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.
486 .. function:: int json_array_clear(json_t *array)
488 Removes all elements from *array*. Returns 0 on sucess and -1 on
491 .. function:: int json_array_extend(json_t *array, json_t *other_array)
493 Appends all elements in *other_array* to the end of *array*.
494 Returns 0 on success and -1 on error.
500 A JSON object is a dictionary of key-value pairs, where the key is a
501 Unicode string and the value is any JSON value.
503 .. function:: json_t *json_object(void)
507 Returns a new JSON object, or *NULL* on error. Initially, the
510 .. function:: size_t json_object_size(const json_t *object)
512 Returns the number of elements in *object*, or 0 if *object* is not
515 .. function:: json_t *json_object_get(const json_t *object, const char *key)
517 .. refcounting:: borrow
519 Get a value corresponding to *key* from *object*. Returns *NULL* if
520 *key* is not found and on error.
522 .. function:: int json_object_set(json_t *object, const char *key, json_t *value)
524 Set the value of *key* to *value* in *object*. *key* must be a
525 valid null terminated UTF-8 encoded Unicode string. If there
526 already is a value for *key*, it is replaced by the new value.
527 Returns 0 on success and -1 on error.
529 .. function:: int json_object_set_nocheck(json_t *object, const char *key, json_t *value)
531 Like :func:`json_object_set`, but doesn't check that *key* is
532 valid UTF-8. Use this function only if you are certain that this
533 really is the case (e.g. you have already checked it by other
536 .. function:: int json_object_set_new(json_t *object, const char *key, json_t *value)
538 Like :func:`json_object_set()` but steals the reference to
539 *value*. This is useful when *value* is newly created and not used
542 .. function:: int json_object_set_new_nocheck(json_t *object, const char *key, json_t *value)
544 Like :func:`json_object_set_new`, but doesn't check that *key* is
545 valid UTF-8. Use this function only if you are certain that this
546 really is the case (e.g. you have already checked it by other
549 .. function:: int json_object_del(json_t *object, const char *key)
551 Delete *key* from *object* if it exists. Returns 0 on success, or
552 -1 if *key* was not found.
555 .. function:: int json_object_clear(json_t *object)
557 Remove all elements from *object*. Returns 0 on success and -1 if
558 *object* is not a JSON object.
560 .. function:: int json_object_update(json_t *object, json_t *other)
562 Update *object* with the key-value pairs from *other*, overwriting
563 existing keys. Returns 0 on success or -1 on error.
566 The following functions implement an iteration protocol for objects,
567 allowing to iterate through all key-value pairs in an object. The
568 items are not returned in any particular order, as this would require
569 sorting due to the internal object representation.
571 .. function:: void *json_object_iter(json_t *object)
573 Returns an opaque iterator which can be used to iterate over all
574 key-value pairs in *object*, or *NULL* if *object* is empty.
576 .. function:: void *json_object_iter_at(json_t *object, const char *key)
578 Like :func:`json_object_iter()`, but returns an iterator to the
579 key-value pair in *object* whose key is equal to *key*, or NULL if
580 *key* is not found in *object*. Iterating forward to the end of
581 *object* only yields all key-value pairs of the object if *key*
582 happens to be the first key in the underlying hash table.
584 .. function:: void *json_object_iter_next(json_t *object, void *iter)
586 Returns an iterator pointing to the next key-value pair in *object*
587 after *iter*, or *NULL* if the whole object has been iterated
590 .. function:: const char *json_object_iter_key(void *iter)
592 Extract the associated key from *iter*.
594 .. function:: json_t *json_object_iter_value(void *iter)
596 .. refcounting:: borrow
598 Extract the associated value from *iter*.
600 .. function:: int json_object_iter_set(json_t *object, void *iter, json_t *value)
602 Set the value of the key-value pair in *object*, that is pointed to
603 by *iter*, to *value*.
605 .. function:: int json_object_iter_set_new(json_t *object, void *iter, json_t *value)
607 Like :func:`json_object_iter_set()`, but steals the reference to
608 *value*. This is useful when *value* is newly created and not used
611 The iteration protocol can be used for example as follows::
613 /* obj is a JSON object */
616 void *iter = json_object_iter(obj);
619 key = json_object_iter_key(iter);
620 value = json_object_iter_value(iter);
621 /* use key and value ... */
622 iter = json_object_iter_next(obj, iter);
629 Jansson uses a single struct type to pass error information to the
630 user. See sections :ref:`apiref-decoding`, :ref:`apiref-pack` and
631 :ref:`apiref-unpack` for functions that pass error information using
634 .. type:: json_error_t
636 .. member:: char text[]
638 The error message (in UTF-8), or an empty string if a message is
641 .. member:: char source[]
643 Source of the error. This can be (a part of) the file name or a
644 special identifier in angle brackers (e.g. ``<string>``).
