Enhance handling of circular references

[jansson.git] / doc / apiref.rst

diff --git a/doc/apiref.rst b/doc/apiref.rst
index 57b5e31..818ec51 100644 (file)
--- a/doc/apiref.rst
+++ b/doc/apiref.rst
@@ -41,6 +41,12 @@ Objects of :ctype:`json_t` are always used through a pointer. There
 are APIs for querying the type, manipulating the reference count, and
 for constructing and manipulating values of different types.
 
 are APIs for querying the type, manipulating the reference count, and
 for constructing and manipulating values of different types.
 

+Unless noted otherwise, all API functions return an error value if an
+error occurs. Depending on the function's signature, the error value
+is either *NULL* or -1. Invalid arguments or invalid input are
+apparent sources for errors. Memory allocation and I/O operations may
+also cause errors.
+

 
 Type
 ----
 
 Type
 ----


@@ -80,8 +86,8 @@ functions:
 .. cfunction:: int json_typeof(const json_t *json)
 
    Return the type of the JSON value (a :ctype:`json_type` cast to
 .. cfunction:: int json_typeof(const json_t *json)
 
    Return the type of the JSON value (a :ctype:`json_type` cast to

-   :ctype:`int`). This function is actually implemented as a macro for
-   speed.
+   :ctype:`int`). *json* MUST NOT be *NULL*. This function is actually
+   implemented as a macro for speed.

 
 .. cfunction:: json_is_object(const json_t *json)
                json_is_array(const json_t *json)
 
 .. cfunction:: json_is_object(const json_t *json)
                json_is_array(const json_t *json)


@@ -93,17 +99,18 @@ functions:
                json_is_null(const json_t *json)
 
    These functions (actually macros) return true (non-zero) for values
                json_is_null(const json_t *json)
 
    These functions (actually macros) return true (non-zero) for values

-   of the given type, and false (zero) for values of other types.
+   of the given type, and false (zero) for values of other types and
+   for *NULL*.

 
 .. cfunction:: json_is_number(const json_t *json)
 
    Returns true for values of types :const:`JSON_INTEGER` and
 
 .. cfunction:: json_is_number(const json_t *json)
 
    Returns true for values of types :const:`JSON_INTEGER` and

-   :const:`JSON_REAL`, and false for other types.
+   :const:`JSON_REAL`, and false for other types and for *NULL*.

 
 .. cfunction:: json_is_boolean(const json_t *json)
 
    Returns true for types :const:`JSON_TRUE` and :const:`JSON_FALSE`,
 
 .. cfunction:: json_is_boolean(const json_t *json)
 
    Returns true for types :const:`JSON_TRUE` and :const:`JSON_FALSE`,

-   and false for values of other types.
+   and false for values of other types and for *NULL*.

 
 
 Reference Count
 
 
 Reference Count


@@ -121,7 +128,8 @@ The following functions are used to manipulate the reference count.
 
 .. cfunction:: json_t *json_incref(json_t *json)
 
 
 .. cfunction:: json_t *json_incref(json_t *json)
 

-   Increment the reference count of *json*.
+   Increment the reference count of *json* if it's not non-*NULL*.
+   Returns *json*.

 
 .. cfunction:: void json_decref(json_t *json)
 
 
 .. cfunction:: void json_decref(json_t *json)
 


@@ -151,29 +159,60 @@ will return a new or borrowed reference or steal a reference to its
 argument.
 
 
 argument.
 
 

+Circular References
+-------------------
+
+A circular reference is created when an object or an array is,
+directly or indirectly, inserted inside itself. The direct case is
+simple::
+
+  json_t *obj = json_object();
+  json_object_set(obj, "foo", obj);
+
+Jansson will refuse to do this, and :cfunc:`json_object_set()` (and
+all the other such functions for objects and arrays) will return with
+an error status. The indirect case is the dangerous one::
+
+  json_t *arr1 = json_array(), *arr2 = json_array();
+  json_array_append(arr1, arr2);
+  json_array_append(arr2, arr1);
+
+In this example, the array ``arr2`` is contained in the array
+``arr1``, and vice versa. Jansson cannot check for this kind of
+indirect circular references without a performance hit, so it's up to
+the user to avoid them.
+
+If a circular reference is created, the memory consumed by the values
+cannot be freed by :cfunc:`json_decref()`. The reference counts never
+drops to zero because the values are keeping the circular reference to
+themselves. Moreover, trying to encode the values with any of the
+encoding functions will fail. The encoder detects circular references
+and returns an error status.
+
+

