API Reference — Jansson v1.1.1 documentation

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# API Reference

## Preliminaries

All declarations are in `jansson.h`, so it’s enough to

    #include <jansson.h>

in each source file.

All constants are prefixed `JSON_` and other identifiers with `json_`. Type names are suffixed with `_t` and `typedef`‘d so that the `struct` keyword need not be used.

## Value Representation

The JSON specification (RFC 4627) defines the following data types: object, array, string, number, boolean, and null. JSON types are used dynamically; arrays and objects can hold any other data type, including themselves. For this reason, Jansson’s type system is also dynamic in nature. There’s one C type to represent all JSON values, and this structure knows the type of the JSON value it holds.

`json_t`
  This data structure is used throughout the library to represent all JSON values. It always contains the type of the JSON value it holds and the value’s reference count. The rest depends on the type of the value.

Objects of `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.

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

The type of a JSON value is queried and tested using the following functions:

enum `json_type`

  The type of a JSON value. The following members are defined:

  `JSON_OBJECT`
  `JSON_ARRAY`
  `JSON_STRING`
  `JSON_INTEGER`
  `JSON_REAL`
  `JSON_TRUE`
  `JSON_FALSE`
  `JSON_NULL`

  These correspond to JSON object, array, string, number, boolean and null. A number is represented by either a value of the type `JSON_INTEGER` or of the type `JSON_REAL`. A true boolean value is represented by a value of the type `JSON_TRUE` and false by a value of the type `JSON_FALSE`.

int `json_typeof`(const json_t *json)
  Return the type of the JSON value (a `json_type` cast to `int`). json MUST NOT be NULL. This function is actually implemented as a macro for speed.

`json_is_object`(const json_t *json)
`json_is_array`(const json_t *json)
`json_is_string`(const json_t *json)
`json_is_integer`(const json_t *json)
`json_is_real`(const json_t *json)
`json_is_true`(const json_t *json)
`json_is_false`(const json_t *json)
`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 and for NULL.

`json_is_number`(const json_t *json)
  Returns true for values of types `JSON_INTEGER` and `JSON_REAL`, and false for other types and for NULL.

`json_is_boolean`(const json_t *json)
  Returns true for types `JSON_TRUE` and `JSON_FALSE`, and false for values of other types and for NULL.

### Reference Count

The reference count is used to track whether a value is still in use or not. When a value is created, it’s reference count is set to 1. If a reference to a value is kept (e.g. a value is stored somewhere for later use), its reference count is incremented, and when the value is no longer needed, the reference count is decremented. When the reference count drops to zero, there are no references left, and the value can be destroyed.

The following functions are used to manipulate the reference count.

json_t *`json_incref`(json_t *json)
  Increment the reference count of json if it’s not non-NULL. Returns json.

void `json_decref`(json_t *json)
  Decrement the reference count of json. As soon as a call to `json_decref()` drops the reference count to zero, the value is destroyed and it can no longer be used.

Functions creating new JSON values set the reference count to 1. These functions are said to return a new reference. Other functions returning (existing) JSON values do not normally increase the reference count. These functions are said to return a borrowed reference. So, if the user will hold a reference to a value returned as a borrowed reference, he must call `json_incref()`. As soon as the value is no longer needed, `json_decref()` should be called to release the reference.

Normally, all functions accepting a JSON value as an argument will manage the reference, i.e. increase and decrease the reference count as needed. However, some functions steal the reference, i.e. they have the same result as if the user called `json_decref()` on the argument right after calling the function. These are usually convenience functions for adding new references to containers and not to worry about the reference count.

In the following sections it is clearly documented whether a function will return a new or borrowed reference or steal a reference to its 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 `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 `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

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

json_t *`json_true`(void)
  Return value: New reference.

  Returns the JSON true value.

json_t *`json_false`(void)
  Return value: New reference.

  Returns the JSON false value.

json_t *`json_null`(void)
  Return value: New reference.

  Returns the JSON null value.

## String

json_t *`json_string`(const char *value)
  Return value: New reference.

  Returns a new JSON string, or NULL on error. value must be a valid UTF-8 encoded Unicode string.

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

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.

  New in version 1.1.

## Number

json_t *`json_integer`(int value)
  Return value: New reference.

  Returns a new JSON integer, or NULL on error.

int `json_integer_value`(const json_t *integer)
  Returns the associated value of integer, or 0 if json is not a JSON integer.

int `json_integer_set`(const json_t *integer, int value)

  Sets the associated value of integer to value. Returns 0 on success and -1 if integer is not a JSON integer.

  New in version 1.1.

json_t *`json_real`(double value)
  Return value: New reference.

  Returns a new JSON real, or NULL on error.

double `json_real_value`(const json_t *real)
  Returns the associated value of real, or 0.0 if real is not a JSON real.

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.

  New in version 1.1.

In addition to the functions above, there’s a common query function for integers and reals:

double `json_number_value`(const json_t *json)
  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

A JSON array is an ordered collection of other JSON values.

json_t *`json_array`(void)
  Return value: New reference.

  Returns a new JSON array, or NULL on error. Initially, the array is empty.

unsigned int `json_array_size`(const json_t *array)
  Returns the number of elements in array, or 0 if array is NULL or not a JSON array.

json_t *`json_array_get`(const json_t *array, unsigned int index)
  Return value: Borrowed reference.

