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.
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.
Returns a value of the type JSON_TRUE, or NULL on error.
Returns a new value of the type JSON_INTEGER, or NULL on error.
Returns a new value of the type JSON_REAL, or NULL on error.
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 value of the type JSON_ARRAY, or NULL on error. Initially, the array is empty.
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 json_array_size() minus 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 value of the type JSON_OBJECT, or NULL on error. Initially, the object is empty.
Get a value corresponding to key from object. Returns NULL if key is not found and on error.
The following functions implement an iteration protocol for objects:
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.
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.