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 .. function:: const char *json_string_value(const json_t *string)
269 Returns the associated value of *string* as a null terminated UTF-8
270 encoded string, or *NULL* if *string* is not a JSON string.
272 .. function:: int json_string_set(const json_t *string, const char *value)
274 Sets the associated value of *string* to *value*. *value* must be a
275 valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
278 .. function:: int json_string_set_nocheck(const json_t *string, const char *value)
280 Like :func:`json_string_set`, but doesn't check that *value* is
281 valid UTF-8. Use this function only if you are certain that this
282 really is the case (e.g. you have already checked it by other
289 The JSON specification only contains one numeric type, "number". The C
290 programming language has distinct types for integer and floating-point
291 numbers, so for practical reasons Jansson also has distinct types for
292 the two. They are called "integer" and "real", respectively. For more
293 information, see :ref:`rfc-conformance`.
297 This is the C type that is used to store JSON integer values. It
298 represents the widest integer type available on your system. In
299 practice it's just a typedef of ``long long`` if your compiler
300 supports it, otherwise ``long``.
302 Usually, you can safely use plain ``int`` in place of
303 ``json_int_t``, and the implicit C integer conversion handles the
304 rest. Only when you know that you need the full 64-bit range, you
305 should use ``json_int_t`` explicitly.
307 ``JSON_INTEGER_IS_LONG_LONG``
309 This is a preprocessor variable that holds the value 1 if
310 :type:`json_int_t` is ``long long``, and 0 if it's ``long``. It
311 can be used as follows::
313 #if JSON_INTEGER_IS_LONG_LONG
314 /* Code specific for long long */
316 /* Code specific for long */
319 ``JSON_INTEGER_FORMAT``
321 This is a macro that expands to a :func:`printf()` conversion
322 specifier that corresponds to :type:`json_int_t`, without the
323 leading ``%`` sign, i.e. either ``"lld"`` or ``"ld"``. This macro
324 is required because the actual type of :type:`json_int_t` can be
325 either ``long`` or ``long long``, and :func:`printf()` reuiqres
326 different length modifiers for the two.
330 json_int_t x = 123123123;
331 printf("x is %" JSON_INTEGER_FORMAT "\n", x);
334 .. function:: json_t *json_integer(json_int_t value)
338 Returns a new JSON integer, or *NULL* on error.
340 .. function:: json_int_t json_integer_value(const json_t *integer)
342 Returns the associated value of *integer*, or 0 if *json* is not a
345 .. function:: int json_integer_set(const json_t *integer, json_int_t value)
347 Sets the associated value of *integer* to *value*. Returns 0 on
348 success and -1 if *integer* is not a JSON integer.
350 .. function:: json_t *json_real(double value)
354 Returns a new JSON real, or *NULL* on error.
356 .. function:: double json_real_value(const json_t *real)
358 Returns the associated value of *real*, or 0.0 if *real* is not a
361 .. function:: int json_real_set(const json_t *real, double value)
363 Sets the associated value of *real* to *value*. Returns 0 on
364 success and -1 if *real* is not a JSON real.
366 In addition to the functions above, there's a common query function
367 for integers and reals:
369 .. function:: double json_number_value(const json_t *json)
371 Returns the associated value of the JSON integer or JSON real
372 *json*, cast to double regardless of the actual type. If *json* is
373 neither JSON real nor JSON integer, 0.0 is returned.
379 A JSON array is an ordered collection of other JSON values.
381 .. function:: json_t *json_array(void)
385 Returns a new JSON array, or *NULL* on error. Initially, the array
388 .. function:: size_t json_array_size(const json_t *array)
390 Returns the number of elements in *array*, or 0 if *array* is NULL
393 .. function:: json_t *json_array_get(const json_t *array, size_t index)
395 .. refcounting:: borrow
397 Returns the element in *array* at position *index*. The valid range
398 for *index* is from 0 to the return value of
399 :func:`json_array_size()` minus 1. If *array* is not a JSON array,
400 if *array* is *NULL*, or if *index* is out of range, *NULL* is
403 .. function:: int json_array_set(json_t *array, size_t index, json_t *value)
405 Replaces the element in *array* at position *index* with *value*.
406 The valid range for *index* is from 0 to the return value of
407 :func:`json_array_size()` minus 1. Returns 0 on success and -1 on
410 .. function:: int json_array_set_new(json_t *array, size_t index, json_t *value)
412 Like :func:`json_array_set()` but steals the reference to *value*.
413 This is useful when *value* is newly created and not used after
416 .. function:: int json_array_append(json_t *array, json_t *value)
418 Appends *value* to the end of *array*, growing the size of *array*
419 by 1. Returns 0 on success and -1 on error.