  Returns the element in array at position index. The valid range for index is from 0 to the return value of `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.

int `json_array_set`(json_t *array, unsigned int index, json_t *value)
  Replaces the element in array at position index with value. The valid range for index is from 0 to the return value of `json_array_size()` minus 1. Returns 0 on success and -1 on error.

int `json_array_set_new`(json_t *array, unsigned int index, json_t *value)

  Like `json_array_set()` but steals the reference to value. This is useful when value is newly created and not used after the call.

  New in version 1.1.

int `json_array_append`(json_t *array, json_t *value)
  Appends value to the end of array, growing the size of array by 1. Returns 0 on success and -1 on error.

int `json_array_append_new`(json_t *array, json_t *value)

  Like `json_array_append()` but steals the reference to value. This is useful when value is newly created and not used after the call.

  New in version 1.1.

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.

  New in version 1.1.

int `json_array_insert_new`(json_t *array, unsigned int index, json_t *value)

  Like `json_array_insert()` but steals the reference to value. This is useful when value is newly created and not used after the call.

  New in version 1.1.

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.

  New in version 1.1.

int `json_array_clear`(json_t *array)

  Removes all elements from array. Returns 0 on sucess and -1 on error.

  New in version 1.1.

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.

  New in version 1.1.

## Object

A JSON object is a dictionary of key-value pairs, where the key is a Unicode string and the value is any JSON value.

json_t *`json_object`(void)
  Return value: New reference.

  Returns a new JSON object, or NULL on error. Initially, the object is empty.

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.

  New in version 1.1.

json_t *`json_object_get`(const json_t *object, const char *key)
  Return value: Borrowed reference.

  Get a value corresponding to key from object. Returns NULL if key is not found and on error.

int `json_object_set`(json_t *object, const char *key, json_t *value)
  Set the value of key to value in object. key must be a valid null terminated UTF-8 encoded Unicode string. If there already is a value for key, it is replaced by the new value. Returns 0 on success and -1 on error.

int `json_object_set_new`(json_t *object, const char *key, json_t *value)

  Like `json_object_set()` but steals the reference to value. This is useful when value is newly created and not used after the call.

  New in version 1.1.

int `json_object_del`(json_t *object, const char *key)
  Delete key from object if it exists. Returns 0 on success, or -1 if key was not found.

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.

  New in version 1.1.

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.

  New in version 1.1.

The following functions implement an iteration protocol for objects:

void *`json_object_iter`(json_t *object)
  Returns an opaque iterator which can be used to iterate over all key-value pairs in object, or NULL if object is empty.

void *`json_object_iter_next`(json_t *object, void *iter)
  Returns an iterator pointing to the next key-value pair in object after iter, or NULL if the whole object has been iterated through.

const char *`json_object_iter_key`(void *iter)
  Extract the associated key from iter.

json_t *`json_object_iter_value`(void *iter)
  Return value: Borrowed reference.

  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

This sections describes the functions that can be used to encode values to JSON. Only objects and arrays can be encoded, since they are the only valid “root” values of a JSON text.

Each function takes a flags parameter that controls some aspects of how the data is encoded. Its default value is 0. The following macros can be ORed together to obtain flags.

`JSON_INDENT(n)`
  Pretty-print the result, indenting arrays and objects by n spaces. The valid range for n is between 0 and 255, other values result in an undefined output. If `JSON_INDENT` is not used or n is 0, no pretty-printing is done and the result is a compact representation.

The following functions perform the actual JSON encoding. The result is in UTF-8.

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 `free()`.

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. If an error occurs, something may have already been written to output. In this case, the output is undefined and most likely not valid JSON.

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 above. Returns 0 on success and -1 on error.

## Decoding

This sections describes the functions that can be used to decode JSON text to the Jansson representation of JSON data. The JSON specification requires that a JSON text is either a serialized array or object, and this requirement is also enforced with the following functions.

The only supported character encoding is UTF-8 (which ASCII is a subset of).

`json_error_t`

  This data structure is used to return information on decoding errors from the decoding functions. Its definition is repeated here:

      #define JSON_ERROR_TEXT_LENGTH  160

      typedef struct {
          char text[JSON_ERROR_TEXT_LENGTH];
          int line;
      } json_error_t;

  line is the line number on which the error occurred, or -1 if this information is not available. text contains the error message (in UTF-8), or an empty string if a message is not available.

  The normal usef of `json_error_t` is to allocate it normally on the stack, and pass a pointer to a decoding function. Example:

      int main() {
          json_t *json;
          json_error_t error;

          json = json_load_file("/path/to/file.json", &error);
          if(!json) {
              /* the error variable contains error information */
          }
          ...
      }

  Also note that if the decoding succeeded (`json != NULL` in the above example), the contents of `error` are unspecified.

  All decoding functions also accept NULL as the `json_error_t` pointer, in which case no error information is returned to the caller.

The following functions perform the actual JSON decoding.

json_t *`json_loads`(const char *input, json_error_t *error)
  Return value: New reference.

  Decodes the JSON string input and returns the array or object it contains, or NULL on error, in which case error is filled with information about the error. See above for discussion on the error parameter.

json_t *`json_loadf`(FILE *input, json_error_t *error)
  Return value: New reference.

  Decodes the JSON text in stream input and returns the array or object it contains, or NULL on error, in which case error is filled with information about the error. See above for discussion on the error parameter.

json_t *`json_load_file`(const char *path, json_error_t *error)
  Return value: New reference.

  Decodes the JSON text in file path and returns the array or object it contains, or NULL on error, in which case error is filled with information about the error. See above for discussion on the error parameter.

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