10 All declarations are in :file:`jansson.h`, so it's enough to
18 All constants are prefixed ``JSON_`` and other identifiers with
19 ``json_``. Type names are suffixed with ``_t`` and ``typedef``\ 'd so
20 that the ``struct`` keyword need not be used.
26 The JSON specification (:rfc:`4627`) defines the following data types:
27 *object*, *array*, *string*, *number*, *boolean*, and *null*. JSON
28 types are used dynamically; arrays and objects can hold any other data
29 type, including themselves. For this reason, Jansson's type system is
30 also dynamic in nature. There's one C type to represent all JSON
31 values, and this structure knows the type of the JSON value it holds.
35 This data structure is used throughout the library to represent all
36 JSON values. It always contains the type of the JSON value it holds
37 and the value's reference count. The rest depends on the type of the
40 Objects of :ctype:`json_t` are always used through a pointer. There
41 are APIs for querying the type, manipulating the reference count, and
42 for constructing and manipulating values of different types.
44 Unless noted otherwise, all API functions return an error value if an
45 error occurs. Depending on the function's signature, the error value
46 is either *NULL* or -1. Invalid arguments or invalid input are
47 apparent sources for errors. Memory allocation and I/O operations may
54 The type of a JSON value is queried and tested using the following
57 .. ctype:: enum json_type
59 The type of a JSON value. The following members are defined:
61 +-------------------------+
62 | :const:`JSON_OBJECT` |
63 +-------------------------+
64 | :const:`JSON_ARRAY` |
65 +-------------------------+
66 | :const:`JSON_STRING` |
67 +-------------------------+
68 | :const:`JSON_INTEGER` |
69 +-------------------------+
70 | :const:`JSON_REAL` |
71 +-------------------------+
72 | :const:`JSON_TRUE` |
73 +-------------------------+
74 | :const:`JSON_FALSE` |
75 +-------------------------+
76 | :const:`JSON_NULL` |
77 +-------------------------+
79 These correspond to JSON object, array, string, number, boolean and
80 null. A number is represented by either a value of the type
81 :const:`JSON_INTEGER` or of the type :const:`JSON_REAL`. A true
82 boolean value is represented by a value of the type
83 :const:`JSON_TRUE` and false by a value of the type
86 .. cfunction:: int json_typeof(const json_t *json)
88 Return the type of the JSON value (a :ctype:`json_type` cast to
89 :ctype:`int`). *json* MUST NOT be *NULL*. This function is actually
90 implemented as a macro for speed.
92 .. cfunction:: json_is_object(const json_t *json)
93 json_is_array(const json_t *json)
94 json_is_string(const json_t *json)
95 json_is_integer(const json_t *json)
96 json_is_real(const json_t *json)
97 json_is_true(const json_t *json)
98 json_is_false(const json_t *json)
99 json_is_null(const json_t *json)
101 These functions (actually macros) return true (non-zero) for values
102 of the given type, and false (zero) for values of other types and
105 .. cfunction:: json_is_number(const json_t *json)
107 Returns true for values of types :const:`JSON_INTEGER` and
108 :const:`JSON_REAL`, and false for other types and for *NULL*.
110 .. cfunction:: json_is_boolean(const json_t *json)
112 Returns true for types :const:`JSON_TRUE` and :const:`JSON_FALSE`,
113 and false for values of other types and for *NULL*.
119 The reference count is used to track whether a value is still in use
120 or not. When a value is created, it's reference count is set to 1. If
121 a reference to a value is kept (e.g. a value is stored somewhere for
122 later use), its reference count is incremented, and when the value is
123 no longer needed, the reference count is decremented. When the
124 reference count drops to zero, there are no references left, and the
125 value can be destroyed.
127 The following functions are used to manipulate the reference count.
129 .. cfunction:: json_t *json_incref(json_t *json)
131 Increment the reference count of *json* if it's not non-*NULL*.
134 .. cfunction:: void json_decref(json_t *json)
136 Decrement the reference count of *json*. As soon as a call to
137 :cfunc:`json_decref()` drops the reference count to zero, the value
138 is destroyed and it can no longer be used.
