12 All declarations are in :file:`jansson.h`, so it's enough to
20 All constants are prefixed with ``JSON_`` (except for those describing
21 the library version, prefixed with ``JANSSON_``). Other identifiers
22 are prefixed with ``json_``. Type names are suffixed with ``_t`` and
23 ``typedef``\ 'd so that the ``struct`` keyword need not be used.
29 The Jansson version is of the form *A.B.C*, where *A* is the major
30 version, *B* is the minor version and *C* is the micro version. If the
31 micro version is zero, it's omitted from the version string, i.e. the
32 version string is just *A.B*.
34 When a new release only fixes bugs and doesn't add new features or
35 functionality, the micro version is incremented. When new features are
36 added in a backwards compatible way, the minor version is incremented
37 and the micro version is set to zero. When there are backwards
38 incompatible changes, the major version is incremented and others are
41 The following preprocessor constants specify the current version of
44 ``JANSSON_VERSION_MAJOR``, ``JANSSON_VERSION_MINOR``, ``JANSSON_VERSION_MICRO``
45 Integers specifying the major, minor and micro versions,
49 A string representation of the current version, e.g. ``"1.2.1"`` or
52 ``JANSSON_VERSION_HEX``
53 A 3-byte hexadecimal representation of the version, e.g.
54 ``0x010201`` for version 1.2.1 and ``0x010300`` for version 1.3.
55 This is useful in numeric comparisions, e.g.::
57 #if JANSSON_VERSION_HEX >= 0x010300
58 /* Code specific to version 1.3 and above */
65 The JSON specification (:rfc:`4627`) defines the following data types:
66 *object*, *array*, *string*, *number*, *boolean*, and *null*. JSON
67 types are used dynamically; arrays and objects can hold any other data
68 type, including themselves. For this reason, Jansson's type system is
69 also dynamic in nature. There's one C type to represent all JSON
70 values, and this structure knows the type of the JSON value it holds.
74 This data structure is used throughout the library to represent all
75 JSON values. It always contains the type of the JSON value it holds
76 and the value's reference count. The rest depends on the type of the
79 Objects of :type:`json_t` are always used through a pointer. There
80 are APIs for querying the type, manipulating the reference count, and
81 for constructing and manipulating values of different types.
83 Unless noted otherwise, all API functions return an error value if an
84 error occurs. Depending on the function's signature, the error value
85 is either *NULL* or -1. Invalid arguments or invalid input are
86 apparent sources for errors. Memory allocation and I/O operations may
93 The type of a JSON value is queried and tested using the following
96 .. type:: enum json_type
98 The type of a JSON value. The following members are defined:
100 +--------------------+
102 +--------------------+
104 +--------------------+
106 +--------------------+
108 +--------------------+
110 +--------------------+
112 +--------------------+
114 +--------------------+
116 +--------------------+
118 These correspond to JSON object, array, string, number, boolean and
119 null. A number is represented by either a value of the type
120 ``JSON_INTEGER`` or of the type ``JSON_REAL``. A true boolean value
121 is represented by a value of the type ``JSON_TRUE`` and false by a
122 value of the type ``JSON_FALSE``.
124 .. function:: int json_typeof(const json_t *json)
126 Return the type of the JSON value (a :type:`json_type` cast to
127 :type:`int`). *json* MUST NOT be *NULL*. This function is actually
128 implemented as a macro for speed.
130 .. function:: json_is_object(const json_t *json)
131 json_is_array(const json_t *json)
132 json_is_string(const json_t *json)
133 json_is_integer(const json_t *json)
134 json_is_real(const json_t *json)
135 json_is_true(const json_t *json)
136 json_is_false(const json_t *json)
137 json_is_null(const json_t *json)
139 These functions (actually macros) return true (non-zero) for values
140 of the given type, and false (zero) for values of other types and
143 .. function:: json_is_number(const json_t *json)
145 Returns true for values of types ``JSON_INTEGER`` and
146 ``JSON_REAL``, and false for other types and for *NULL*.
148 .. function:: json_is_boolean(const json_t *json)
150 Returns true for types ``JSON_TRUE`` and ``JSON_FALSE``, and false
151 for values of other types and for *NULL*.
154 .. _apiref-reference-count:
159 The reference count is used to track whether a value is still in use
160 or not. When a value is created, it's reference count is set to 1. If
161 a reference to a value is kept (e.g. a value is stored somewhere for
162 later use), its reference count is incremented, and when the value is
163 no longer needed, the reference count is decremented. When the
164 reference count drops to zero, there are no references left, and the
165 value can be destroyed.
167 The following functions are used to manipulate the reference count.
169 .. function:: json_t *json_incref(json_t *json)
171 Increment the reference count of *json* if it's not non-*NULL*.
174 .. function:: void json_decref(json_t *json)
176 Decrement the reference count of *json*. As soon as a call to
177 :func:`json_decref()` drops the reference count to zero, the value
178 is destroyed and it can no longer be used.
180 Functions creating new JSON values set the reference count to 1. These
181 functions are said to return a **new reference**. Other functions
182 returning (existing) JSON values do not normally increase the
183 reference count. These functions are said to return a **borrowed
184 reference**. So, if the user will hold a reference to a value returned
185 as a borrowed reference, he must call :func:`json_incref`. As soon as
186 the value is no longer needed, :func:`json_decref` should be called
187 to release the reference.