421 .. function:: int json_array_append_new(json_t *array, json_t *value)
423 Like :func:`json_array_append()` but steals the reference to
424 *value*. This is useful when *value* is newly created and not used
427 .. function:: int json_array_insert(json_t *array, size_t index, json_t *value)
429 Inserts *value* to *array* at position *index*, shifting the
430 elements at *index* and after it one position towards the end of
431 the array. Returns 0 on success and -1 on error.
433 .. function:: int json_array_insert_new(json_t *array, size_t index, json_t *value)
435 Like :func:`json_array_insert()` but steals the reference to
436 *value*. This is useful when *value* is newly created and not used
439 .. function:: int json_array_remove(json_t *array, size_t index)
441 Removes the element in *array* at position *index*, shifting the
442 elements after *index* one position towards the start of the array.
443 Returns 0 on success and -1 on error.
445 .. function:: int json_array_clear(json_t *array)
447 Removes all elements from *array*. Returns 0 on sucess and -1 on
450 .. function:: int json_array_extend(json_t *array, json_t *other_array)
452 Appends all elements in *other_array* to the end of *array*.
453 Returns 0 on success and -1 on error.
459 A JSON object is a dictionary of key-value pairs, where the key is a
460 Unicode string and the value is any JSON value.
462 .. function:: json_t *json_object(void)
466 Returns a new JSON object, or *NULL* on error. Initially, the
469 .. function:: size_t json_object_size(const json_t *object)
471 Returns the number of elements in *object*, or 0 if *object* is not
474 .. function:: json_t *json_object_get(const json_t *object, const char *key)
476 .. refcounting:: borrow
478 Get a value corresponding to *key* from *object*. Returns *NULL* if
479 *key* is not found and on error.
481 .. function:: int json_object_set(json_t *object, const char *key, json_t *value)
483 Set the value of *key* to *value* in *object*. *key* must be a
484 valid null terminated UTF-8 encoded Unicode string. If there
485 already is a value for *key*, it is replaced by the new value.
486 Returns 0 on success and -1 on error.
488 .. function:: int json_object_set_nocheck(json_t *object, const char *key, json_t *value)
490 Like :func:`json_object_set`, but doesn't check that *key* is
491 valid UTF-8. Use this function only if you are certain that this
492 really is the case (e.g. you have already checked it by other
495 .. function:: int json_object_set_new(json_t *object, const char *key, json_t *value)
497 Like :func:`json_object_set()` but steals the reference to
498 *value*. This is useful when *value* is newly created and not used
501 .. function:: int json_object_set_new_nocheck(json_t *object, const char *key, json_t *value)
503 Like :func:`json_object_set_new`, but doesn't check that *key* is
504 valid UTF-8. Use this function only if you are certain that this
505 really is the case (e.g. you have already checked it by other
508 .. function:: int json_object_del(json_t *object, const char *key)
510 Delete *key* from *object* if it exists. Returns 0 on success, or
511 -1 if *key* was not found.
514 .. function:: int json_object_clear(json_t *object)
516 Remove all elements from *object*. Returns 0 on success and -1 if
517 *object* is not a JSON object.
519 .. function:: int json_object_update(json_t *object, json_t *other)
521 Update *object* with the key-value pairs from *other*, overwriting
522 existing keys. Returns 0 on success or -1 on error.
525 The following functions implement an iteration protocol for objects:
527 .. function:: void *json_object_iter(json_t *object)
529 Returns an opaque iterator which can be used to iterate over all
530 key-value pairs in *object*, or *NULL* if *object* is empty.
