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.
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.
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.
The type of a JSON value is queried and tested using the following functions:
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.
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.
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.
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.
These values are implemented as singletons, so each of these functions returns the same value each time.
Returns a new JSON string, or NULL on error. value must be a valid UTF-8 encoded Unicode string.
Like json_string(), but doesn’t check that value is valid UTF-8. Use this function only if you are certain that this really is the case (e.g. you have already checked it by other means).
New in version 1.2.
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.
Like json_string_set(), but doesn’t check that value is valid UTF-8. Use this function only if you are certain that this really is the case (e.g. you have already checked it by other means).
New in version 1.2.
Returns a new JSON integer, or NULL on error.
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.
Returns a new JSON real, or NULL on error.
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:
A JSON array is an ordered collection of other JSON values.
Returns a new JSON array, or NULL on error. Initially, the array is empty.
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.
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.
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.
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.
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.
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.
A JSON object is a dictionary of key-value pairs, where the key is a Unicode string and the value is any JSON value.
Returns a new JSON object, or NULL on error. Initially, the object is empty.
Returns the number of elements in object, or 0 if object is not a JSON object.
New in version 1.1.
Get a value corresponding to key from object. Returns NULL if key is not found and on error.
Like json_object_set(), but doesn’t check that key is valid UTF-8. Use this function only if you are certain that this really is the case (e.g. you have already checked it by other means).
New in version 1.2.
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.
Like json_object_set_new(), but doesn’t check that key is valid UTF-8. Use this function only if you are certain that this really is the case (e.g. you have already checked it by other means).
New in version 1.2.
Remove all elements from object. Returns 0 on success and -1 if object is not a JSON object.
New in version 1.1.
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:
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);
}
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.
By default, the output has no newlines, and spaces are used between array and object elements for a readable output. This behavior can be altered by using the JSON_INDENT and JSON_COMPACT flags described below. A newline is never appended to the end of the encoded JSON data.
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.
This flag enables a compact representation, i.e. sets the separator between array and object items to "," and between object keys and values to ":". Without this flag, the corresponding separators are ", " and ": " for more readable output.
New in version 1.2.
If this flag is used, the output is guaranteed to consist only of ASCII characters. This is achived by escaping all Unicode characters outside the ASCII range.
New in version 1.2.
If this flag is used, all the objects in output are sorted by key. This is useful e.g. if two JSON texts are diffed or visually compared.
New in version 1.2.
The following functions perform the actual JSON encoding. The result is in UTF-8.
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).
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.
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.
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.
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.
Testing for equality of two JSON values cannot, in general, be achieved using the == operator. Equality in the terms of the == operator states that the two json_t pointers point to exactly the same JSON value. However, two JSON values can be equal not only if they are exactly the same value, but also if they have equal “contents”:
The following function can be used to test whether two JSON values are equal.
Because of reference counting, passing JSON values around doesn’t require copying them. But sometimes a fresh copy of a JSON value is needed. For example, if you need to modify an array, but still want to use the original afterwards, you should take a copy of it first.
Jansson supports two kinds of copying: shallow and deep. There is a difference between these methods only for arrays and objects. Shallow copying only copies the first level value (array or object) and uses the same child values in the copied value. Deep copying makes a fresh copy of the child values, too. Moreover, all the child values are deep copied in a recursive fashion.