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 *json)
199 Returns the associated value of the JSON string *json* as a null
200 terminated UTF-8 encoded string, or *NULL* if *json* is not a JSON
207 .. cfunction:: json_t *json_integer(int value)
211 Returns a new JSON integer, or *NULL* on error.
213 .. cfunction:: int json_integer_value(const json_t *json)
215 Returns the associated value the JSON integer *json*. If *json* is
216 *NULL* or not a JSON integer, 0 is returned.
218 .. cfunction:: json_t *json_real(double value)
222 Returns a new JSON real, or *NULL* on error.
224 .. cfunction:: double json_real_value(const json_t *json)
226 Returns the associated value of the JSON real *json*. If *json* is
227 *NULL* or not a JSON real, 0.0 is returned.
229 In addition to the functions above, there's a common query function
230 for integers and reals:
232 .. cfunction:: double json_number_value(const json_t *json)
234 Returns the associated value of the JSON integer or JSON real
235 *json*, cast to double regardless of the actual type. If *json* is
236 neither JSON real nor JSON integer, 0.0 is returned.
242 A JSON array is an ordered collection of other JSON values.
244 .. cfunction:: json_t *json_array(void)
248 Returns a new JSON array, or *NULL* on error. Initially, the array
251 .. cfunction:: unsigned int json_array_size(const json_t *array)
253 Returns the number of elements in *array*, or 0 if *array* is NULL
256 .. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
258 .. refcounting:: borrow
260 Returns the element in *array* at position *index*. The valid range
261 for *index* is from 0 to the return value of
262 :cfunc:`json_array_size()` minus 1. If *array* is not a JSON array,
263 if *array* is *NULL*, or if *index* is out of range, *NULL* is
266 .. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
268 Replaces the element in *array* at position *index* with *value*.
269 The valid range for *index* is from 0 to the return value of
270 :cfunc:`json_array_size()` minus 1. Returns 0 on success and -1 on
273 .. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
275 Like :cfunc:`json_array_set()` but steals the reference to *value*.
276 This is useful when *value* is newly created and not used after
279 .. versionadded:: 1.1
281 .. cfunction:: int json_array_append(json_t *array, json_t *value)
283 Appends *value* to the end of *array*, growing the size of *array*
284 by 1. Returns 0 on success and -1 on error.
286 .. cfunction:: int json_array_append_new(json_t *array, json_t *value)
288 Like :cfunc:`json_array_append()` but steals the reference to
289 *value*. This is useful when *value* is newly created and not used
292 .. versionadded:: 1.1
298 A JSON object is a dictionary of key-value pairs, where the key is a
299 Unicode string and the value is any JSON value.
301 .. cfunction:: json_t *json_object(void)
305 Returns a new JSON object, or *NULL* on error. Initially, the
308 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
310 .. refcounting:: borrow
312 Get a value corresponding to *key* from *object*. Returns *NULL* if
313 *key* is not found and on error.
315 .. cfunction:: int json_object_set(json_t *object, const char *key, json_t *value)
317 Set the value of *key* to *value* in *object*. *key* must be a
318 valid null terminated UTF-8 encoded Unicode string. If there
319 already is a value for *key*, it is replaced by the new value.
320 Returns 0 on success and -1 on error.
322 .. cfunction:: int json_object_set_new(json_t *object, const char *key, json_t *value)
324 Like :cfunc:`json_object_set()` but steals the reference to
325 *value*. This is useful when *value* is newly created and not used
328 .. versionadded:: 1.1
330 .. cfunction:: int json_object_del(json_t *object, const char *key)
332 Delete *key* from *object* if it exists. Returns 0 on success, or
333 -1 if *key* was not found.
336 The following functions implement an iteration protocol for objects:
338 .. cfunction:: void *json_object_iter(json_t *object)
340 Returns an opaque iterator which can be used to iterate over all
341 key-value pairs in *object*, or *NULL* if *object* is empty.