140 Functions creating new JSON values set the reference count to 1. These
141 functions are said to return a **new reference**. Other functions
142 returning (existing) JSON values do not normally increase the
143 reference count. These functions are said to return a **borrowed
144 reference**. So, if the user will hold a reference to a value returned
145 as a borrowed reference, he must call :cfunc:`json_incref`. As soon as
146 the value is no longer needed, :cfunc:`json_decref` should be called
147 to release the reference.
149 Normally, all functions accepting a JSON value as an argument will
150 nmanage the reference, i.e. increase and decrease the reference count
151 as needed. However, some functions **steal** the reference, i.e. they
152 have the same result as if the user called :cfunc:`json_decref()` on
153 the argument right after calling the function. These are usually
154 convenience functions for adding new references to containers and not
155 to worry about the reference count.
157 In the following sections it is clearly documented whether a function
158 will return a new or borrowed reference or steal a reference to its
165 These values are implemented as singletons, so each of these functions
166 returns the same value each time.
168 .. cfunction:: json_t *json_true(void)
172 Returns the JSON true value.
174 .. cfunction:: json_t *json_false(void)
178 Returns the JSON false value.
180 .. cfunction:: json_t *json_null(void)
184 Returns the JSON null value.
190 .. cfunction:: json_t *json_string(const char *value)
194 Returns a new JSON string, or *NULL* on error. *value* must be a
195 valid UTF-8 encoded Unicode string.
197 .. cfunction:: const char *json_string_value(const json_t *string)
199 Returns the associated value of *string* as a null terminated UTF-8
200 encoded string, or *NULL* if *string* is not a JSON string.
202 .. cfunction:: int json_string_set(const json_t *string, const char *value)
204 Sets the associated value of *string* to *value*. *value* must be a
205 valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
208 .. versionadded:: 1.1
214 .. cfunction:: json_t *json_integer(int value)
218 Returns a new JSON integer, or *NULL* on error.
220 .. cfunction:: int json_integer_value(const json_t *integer)
222 Returns the associated value of *integer*, or 0 if *json* is not a
225 .. cfunction:: int json_integer_set(const json_t *integer, int value)
227 Sets the associated value of *integer* to *value*. Returns 0 on
228 success and -1 if *integer* is not a JSON integer.
230 .. versionadded:: 1.1
232 .. cfunction:: json_t *json_real(double value)
236 Returns a new JSON real, or *NULL* on error.
238 .. cfunction:: double json_real_value(const json_t *real)
240 Returns the associated value of *real*, or 0.0 if *real* is not a
243 .. cfunction:: int json_real_set(const json_t *real, double value)
245 Sets the associated value of *real* to *value*. Returns 0 on
246 success and -1 if *real* is not a JSON real.
248 .. versionadded:: 1.1
250 In addition to the functions above, there's a common query function
251 for integers and reals:
253 .. cfunction:: double json_number_value(const json_t *json)
255 Returns the associated value of the JSON integer or JSON real
256 *json*, cast to double regardless of the actual type. If *json* is
257 neither JSON real nor JSON integer, 0.0 is returned.
263 A JSON array is an ordered collection of other JSON values.
265 .. cfunction:: json_t *json_array(void)
269 Returns a new JSON array, or *NULL* on error. Initially, the array
272 .. cfunction:: unsigned int json_array_size(const json_t *array)
274 Returns the number of elements in *array*, or 0 if *array* is NULL
277 .. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
279 .. refcounting:: borrow
281 Returns the element in *array* at position *index*. The valid range
282 for *index* is from 0 to the return value of
283 :cfunc:`json_array_size()` minus 1. If *array* is not a JSON array,
284 if *array* is *NULL*, or if *index* is out of range, *NULL* is
287 .. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
289 Replaces the element in *array* at position *index* with *value*.
290 The valid range for *index* is from 0 to the return value of
291 :cfunc:`json_array_size()` minus 1. Returns 0 on success and -1 on
294 .. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
296 Like :cfunc:`json_array_set()` but steals the reference to *value*.
297 This is useful when *value* is newly created and not used after
300 .. versionadded:: 1.1
302 .. cfunction:: int json_array_append(json_t *array, json_t *value)
304 Appends *value* to the end of *array*, growing the size of *array*
305 by 1. Returns 0 on success and -1 on error.