648 The line number on which the error occurred.
650 .. member:: int column
652 The column on which the error occurred. Note that this is the
653 *character column*, not the byte column, i.e. a multibyte UTF-8
654 character counts as one column.
656 .. member:: size_t position
658 The position in bytes from the start of the input. This is
659 useful for debugging Unicode encoding problems.
661 The normal use of :type:`json_error_t` is to allocate it on the stack,
662 and pass a pointer to a function. Example::
668 json = json_load_file("/path/to/file.json", 0, &error);
670 /* the error variable contains error information */
675 Also note that if the call succeeded (``json != NULL`` in the above
676 example), the contents of ``error`` are unspecified.
678 All functions also accept *NULL* as the :type:`json_error_t` pointer,
679 in which case no error information is returned to the caller.
685 This sections describes the functions that can be used to encode
686 values to JSON. By default, only objects and arrays can be encoded
687 directly, since they are the only valid *root* values of a JSON text.
688 To encode any JSON value, use the ``JSON_ENCODE_ANY`` flag (see
691 By default, the output has no newlines, and spaces are used between
692 array and object elements for a readable output. This behavior can be
693 altered by using the ``JSON_INDENT`` and ``JSON_COMPACT`` flags
694 described below. A newline is never appended to the end of the encoded
697 Each function takes a *flags* parameter that controls some aspects of
698 how the data is encoded. Its default value is 0. The following macros
699 can be ORed together to obtain *flags*.
702 Pretty-print the result, using newlines between array and object
703 items, and indenting with *n* spaces. The valid range for *n* is
704 between 0 and 32, other values result in an undefined output. If
705 ``JSON_INDENT`` is not used or *n* is 0, no newlines are inserted
706 between array and object items.
709 This flag enables a compact representation, i.e. sets the separator
710 between array and object items to ``","`` and between object keys
711 and values to ``":"``. Without this flag, the corresponding
712 separators are ``", "`` and ``": "`` for more readable output.
714 ``JSON_ENSURE_ASCII``
715 If this flag is used, the output is guaranteed to consist only of
716 ASCII characters. This is achived by escaping all Unicode
717 characters outside the ASCII range.
720 If this flag is used, all the objects in output are sorted by key.
721 This is useful e.g. if two JSON texts are diffed or visually
724 ``JSON_PRESERVE_ORDER``
725 If this flag is used, object keys in the output are sorted into the
726 same order in which they were first inserted to the object. For
727 example, decoding a JSON text and then encoding with this flag
728 preserves the order of object keys.
731 Specifying this flag makes it possible to encode any JSON value on
732 its own. Without it, only objects and arrays can be passed as the
733 *root* value to the encoding functions.
735 **Note:** Encoding any value may be useful in some scenarios, but
736 it's generally discouraged as it violates strict compatiblity with
737 :rfc:`4627`. If you use this flag, don't expect interoperatibility
738 with other JSON systems. Even Jansson itself doesn't have any means
739 to decode JSON texts whose root value is not object or array.
741 .. versionadded:: 2.1
743 The following functions perform the actual JSON encoding. The result
746 .. function:: char *json_dumps(const json_t *root, size_t flags)
748 Returns the JSON representation of *root* as a string, or *NULL* on
749 error. *flags* is described above. The return value must be freed
750 by the caller using :func:`free()`.
752 .. function:: int json_dumpf(const json_t *root, FILE *output, size_t flags)
754 Write the JSON representation of *root* to the stream *output*.
755 *flags* is described above. Returns 0 on success and -1 on error.
756 If an error occurs, something may have already been written to
757 *output*. In this case, the output is undefined and most likely not
760 .. function:: int json_dump_file(const json_t *json, const char *path, size_t flags)
762 Write the JSON representation of *root* to the file *path*. If
763 *path* already exists, it is overwritten. *flags* is described
764 above. Returns 0 on success and -1 on error.
772 This sections describes the functions that can be used to decode JSON
773 text to the Jansson representation of JSON data. The JSON
774 specification requires that a JSON text is either a serialized array
775 or object, and this requirement is also enforced with the following
776 functions. In other words, the top level value in the JSON text being
777 decoded must be either array or object.
779 See :ref:`rfc-conformance` for a discussion on Jansson's conformance
780 to the JSON specification. It explains many design decisions that
781 affect especially the behavior of the decoder.
783 Each function takes a *flags* parameter that can be used to control
784 the behavior of the decoder. Its default value is 0. The following
785 macros can be ORed together to obtain *flags*.