189 Normally, all functions accepting a JSON value as an argument will
190 manage the reference, i.e. increase and decrease the reference count
191 as needed. However, some functions **steal** the reference, i.e. they
192 have the same result as if the user called :func:`json_decref()` on
193 the argument right after calling the function. These functions are
194 suffixed with ``_new`` or have ``_new_`` somewhere in their name.
196 For example, the following code creates a new JSON array and appends
199 json_t *array, *integer;
201 array = json_array();
202 integer = json_integer(42);
204 json_array_append(array, integer);
205 json_decref(integer);
207 Note how the caller has to release the reference to the integer value
208 by calling :func:`json_decref()`. By using a reference stealing
209 function :func:`json_array_append_new()` instead of
210 :func:`json_array_append()`, the code becomes much simpler::
212 json_t *array = json_array();
213 json_array_append_new(array, json_integer(42));
215 In this case, the user doesn't have to explicitly release the
216 reference to the integer value, as :func:`json_array_append_new()`
217 steals the reference when appending the value to the array.
219 In the following sections it is clearly documented whether a function
220 will return a new or borrowed reference or steal a reference to its
227 A circular reference is created when an object or an array is,
228 directly or indirectly, inserted inside itself. The direct case is
231 json_t *obj = json_object();
232 json_object_set(obj, "foo", obj);
234 Jansson will refuse to do this, and :func:`json_object_set()` (and
235 all the other such functions for objects and arrays) will return with
236 an error status. The indirect case is the dangerous one::
238 json_t *arr1 = json_array(), *arr2 = json_array();
239 json_array_append(arr1, arr2);
240 json_array_append(arr2, arr1);
242 In this example, the array ``arr2`` is contained in the array
243 ``arr1``, and vice versa. Jansson cannot check for this kind of
244 indirect circular references without a performance hit, so it's up to
245 the user to avoid them.
247 If a circular reference is created, the memory consumed by the values
248 cannot be freed by :func:`json_decref()`. The reference counts never
249 drops to zero because the values are keeping the references to each
250 other. Moreover, trying to encode the values with any of the encoding
251 functions will fail. The encoder detects circular references and
252 returns an error status.
258 These values are implemented as singletons, so each of these functions
259 returns the same value each time.
261 .. function:: json_t *json_true(void)
265 Returns the JSON true value.
267 .. function:: json_t *json_false(void)
271 Returns the JSON false value.
273 .. function:: json_t *json_null(void)
277 Returns the JSON null value.
283 Jansson uses UTF-8 as the character encoding. All JSON strings must be
284 valid UTF-8 (or ASCII, as it's a subset of UTF-8). Normal null
285 terminated C strings are used, so JSON strings may not contain
286 embedded null characters. All other Unicode codepoints U+0001 through
287 U+10FFFF are allowed.
289 .. function:: json_t *json_string(const char *value)
293 Returns a new JSON string, or *NULL* on error. *value* must be a
294 valid UTF-8 encoded Unicode string.
296 .. function:: json_t *json_string_nocheck(const char *value)
300 Like :func:`json_string`, but doesn't check that *value* is valid
301 UTF-8. Use this function only if you are certain that this really
302 is the case (e.g. you have already checked it by other means).
304 .. function:: const char *json_string_value(const json_t *string)
306 Returns the associated value of *string* as a null terminated UTF-8
307 encoded string, or *NULL* if *string* is not a JSON string.
309 The retuned value is read-only and must not be modified or freed by
310 the user. It is valid as long as *string* exists, i.e. as long as
311 its reference count has not dropped to zero.
313 .. function:: int json_string_set(const json_t *string, const char *value)
315 Sets the associated value of *string* to *value*. *value* must be a
316 valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
319 .. function:: int json_string_set_nocheck(const json_t *string, const char *value)
321 Like :func:`json_string_set`, but doesn't check that *value* is
322 valid UTF-8. Use this function only if you are certain that this
323 really is the case (e.g. you have already checked it by other
330 The JSON specification only contains one numeric type, "number". The C
331 programming language has distinct types for integer and floating-point
332 numbers, so for practical reasons Jansson also has distinct types for
333 the two. They are called "integer" and "real", respectively. For more
334 information, see :ref:`rfc-conformance`.
338 This is the C type that is used to store JSON integer values. It
339 represents the widest integer type available on your system. In
340 practice it's just a typedef of ``long long`` if your compiler
341 supports it, otherwise ``long``.
343 Usually, you can safely use plain ``int`` in place of
344 ``json_int_t``, and the implicit C integer conversion handles the
345 rest. Only when you know that you need the full 64-bit range, you
346 should use ``json_int_t`` explicitly.
348 ``JSON_INTEGER_IS_LONG_LONG``
350 This is a preprocessor variable that holds the value 1 if
351 :type:`json_int_t` is ``long long``, and 0 if it's ``long``. It
352 can be used as follows::
354 #if JSON_INTEGER_IS_LONG_LONG
355 /* Code specific for long long */
357 /* Code specific for long */
360 ``JSON_INTEGER_FORMAT``
362 This is a macro that expands to a :func:`printf()` conversion
363 specifier that corresponds to :type:`json_int_t`, without the
364 leading ``%`` sign, i.e. either ``"lld"`` or ``"ld"``. This macro
365 is required because the actual type of :type:`json_int_t` can be
366 either ``long`` or ``long long``, and :func:`printf()` reuiqres
367 different length modifiers for the two.
