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 functions implement an iteration protocol for objects,
569 allowing to iterate through all key-value pairs in an object. The
570 items are not returned in any particular order, as this would require
571 sorting due to the internal hashtable implementation.
573 .. function:: void *json_object_iter(json_t *object)
575 Returns an opaque iterator which can be used to iterate over all
576 key-value pairs in *object*, or *NULL* if *object* is empty.
578 .. function:: void *json_object_iter_at(json_t *object, const char *key)
580 Like :func:`json_object_iter()`, but returns an iterator to the
581 key-value pair in *object* whose key is equal to *key*, or NULL if
582 *key* is not found in *object*. Iterating forward to the end of
583 *object* only yields all key-value pairs of the object if *key*
584 happens to be the first key in the underlying hash table.
586 .. function:: void *json_object_iter_next(json_t *object, void *iter)
588 Returns an iterator pointing to the next key-value pair in *object*
589 after *iter*, or *NULL* if the whole object has been iterated
592 .. function:: const char *json_object_iter_key(void *iter)
594 Extract the associated key from *iter*.
596 .. function:: json_t *json_object_iter_value(void *iter)
598 .. refcounting:: borrow
600 Extract the associated value from *iter*.
602 .. function:: int json_object_iter_set(json_t *object, void *iter, json_t *value)
604 Set the value of the key-value pair in *object*, that is pointed to
605 by *iter*, to *value*.
607 .. function:: int json_object_iter_set_new(json_t *object, void *iter, json_t *value)
609 Like :func:`json_object_iter_set()`, but steals the reference to
610 *value*. This is useful when *value* is newly created and not used
613 The iteration protocol can be used for example as follows::
615 /* obj is a JSON object */
618 void *iter = json_object_iter(obj);
621 key = json_object_iter_key(iter);
622 value = json_object_iter_value(iter);
623 /* use key and value ... */
624 iter = json_object_iter_next(obj, iter);
631 Jansson uses a single struct type to pass error information to the
632 user. See sections :ref:`apiref-decoding`, :ref:`apiref-pack` and
633 :ref:`apiref-unpack` for functions that pass error information using
636 .. type:: json_error_t
638 .. member:: char text[]
640 The error message (in UTF-8), or an empty string if a message is
643 .. member:: char source[]
645 Source of the error. This can be (a part of) the file name or a
646 special identifier in angle brackers (e.g. ``<string>``).
650 The line number on which the error occurred.
652 .. member:: int column
654 The column on which the error occurred. Note that this is the
655 *character column*, not the byte column, i.e. a multibyte UTF-8
656 character counts as one column.
658 .. member:: size_t position
660 The position in bytes from the start of the input. This is
661 useful for debugging Unicode encoding problems.
663 The normal use of :type:`json_error_t` is to allocate it on the stack,
664 and pass a pointer to a function. Example::
670 json = json_load_file("/path/to/file.json", 0, &error);
672 /* the error variable contains error information */
677 Also note that if the call succeeded (``json != NULL`` in the above
678 example), the contents of ``error`` are unspecified.
680 All functions also accept *NULL* as the :type:`json_error_t` pointer,
681 in which case no error information is returned to the caller.
687 This sections describes the functions that can be used to encode
688 values to JSON. By default, only objects and arrays can be encoded
689 directly, since they are the only valid *root* values of a JSON text.
690 To encode any JSON value, use the ``JSON_ENCODE_ANY`` flag (see
693 By default, the output has no newlines, and spaces are used between
694 array and object elements for a readable output. This behavior can be
695 altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
696 described below. A newline is never appended to the end of the encoded
699 Each function takes a *flags* parameter that controls some aspects of
700 how the data is encoded. Its default value is 0. The following macros
701 can be ORed together to obtain *flags*.
704 Pretty-print the result, using newlines between array and object
705 items, and indenting with *n* spaces. The valid range for *n* is
706 between 0 and 32, other values result in an undefined output. If
707 ``JSON_INDENT`` is not used or *n* is 0, no newlines are inserted
708 between array and object items.
711 This flag enables a compact representation, i.e. sets the separator
712 between array and object items to ``","`` and between object keys
713 and values to ``":"``. Without this flag, the corresponding
714 separators are ``", "`` and ``": "`` for more readable output.
