All declarations are in jansson.h, so it’s enough to
#include <jansson.h>
in each source file.
All constants are prefixed with JSON_ (except for those describing the library version, prefixed with JANSSON_). Other identifiers are prefixed with json_. Type names are suffixed with _t and typedef‘d so that the struct keyword need not be used.
The Jansson version is of the form A.B.C, where A is the major version, B is the minor version and C is the micro version. If the micro version is zero, it’s omitted from the version string, i.e. the version string is just A.B.
When a new release only fixes bugs and doesn’t add new features or functionality, the micro version is incremented. When new features are added in a backwards compatible way, the minor version is incremented and the micro version is set to zero. When there are backwards incompatible changes, the major version is incremented and others are set to zero.
The following preprocessor constants specify the current version of the library:
A 3-byte hexadecimal representation of the version, e.g. 0x010201 for version 1.2.1 and 0x010300 for version 1.3. This is useful in numeric comparisions, e.g.:
#if JANSSON_VERSION_HEX >= 0x010300
/* Code specific to version 1.3 and above */
#endif
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.
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.
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.
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.
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.
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.
Increment the reference count of json if it’s not non-NULL. Returns 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 functions are suffixed with _new or have _new_ somewhere in their name.
For example, the following code creates a new JSON array and appends an integer to it:
json_t *array, *integer;
array = json_array();
integer = json_integer(42);
json_array_append(array, integer);
json_decref(integer);
Note how the caller has to release the reference to the integer value by calling json_decref(). By using a reference stealing function json_array_append_new() instead of json_array_append(), the code becomes much simpler:
json_t *array = json_array();
json_array_append_new(array, json_integer(42));
In this case, the user doesn’t have to explicitly release the reference to the integer value, as json_array_append_new() steals the reference when appending the value to the array.
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 references to each other. 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.
Jansson uses UTF-8 as the character encoding. All JSON strings must be valid UTF-8 (or ASCII, as it’s a subset of UTF-8). Normal null terminated C strings are used, so JSON strings may not contain embedded null characters. All other Unicode codepoints U+0001 through U+10FFFF are allowed.
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).
Returns the associated value of string as a null terminated UTF-8 encoded string, or NULL if string is not a JSON string.
The retuned value is read-only and must not be modified or freed by the user. It is valid as long as string exists, i.e. as long as its reference count has not dropped to zero.
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.
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).
The JSON specification only contains one numeric type, “number”. The C programming language has distinct types for integer and floating-point numbers, so for practical reasons Jansson also has distinct types for the two. They are called “integer” and “real”, respectively. For more information, see RFC Conformance.
This is the C type that is used to store JSON integer values. It represents the widest integer type available on your system. In practice it’s just a typedef of long long if your compiler supports it, otherwise long.
Usually, you can safely use plain int in place of json_int_t, and the implicit C integer conversion handles the rest. Only when you know that you need the full 64-bit range, you should use json_int_t explicitly.
JSON_INTEGER_IS_LONG_LONG
This is a preprocessor variable that holds the value 1 if json_int_t is long long, and 0 if it’s long. It can be used as follows:
#if JSON_INTEGER_IS_LONG_LONG /* Code specific for long long */ #else /* Code specific for long */ #endif
JSON_INTEGER_FORMAT
This is a macro that expands to a printf() conversion specifier that corresponds to json_int_t, without the leading % sign, i.e. either "lld" or "ld". This macro is required because the actual type of json_int_t can be either long or long long, and printf() reuiqres different length modifiers for the two.
Example:
json_int_t x = 123123123; printf("x is %" JSON_INTEGER_FORMAT "\n", x);
Returns a new JSON integer, or NULL on error.
Returns the associated value of integer, or 0 if json is not a JSON integer.
Sets the associated value of integer to value. Returns 0 on success and -1 if integer is not a JSON integer.
Returns a new JSON real, or NULL on error.
Returns the associated value of real, or 0.0 if real is not a JSON real.
Sets the associated value of real to value. Returns 0 on success and -1 if real is not a JSON real.
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 number of elements in array, or 0 if array is NULL or not a JSON array.
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.
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.
Like json_array_set() but steals the reference to value. This is useful when value is newly created and not used after the call.
Appends value to the end of array, growing the size of array by 1. Returns 0 on success and -1 on error.
Like json_array_append() but steals the reference to value. This is useful when value is newly created and not used after the call.
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.
Like json_array_insert() but steals the reference to value. This is useful when value is newly created and not used after the call.
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.
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.
Get a value corresponding to key from object. Returns NULL if key is not found and on error.
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.
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).
Like json_object_set() but steals the reference to value. This is useful when value is newly created and not used after the call.
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).
Delete key from object if it exists. Returns 0 on success, or -1 if key was not found.
Remove all elements from object. Returns 0 on success and -1 if object is not a JSON object.
Update object with the key-value pairs from other, overwriting existing keys. Returns 0 on success or -1 on error.
The following functions implement an iteration protocol for objects, allowing to iterate through all key-value pairs in an object. The items are not returned in any particular order, as this would require sorting due to the internal object representation.
Returns an opaque iterator which can be used to iterate over all key-value pairs in object, or NULL if object is empty.
Like json_object_iter(), but returns an iterator to the key-value pair in object whose key is equal to key, or NULL if key is not found in object. Iterating forward to the end of object only yields all key-value pairs of the object if key happens to be the first key in the underlying hash table.
Returns an iterator pointing to the next key-value pair in object after iter, or NULL if the whole object has been iterated through.