532 .. function:: void *json_object_iter_at(json_t *object, const char *key)
534 Like :func:`json_object_iter()`, but returns an iterator to the
535 key-value pair in *object* whose key is equal to *key*, or NULL if
536 *key* is not found in *object*. Iterating forward to the end of
537 *object* only yields all key-value pairs of the object if *key*
538 happens to be the first key in the underlying hash table.
540 .. function:: 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 .. function:: const char *json_object_iter_key(void *iter)
548 Extract the associated key from *iter*.
550 .. function:: json_t *json_object_iter_value(void *iter)
552 .. refcounting:: borrow
554 Extract the associated value from *iter*.
556 .. function:: 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 .. function:: int json_object_iter_set_new(json_t *object, void *iter, json_t *value)
563 Like :func:`json_object_iter_set()`, but steals the reference to
564 *value*. This is useful when *value* is newly created and not used
567 The iteration protocol can be used for example as follows::
569 /* obj is a JSON object */
572 void *iter = json_object_iter(obj);
575 key = json_object_iter_key(iter);
576 value = json_object_iter_value(iter);
577 /* use key and value ... */
578 iter = json_object_iter_next(obj, iter);
585 This sections describes the functions that can be used to encode
586 values to JSON. Only objects and arrays can be encoded, since they are
587 the only valid "root" values of a JSON text.
589 By default, the output has no newlines, and spaces are used between
590 array and object elements for a readable output. This behavior can be
591 altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
592 described below. A newline is never appended to the end of the encoded
595 Each function takes a *flags* parameter that controls some aspects of
596 how the data is encoded. Its default value is 0. The following macros
597 can be ORed together to obtain *flags*.
600 Pretty-print the result, using newlines between array and object
601 items, and indenting with *n* spaces. The valid range for *n* is
602 between 0 and 32, other values result in an undefined output. If
603 ``JSON_INDENT`` is not used or *n* is 0, no newlines are inserted
604 between array and object items.
607 This flag enables a compact representation, i.e. sets the separator
608 between array and object items to ``","`` and between object keys
609 and values to ``":"``. Without this flag, the corresponding
610 separators are ``", "`` and ``": "`` for more readable output.
612 ``JSON_ENSURE_ASCII``
613 If this flag is used, the output is guaranteed to consist only of
614 ASCII characters. This is achived by escaping all Unicode
615 characters outside the ASCII range.
618 If this flag is used, all the objects in output are sorted by key.
619 This is useful e.g. if two JSON texts are diffed or visually
622 ``JSON_PRESERVE_ORDER``
623 If this flag is used, object keys in the output are sorted into the
624 same order in which they were first inserted to the object. For
625 example, decoding a JSON text and then encoding with this flag
626 preserves the order of object keys.
628 The following functions perform the actual JSON encoding. The result
631 .. function:: char *json_dumps(const json_t *root, size_t flags)
633 Returns the JSON representation of *root* as a string, or *NULL* on
634 error. *flags* is described above. The return value must be freed
635 by the caller using :func:`free()`.
637 .. function:: int json_dumpf(const json_t *root, FILE *output, size_t flags)
639 Write the JSON representation of *root* to the stream *output*.
640 *flags* is described above. Returns 0 on success and -1 on error.
641 If an error occurs, something may have already been written to
642 *output*. In this case, the output is undefined and most likely not
645 .. function:: int json_dump_file(const json_t *json, const char *path, size_t flags)
647 Write the JSON representation of *root* to the file *path*. If
648 *path* already exists, it is overwritten. *flags* is described
649 above. Returns 0 on success and -1 on error.
655 This sections describes the functions that can be used to decode JSON
656 text to the Jansson representation of JSON data. The JSON
657 specification requires that a JSON text is either a serialized array
658 or object, and this requirement is also enforced with the following
659 functions. In other words, the top level value in the JSON text being
660 decoded must be either array or object.
662 See :ref:`rfc-conformance` for a discussion on Jansson's conformance
663 to the JSON specification. It explains many design decisions that
664 affect especially the behavior of the decoder.
666 .. type:: json_error_t
668 This opaque structure is used to return information on errors from
669 the decoding functions. See below for more discussion on error
672 The following functions perform the JSON decoding:
674 .. function:: json_t *json_loads(const char *input, size_t flags, json_error_t **error)
678 Decodes the JSON string *input* and returns the array or object it
679 contains, or *NULL* on error. If *error* is non-*NULL*, it's used
680 to return error information. See below for more discussion on error
681 reporting. *flags* is currently unused, and should be set to 0.
