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.
48 The type of a JSON value is queried and tested using the following
51 .. ctype:: enum json_type
53 The type of a JSON value. The following members are defined:
55 +-------------------------+
56 | :const:`JSON_OBJECT` |
57 +-------------------------+
58 | :const:`JSON_ARRAY` |
59 +-------------------------+
60 | :const:`JSON_STRING` |
61 +-------------------------+
62 | :const:`JSON_INTEGER` |
63 +-------------------------+
64 | :const:`JSON_REAL` |
65 +-------------------------+
66 | :const:`JSON_TRUE` |
67 +-------------------------+
68 | :const:`JSON_FALSE` |
69 +-------------------------+
70 | :const:`JSON_NULL` |
71 +-------------------------+
73 These correspond to JSON object, array, string, number, boolean and
74 null. A number is represented by either a value of the type
75 :const:`JSON_INTEGER` or of the type :const:`JSON_REAL`. A true
76 boolean value is represented by a value of the type
77 :const:`JSON_TRUE` and false by a value of the type
80 .. cfunction:: int json_typeof(const json_t *json)
82 Return the type of the JSON value (a :ctype:`json_type` cast to
83 :ctype:`int`). This function is actually implemented as a macro for
86 .. cfunction:: json_is_object(const json_t *json)
87 json_is_array(const json_t *json)
88 json_is_string(const json_t *json)
89 json_is_integer(const json_t *json)
90 json_is_real(const json_t *json)
91 json_is_true(const json_t *json)
92 json_is_false(const json_t *json)
93 json_is_null(const json_t *json)
95 These functions (actually macros) return true (non-zero) for values
96 of the given type, and false (zero) for values of other types.
98 .. cfunction:: json_is_number(const json_t *json)
100 Returns true for values of types :const:`JSON_INTEGER` and
101 :const:`JSON_REAL`, and false for other types.
103 .. cfunction:: json_is_boolean(const json_t *json)
105 Returns true for types :const:`JSON_TRUE` and :const:`JSON_FALSE`,
106 and false for values of other types.
112 The reference count is used to track whether a value is still in use
113 or not. When a value is created, it's reference count is set to 1. If
114 a reference to a value is kept (e.g. a value is stored somewhere for
115 later use), its reference count is incremented, and when the value is
116 no longer needed, the reference count is decremented. When the
117 reference count drops to zero, there are no references left, and the
118 value can be destroyed.
120 The following functions are used to manipulate the reference count.
122 .. cfunction:: json_t *json_incref(json_t *json)
124 Increment the reference count of *json*.
126 .. cfunction:: void json_decref(json_t *json)
128 Decrement the reference count of *json*. As soon as a call to
129 :cfunc:`json_decref()` drops the reference count to zero, the value
130 is destroyed and it can no longer be used.
132 Functions creating new JSON values set the reference count to 1. These
133 functions are said to return a **new reference**. Other functions
134 returning (existing) JSON values do not normally increase the
135 reference count. These functions are said to return a **borrowed
136 reference**. So, if the user will hold a reference to a value returned
137 as a borrowed reference, he must call :cfunc:`json_incref`. As soon as
138 the value is no longer needed, :cfunc:`json_decref` should be called
139 to release the reference.
141 Normally, all functions accepting a JSON value as an argument will
142 manage the reference, i.e. increase and decrease the reference count
143 as needed. However, some functions **steal** the reference, i.e. they
144 have the same result as if the user called :cfunc:`json_decref()` on
145 the argument right after calling the function. These are usually
146 convenience functions for adding new references to containers and not
147 to worry about the reference count.
149 In the following sections it is clearly documented whether a function
150 will return a new or borrowed reference or steal a reference to its
157 .. cfunction:: json_t *json_true(void)
161 Returns a value of the type :const:`JSON_TRUE`, or *NULL* on
164 .. cfunction:: json_t *json_false(void)
168 Returns a value of the type :const:`JSON_FALSE`, or *NULL* on
171 .. cfunction:: json_t *json_null(void)
175 Returns a value of the type :const:`JSON_NULL`, or *NULL* on
182 .. cfunction:: json_t *json_string(const char *value)
186 Returns a new value of the type :const:`JSON_STRING`, or *NULL* on
187 error. *value* must be a valid UTF-8 encoded Unicode string.