371 json_int_t x = 123123123;
372 printf("x is %" JSON_INTEGER_FORMAT "\n", x);
375 .. function:: json_t *json_integer(json_int_t value)
379 Returns a new JSON integer, or *NULL* on error.
381 .. function:: json_int_t json_integer_value(const json_t *integer)
383 Returns the associated value of *integer*, or 0 if *json* is not a
386 .. function:: int json_integer_set(const json_t *integer, json_int_t value)
388 Sets the associated value of *integer* to *value*. Returns 0 on
389 success and -1 if *integer* is not a JSON integer.
391 .. function:: json_t *json_real(double value)
395 Returns a new JSON real, or *NULL* on error.
397 .. function:: double json_real_value(const json_t *real)
399 Returns the associated value of *real*, or 0.0 if *real* is not a
402 .. function:: int json_real_set(const json_t *real, double value)
404 Sets the associated value of *real* to *value*. Returns 0 on
405 success and -1 if *real* is not a JSON real.
407 In addition to the functions above, there's a common query function
408 for integers and reals:
410 .. function:: double json_number_value(const json_t *json)
412 Returns the associated value of the JSON integer or JSON real
413 *json*, cast to double regardless of the actual type. If *json* is
414 neither JSON real nor JSON integer, 0.0 is returned.
420 A JSON array is an ordered collection of other JSON values.
422 .. function:: json_t *json_array(void)
426 Returns a new JSON array, or *NULL* on error. Initially, the array
429 .. function:: size_t json_array_size(const json_t *array)
431 Returns the number of elements in *array*, or 0 if *array* is NULL
434 .. function:: json_t *json_array_get(const json_t *array, size_t index)
436 .. refcounting:: borrow
438 Returns the element in *array* at position *index*. The valid range
439 for *index* is from 0 to the return value of
440 :func:`json_array_size()` minus 1. If *array* is not a JSON array,
441 if *array* is *NULL*, or if *index* is out of range, *NULL* is
444 .. function:: int json_array_set(json_t *array, size_t index, json_t *value)
446 Replaces the element in *array* at position *index* with *value*.
447 The valid range for *index* is from 0 to the return value of
448 :func:`json_array_size()` minus 1. Returns 0 on success and -1 on
451 .. function:: int json_array_set_new(json_t *array, size_t index, json_t *value)
453 Like :func:`json_array_set()` but steals the reference to *value*.
454 This is useful when *value* is newly created and not used after
457 .. function:: int json_array_append(json_t *array, json_t *value)
459 Appends *value* to the end of *array*, growing the size of *array*
460 by 1. Returns 0 on success and -1 on error.
462 .. function:: int json_array_append_new(json_t *array, json_t *value)
464 Like :func:`json_array_append()` but steals the reference to
465 *value*. This is useful when *value* is newly created and not used
468 .. function:: int json_array_insert(json_t *array, size_t index, json_t *value)
470 Inserts *value* to *array* at position *index*, shifting the
471 elements at *index* and after it one position towards the end of
472 the array. Returns 0 on success and -1 on error.
474 .. function:: int json_array_insert_new(json_t *array, size_t index, json_t *value)
476 Like :func:`json_array_insert()` but steals the reference to
477 *value*. This is useful when *value* is newly created and not used
480 .. function:: int json_array_remove(json_t *array, size_t index)
482 Removes the element in *array* at position *index*, shifting the
483 elements after *index* one position towards the start of the array.
484 Returns 0 on success and -1 on error. The reference count of the
485 removed value is decremented.
487 .. function:: int json_array_clear(json_t *array)
489 Removes all elements from *array*. Returns 0 on sucess and -1 on
490 error. The reference count of all removed values are decremented.
492 .. function:: int json_array_extend(json_t *array, json_t *other_array)
494 Appends all elements in *other_array* to the end of *array*.
495 Returns 0 on success and -1 on error.
501 A JSON object is a dictionary of key-value pairs, where the key is a
502 Unicode string and the value is any JSON value.
504 .. function:: json_t *json_object(void)
508 Returns a new JSON object, or *NULL* on error. Initially, the
511 .. function:: size_t json_object_size(const json_t *object)
513 Returns the number of elements in *object*, or 0 if *object* is not
516 .. function:: json_t *json_object_get(const json_t *object, const char *key)
518 .. refcounting:: borrow
520 Get a value corresponding to *key* from *object*. Returns *NULL* if
521 *key* is not found and on error.
523 .. function:: int json_object_set(json_t *object, const char *key, json_t *value)
525 Set the value of *key* to *value* in *object*. *key* must be a
526 valid null terminated UTF-8 encoded Unicode string. If there
527 already is a value for *key*, it is replaced by the new value.
