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.
104 .. cfunction:: json_is_number(const json_t *json)
106 Returns true for values of types :const:`JSON_INTEGER` and
107 :const:`JSON_REAL`, and false for other types.
109 .. cfunction:: json_is_boolean(const json_t *json)
111 Returns true for types :const:`JSON_TRUE` and :const:`JSON_FALSE`,
112 and false for values of other types.
118 The reference count is used to track whether a value is still in use
119 or not. When a value is created, it's reference count is set to 1. If
120 a reference to a value is kept (e.g. a value is stored somewhere for
121 later use), its reference count is incremented, and when the value is
122 no longer needed, the reference count is decremented. When the
123 reference count drops to zero, there are no references left, and the
124 value can be destroyed.
126 The following functions are used to manipulate the reference count.
128 .. cfunction:: json_t *json_incref(json_t *json)
130 Increment the reference count of *json*. Returns *json*.
132 .. cfunction:: void json_decref(json_t *json)
134 Decrement the reference count of *json*. As soon as a call to
135 :cfunc:`json_decref()` drops the reference count to zero, the value
136 is destroyed and it can no longer be used.
138 Functions creating new JSON values set the reference count to 1. These
139 functions are said to return a **new reference**. Other functions
140 returning (existing) JSON values do not normally increase the
141 reference count. These functions are said to return a **borrowed
142 reference**. So, if the user will hold a reference to a value returned
143 as a borrowed reference, he must call :cfunc:`json_incref`. As soon as
144 the value is no longer needed, :cfunc:`json_decref` should be called
145 to release the reference.
147 Normally, all functions accepting a JSON value as an argument will
148 manage the reference, i.e. increase and decrease the reference count
149 as needed. However, some functions **steal** the reference, i.e. they
150 have the same result as if the user called :cfunc:`json_decref()` on
151 the argument right after calling the function. These are usually
152 convenience functions for adding new references to containers and not
153 to worry about the reference count.
155 In the following sections it is clearly documented whether a function
156 will return a new or borrowed reference or steal a reference to its
163 .. cfunction:: json_t *json_true(void)
167 Returns a value of the type :const:`JSON_TRUE`, or *NULL* on
170 .. cfunction:: json_t *json_false(void)
174 Returns a value of the type :const:`JSON_FALSE`, or *NULL* on
177 .. cfunction:: json_t *json_null(void)
181 Returns a value of the type :const:`JSON_NULL`, or *NULL* on
188 .. cfunction:: json_t *json_string(const char *value)
192 Returns a new value of the type :const:`JSON_STRING`, or *NULL* on
193 error. *value* must be a valid UTF-8 encoded Unicode string.
195 .. cfunction:: const char *json_string_value(const json_t *json)
197 Returns the associated value of a :const:`JSON_STRING` value as a
198 null terminated UTF-8 encoded string.
204 .. cfunction:: json_t *json_integer(int value)
208 Returns a new value of the type :const:`JSON_INTEGER`, or *NULL* on
211 .. cfunction:: int json_integer_value(const json_t *json)
213 Returns the associated integer value of values of the type
214 :const:`JSON_INTEGER`, or 0 for values of other types.
216 .. cfunction:: json_t *json_real(double value)
220 Returns a new value of the type :const:`JSON_REAL`, or *NULL* on
223 .. cfunction:: double json_real_value(const json_t *json)
225 Returns the associated real value of values of the type
226 :const:`JSON_INTEGER`, or 0 for values of other types.
228 In addition to the functions above, there's a common query function
229 for integers and reals:
231 .. cfunction:: double json_number_value(const json_t *json)
233 Returns the value of either ``JSON_INTEGER`` or ``JSON_REAL``, cast
234 to double regardless of the actual type.
240 A JSON array is an ordered collection of other JSON values.
242 .. cfunction:: json_t *json_array(void)
246 Returns a new value of the type :const:`JSON_ARRAY`, or *NULL* on
247 error. Initially, the array is empty.
249 .. cfunction:: unsigned int json_array_size(const json_t *array)
251 Returns the number of elements in *array*.
253 .. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
255 .. refcounting:: borrow
257 Returns the element in *array* at position *index*, or *NULL* if
258 *index* is out of range. The valid range for *index* is from 0 to
259 the return value of :cfunc:`json_array_size()` minus 1.
261 .. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
263 Replaces the element in *array* at position *index* with *value*.
264 Returns 0 on success, or -1 if *index* is out of range. The valid
265 range for *index* is from 0 to the return value of
266 :cfunc:`json_array_size()` minus 1.
268 .. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
270 Like :cfunc:`json_array_set()` but steals the reference to *value*.
271 This is useful when *value* is newly created and not used after
274 .. versionadded:: 1.1
276 .. cfunction:: int json_array_append(json_t *array, json_t *value)
278 Appends *value* to the end of *array*, growing the size of *array*
279 by 1. Returns 0 on success and -1 on error.
281 .. cfunction:: int json_array_append_new(json_t *array, json_t *value)
283 Like :cfunc:`json_array_append()` but steals the reference to
284 *value*. This is useful when *value* is newly created and not used
287 .. versionadded:: 1.1
293 A JSON object is a dictionary of key-value pairs, where the key is a
294 Unicode string and the value is any JSON value.
296 .. cfunction:: json_t *json_object(void)
300 Returns a new value of the type :const:`JSON_OBJECT`, or *NULL* on
301 error. Initially, the object is empty.