307 .. cfunction:: int json_array_append_new(json_t *array, json_t *value)
309 Like :cfunc:`json_array_append()` but steals the reference to
310 *value*. This is useful when *value* is newly created and not used
313 .. versionadded:: 1.1
315 .. cfunction:: int json_array_insert(json_t *array, unsigned int index, json_t *value)
317 Inserts *value* to *array* at position *index*, shifting the
318 elements at *index* and after it one position towards the end of
319 the array. Returns 0 on success and -1 on error.
321 .. versionadded:: 1.1
323 .. cfunction:: int json_array_insert_new(json_t *array, unsigned int index, json_t *value)
325 Like :cfunc:`json_array_insert()` but steals the reference to
326 *value*. This is useful when *value* is newly created and not used
329 .. versionadded:: 1.1
331 .. cfunction:: int json_array_remove(json_t *array, unsigned int index)
333 Removes the element in *array* at position *index*, shifting the
334 elements after *index* one position towards the start of the array.
335 Returns 0 on success and -1 on error.
337 .. versionadded:: 1.1
339 .. cfunction:: int json_array_clear(json_t *array)
341 Removes all elements from *array*. Returns 0 on sucess and -1 on
344 .. versionadded:: 1.1
346 .. cfunction:: int json_array_extend(json_t *array, json_t *other_array)
348 Appends all elements in *other_array* to the end of *array*.
349 Returns 0 on success and -1 on error.
351 .. versionadded:: 1.1
357 A JSON object is a dictionary of key-value pairs, where the key is a
358 Unicode string and the value is any JSON value.
360 .. cfunction:: json_t *json_object(void)
364 Returns a new JSON object, or *NULL* on error. Initially, the
367 .. cfunction:: unsigned int json_object_size(const json_t *object)
369 Returns the number of elements in *object*, or 0 if *object* is not
372 .. versionadded:: 1.1
374 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
376 .. refcounting:: borrow
378 Get a value corresponding to *key* from *object*. Returns *NULL* if
379 *key* is not found and on error.
381 .. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)
383 Set the value of *key* to *value* in *object*. *key* must be a
384 valid null terminated UTF-8 encoded Unicode string. If there
385 already is a value for *key*, it is replaced by the new value.
386 Returns 0 on success and -1 on error.
388 .. cfunction:: int json_object_set_new(json_t *object, const char *key, json_t *value)
390 Like :cfunc:`json_object_set()` but steals the reference to
391 *value*. This is useful when *value* is newly created and not used
394 .. versionadded:: 1.1
396 .. cfunction:: int json_object_del(json_t *object, const char *key)
398 Delete *key* from *object* if it exists. Returns 0 on success, or
399 -1 if *key* was not found.
402 .. cfunction:: int json_object_clear(json_t *object)
404 Remove all elements from *object*. Returns 0 on success and -1 if
405 *object* is not a JSON object.
407 .. versionadded:: 1.1
409 .. cfunction:: int json_object_update(json_t *object, json_t *other)
411 Update *object* with the key-value pairs from *other*, overwriting
412 existing keys. Returns 0 on success or -1 on error.
414 .. versionadded:: 1.1
417 The following functions implement an iteration protocol for objects:
419 .. cfunction:: void *json_object_iter(json_t *object)
421 Returns an opaque iterator which can be used to iterate over all
422 key-value pairs in *object*, or *NULL* if *object* is empty.