716 ``JSON_ENSURE_ASCII``
717 If this flag is used, the output is guaranteed to consist only of
718 ASCII characters. This is achived by escaping all Unicode
719 characters outside the ASCII range.
722 If this flag is used, all the objects in output are sorted by key.
723 This is useful e.g. if two JSON texts are diffed or visually
726 ``JSON_PRESERVE_ORDER``
727 If this flag is used, object keys in the output are sorted into the
728 same order in which they were first inserted to the object. For
729 example, decoding a JSON text and then encoding with this flag
730 preserves the order of object keys.
733 Specifying this flag makes it possible to encode any JSON value on
734 its own. Without it, only objects and arrays can be passed as the
735 *root* value to the encoding functions.
737 **Note:** Encoding any value may be useful in some scenarios, but
738 it's generally discouraged as it violates strict compatiblity with
739 :rfc:`4627`. If you use this flag, don't expect interoperatibility
740 with other JSON systems. Even Jansson itself doesn't have any means
741 to decode JSON texts whose root value is not object or array.
743 .. versionadded:: 2.1
745 The following functions perform the actual JSON encoding. The result
748 .. function:: char *json_dumps(const json_t *root, size_t flags)
750 Returns the JSON representation of *root* as a string, or *NULL* on
751 error. *flags* is described above. The return value must be freed
752 by the caller using :func:`free()`.
754 .. function:: int json_dumpf(const json_t *root, FILE *output, size_t flags)
756 Write the JSON representation of *root* to the stream *output*.
757 *flags* is described above. Returns 0 on success and -1 on error.
758 If an error occurs, something may have already been written to
759 *output*. In this case, the output is undefined and most likely not
762 .. function:: int json_dump_file(const json_t *json, const char *path, size_t flags)
764 Write the JSON representation of *root* to the file *path*. If
765 *path* already exists, it is overwritten. *flags* is described
766 above. Returns 0 on success and -1 on error.
774 This sections describes the functions that can be used to decode JSON
775 text to the Jansson representation of JSON data. The JSON
776 specification requires that a JSON text is either a serialized array
777 or object, and this requirement is also enforced with the following
778 functions. In other words, the top level value in the JSON text being
779 decoded must be either array or object.
781 See :ref:`rfc-conformance` for a discussion on Jansson's conformance
782 to the JSON specification. It explains many design decisions that
783 affect especially the behavior of the decoder.
785 Each function takes a *flags* parameter that can be used to control
786 the behavior of the decoder. Its default value is 0. The following
787 macros can be ORed together to obtain *flags*.
789 ``JSON_REJECT_DUPLICATES``
790 Issue a decoding error if any JSON object in the input text
791 contains duplicate keys. Without this flag, the value of the last
792 occurence of each key ends up in the result. Key equivalence is
793 checked byte-by-byte, without special Unicode comparison
796 .. versionadded:: 2.1
798 ``JSON_DISABLE_EOF_CHECK``
799 By default, the decoder expects that its whole input constitutes a
800 valid JSON text, and issues an error if there's extra data after
801 the otherwise valid JSON input. With this flag enabled, the decoder
802 stops after decoding a valid JSON array or object, and thus allows
803 extra data after the JSON text.
805 .. versionadded:: 2.1
807 The following functions perform the actual JSON decoding.
809 .. function:: json_t *json_loads(const char *input, size_t flags, json_error_t *error)
813 Decodes the JSON string *input* and returns the array or object it
814 contains, or *NULL* on error, in which case *error* is filled with
815 information about the error. *flags* is described above.
817 .. function:: json_t *json_loadb(const char *buffer, size_t buflen, size_t flags, json_error_t *error)
821 Decodes the JSON string *buffer*, whose length is *buflen*, and
822 returns the array or object it contains, or *NULL* on error, in
823 which case *error* is filled with information about the error. This
824 is similar to :func:`json_loads()` except that the string doesn't
825 need to be null-terminated. *flags* is described above.