343 .. cfunction:: void *json_object_iter_next(json_t *object, void *iter)
345 Returns an iterator pointing to the next key-value pair in *object*
346 after *iter*, or *NULL* if the whole object has been iterated
349 .. cfunction:: const char *json_object_iter_key(void *iter)
351 Extract the associated key from *iter*.
353 .. cfunction:: json_t *json_object_iter_value(void *iter)
355 .. refcounting:: borrow
357 Extract the associated value from *iter*.
359 The iteration protocol can be used for example as follows::
361 /* obj is a JSON object */
364 void *iter = json_object_iter(obj);
367 key = json_object_iter_key(iter);
368 value = json_object_iter_value(iter);
369 /* use key and value ... */
370 iter = json_object_iter_next(obj, iter);
377 This sections describes the functions that can be used to encode
378 values to JSON. Only objects and arrays can be encoded, since they are
379 the only valid "root" values of a JSON text.
381 Each function takes a *flags* parameter that controls some aspects of
382 how the data is encoded. Its default value is 0. The following macros
383 can be ORed together to obtain *flags*.
386 Pretty-print the result, indenting arrays and objects by *n*
387 spaces. The valid range for *n* is between 0 and 255, other values
388 result in an undefined output. If ``JSON_INDENT`` is not used or
389 *n* is 0, no pretty-printing is done and the result is a compact
392 The following functions perform the actual JSON encoding. The result
395 .. cfunction:: char *json_dumps(const json_t *root, unsigned long flags)
397 Returns the JSON representation of *root* as a string, or *NULL* on
398 error. *flags* is described above. The return value must be freed
399 by the caller using :cfunc:`free()`.
401 .. cfunction:: int json_dumpf(const json_t *root, FILE *output, unsigned long flags)
403 Write the JSON representation of *root* to the stream *output*.
404 *flags* is described above. Returns 0 on success and -1 on error.
406 .. cfunction:: int json_dump_file(const json_t *json, const char *path, unsigned long flags)
408 Write the JSON representation of *root* to the file *path*. If
409 *path* already exists, it is overwritten. *flags* is described
410 above. Returns 0 on success and -1 on error.
416 This sections describes the functions that can be used to decode JSON
417 text to the Jansson representation of JSON data. The JSON
418 specification requires that a JSON text is either a serialized array
419 or object, and this requirement is also enforced with the following
422 The only supported character encoding is UTF-8 (which ASCII is a
425 .. ctype:: json_error_t
427 This data structure is used to return information on decoding
428 errors from the decoding functions. Its definition is repeated
431 #define JSON_ERROR_TEXT_LENGTH 160
434 char text[JSON_ERROR_TEXT_LENGTH];
438 *line* is the line number on which the error occurred, or -1 if
439 this information is not available. *text* contains the error
440 message (in UTF-8), or an empty string if a message is not
443 The normal usef of :ctype:`json_error_t` is to allocate it normally
444 on the stack, and pass a pointer to a decoding function. Example::
450 json = json_load_file("/path/to/file.json", &error);
452 /* the error variable contains error information */
457 Also note that if the decoding succeeded (``json != NULL`` in the
458 above example), the contents of ``error`` are unspecified.
460 All decoding functions also accept *NULL* as the
461 :ctype:`json_error_t` pointer, in which case no error information
462 is returned to the caller.
464 The following functions perform the actual JSON decoding.
466 .. cfunction:: json_t *json_loads(const char *input, json_error_t *error)
470 Decodes the JSON string *input* and returns the array or object it
471 contains, or *NULL* on error, in which case *error* is filled with
472 information about the error. See above for discussion on the
475 .. cfunction:: json_t *json_loadf(FILE *input, json_error_t *error)
479 Decodes the JSON text in stream *input* and returns the array or
480 object it contains, or *NULL* on error, in which case *error* is
481 filled with information about the error. See above for discussion
482 on the *error* parameter.
484 .. cfunction:: json_t *json_load_file(const char *path, json_error_t *error)
488 Decodes the JSON text in file *path* and returns the array or
489 object it contains, or *NULL* on error, in which case *error* is
490 filled with information about the error. See above for discussion
491 on the *error* parameter.