528 Returns 0 on success and -1 on error.
530 .. function:: int json_object_set_nocheck(json_t *object, const char *key, json_t *value)
532 Like :func:`json_object_set`, but doesn't check that *key* is
533 valid UTF-8. Use this function only if you are certain that this
534 really is the case (e.g. you have already checked it by other
537 .. function:: int json_object_set_new(json_t *object, const char *key, json_t *value)
539 Like :func:`json_object_set()` but steals the reference to
540 *value*. This is useful when *value* is newly created and not used
543 .. function:: int json_object_set_new_nocheck(json_t *object, const char *key, json_t *value)
545 Like :func:`json_object_set_new`, but doesn't check that *key* is
546 valid UTF-8. Use this function only if you are certain that this
547 really is the case (e.g. you have already checked it by other
550 .. function:: int json_object_del(json_t *object, const char *key)
552 Delete *key* from *object* if it exists. Returns 0 on success, or
553 -1 if *key* was not found. The reference count of the removed value
556 .. function:: int json_object_clear(json_t *object)
558 Remove all elements from *object*. Returns 0 on success and -1 if
559 *object* is not a JSON object. The reference count of all removed
560 values are decremented.
562 .. function:: int json_object_update(json_t *object, json_t *other)
564 Update *object* with the key-value pairs from *other*, overwriting
565 existing keys. Returns 0 on success or -1 on error.
568 The following macro can be used to iterate through all key-value pairs
571 .. function:: json_object_foreach(object, key, value)
573 Iterate over every key-value pair of ``object``, running the block
574 of code that follows each time with the proper values set to
575 variables ``key`` and ``value``, of types :type:`const char *` and
576 :type:`json_t *` respectively. Example::
578 /* obj is a JSON object */
582 json_object_foreach(obj, key, value) {
583 /* block of code that uses key and value */
586 The items are not returned in any particular order.
588 This macro expands to an ordinary ``for`` statement upon
589 preprocessing, so its performance is equivalent to that of
590 hand-written iteration code using the object iteration protocol
591 (see below). The main advantage of this macro is that it abstracts
592 away the complexity behind iteration, and makes for shorter, more
595 .. versionadded:: 2.3
598 The following functions implement an iteration protocol for objects,
599 allowing to iterate through all key-value pairs in an object. The
600 items are not returned in any particular order, as this would require
601 sorting due to the internal hashtable implementation.
603 .. function:: void *json_object_iter(json_t *object)
605 Returns an opaque iterator which can be used to iterate over all
606 key-value pairs in *object*, or *NULL* if *object* is empty.
608 .. function:: void *json_object_iter_at(json_t *object, const char *key)
610 Like :func:`json_object_iter()`, but returns an iterator to the
611 key-value pair in *object* whose key is equal to *key*, or NULL if
612 *key* is not found in *object*. Iterating forward to the end of
613 *object* only yields all key-value pairs of the object if *key*
614 happens to be the first key in the underlying hash table.
616 .. function:: void *json_object_iter_next(json_t *object, void *iter)
618 Returns an iterator pointing to the next key-value pair in *object*
619 after *iter*, or *NULL* if the whole object has been iterated
622 .. function:: const char *json_object_iter_key(void *iter)
624 Extract the associated key from *iter*.
626 .. function:: json_t *json_object_iter_value(void *iter)
628 .. refcounting:: borrow
630 Extract the associated value from *iter*.
632 .. function:: int json_object_iter_set(json_t *object, void *iter, json_t *value)
634 Set the value of the key-value pair in *object*, that is pointed to
635 by *iter*, to *value*.
637 .. function:: int json_object_iter_set_new(json_t *object, void *iter, json_t *value)
639 Like :func:`json_object_iter_set()`, but steals the reference to
640 *value*. This is useful when *value* is newly created and not used
643 .. function:: void *json_object_key_to_iter(const char *key)
645 Like :func:`json_object_iter_at()`, but much faster. Only works for
646 values returned by :func:`json_object_iter_key()`. Using other keys
647 will lead to segfaults. This function is used internally to
648 implement :func:`json_object_foreach`.
650 .. versionadded:: 2.3
652 The iteration protocol can be used for example as follows::
654 /* obj is a JSON object */
658 void *iter = json_object_iter(obj);
661 key = json_object_iter_key(iter);
662 value = json_object_iter_value(iter);
663 /* use key and value ... */
664 iter = json_object_iter_next(obj, iter);
671 Jansson uses a single struct type to pass error information to the
672 user. See sections :ref:`apiref-decoding`, :ref:`apiref-pack` and
673 :ref:`apiref-unpack` for functions that pass error information using
676 .. type:: json_error_t
678 .. member:: char text[]
680 The error message (in UTF-8), or an empty string if a message is
683 .. member:: char source[]
685 Source of the error. This can be (a part of) the file name or a
686 special identifier in angle brackers (e.g. ``<string>``).
690 The line number on which the error occurred.
692 .. member:: int column
694 The column on which the error occurred. Note that this is the
695 *character column*, not the byte column, i.e. a multibyte UTF-8
696 character counts as one column.
698 .. member:: size_t position
700 The position in bytes from the start of the input. This is
701 useful for debugging Unicode encoding problems.
703 The normal use of :type:`json_error_t` is to allocate it on the stack,
704 and pass a pointer to a function. Example::
710 json = json_load_file("/path/to/file.json", 0, &error);
712 /* the error variable contains error information */
717 Also note that if the call succeeded (``json != NULL`` in the above
718 example), the contents of ``error`` are generally left unspecified.
719 The decoding functions write to the ``position`` member also on
720 success. See :ref:`apiref-decoding` for more info.
722 All functions also accept *NULL* as the :type:`json_error_t` pointer,
723 in which case no error information is returned to the caller.
729 This sections describes the functions that can be used to encode
730 values to JSON. By default, only objects and arrays can be encoded
731 directly, since they are the only valid *root* values of a JSON text.