189 .. cfunction:: const char *json_string_value(const json_t *json)
191 Returns the associated value of a :const:`JSON_STRING` value as a
192 null terminated UTF-8 encoded string.
198 .. cfunction:: json_t *json_integer(int value)
202 Returns a new value of the type :const:`JSON_INTEGER`, or *NULL* on
205 .. cfunction:: int json_integer_value(const json_t *json)
207 Returns the associated integer value of values of the type
208 :const:`JSON_INTEGER`, or 0 for values of other types.
210 .. cfunction:: json_t *json_real(double value)
214 Returns a new value of the type :const:`JSON_REAL`, or *NULL* on
217 .. cfunction:: double json_real_value(const json_t *json)
219 Returns the associated real value of values of the type
220 :const:`JSON_INTEGER`, or 0 for values of other types.
222 In addition to the functions above, there's a common query function
223 for integers and reals:
225 .. cfunction:: double json_number_value(const json_t *json)
227 Returns the value of either ``JSON_INTEGER`` or ``JSON_REAL``, cast
228 to double regardless of the actual type.
234 A JSON array is an ordered collection of other JSON values.
236 .. cfunction:: json_t *json_array(void)
240 Returns a new value of the type :const:`JSON_ARRAY`, or *NULL* on
241 error. Initially, the array is empty.
243 .. cfunction:: unsigned int json_array_size(const json_t *array)
245 Returns the number of elements in *array*.
247 .. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
249 .. refcounting:: borrow
251 Returns the element in *array* at position *index*, or *NULL* if
252 *index* is out of range. The valid range for *index* is from 0 to
253 the return value of :cfunc:`json_array_size()` minus 1.
255 .. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
257 Replaces the element in *array* at position *index* with *value*.
258 Returns 0 on success, or -1 if *index* is out of range. The valid
259 range for *index* is from 0 to the return value of
260 :cfunc:`json_array_size()` minus 1.
262 .. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
264 Like :cfunc:`json_array_set()` but steals the reference to *value*.
265 This is useful when *value* is newly created and not used after
268 .. versionadded:: 1.1
270 .. cfunction:: int json_array_append(json_t *array, json_t *value)
272 Appends *value* to the end of *array*, growing the size of *array*
273 by 1. Returns 0 on success and -1 on error.
275 .. cfunction:: int json_array_append_new(json_t *array, json_t *value)
277 Like :cfunc:`json_array_append()` but steals the reference to
278 *value*. This is useful when *value* is newly created and not used
281 .. versionadded:: 1.1
287 A JSON object is a dictionary of key-value pairs, where the key is a
288 Unicode string and the value is any JSON value.
290 .. cfunction:: json_t *json_object(void)
294 Returns a new value of the type :const:`JSON_OBJECT`, or *NULL* on
295 error. Initially, the object is empty.
297 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
299 .. refcounting:: borrow
301 Get a value corresponding to *key* from *object*. Returns *NULL* if
302 *key* is not found and on error.
304 .. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)
306 Set the value of *key* to *value* in *object*. *key* must be a
307 valid terminated UTF-8 encoded Unicode string. If there already is
308 a value for *key*, it is replaced by the new value. Returns 0 on
309 success and -1 on error.
311 .. cfunction:: int json_object_set_new(json_t *object, const char *key, json_t *value)
313 Like :cfunc:`json_object_set()` but steals the reference to
314 *value*. This is useful when *value* is newly created and not used
317 .. versionadded:: 1.1
319 .. cfunction:: int json_object_del(json_t *object, const char *key)
321 Delete *key* from *object* if it exists. Returns 0 on success, or
322 -1 if *key* was not found.