827 .. versionadded:: 2.1
829 .. function:: json_t *json_loadf(FILE *input, size_t flags, json_error_t *error)
833 Decodes the JSON text in stream *input* and returns the array or
834 object it contains, or *NULL* on error, in which case *error* is
835 filled with information about the error. *flags* is described
838 .. function:: json_t *json_load_file(const char *path, size_t flags, json_error_t *error)
842 Decodes the JSON text in file *path* and returns the array or
843 object it contains, or *NULL* on error, in which case *error* is
844 filled with information about the error. *flags* is described
853 This section describes functions that help to create, or *pack*,
854 complex JSON values, especially nested objects and arrays. Value
855 building is based on a *format string* that is used to tell the
856 functions about the expected arguments.
858 For example, the format string ``"i"`` specifies a single integer
859 value, while the format string ``"[ssb]"`` or the equivalent ``"[s, s,
860 b]"`` specifies an array value with two strings and a boolean as its
863 /* Create the JSON integer 42 */
866 /* Create the JSON array ["foo", "bar", true] */
867 json_pack("[ssb]", "foo", "bar", 1);
869 Here's the full list of format characters. The type in parentheses
870 denotes the resulting JSON type, and the type in brackets (if any)
871 denotes the C type that is expected as the corresponding argument.
873 ``s`` (string) [const char \*]
874 Convert a NULL terminated UTF-8 string to a JSON string.
877 Output a JSON null value. No argument is consumed.
879 ``b`` (boolean) [int]
880 Convert a C :type:`int` to JSON boolean value. Zero is converted
881 to ``false`` and non-zero to ``true``.
883 ``i`` (integer) [int]
884 Convert a C :type:`int` to JSON integer.
886 ``I`` (integer) [json_int_t]
887 Convert a C :type:`json_int_t` to JSON integer.
889 ``f`` (real) [double]
890 Convert a C :type:`double` to JSON real.
892 ``o`` (any value) [json_t \*]
893 Output any given JSON value as-is. If the value is added to an
894 array or object, the reference to the value passed to ``o`` is
895 stolen by the container.
897 ``O`` (any value) [json_t \*]
898 Like ``o``, but the argument's reference count is incremented.
899 This is useful if you pack into an array or object and want to
900 keep the reference for the JSON value consumed by ``O`` to
904 Build an array with contents from the inner format string. ``fmt``
905 may contain objects and arrays, i.e. recursive value building is
909 Build an object with contents from the inner format string
910 ``fmt``. The first, third, etc. format character represent a key,
911 and must be ``s`` (as object keys are always strings). The second,
912 fourth, etc. format character represent a value. Any value may be
913 an object or array, i.e. recursive value building is supported.
915 The following functions compose the value building API:
917 .. function:: json_t *json_pack(const char *fmt, ...)
921 Build a new JSON value according to the format string *fmt*. For
922 each format character (except for ``{}[]n``), one argument is
923 consumed and used to build the corresponding value. Returns *NULL*
926 .. function:: json_t *json_pack_ex(json_error_t *error, size_t flags, const char *fmt, ...)
927 json_t *json_vpack_ex(json_error_t *error, size_t flags, const char *fmt, va_list ap)
931 Like :func:`json_pack()`, but an in the case of an error, an error
932 message is written to *error*, if it's not *NULL*. The *flags*
933 parameter is currently unused and should be set to 0.
935 As only the errors in format string (and out-of-memory errors) can
936 be caught by the packer, these two functions are most likely only
937 useful for debugging format strings.
941 /* Build an empty JSON object */
944 /* Build the JSON object {"foo": 42, "bar": 7} */
945 json_pack("{sisi}", "foo", 42, "bar", 7);
947 /* Like above, ':', ',' and whitespace are ignored */
948 json_pack("{s:i, s:i}", "foo", 42, "bar", 7);
950 /* Build the JSON array [[1, 2], {"cool": true}] */
951 json_pack("[[i,i],{s:b]]", 1, 2, "cool", 1);
956 Parsing and Validating Values
957 =============================
959 This sectinon describes functions that help to validate complex values
960 and extract, or *unpack*, data from them. Like :ref:`building values
961 <apiref-pack>`, this is also based on format strings.
963 While a JSON value is unpacked, the type specified in the format
964 string is checked to match that of the JSON value. This is the
965 validation part of the process. In addition to this, the unpacking
966 functions can also check that all items of arrays and objects are
967 unpacked. This check be enabled with the format character ``!`` or by
968 using the flag ``JSON_STRICT``. See below for details.