 True, False and Null
 ====================
 
 True, False and Null
 ====================
 

+These values are implemented as singletons, so each of these functions
+returns the same value each time.
+

 .. cfunction:: json_t *json_true(void)
 
    .. refcounting:: new
 
 .. cfunction:: json_t *json_true(void)
 
    .. refcounting:: new
 

-   Returns a value of the type :const:`JSON_TRUE`, or *NULL* on
-   error.
+   Returns the JSON true value.

 
 .. cfunction:: json_t *json_false(void)
 
    .. refcounting:: new
 
 
 .. cfunction:: json_t *json_false(void)
 
    .. refcounting:: new
 

-   Returns a value of the type :const:`JSON_FALSE`, or *NULL* on
-   error.
+   Returns the JSON false value.

 
 .. cfunction:: json_t *json_null(void)
 
    .. refcounting:: new
 
 
 .. cfunction:: json_t *json_null(void)
 
    .. refcounting:: new
 

-   Returns a value of the type :const:`JSON_NULL`, or *NULL* on
-   error.
+   Returns the JSON null value.

 
 
 String
 
 
 String


@@ -183,13 +222,21 @@ String
 
    .. refcounting:: new
 
 
    .. refcounting:: new
 

-   Returns a new value of the type :const:`JSON_STRING`, or *NULL* on
-   error. *value* must be a valid UTF-8 encoded Unicode string.
+   Returns a new JSON string, or *NULL* on error. *value* must be a
+   valid UTF-8 encoded Unicode string.
+
+.. cfunction:: const char *json_string_value(const json_t *string)

 
 

-.. cfunction:: const char *json_string_value(const json_t *json)
+   Returns the associated value of *string* as a null terminated UTF-8
+   encoded string, or *NULL* if *string* is not a JSON string.

 
 

-   Returns the associated value of a :const:`JSON_STRING` value as a
-   null terminated UTF-8 encoded string.
+.. cfunction:: int json_string_set(const json_t *string, const char *value)
+
+   Sets the associated value of *string* to *value*. *value* must be a
+   valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on
+   error.
+
+   .. versionadded:: 1.1

 
 
 Number
 
 
 Number


@@ -199,33 +246,46 @@ Number
 
    .. refcounting:: new
 
 
    .. refcounting:: new
 

-   Returns a new value of the type :const:`JSON_INTEGER`, or *NULL* on
-   error.
+   Returns a new JSON integer, or *NULL* on error.
+
+.. cfunction:: int json_integer_value(const json_t *integer)
+
+   Returns the associated value of *integer*, or 0 if *json* is not a
+   JSON integer.

 
 

-.. cfunction:: int json_integer_value(const json_t *json)
+.. cfunction:: int json_integer_set(const json_t *integer, int value)

 
 

-   Returns the associated integer value of values of the type
-   :const:`JSON_INTEGER`, or 0 for values of other types.
+   Sets the associated value of *integer* to *value*. Returns 0 on
+   success and -1 if *integer* is not a JSON integer.
+
+   .. versionadded:: 1.1

 
 .. cfunction:: json_t *json_real(double value)
 
    .. refcounting:: new
 
 
 .. cfunction:: json_t *json_real(double value)
 
    .. refcounting:: new
 

-   Returns a new value of the type :const:`JSON_REAL`, or *NULL* on
-   error.
+   Returns a new JSON real, or *NULL* on error.
+
+.. cfunction:: double json_real_value(const json_t *real)

 
 

-.. cfunction:: double json_real_value(const json_t *json)
+   Returns the associated value of *real*, or 0.0 if *real* is not a
+   JSON real.