325 The following functions implement an iteration protocol for objects:
327 .. cfunction:: void *json_object_iter(json_t *object)
329 Returns an opaque iterator which can be used to iterate over all
330 key-value pairs in *object*, or *NULL* if *object* is empty.
332 .. cfunction:: void *json_object_iter_next(json_t *object, void *iter)
334 Returns an iterator pointing to the next key-value pair in *object*
335 after *iter*, or *NULL* if the whole object has been iterated
338 .. cfunction:: const char *json_object_iter_key(void *iter)
340 Extract the associated key from *iter*.
342 .. cfunction:: json_t *json_object_iter_value(void *iter)
344 .. refcounting:: borrow
346 Extract the associated value from *iter*.
352 This sections describes the functions that can be used to encode
353 values to JSON. Only objects and arrays can be encoded, since they are
354 the only valid "root" values of a JSON text.
356 Each function takes a *flags* parameter that controls some aspects of
357 how the data is encoded. Its default value is 0. The following macros
358 can be ORed together to obtain *flags*.
361 Pretty-print the result, indenting arrays and objects by *n*
362 spaces. The valid range for *n* is between 0 and 255, other values
363 result in an undefined output. If ``JSON_INDENT`` is not used or
364 *n* is 0, no pretty-printing is done and the result is a compact
367 The following functions perform the actual JSON encoding. The result
370 .. cfunction:: char *json_dumps(const json_t *root, uint32_t flags)
372 Returns the JSON representation of *root* as a string, or *NULL* on
373 error. *flags* is described above. The return value must be freed
374 by the caller using :cfunc:`free()`.
376 .. cfunction:: int json_dumpf(const json_t *root, FILE *output, uint32_t flags)
378 Write the JSON representation of *root* to the stream *output*.
379 *flags* is described above. Returns 0 on success and -1 on error.
381 .. cfunction:: int json_dump_file(const json_t *json, const char *path, uint32_t flags)
383 Write the JSON representation of *root* to the file *path*. If
384 *path* already exists, it is overwritten. *flags* is described
385 above. Returns 0 on success and -1 on error.
391 This sections describes the functions that can be used to decode JSON
392 text to the Jansson representation of JSON data. The JSON
393 specification requires that a JSON text is either a serialized array
394 or object, and this requirement is also enforced with the following
397 The only supported character encoding is UTF-8 (which ASCII is a
400 .. ctype:: json_error_t
402 This data structure is used to return information on decoding
403 errors from the decoding functions. Its definition is repeated
406 #define JSON_ERROR_TEXT_LENGTH 160
409 char text[JSON_ERROR_TEXT_LENGTH];
413 *line* is the line number on which the error occurred, or -1 if
414 this information is not available. *text* contains the error
415 message (in UTF-8), or an empty string if a message is not
418 The normal usef of :ctype:`json_error_t` is to allocate it normally
419 on the stack, and pass a pointer to a decoding function. Example::
425 json = json_load_file("/path/to/file.json", &error);
427 /* the error variable contains error information */
432 Also note that if the decoding succeeded (``json != NULL`` in the
433 above example), the contents of ``error`` are unspecified.
435 All decoding functions also accept *NULL* as the
436 :ctype:`json_error_t` pointer, in which case no error information
437 is returned to the caller.
439 The following functions perform the actual JSON decoding.
441 .. cfunction:: json_t *json_loads(const char *input, json_error_t *error)
445 Decodes the JSON string *input* and returns the array or object it
446 contains, or *NULL* on error, in which case *error* is filled with
447 information about the error. See above for discussion on the
450 .. cfunction:: json_t *json_loadf(FILE *input, json_error_t *error)
454 Decodes the JSON text in stream *input* and returns the array or
455 object it contains, or *NULL* on error, in which case *error* is
456 filled with information about the error. See above for discussion
457 on the *error* parameter.
459 .. cfunction:: json_t *json_load_file(const char *path, json_error_t *error)
463 Decodes the JSON text in file *path* and returns the array or
464 object it contains, or *NULL* on error, in which case *error* is
465 filled with information about the error. See above for discussion
466 on the *error* parameter.