970 Here's the full list of format characters. The type in parentheses
971 denotes the JSON type, and the type in brackets (if any) denotes the C
972 type whose address should be passed.
974 ``s`` (string) [const char \*]
975 Convert a JSON string to a pointer to a NULL terminated UTF-8
979 Expect a JSON null value. Nothing is extracted.
981 ``b`` (boolean) [int]
982 Convert a JSON boolean value to a C :type:`int`, so that ``true``
983 is converted to 1 and ``false`` to 0.
985 ``i`` (integer) [int]
986 Convert a JSON integer to C :type:`int`.
988 ``I`` (integer) [json_int_t]
989 Convert a JSON integer to C :type:`json_int_t`.
991 ``f`` (real) [double]
992 Convert a JSON real to C :type:`double`.
994 ``F`` (integer or real) [double]
995 Convert a JSON number (integer or real) to C :type:`double`.
997 ``o`` (any value) [json_t \*]
998 Store a JSON value with no conversion to a :type:`json_t` pointer.
1000 ``O`` (any value) [json_t \*]
1001 Like ``O``, but the JSON value's reference count is incremented.
1004 Convert each item in the JSON array according to the inner format
1005 string. ``fmt`` may contain objects and arrays, i.e. recursive
1006 value extraction is supporetd.
1009 Convert each item in the JSON object according to the inner format
1010 string ``fmt``. The first, third, etc. format character represent
1011 a key, and must be ``s``. The corresponding argument to unpack
1012 functions is read as the object key. The second fourth, etc.
1013 format character represent a value and is written to the address
1014 given as the corresponding argument. **Note** that every other
1015 argument is read from and every other is written to.
1017 ``fmt`` may contain objects and arrays as values, i.e. recursive
1018 value extraction is supporetd.
1021 This special format character is used to enable the check that
1022 all object and array items are accessed, on a per-value basis. It
1023 must appear inside an array or object as the last format character
1024 before the closing bracket or brace. To enable the check globally,
1025 use the ``JSON_STRICT`` unpacking flag.
1028 This special format character is the opposite of ``!``. If the
1029 ``JSON_STRICT`` flag is used, ``*`` can be used to disable the
1030 strict check on a per-value basis. It must appear inside an array
1031 or object as the last format character before the closing bracket
1034 The following functions compose the parsing and validation API:
1036 .. function:: int json_unpack(json_t *root, const char *fmt, ...)
1038 Validate and unpack the JSON value *root* according to the format
1039 string *fmt*. Returns 0 on success and -1 on failure.
1041 .. function:: int json_unpack_ex(json_t *root, json_error_t *error, size_t flags, const char *fmt, ...)
1042 int json_vunpack_ex(json_t *root, json_error_t *error, size_t flags, const char *fmt, va_list ap)
1044 Validate and unpack the JSON value *root* according to the format
1045 string *fmt*. If an error occurs and *error* is not *NULL*, write
1046 error information to *error*. *flags* can be used to control the
1047 behaviour of the unpacker, see below for the flags. Returns 0 on
1048 success and -1 on failure.
1050 The following unpacking flags are available:
1053 Enable the extra validation step checking that all object and
1054 array items are unpacked. This is equivalent to appending the
1055 format character ``!`` to the end of every array and object in the
1058 ``JSON_VALIDATE_ONLY``
1059 Don't extract any data, just validate the JSON value against the
1060 given format string. Note that object keys must still be specified
1061 after the format string.