 
 

-   Returns the associated real value of values of the type
-   :const:`JSON_INTEGER`, or 0 for values of other types.
+.. cfunction:: int json_real_set(const json_t *real, double value)
+
+   Sets the associated value of *real* to *value*. Returns 0 on
+   success and -1 if *real* is not a JSON real.
+
+   .. versionadded:: 1.1

 
 In addition to the functions above, there's a common query function
 for integers and reals:
 
 .. cfunction:: double json_number_value(const json_t *json)
 
 
 In addition to the functions above, there's a common query function
 for integers and reals:
 
 .. cfunction:: double json_number_value(const json_t *json)
 

-   Returns the value of either ``JSON_INTEGER`` or ``JSON_REAL``, cast
-   to double regardless of the actual type.
+   Returns the associated value of the JSON integer or JSON real
+   *json*, cast to double regardless of the actual type. If *json* is
+   neither JSON real nor JSON integer, 0.0 is returned.

 
 
 Array
 
 
 Array


@@ -237,27 +297,30 @@ A JSON array is an ordered collection of other JSON values.
 
    .. refcounting:: new
 
 
    .. refcounting:: new
 

-   Returns a new value of the type :const:`JSON_ARRAY`, or *NULL* on
-   error. Initially, the array is empty.
+   Returns a new JSON array, or *NULL* on error. Initially, the array
+   is empty.

 
 .. cfunction:: unsigned int json_array_size(const json_t *array)
 
 
 .. cfunction:: unsigned int json_array_size(const json_t *array)
 

-   Returns the number of elements in *array*.
+   Returns the number of elements in *array*, or 0 if *array* is NULL
+   or not a JSON array.

 
 .. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
 
    .. refcounting:: borrow
 
 
 .. cfunction:: json_t *json_array_get(const json_t *array, unsigned int index)
 
    .. refcounting:: borrow
 

-   Returns the element in *array* at position *index*, or *NULL* if
-   *index* is out of range. The valid range for *index* is from 0 to
-   the return value of :cfunc:`json_array_size()` minus 1.
+   Returns the element in *array* at position *index*. The valid range
+   for *index* is from 0 to the return value of
+   :cfunc:`json_array_size()` minus 1. If *array* is not a JSON array,
+   if *array* is *NULL*, or if *index* is out of range, *NULL* is
+   returned.

 
 .. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
 
    Replaces the element in *array* at position *index* with *value*.
 
 .. cfunction:: int json_array_set(json_t *array, unsigned int index, json_t *value)
 
    Replaces the element in *array* at position *index* with *value*.

-   Returns 0 on success, or -1 if *index* is out of range. The valid
-   range for *index* is from 0 to the return value of
-   :cfunc:`json_array_size()` minus 1.
+   The valid range for *index* is from 0 to the return value of
+   :cfunc:`json_array_size()` minus 1. Returns 0 on success and -1 on
+   error.

 
 .. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
 
 
 .. cfunction:: int json_array_set_new(json_t *array, unsigned int index, json_t *value)
 


@@ -280,6 +343,44 @@ A JSON array is an ordered collection of other JSON values.
 
    .. versionadded:: 1.1
 
 
    .. versionadded:: 1.1
 

+.. cfunction:: int json_array_insert(json_t *array, unsigned int index, json_t *value)
+
+   Inserts *value* to *array* at position *index*, shifting the
+   elements at *index* and after it one position towards the end of
+   the array. Returns 0 on success and -1 on error.
+
+   .. versionadded:: 1.1
+
+.. cfunction:: int json_array_insert_new(json_t *array, unsigned int index, json_t *value)
+
+   Like :cfunc:`json_array_insert()` but steals the reference to
+   *value*. This is useful when *value* is newly created and not used
+   after the call.
+
+   .. versionadded:: 1.1
+
+.. cfunction:: int json_array_remove(json_t *array, unsigned int index)
+
+   Removes the element in *array* at position *index*, shifting the
+   elements after *index* one position towards the start of the array.
+   Returns 0 on success and -1 on error.
+
+   .. versionadded:: 1.1
+
+.. cfunction:: int json_array_clear(json_t *array)
+
+   Removes all elements from *array*. Returns 0 on sucess and -1 on
+   error.
+
+   .. versionadded:: 1.1
+
+.. cfunction:: int json_array_extend(json_t *array, json_t *other_array)
+
+   Appends all elements in *other_array* to the end of *array*.
+   Returns 0 on success and -1 on error.
+
+   .. versionadded:: 1.1
+

 
 Object
 ======
 
 Object
 ======


@@ -291,8 +392,15 @@ Unicode string and the value is any JSON value.
 