424 .. cfunction:: void *json_object_iter_next(json_t *object, void *iter)
426 Returns an iterator pointing to the next key-value pair in *object*
427 after *iter*, or *NULL* if the whole object has been iterated
430 .. cfunction:: const char *json_object_iter_key(void *iter)
432 Extract the associated key from *iter*.
434 .. cfunction:: json_t *json_object_iter_value(void *iter)
436 .. refcounting:: borrow
438 Extract the associated value from *iter*.
440 The iteration protocol can be used for example as follows::
442 /* obj is a JSON object */
445 void *iter = json_object_iter(obj);
448 key = json_object_iter_key(iter);
449 value = json_object_iter_value(iter);
450 /* use key and value ... */
451 iter = json_object_iter_next(obj, iter);
458 This sections describes the functions that can be used to encode
459 values to JSON. Only objects and arrays can be encoded, since they are
460 the only valid "root" values of a JSON text.
462 Each function takes a *flags* parameter that controls some aspects of
463 how the data is encoded. Its default value is 0. The following macros
464 can be ORed together to obtain *flags*.
467 Pretty-print the result, indenting arrays and objects by *n*
468 spaces. The valid range for *n* is between 0 and 255, other values
469 result in an undefined output. If ``JSON_INDENT`` is not used or
470 *n* is 0, no pretty-printing is done and the result is a compact
473 The following functions perform the actual JSON encoding. The result
476 .. cfunction:: char *json_dumps(const json_t *root, unsigned long flags)
478 Returns the JSON representation of *root* as a string, or *NULL* on
479 error. *flags* is described above. The return value must be freed
480 by the caller using :cfunc:`free()`.
482 .. cfunction:: int json_dumpf(const json_t *root, FILE *output, unsigned long flags)
484 Write the JSON representation of *root* to the stream *output*.
485 *flags* is described above. Returns 0 on success and -1 on error.
487 .. cfunction:: int json_dump_file(const json_t *json, const char *path, unsigned long flags)
489 Write the JSON representation of *root* to the file *path*. If
490 *path* already exists, it is overwritten. *flags* is described
491 above. Returns 0 on success and -1 on error.
497 This sections describes the functions that can be used to decode JSON
498 text to the Jansson representation of JSON data. The JSON
499 specification requires that a JSON text is either a serialized array
500 or object, and this requirement is also enforced with the following
503 The only supported character encoding is UTF-8 (which ASCII is a
506 .. ctype:: json_error_t
508 This data structure is used to return information on decoding
509 errors from the decoding functions. Its definition is repeated
512 #define JSON_ERROR_TEXT_LENGTH 160
515 char text[JSON_ERROR_TEXT_LENGTH];
519 *line* is the line number on which the error occurred, or -1 if
520 this information is not available. *text* contains the error
521 message (in UTF-8), or an empty string if a message is not
524 The normal usef of :ctype:`json_error_t` is to allocate it normally
525 on the stack, and pass a pointer to a decoding function. Example::
531 json = json_load_file("/path/to/file.json", &error);
533 /* the error variable contains error information */
538 Also note that if the decoding succeeded (``json != NULL`` in the
539 above example), the contents of ``error`` are unspecified.
541 All decoding functions also accept *NULL* as the
542 :ctype:`json_error_t` pointer, in which case no error information
543 is returned to the caller.
545 The following functions perform the actual JSON decoding.
547 .. cfunction:: json_t *json_loads(const char *input, json_error_t *error)
551 Decodes the JSON string *input* and returns the array or object it
552 contains, or *NULL* on error, in which case *error* is filled with
553 information about the error. See above for discussion on the
556 .. cfunction:: json_t *json_loadf(FILE *input, json_error_t *error)
560 Decodes the JSON text in stream *input* and returns the array or
561 object it contains, or *NULL* on error, in which case *error* is
562 filled with information about the error. See above for discussion
563 on the *error* parameter.
565 .. cfunction:: json_t *json_load_file(const char *path, json_error_t *error)
569 Decodes the JSON text in file *path* and returns the array or
570 object it contains, or *NULL* on error, in which case *error* is
571 filled with information about the error. See above for discussion
572 on the *error* parameter.