732 To encode any JSON value, use the ``JSON_ENCODE_ANY`` flag (see
735 By default, the output has no newlines, and spaces are used between
736 array and object elements for a readable output. This behavior can be
737 altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
738 described below. A newline is never appended to the end of the encoded
741 Each function takes a *flags* parameter that controls some aspects of
742 how the data is encoded. Its default value is 0. The following macros
743 can be ORed together to obtain *flags*.
746 Pretty-print the result, using newlines between array and object
747 items, and indenting with *n* spaces. The valid range for *n* is
748 between 0 and 31 (inclusive), other values result in an undefined
749 output. If ``JSON_INDENT`` is not used or *n* is 0, no newlines are
750 inserted between array and object items.
753 This flag enables a compact representation, i.e. sets the separator
754 between array and object items to ``","`` and between object keys
755 and values to ``":"``. Without this flag, the corresponding
756 separators are ``", "`` and ``": "`` for more readable output.
758 ``JSON_ENSURE_ASCII``
759 If this flag is used, the output is guaranteed to consist only of
760 ASCII characters. This is achived by escaping all Unicode
761 characters outside the ASCII range.
764 If this flag is used, all the objects in output are sorted by key.
765 This is useful e.g. if two JSON texts are diffed or visually
768 ``JSON_PRESERVE_ORDER``
769 If this flag is used, object keys in the output are sorted into the
770 same order in which they were first inserted to the object. For
771 example, decoding a JSON text and then encoding with this flag
772 preserves the order of object keys.
775 Specifying this flag makes it possible to encode any JSON value on
776 its own. Without it, only objects and arrays can be passed as the
777 *root* value to the encoding functions.
779 **Note:** Encoding any value may be useful in some scenarios, but
780 it's generally discouraged as it violates strict compatiblity with
781 :rfc:`4627`. If you use this flag, don't expect interoperatibility
782 with other JSON systems.
784 .. versionadded:: 2.1
786 The following functions perform the actual JSON encoding. The result
789 .. function:: char *json_dumps(const json_t *root, size_t flags)
791 Returns the JSON representation of *root* as a string, or *NULL* on
792 error. *flags* is described above. The return value must be freed
793 by the caller using :func:`free()`.
795 .. function:: int json_dumpf(const json_t *root, FILE *output, size_t flags)
797 Write the JSON representation of *root* to the stream *output*.
798 *flags* is described above. Returns 0 on success and -1 on error.
799 If an error occurs, something may have already been written to
800 *output*. In this case, the output is undefined and most likely not
803 .. function:: int json_dump_file(const json_t *json, const char *path, size_t flags)
805 Write the JSON representation of *root* to the file *path*. If
806 *path* already exists, it is overwritten. *flags* is described
807 above. Returns 0 on success and -1 on error.
809 .. type:: json_dump_callback_t
811 A typedef for a function that's called by
812 :func:`json_dump_callback()`::
814 typedef int (*json_dump_callback_t)(const char *buffer, size_t size, void *data);
816 *buffer* points to a buffer containing a chunk of output, *size* is
817 the length of the buffer, and *data* is the corresponding
818 :func:`json_dump_callback()` argument passed through.
820 On error, the function should return -1 to stop the encoding
821 process. On success, it should return 0.
823 .. versionadded:: 2.2
825 .. function:: int json_dump_callback(const json_t *json, json_dump_callback_t callback, void *data, size_t flags)
827 Call *callback* repeatedly, passing a chunk of the JSON
828 representation of *root* each time. *flags* is described above.
829 Returns 0 on success and -1 on error.
831 .. versionadded:: 2.2
839 This sections describes the functions that can be used to decode JSON
840 text to the Jansson representation of JSON data. The JSON
841 specification requires that a JSON text is either a serialized array
842 or object, and this requirement is also enforced with the following
843 functions. In other words, the top level value in the JSON text being
844 decoded must be either array or object. To decode any JSON value, use
845 the ``JSON_DECODE_ANY`` flag (see below).
847 See :ref:`rfc-conformance` for a discussion on Jansson's conformance
848 to the JSON specification. It explains many design decisions that
849 affect especially the behavior of the decoder.
851 Each function takes a *flags* parameter that can be used to control
852 the behavior of the decoder. Its default value is 0. The following
853 macros can be ORed together to obtain *flags*.
855 ``JSON_REJECT_DUPLICATES``
856 Issue a decoding error if any JSON object in the input text
857 contains duplicate keys. Without this flag, the value of the last
858 occurence of each key ends up in the result. Key equivalence is
859 checked byte-by-byte, without special Unicode comparison
862 .. versionadded:: 2.1
865 By default, the decoder expects an array or object as the input.
866 With this flag enabled, the decoder accepts any valid JSON value.
868 **Note:** Decoding any value may be useful in some scenarios, but
869 it's generally discouraged as it violates strict compatiblity with
870 :rfc:`4627`. If you use this flag, don't expect interoperatibility
871 with other JSON systems.
873 .. versionadded:: 2.3
875 ``JSON_DISABLE_EOF_CHECK``
876 By default, the decoder expects that its whole input constitutes a
877 valid JSON text, and issues an error if there's extra data after
878 the otherwise valid JSON input. With this flag enabled, the decoder
879 stops after decoding a valid JSON array or object, and thus allows
880 extra data after the JSON text.