787 ``JSON_REJECT_DUPLICATES``
788 Issue a decoding error if any JSON object in the input text
789 contains duplicate keys. Without this flag, the value of the last
790 occurence of each key ends up in the result. Key equivalence is
791 checked byte-by-byte, without special Unicode comparison
794 .. versionadded:: 2.1
796 The following functions perform the actual JSON decoding.
798 .. function:: json_t *json_loads(const char *input, size_t flags, json_error_t *error)
802 Decodes the JSON string *input* and returns the array or object it
803 contains, or *NULL* on error, in which case *error* is filled with
804 information about the error. *flags* is described above.
806 .. function:: json_t *json_loadb(const char *buffer, size_t buflen, size_t flags, json_error_t *error)
810 Decodes the JSON string *buffer*, whose length is *buflen*, and
811 returns the array or object it contains, or *NULL* on error, in
812 which case *error* is filled with information about the error. This
813 is similar to :func:`json_loads()` except that the string doesn't
814 need to be null-terminated. *flags* is described above.
816 .. versionadded:: 2.1
818 .. function:: json_t *json_loadf(FILE *input, size_t flags, json_error_t *error)
822 Decodes the JSON text in stream *input* and returns the array or
823 object it contains, or *NULL* on error, in which case *error* is
824 filled with information about the error. *flags* is described
827 .. function:: json_t *json_load_file(const char *path, size_t flags, json_error_t *error)
831 Decodes the JSON text in file *path* and returns the array or
832 object it contains, or *NULL* on error, in which case *error* is
833 filled with information about the error. *flags* is described
842 This sectinon describes functions that help to create, or *pack*,
843 complex JSON values, especially nested objects and arrays. Value
844 building is based on a *format string* that is used to tell the
845 functions about the expected arguments.
847 For example, the format string ``"i"`` specifies a single integer
848 value, while the format string ``"[ssb]"`` or the equivalent ``"[s, s,
849 b]"`` specifies an array value with two integers and a boolean as its
852 /* Create the JSON integer 42 */
855 /* Create the JSON array ["foo", "bar", true] */
856 json_pack("[ssb]", "foo", "bar", 1);
858 Here's the full list of format characters. The type in parentheses
859 denotes the resulting JSON type, and the type in brackets (if any)
860 denotes the C type that is expected as the corresponding argument.
862 ``s`` (string) [const char \*]
863 Convert a NULL terminated UTF-8 string to a JSON string.
866 Output a JSON null value. No argument is consumed.
868 ``b`` (boolean) [int]
869 Convert a C :type:`int` to JSON boolean value. Zero is converted
870 to ``false`` and non-zero to ``true``.
872 ``i`` (integer) [int]
873 Convert a C :type:`int` to JSON integer.
875 ``I`` (integer) [json_int_t]
876 Convert a C :type:`json_int_t` to JSON integer.
878 ``f`` (real) [double]
879 Convert a C :type:`double` to JSON real.
881 ``o`` (any value) [json_t \*]
882 Output any given JSON value as-is. If the value is added to an
883 array or object, the reference to the value passed to ``o`` is
884 stealed by the container.
886 ``O`` (any value) [json_t \*]
887 Like ``o``, but the argument's reference count is incremented.
888 This is useful if you pack into an array or object and want to
889 keep the reference for the JSON value consumed by ``O`` to
893 Build an array with contents from the inner format string. ``fmt``
894 may contain objects and arrays, i.e. recursive value building is
898 Build an object with contents from the inner format string
899 ``fmt``. The first, third, etc. format character represent a key,
900 and must be ``s`` (as object keys are always strings). The second,
901 fourth, etc. format character represent a value. Any value may be
902 an object or array, i.e. recursive value building is supported.
904 The following functions compose the value building API:
906 .. function:: json_t *json_pack(const char *fmt, ...)
910 Build a new JSON value according to the format string *fmt*. For
911 each format character (except for ``{}[]n``), one argument is
912 consumed and used to build the corresponding value. Returns *NULL*
915 .. function:: json_t *json_pack_ex(json_error_t *error, size_t flags, const char *fmt, ...)
916 json_t *json_vpack_ex(json_error_t *error, size_t flags, const char *fmt, va_list ap)
920 Like :func:`json_pack()`, but an in the case of an error, an error
921 message is written to *error*, if it's not *NULL*. The *flags*
922 parameter is currently unused and should be set to 0.
924 As only the errors in format string (and out-of-memory errors) can
925 be caught by the packer, these two functions are most likely only
926 useful for debugging format strings.