303 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
305 .. refcounting:: borrow
307 Get a value corresponding to *key* from *object*. Returns *NULL* if
308 *key* is not found and on error.
310 .. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)
312 Set the value of *key* to *value* in *object*. *key* must be a
313 valid null terminated UTF-8 encoded Unicode string. If there
314 already is a value for *key*, it is replaced by the new value.
315 Returns 0 on success and -1 on error.
317 .. cfunction:: int json_object_set_new(json_t *object, const char *key, json_t *value)
319 Like :cfunc:`json_object_set()` but steals the reference to
320 *value*. This is useful when *value* is newly created and not used
323 .. versionadded:: 1.1
325 .. cfunction:: int json_object_del(json_t *object, const char *key)
327 Delete *key* from *object* if it exists. Returns 0 on success, or
328 -1 if *key* was not found.
331 The following functions implement an iteration protocol for objects:
333 .. cfunction:: void *json_object_iter(json_t *object)
335 Returns an opaque iterator which can be used to iterate over all
336 key-value pairs in *object*, or *NULL* if *object* is empty.
338 .. cfunction:: void *json_object_iter_next(json_t *object, void *iter)
340 Returns an iterator pointing to the next key-value pair in *object*
341 after *iter*, or *NULL* if the whole object has been iterated
344 .. cfunction:: const char *json_object_iter_key(void *iter)
346 Extract the associated key from *iter*.
348 .. cfunction:: json_t *json_object_iter_value(void *iter)
350 .. refcounting:: borrow
352 Extract the associated value from *iter*.
358 This sections describes the functions that can be used to encode
359 values to JSON. Only objects and arrays can be encoded, since they are
360 the only valid "root" values of a JSON text.
362 Each function takes a *flags* parameter that controls some aspects of
363 how the data is encoded. Its default value is 0. The following macros
364 can be ORed together to obtain *flags*.
367 Pretty-print the result, indenting arrays and objects by *n*
368 spaces. The valid range for *n* is between 0 and 255, other values
369 result in an undefined output. If ``JSON_INDENT`` is not used or
370 *n* is 0, no pretty-printing is done and the result is a compact
373 The following functions perform the actual JSON encoding. The result
376 .. cfunction:: char *json_dumps(const json_t *root, uint32_t flags)
378 Returns the JSON representation of *root* as a string, or *NULL* on
379 error. *flags* is described above. The return value must be freed
380 by the caller using :cfunc:`free()`.
382 .. cfunction:: int json_dumpf(const json_t *root, FILE *output, uint32_t flags)
384 Write the JSON representation of *root* to the stream *output*.
385 *flags* is described above. Returns 0 on success and -1 on error.
387 .. cfunction:: int json_dump_file(const json_t *json, const char *path, uint32_t flags)
389 Write the JSON representation of *root* to the file *path*. If
390 *path* already exists, it is overwritten. *flags* is described
391 above. Returns 0 on success and -1 on error.
397 This sections describes the functions that can be used to decode JSON
398 text to the Jansson representation of JSON data. The JSON
399 specification requires that a JSON text is either a serialized array
400 or object, and this requirement is also enforced with the following
403 The only supported character encoding is UTF-8 (which ASCII is a
406 .. ctype:: json_error_t
408 This data structure is used to return information on decoding
409 errors from the decoding functions. Its definition is repeated
412 #define JSON_ERROR_TEXT_LENGTH 160
415 char text[JSON_ERROR_TEXT_LENGTH];
419 *line* is the line number on which the error occurred, or -1 if
420 this information is not available. *text* contains the error
421 message (in UTF-8), or an empty string if a message is not
424 The normal usef of :ctype:`json_error_t` is to allocate it normally
425 on the stack, and pass a pointer to a decoding function. Example::
431 json = json_load_file("/path/to/file.json", &error);
433 /* the error variable contains error information */
438 Also note that if the decoding succeeded (``json != NULL`` in the
439 above example), the contents of ``error`` are unspecified.
441 All decoding functions also accept *NULL* as the
442 :ctype:`json_error_t` pointer, in which case no error information
443 is returned to the caller.
445 The following functions perform the actual JSON decoding.
447 .. cfunction:: json_t *json_loads(const char *input, json_error_t *error)
451 Decodes the JSON string *input* and returns the array or object it
452 contains, or *NULL* on error, in which case *error* is filled with
453 information about the error. See above for discussion on the
456 .. cfunction:: json_t *json_loadf(FILE *input, json_error_t *error)
460 Decodes the JSON text in stream *input* and returns the array or
461 object it contains, or *NULL* on error, in which case *error* is
462 filled with information about the error. See above for discussion
463 on the *error* parameter.
465 .. cfunction:: json_t *json_load_file(const char *path, json_error_t *error)
469 Decodes the JSON text in file *path* and returns the array or
470 object it contains, or *NULL* on error, in which case *error* is
471 filled with information about the error. See above for discussion
472 on the *error* parameter.