882 Normally, reading will stop when the last ``]`` or ``}`` in the
883 JSON input is encountered. If both ``JSON_DISABLE_EOF_CHECK`` and
884 ``JSON_DECODE_ANY`` flags are used, the decoder may read one extra
885 UTF-8 code unit (up to 4 bytes of input). For example, decoding
886 ``4true`` correctly decodes the integer 4, but also reads the
887 ``t``. For this reason, if reading multiple consecutive values that
888 are not arrays or objects, they should be separated by at least one
889 whitespace character.
891 .. versionadded:: 2.1
893 Each function also takes an optional :type:`json_error_t` parameter
894 that is filled with error information if decoding fails. It's also
895 updated on success; the number of bytes of input read is written to
896 its ``position`` field. This is especially useful when using
897 ``JSON_DISABLE_EOF_CHECK`` to read multiple consecutive JSON texts.
899 .. versionadded:: 2.3
900 Number of bytes of input read is written to the ``position`` field
901 of the :type:`json_error_t` structure.
903 If no error or position information is needed, you can pass *NULL*.
905 The following functions perform the actual JSON decoding.
907 .. function:: json_t *json_loads(const char *input, size_t flags, json_error_t *error)
911 Decodes the JSON string *input* and returns the array or object it
912 contains, or *NULL* on error, in which case *error* is filled with
913 information about the error. *flags* is described above.
915 .. function:: json_t *json_loadb(const char *buffer, size_t buflen, size_t flags, json_error_t *error)
919 Decodes the JSON string *buffer*, whose length is *buflen*, and
920 returns the array or object it contains, or *NULL* on error, in
921 which case *error* is filled with information about the error. This
922 is similar to :func:`json_loads()` except that the string doesn't
923 need to be null-terminated. *flags* is described above.
925 .. versionadded:: 2.1
927 .. function:: json_t *json_loadf(FILE *input, size_t flags, json_error_t *error)
931 Decodes the JSON text in stream *input* and returns the array or
932 object it contains, or *NULL* on error, in which case *error* is
933 filled with information about the error. *flags* is described
936 This function will start reading the input from whatever position
937 the input file was, without attempting to seek first. If an error
938 occurs, the file position will be left indeterminate. On success,
939 the file position will be at EOF, unless ``JSON_DISABLE_EOF_CHECK``
940 flag was used. In this case, the file position will be at the first
941 character after the last ``]`` or ``}`` in the JSON input. This
942 allows calling :func:`json_loadf()` on the same ``FILE`` object
943 multiple times, if the input consists of consecutive JSON texts,
944 possibly separated by whitespace.
946 .. function:: json_t *json_load_file(const char *path, size_t flags, json_error_t *error)
950 Decodes the JSON text in file *path* and returns the array or
951 object it contains, or *NULL* on error, in which case *error* is
952 filled with information about the error. *flags* is described
961 This section describes functions that help to create, or *pack*,
962 complex JSON values, especially nested objects and arrays. Value
963 building is based on a *format string* that is used to tell the
964 functions about the expected arguments.
966 For example, the format string ``"i"`` specifies a single integer
967 value, while the format string ``"[ssb]"`` or the equivalent ``"[s, s,
968 b]"`` specifies an array value with two strings and a boolean as its
971 /* Create the JSON integer 42 */
974 /* Create the JSON array ["foo", "bar", true] */
975 json_pack("[ssb]", "foo", "bar", 1);
977 Here's the full list of format characters. The type in parentheses
978 denotes the resulting JSON type, and the type in brackets (if any)
979 denotes the C type that is expected as the corresponding argument.
981 ``s`` (string) [const char \*]
982 Convert a NULL terminated UTF-8 string to a JSON string.
985 Output a JSON null value. No argument is consumed.
987 ``b`` (boolean) [int]
988 Convert a C :type:`int` to JSON boolean value. Zero is converted
989 to ``false`` and non-zero to ``true``.
991 ``i`` (integer) [int]
992 Convert a C :type:`int` to JSON integer.
994 ``I`` (integer) [json_int_t]
995 Convert a C :type:`json_int_t` to JSON integer.
997 ``f`` (real) [double]
998 Convert a C :type:`double` to JSON real.
1000 ``o`` (any value) [json_t \*]
1001 Output any given JSON value as-is. If the value is added to an
1002 array or object, the reference to the value passed to ``o`` is
1003 stolen by the container.
1005 ``O`` (any value) [json_t \*]
1006 Like ``o``, but the argument's reference count is incremented.
1007 This is useful if you pack into an array or object and want to
1008 keep the reference for the JSON value consumed by ``O`` to
1012 Build an array with contents from the inner format string. ``fmt``
1013 may contain objects and arrays, i.e. recursive value building is
1017 Build an object with contents from the inner format string
1018 ``fmt``. The first, third, etc. format character represent a key,
1019 and must be ``s`` (as object keys are always strings). The second,
1020 fourth, etc. format character represent a value. Any value may be
1021 an object or array, i.e. recursive value building is supported.
1023 The following functions compose the value building API:
1025 .. function:: json_t *json_pack(const char *fmt, ...)
1027 .. refcounting:: new
1029 Build a new JSON value according to the format string *fmt*. For
1030 each format character (except for ``{}[]n``), one argument is
1031 consumed and used to build the corresponding value. Returns *NULL*
1034 .. function:: json_t *json_pack_ex(json_error_t *error, size_t flags, const char *fmt, ...)