683 .. function:: json_t *json_loadf(FILE *input, size_t flags, json_error_t **error)
687 Decodes the JSON text in stream *input* and returns the array or
688 object it contains, or *NULL* on error. If *error* is non-*NULL*,
689 it's used to return error information. See below for more
690 discussion on error reporting. *flags* is currently unused, and
693 .. function:: json_t *json_load_file(const char *path, size_t flags, json_error_t **error)
697 Decodes the JSON text in file *path* and returns the array or
698 object it contains, or *NULL* on error. If *error* is non-*NULL*,
699 it's used to return error information. See below for more
700 discussion on error reporting. *flags* is currently unused, and
704 The :type:`json_error_t` parameter, that all decoding function accept
705 as their last parameter, is used to return information on decoding
706 errors to the caller. It is used by having a ``json_error_t *``
707 variable and passing a pointer to this variable to a decoding
714 json = json_load_file("/path/to/file.json", 0, &error);
716 /* the error variable contains error information */
717 fprintf(stderr, "Decoding error occured on line %d: %s\n", json_error_line(error), json_error_msg(error));
724 Note that **the caller must free the error structure** after use if a
725 decoding error occurs. If decoding is succesfully finished, *error* is
726 simply set to *NULL* by the decoding function.
728 All decoding functions also accept *NULL* as the :type:`json_error_t`
729 pointer, in which case no error information is returned to the caller.
735 json = json_load_file("/path/to/file.json", 0, NULL);
737 /* A decoding error occured but no error information is available */
743 :type:`json_error_t` is totally opaque and must be queried using the
746 .. function:: const char *json_error_msg(const json_error_t *error)
748 Return a pointer to an UTF-8 encoded string that describes the
749 error in human-readable text, or *NULL* if *error* is *NULL*.
751 .. function:: int json_error_line(const json_error_t *error)
753 Return the line numer on which the error occurred, or -1 if this
754 information is not available or if *error* is *NULL*.
760 Testing for equality of two JSON values cannot, in general, be
761 achieved using the ``==`` operator. Equality in the terms of the
762 ``==`` operator states that the two :type:`json_t` pointers point to
763 exactly the same JSON value. However, two JSON values can be equal not
764 only if they are exactly the same value, but also if they have equal
767 * Two integer or real values are equal if their contained numeric
768 values are equal. An integer value is never equal to a real value,
771 * Two strings are equal if their contained UTF-8 strings are equal,
772 byte by byte. Unicode comparison algorithms are not implemented.
774 * Two arrays are equal if they have the same number of elements and
775 each element in the first array is equal to the corresponding
776 element in the second array.
778 * Two objects are equal if they have exactly the same keys and the
779 value for each key in the first object is equal to the value of the
780 corresponding key in the second object.
782 * Two true, false or null values have no "contents", so they are equal
783 if their types are equal. (Because these values are singletons,
784 their equality can actually be tested with ``==``.)
786 The following function can be used to test whether two JSON values are
789 .. function:: int json_equal(json_t *value1, json_t *value2)
791 Returns 1 if *value1* and *value2* are equal, as defined above.
792 Returns 0 if they are inequal or one or both of the pointers are
799 Because of reference counting, passing JSON values around doesn't
800 require copying them. But sometimes a fresh copy of a JSON value is
801 needed. For example, if you need to modify an array, but still want to
802 use the original afterwards, you should take a copy of it first.
804 Jansson supports two kinds of copying: shallow and deep. There is a
805 difference between these methods only for arrays and objects. Shallow
806 copying only copies the first level value (array or object) and uses
807 the same child values in the copied value. Deep copying makes a fresh
808 copy of the child values, too. Moreover, all the child values are deep
809 copied in a recursive fashion.
811 .. function:: json_t *json_copy(json_t *value)
815 Returns a shallow copy of *value*, or *NULL* on error.
817 .. function:: json_t *json_deep_copy(json_t *value)
821 Returns a deep copy of *value*, or *NULL* on error.