child values in the copied value. Deep copying makes a fresh
832 copy of the child values, too. Moreover, all the child values are deep
833 copied in a recursive fashion.
835 .. function:: json_t *json_copy(json_t *value)
839 Returns a shallow copy of *value*, or *NULL* on error.
841 .. function:: json_t *json_deep_copy(json_t *value)
845 Returns a deep copy of *value*, or *NULL* on error.