1035 json_t *json_vpack_ex(json_error_t *error, size_t flags, const char *fmt, va_list ap)
1037 .. refcounting:: new
1039 Like :func:`json_pack()`, but an in the case of an error, an error
1040 message is written to *error*, if it's not *NULL*. The *flags*
1041 parameter is currently unused and should be set to 0.
1043 As only the errors in format string (and out-of-memory errors) can
1044 be caught by the packer, these two functions are most likely only
1045 useful for debugging format strings.
1049 /* Build an empty JSON object */
1052 /* Build the JSON object {"foo": 42, "bar": 7} */
1053 json_pack("{sisi}", "foo", 42, "bar", 7);
1055 /* Like above, ':', ',' and whitespace are ignored */
1056 json_pack("{s:i, s:i}", "foo", 42, "bar", 7);
1058 /* Build the JSON array [[1, 2], {"cool": true}] */
1059 json_pack("[[i,i],{s:b]]", 1, 2, "cool", 1);
1064 Parsing and Validating Values
1065 =============================
1067 This sectinon describes functions that help to validate complex values
1068 and extract, or *unpack*, data from them. Like :ref:`building values
1069 <apiref-pack>`, this is also based on format strings.
1071 While a JSON value is unpacked, the type specified in the format
1072 string is checked to match that of the JSON value. This is the
1073 validation part of the process. In addition to this, the unpacking
1074 functions can also check that all items of arrays and objects are
1075 unpacked. This check be enabled with the format character ``!`` or by
1076 using the flag ``JSON_STRICT``. See below for details.
1078 Here's the full list of format characters. The type in parentheses
1079 denotes the JSON type, and the type in brackets (if any) denotes the C
1080 type whose address should be passed.
1082 ``s`` (string) [const char \*]
1083 Convert a JSON string to a pointer to a NULL terminated UTF-8
1084 string. The resulting string is extracted by using
1085 :func:`json_string_value()` internally, so it exists as long as
1086 there are still references to the corresponding JSON string.
1089 Expect a JSON null value. Nothing is extracted.
1091 ``b`` (boolean) [int]
1092 Convert a JSON boolean value to a C :type:`int`, so that ``true``
1093 is converted to 1 and ``false`` to 0.
1095 ``i`` (integer) [int]
1096 Convert a JSON integer to C :type:`int`.
1098 ``I`` (integer) [json_int_t]
1099 Convert a JSON integer to C :type:`json_int_t`.
1101 ``f`` (real) [double]
1102 Convert a JSON real to C :type:`double`.
1104 ``F`` (integer or real) [double]
1105 Convert a JSON number (integer or real) to C :type:`double`.
1107 ``o`` (any value) [json_t \*]
1108 Store a JSON value with no conversion to a :type:`json_t` pointer.
1110 ``O`` (any value) [json_t \*]
1111 Like ``O``, but the JSON value's reference count is incremented.
1114 Convert each item in the JSON array according to the inner format
1115 string. ``fmt`` may contain objects and arrays, i.e. recursive
1116 value extraction is supporetd.
1119 Convert each item in the JSON object according to the inner format
1120 string ``fmt``. The first, third, etc. format character represent
1121 a key, and must be ``s``. The corresponding argument to unpack
1122 functions is read as the object key. The second fourth, etc.
1123 format character represent a value and is written to the address
1124 given as the corresponding argument. **Note** that every other
1125 argument is read from and every other is written to.
1127 ``fmt`` may contain objects and arrays as values, i.e. recursive
1128 value extraction is supporetd.
1131 This special format character is used to enable the check that
1132 all object and array items are accessed, on a per-value basis. It
1133 must appear inside an array or object as the last format character
1134 before the closing bracket or brace. To enable the check globally,
1135 use the ``JSON_STRICT`` unpacking flag.
1138 This special format character is the opposite of ``!``. If the
1139 ``JSON_STRICT`` flag is used, ``*`` can be used to disable the
1140 strict check on a per-value basis. It must appear inside an array
1141 or object as the last format character before the closing bracket
1144 The following functions compose the parsing and validation API:
1146 .. function:: int json_unpack(json_t *root, const char *fmt, ...)
1148 Validate and unpack the JSON value *root* according to the format
1149 string *fmt*. Returns 0 on success and -1 on failure.
1151 .. function:: int json_unpack_ex(json_t *root, json_error_t *error, size_t flags, const char *fmt, ...)
1152 int json_vunpack_ex(json_t *root, json_error_t *error, size_t flags, const char *fmt, va_list ap)
1154 Validate and unpack the JSON value *root* according to the format
1155 string *fmt*. If an error occurs and *error* is not *NULL*, write
1156 error information to *error*. *flags* can be used to control the
1157 behaviour of the unpacker, see below for the flags. Returns 0 on
1158 success and -1 on failure.
1162 The first argument of all unpack functions is ``json_t *root``
1163 instead of ``const json_t *root``, because the use of ``O`` format
1164 character causes the reference count of ``root``, or some value
1165 reachable from ``root``, to be increased. Furthermore, the ``o``
1166 format character may be used to extract a value as-is, which allows
1167 modifying the structure or contents of a value reachable from
1170 If the ``O`` and ``o`` format character are not used, it's
1171 perfectly safe to cast a ``const json_t *`` variable to plain
1172 ``json_t *`` when used with these functions.