930 /* Build an empty JSON object */
933 /* Build the JSON object {"foo": 42, "bar": 7} */
934 json_pack("{sisb}", "foo", 42, "bar", 7);
936 /* Like above, ':', ',' and whitespace are ignored */
937 json_pack("{s:i, s:b}", "foo", 42, "bar", 7);
939 /* Build the JSON array [[1, 2], {"cool": true}] */
940 json_pack("[[i,i],{s:b]]", 1, 2, "cool", 1);
945 Parsing and Validating Values
946 =============================
948 This sectinon describes functions that help to validate complex values
949 and extract, or *unpack*, data from them. Like :ref:`building values
950 <apiref-pack>`, this is also based on format strings.
952 While a JSON value is unpacked, the type specified in the format
953 string is checked to match that of the JSON value. This is the
954 validation part of the process. In addition to this, the unpacking
955 functions can also check that all items of arrays and objects are
956 unpacked. This check be enabled with the format character ``!`` or by
957 using the flag ``JSON_STRICT``. See below for details.
959 Here's the full list of format characters. The type in parentheses
960 denotes the JSON type, and the type in brackets (if any) denotes the C
961 type whose address should be passed.
963 ``s`` (string) [const char \*]
964 Convert a JSON string to a pointer to a NULL terminated UTF-8
968 Expect a JSON null value. Nothing is extracted.
970 ``b`` (boolean) [int]
971 Convert a JSON boolean value to a C :type:`int`, so that ``true``
972 is converted to 1 and ``false`` to 0.
974 ``i`` (integer) [int]
975 Convert a JSON integer to C :type:`int`.
977 ``I`` (integer) [json_int_t]
978 Convert a JSON integer to C :type:`json_int_t`.
980 ``f`` (real) [double]
981 Convert a JSON real to C :type:`double`.
983 ``F`` (integer or real) [double]
984 Convert a JSON number (integer or real) to C :type:`double`.
986 ``o`` (any value) [json_t \*]
987 Store a JSON value with no conversion to a :type:`json_t` pointer.
989 ``O`` (any value) [json_t \*]
990 Like ``O``, but the JSON value's reference count is incremented.
993 Convert each item in the JSON array according to the inner format
994 string. ``fmt`` may contain objects and arrays, i.e. recursive
995 value extraction is supporetd.
998 Convert each item in the JSON object according to the inner format
999 string ``fmt``. The first, third, etc. format character represent
1000 a key, and must be ``s``. The corresponding argument to unpack
1001 functions is read as the object key. The second fourth, etc.
1002 format character represent a value and is written to the address
1003 given as the corresponding argument. **Note** that every other
1004 argument is read from and every other is written to.
1006 ``fmt`` may contain objects and arrays as values, i.e. recursive
1007 value extraction is supporetd.
1010 This special format character is used to enable the check that
1011 all object and array items are accessed, on a per-value basis. It
1012 must appear inside an array or object as the last format character
1013 before the closing bracket or brace. To enable the check globally,
1014 use the ``JSON_STRICT`` unpacking flag.
1017 This special format character is the opposite of ``!``. If the
1018 ``JSON_STRICT`` flag is used, ``*`` can be used to disable the
1019 strict check on a per-value basis. It must appear inside an array
1020 or object as the last format character before the closing bracket
1023 The following functions compose the parsing and validation API:
1025 .. function:: int json_unpack(json_t *root, const char *fmt, ...)
1027 Validate and unpack the JSON value *root* according to the format
1028 string *fmt*. Returns 0 on success and -1 on failure.
1030 .. function:: int json_unpack_ex(json_t *root, json_error_t *error, size_t flags, const char *fmt, ...)
1031 int json_vunpack_ex(json_t *root, json_error_t *error, size_t flags, const char *fmt, va_list ap)
1033 Validate and unpack the JSON value *root* according to the format
1034 string *fmt*. If an error occurs and *error* is not *NULL*, write
1035 error information to *error*. *flags* can be used to control the
1036 behaviour of the unpacker, see below for the flags. Returns 0 on
1037 success and -1 on failure.
1039 The following unpacking flags are available:
1042 Enable the extra validation step checking that all object and
1043 array items are unpacked. This is equivalent to appending the
1044 format character ``!`` to the end of every array and object in the
1047 ``JSON_VALIDATE_ONLY``
1048 Don't extract any data, just validate the JSON value against the
1049 given format string. Note that object keys must still be specified
1050 after the format string.