    .. refcounting:: new
 
 
    .. refcounting:: new
 

-   Returns a new value of the type :const:`JSON_OBJECT`, or *NULL* on
-   error. Initially, the object is empty.
+   Returns a new JSON object, or *NULL* on error. Initially, the
+   object is empty.
+
+.. cfunction:: unsigned int json_object_size(const json_t *object)
+
+   Returns the number of elements in *object*, or 0 if *object* is not
+   a JSON object.
+
+   .. versionadded:: 1.1

 
 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
 
 
 .. cfunction:: json_t *json_object_get(const json_t *object, const char *key)
 


@@ -322,6 +430,21 @@ Unicode string and the value is any JSON value.
    -1 if *key* was not found.
 
 
    -1 if *key* was not found.
 
 

+.. cfunction:: int json_object_clear(json_t *object)
+
+   Remove all elements from *object*. Returns 0 on success and -1 if
+   *object* is not a JSON object.
+
+   .. versionadded:: 1.1
+
+.. cfunction:: int json_object_update(json_t *object, json_t *other)
+
+   Update *object* with the key-value pairs from *other*, overwriting
+   existing keys. Returns 0 on success or -1 on error.
+
+   .. versionadded:: 1.1
+
+

 The following functions implement an iteration protocol for objects:
 
 .. cfunction:: void *json_object_iter(json_t *object)
 The following functions implement an iteration protocol for objects:
 
 .. cfunction:: void *json_object_iter(json_t *object)


@@ -345,6 +468,20 @@ The following functions implement an iteration protocol for objects:
 
    Extract the associated value from *iter*.
 
 
    Extract the associated value from *iter*.
 

+The iteration protocol can be used for example as follows::
+
+   /* obj is a JSON object */
+   const char *key;
+   json_t *value;
+   void *iter = json_object_iter(obj);
+   while(iter)
+   {
+       key = json_object_iter_key(iter);
+       value = json_object_iter_value(iter);
+       /* use key and value ... */
+       iter = json_object_iter_next(obj, iter);
+   }
+

 
 Encoding
 ========
 
 Encoding
 ========


@@ -367,18 +504,18 @@ can be ORed together to obtain *flags*.
 The following functions perform the actual JSON encoding. The result
 is in UTF-8.
 
 The following functions perform the actual JSON encoding. The result
 is in UTF-8.
 

-.. cfunction:: char *json_dumps(const json_t *root, uint32_t flags)
+.. cfunction:: char *json_dumps(const json_t *root, unsigned long flags)

 
    Returns the JSON representation of *root* as a string, or *NULL* on
    error. *flags* is described above. The return value must be freed
    by the caller using :cfunc:`free()`.
 
 
    Returns the JSON representation of *root* as a string, or *NULL* on
    error. *flags* is described above. The return value must be freed
    by the caller using :cfunc:`free()`.
 

-.. cfunction:: int json_dumpf(const json_t *root, FILE *output, uint32_t flags)
+.. cfunction:: int json_dumpf(const json_t *root, FILE *output, unsigned long flags)

 
    Write the JSON representation of *root* to the stream *output*.
    *flags* is described above. Returns 0 on success and -1 on error.
 
 
    Write the JSON representation of *root* to the stream *output*.
    *flags* is described above. Returns 0 on success and -1 on error.
 

-.. cfunction:: int json_dump_file(const json_t *json, const char *path, uint32_t flags)
+.. cfunction:: int json_dump_file(const json_t *json, const char *path, unsigned long flags)

 
    Write the JSON representation of *root* to the file *path*. If
    *path* already exists, it is overwritten. *flags* is described
 
    Write the JSON representation of *root* to the file *path*. If
    *path* already exists, it is overwritten. *flags* is described