1174 The following unpacking flags are available:
1177 Enable the extra validation step checking that all object and
1178 array items are unpacked. This is equivalent to appending the
1179 format character ``!`` to the end of every array and object in the
1182 ``JSON_VALIDATE_ONLY``
1183 Don't extract any data, just validate the JSON value against the
1184 given format string. Note that object keys must still be specified
1185 after the format string.
1189 /* root is the JSON integer 42 */
1191 json_unpack(root, "i", &myint);
1192 assert(myint == 42);
1194 /* root is the JSON object {"foo": "bar", "quux": true} */
1197 json_unpack(root, "{s:s, s:b}", "foo", &str, "quux", &boolean);
1198 assert(strcmp(str, "bar") == 0 && boolean == 1);
1200 /* root is the JSON array [[1, 2], {"baz": null} */
1202 json_unpack_ex(root, &error, JSON_VALIDATE_ONLY, "[[i,i], {s:n}]", "baz");
1203 /* returns 0 for validation success, nothing is extracted */
1205 /* root is the JSON array [1, 2, 3, 4, 5] */
1207 json_unpack(root, "[ii!]", &myint1, &myint2);
1208 /* returns -1 for failed validation */
1214 Testing for equality of two JSON values cannot, in general, be
1215 achieved using the ``==`` operator. Equality in the terms of the
1216 ``==`` operator states that the two :type:`json_t` pointers point to
1217 exactly the same JSON value. However, two JSON values can be equal not
1218 only if they are exactly the same value, but also if they have equal
1221 * Two integer or real values are equal if their contained numeric
1222 values are equal. An integer value is never equal to a real value,
1225 * Two strings are equal if their contained UTF-8 strings are equal,
1226 byte by byte. Unicode comparison algorithms are not implemented.
1228 * Two arrays are equal if they have the same number of elements and
1229 each element in the first array is equal to the corresponding
1230 element in the second array.
1232 * Two objects are equal if they have exactly the same keys and the
1233 value for each key in the first object is equal to the value of the
1234 corresponding key in the second object.
1236 * Two true, false or null values have no "contents", so they are equal
1237 if their types are equal. (Because these values are singletons,
1238 their equality can actually be tested with ``==``.)
1240 The following function can be used to test whether two JSON values are
1243 .. function:: int json_equal(json_t *value1, json_t *value2)
1245 Returns 1 if *value1* and *value2* are equal, as defined above.
1246 Returns 0 if they are inequal or one or both of the pointers are
1253 Because of reference counting, passing JSON values around doesn't
1254 require copying them. But sometimes a fresh copy of a JSON value is
1255 needed. For example, if you need to modify an array, but still want to
1256 use the original afterwards, you should take a copy of it first.
1258 Jansson supports two kinds of copying: shallow and deep. There is a
1259 difference between these methods only for arrays and objects. Shallow
1260 copying only copies the first level value (array or object) and uses
1261 the same child values in the copied value. Deep copying makes a fresh
1262 copy of the child values, too. Moreover, all the child values are deep
1263 copied in a recursive fashion.
1265 .. function:: json_t *json_copy(json_t *value)
1267 .. refcounting:: new
1269 Returns a shallow copy of *value*, or *NULL* on error.
1271 .. function:: json_t *json_deep_copy(json_t *value)
1273 .. refcounting:: new
1275 Returns a deep copy of *value*, or *NULL* on error.
1278 .. _apiref-custom-memory-allocation:
1280 Custom Memory Allocation
1281 ========================
1283 By default, Jansson uses :func:`malloc()` and :func:`free()` for
1284 memory allocation. These functions can be overridden if custom
1287 .. type:: json_malloc_t
1289 A typedef for a function pointer with :func:`malloc()`'s
1292 typedef void *(*json_malloc_t)(size_t);
1294 .. type:: json_free_t
1296 A typedef for a function pointer with :func:`free()`'s
1299 typedef void (*json_free_t)(void *);
1301 .. function:: void json_set_alloc_funcs(json_malloc_t malloc_fn, json_free_t free_fn)
1303 Use *malloc_fn* instead of :func:`malloc()` and *free_fn* instead
1304 of :func:`free()`. This function has to be called before any other
1305 Jansson's API functions to ensure that all memory operations use
1310 Use the `Boehm's conservative garbage collector`_ for memory
1313 json_set_alloc_funcs(GC_malloc, GC_free);
1315 .. _Boehm's conservative garbage collector: http://www.hpl.hp.com/personal/Hans_Boehm/gc/
1317 Allow storing sensitive data (e.g. passwords or encryption keys) in
1318 JSON structures by zeroing all memory when freed::
1320 static void *secure_malloc(size_t size)
1322 /* Store the memory area size in the beginning of the block */
1323 void *ptr = malloc(size + 8);
1324 *((size_t *)ptr) = size;
1328 static void secure_free(void *ptr)
1333 size = *((size_t *)ptr);
1335 guaranteed_memset(ptr, 0, size);
1341 json_set_alloc_funcs(secure_malloc, secure_free);
1345 For more information about the issues of storing sensitive data in
1347 http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/protect-secrets.html.
1348 The page also explains the :func:`guaranteed_memset()` function used
1349 in the example and gives a sample implementation for it.