1054 /* root is the JSON integer 42 */
1056 json_unpack(root, "i", &myint);
1057 assert(myint == 42);
1059 /* root is the JSON object {"foo": "bar", "quux": true} */
1062 json_unpack(root, "{s:s, s:b}", "foo", &str, "quux", &boolean);
1063 assert(strcmp(str, "bar") == 0 && boolean == 1);
1065 /* root is the JSON array [[1, 2], {"baz": null} */
1067 json_unpack_ex(root, &error, JSON_VALIDATE_ONLY, "[[i,i], {s:n}]", "baz");
1068 /* returns 0 for validation success, nothing is extracted */
1070 /* root is the JSON array [1, 2, 3, 4, 5] */
1072 json_unpack(root, "[ii!]", &myint1, &myint2);
1073 /* returns -1 for failed validation */
1079 Testing for equality of two JSON values cannot, in general, be
1080 achieved using the ``==`` operator. Equality in the terms of the
1081 ``==`` operator states that the two :type:`json_t` pointers point to
1082 exactly the same JSON value. However, two JSON values can be equal not
1083 only if they are exactly the same value, but also if they have equal
1086 * Two integer or real values are equal if their contained numeric
1087 values are equal. An integer value is never equal to a real value,
1090 * Two strings are equal if their contained UTF-8 strings are equal,
1091 byte by byte. Unicode comparison algorithms are not implemented.
1093 * Two arrays are equal if they have the same number of elements and
1094 each element in the first array is equal to the corresponding
1095 element in the second array.
1097 * Two objects are equal if they have exactly the same keys and the
1098 value for each key in the first object is equal to the value of the
1099 corresponding key in the second object.
1101 * Two true, false or null values have no "contents", so they are equal
1102 if their types are equal. (Because these values are singletons,
1103 their equality can actually be tested with ``==``.)
1105 The following function can be used to test whether two JSON values are
1108 .. function:: int json_equal(json_t *value1, json_t *value2)
1110 Returns 1 if *value1* and *value2* are equal, as defined above.
1111 Returns 0 if they are inequal or one or both of the pointers are
1118 Because of reference counting, passing JSON values around doesn't
1119 require copying them. But sometimes a fresh copy of a JSON value is
1120 needed. For example, if you need to modify an array, but still want to
1121 use the original afterwards, you should take a copy of it first.
1123 Jansson supports two kinds of copying: shallow and deep. There is a
1124 difference between these methods only for arrays and objects. Shallow
1125 copying only copies the first level value (array or object) and uses
1126 the same child values in the copied value. Deep copying makes a fresh
1127 copy of the child values, too. Moreover, all the child values are deep
1128 copied in a recursive fashion.
1130 .. function:: json_t *json_copy(json_t *value)
1132 .. refcounting:: new
1134 Returns a shallow copy of *value*, or *NULL* on error.
1136 .. function:: json_t *json_deep_copy(json_t *value)
1138 .. refcounting:: new
1140 Returns a deep copy of *value*, or *NULL* on error.
1143 Custom Memory Allocation
1144 ========================
1146 By default, Jansson uses :func:`malloc()` and :func:`free()` for
1147 memory allocation. These functions can be overridden if custom
1150 .. type:: json_malloc_t
1152 A typedef for a function pointer with :func:`malloc()`'s
1155 typedef void *(*json_malloc_t)(size_t);
1157 .. type:: json_free_t
1159 A typedef for a function pointer with :func:`free()`'s
1162 typedef void (*json_free_t)(void *);
1164 .. function:: void json_set_alloc_funcs(json_malloc_t malloc_fn, json_free_t free_fn)
1166 Use *malloc_fn* instead of :func:`malloc()` and *free_fn* instead
1167 of :func:`free()`. This function has to be called before any other
1168 Jansson's API functions to ensure that all memory operations use
1173 Use the `Boehm's conservative garbage collector`_ for memory
1176 json_set_alloc_funcs(GC_malloc, GC_free);
1178 .. _Boehm's conservative garbage collector: http://www.hpl.hp.com/personal/Hans_Boehm/gc/
1180 Allow storing sensitive data (e.g. passwords or encryption keys) in
1181 JSON structures by zeroing all memory when freed::
1183 static void *secure_malloc(size_t size)
1185 /* Store the memory area size in the beginning of the block */
1186 void *ptr = malloc(size + 8);
1187 *((size_t *)ptr) = size;
1191 static void secure_free(void *ptr)
1196 size = *((size_t *)ptr);
1198 guaranteed_memset(ptr, 0, size);
1204 json_set_alloc_funcs(secure_malloc, secure_free);
1208 For more information about the issues of storing sensitive data in
1210 http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/protect-secrets.html.
1211 The page also examplains the :func:`guaranteed_memset()` function used
1212 in the example and gives a sample implementation for it.