1065 /* root is the JSON integer 42 */
1067 json_unpack(root, "i", &myint);
1068 assert(myint == 42);
1070 /* root is the JSON object {"foo": "bar", "quux": true} */
1073 json_unpack(root, "{s:s, s:b}", "foo", &str, "quux", &boolean);
1074 assert(strcmp(str, "bar") == 0 && boolean == 1);
1076 /* root is the JSON array [[1, 2], {"baz": null} */
1078 json_unpack_ex(root, &error, JSON_VALIDATE_ONLY, "[[i,i], {s:n}]", "baz");
1079 /* returns 0 for validation success, nothing is extracted */
1081 /* root is the JSON array [1, 2, 3, 4, 5] */
1083 json_unpack(root, "[ii!]", &myint1, &myint2);
1084 /* returns -1 for failed validation */
1090 Testing for equality of two JSON values cannot, in general, be
1091 achieved using the ``==`` operator. Equality in the terms of the
1092 ``==`` operator states that the two :type:`json_t` pointers point to
1093 exactly the same JSON value. However, two JSON values can be equal not
1094 only if they are exactly the same value, but also if they have equal
1097 * Two integer or real values are equal if their contained numeric
1098 values are equal. An integer value is never equal to a real value,
1101 * Two strings are equal if their contained UTF-8 strings are equal,
1102 byte by byte. Unicode comparison algorithms are not implemented.
1104 * Two arrays are equal if they have the same number of elements and
1105 each element in the first array is equal to the corresponding
1106 element in the second array.
1108 * Two objects are equal if they have exactly the same keys and the
1109 value for each key in the first object is equal to the value of the
1110 corresponding key in the second object.
1112 * Two true, false or null values have no "contents", so they are equal
1113 if their types are equal. (Because these values are singletons,
1114 their equality can actually be tested with ``==``.)
1116 The following function can be used to test whether two JSON values are
1119 .. function:: int json_equal(json_t *value1, json_t *value2)
1121 Returns 1 if *value1* and *value2* are equal, as defined above.
1122 Returns 0 if they are inequal or one or both of the pointers are
1129 Because of reference counting, passing JSON values around doesn't
1130 require copying them. But sometimes a fresh copy of a JSON value is
1131 needed. For example, if you need to modify an array, but still want to
1132 use the original afterwards, you should take a copy of it first.
1134 Jansson supports two kinds of copying: shallow and deep. There is a
1135 difference between these methods only for arrays and objects. Shallow
1136 copying only copies the first level value (array or object) and uses
1137 the same child values in the copied value. Deep copying makes a fresh
1138 copy of the child values, too. Moreover, all the child values are deep
1139 copied in a recursive fashion.
1141 .. function:: json_t *json_copy(json_t *value)
1143 .. refcounting:: new
1145 Returns a shallow copy of *value*, or *NULL* on error.
1147 .. function:: json_t *json_deep_copy(json_t *value)
1149 .. refcounting:: new
1151 Returns a deep copy of *value*, or *NULL* on error.
1154 Custom Memory Allocation
1155 ========================
1157 By default, Jansson uses :func:`malloc()` and :func:`free()` for
1158 memory allocation. These functions can be overridden if custom
1161 .. type:: json_malloc_t
1163 A typedef for a function pointer with :func:`malloc()`'s
1166 typedef void *(*json_malloc_t)(size_t);
1168 .. type:: json_free_t
1170 A typedef for a function pointer with :func:`free()`'s
1173 typedef void (*json_free_t)(void *);
1175 .. function:: void json_set_alloc_funcs(json_malloc_t malloc_fn, json_free_t free_fn)
1177 Use *malloc_fn* instead of :func:`malloc()` and *free_fn* instead
1178 of :func:`free()`. This function has to be called before any other
1179 Jansson's API functions to ensure that all memory operations use
1184 Use the `Boehm's conservative garbage collector`_ for memory
1187 json_set_alloc_funcs(GC_malloc, GC_free);
1189 .. _Boehm's conservative garbage collector: http://www.hpl.hp.com/personal/Hans_Boehm/gc/
1191 Allow storing sensitive data (e.g. passwords or encryption keys) in
1192 JSON structures by zeroing all memory when freed::
1194 static void *secure_malloc(size_t size)
1196 /* Store the memory area size in the beginning of the block */
1197 void *ptr = malloc(size + 8);
1198 *((size_t *)ptr) = size;
1202 static void secure_free(void *ptr)
1207 size = *((size_t *)ptr);
1209 guaranteed_memset(ptr, 0, size);
1215 json_set_alloc_funcs(secure_malloc, secure_free);
1219 For more information about the issues of storing sensitive data in
1221 http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/protect-secrets.html.
1222 The page also examplains the :func:`guaranteed_memset()` function used
1223 in the example and gives a sample implementation for it.