Extract the associated key from iter.
Extract the associated value from iter.
Set the value of the key-value pair in object, that is pointed to by iter, to value.
Like json_object_iter_set(), but steals the reference to value. This is useful when value is newly created and not used after the call.
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);
}
Jansson uses a single struct type to pass error information to the user. See sections Decoding, Building Values and Parsing and Validating Values for functions that pass error information using this struct.
The error message (in UTF-8), or an empty string if a message is not available.
Source of the error. This can be (a part of) the file name or a special identifier in angle brackers (e.g. <string>).
The line number on which the error occurred.
The column on which the error occurred. Note that this is the character column, not the byte column, i.e. a multibyte UTF-8 character counts as one column.
The position in bytes from the start of the input. This is useful for debugging Unicode encoding problems.
The normal use of json_error_t is to allocate it on the stack, and pass a pointer to a function. Example:
int main() {
json_t *json;
json_error_t error;
json = json_load_file("/path/to/file.json", 0, &error);
if(!json) {
/* the error variable contains error information */
}
...
}
Also note that if the call succeeded (json != NULL in the above example), the contents of error are unspecified.
All functions also accept NULL as the json_error_t pointer, in which case no error information is returned to the caller.
This sections describes the functions that can be used to encode values to JSON. By default, only objects and arrays can be encoded directly, since they are the only valid root values of a JSON text. To encode any JSON value, use the JSON_ENCODE_ANY flag (see below).
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.
Specifying this flag makes it possible to encode any JSON value on its own. Without it, only objects and arrays can be passed as the root value to the encoding functions.
Note: Encoding any value may be useful in some scenarios, but it’s generally discouraged as it violates strict compatiblity with RFC 4627. If you use this flag, don’t expect interoperatibility with other JSON systems. Even Jansson itself doesn’t have any means to decode JSON texts whose root value is not object or array.
New in version 2.1.
The following functions perform the actual JSON encoding. The result is in UTF-8.
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().
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.
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. In other words, the top level value in the JSON text being decoded must be either array or object.
See RFC Conformance for a discussion on Jansson’s conformance to the JSON specification. It explains many design decisions that affect especially the behavior of the decoder.
Each function takes a flags parameter that can be used to control the behavior of the decoder. Its default value is 0. The following macros can be ORed together to obtain flags.
Issue a decoding error if any JSON object in the input text contains duplicate keys. Without this flag, the value of the last occurence of each key ends up in the result. Key equivalence is checked byte-by-byte, without special Unicode comparison algorithms.
New in version 2.1.
By default, the decoder expects that its whole input constitutes a valid JSON text, and issues an error if there’s extra data after the otherwise valid JSON input. With this flag enabled, the decoder stops after decoding a valid JSON array or object, and thus allows extra data after the JSON text.
New in version 2.1.
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. flags is described above.
Decodes the JSON string buffer, whose length is buflen, and returns the array or object it contains, or NULL on error, in which case error is filled with information about the error. This is similar to json_loads() except that the string doesn’t need to be null-terminated. flags is described above.
New in version 2.1.
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. flags is described above.
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. flags is described above.
This sectinon describes functions that help to create, or pack, complex JSON values, especially nested objects and arrays. Value building is based on a format string that is used to tell the functions about the expected arguments.
For example, the format string "i" specifies a single integer value, while the format string "[ssb]" or the equivalent "[s, s, b]" specifies an array value with two integers and a boolean as its items:
/* Create the JSON integer 42 */
json_pack("i", 42);
/* Create the JSON array ["foo", "bar", true] */
json_pack("[ssb]", "foo", "bar", 1);
Here’s the full list of format characters. The type in parentheses denotes the resulting JSON type, and the type in brackets (if any) denotes the C type that is expected as the corresponding argument.
The following functions compose the value building API:
Build a new JSON value according to the format string fmt. For each format character (except for {}[]n), one argument is consumed and used to build the corresponding value. Returns NULL on error.
Like json_pack(), but an in the case of an error, an error message is written to error, if it’s not NULL. The flags parameter is currently unused and should be set to 0.
As only the errors in format string (and out-of-memory errors) can be caught by the packer, these two functions are most likely only useful for debugging format strings.
More examples:
/* Build an empty JSON object */
json_pack("{}");
/* Build the JSON object {"foo": 42, "bar": 7} */
json_pack("{sisb}", "foo", 42, "bar", 7);
/* Like above, ':', ',' and whitespace are ignored */
json_pack("{s:i, s:b}", "foo", 42, "bar", 7);
/* Build the JSON array [[1, 2], {"cool": true}] */
json_pack("[[i,i],{s:b]]", 1, 2, "cool", 1);
This sectinon describes functions that help to validate complex values and extract, or unpack, data from them. Like building values, this is also based on format strings.
While a JSON value is unpacked, the type specified in the format string is checked to match that of the JSON value. This is the validation part of the process. In addition to this, the unpacking functions can also check that all items of arrays and objects are unpacked. This check be enabled with the format character ! or by using the flag JSON_STRICT. See below for details.
Here’s the full list of format characters. The type in parentheses denotes the JSON type, and the type in brackets (if any) denotes the C type whose address should be passed.
Convert each item in the JSON object according to the inner format string fmt. The first, third, etc. format character represent a key, and must be s. The corresponding argument to unpack functions is read as the object key. The second fourth, etc. format character represent a value and is written to the address given as the corresponding argument. Note that every other argument is read from and every other is written to.
fmt may contain objects and arrays as values, i.e. recursive value extraction is supporetd.
The following functions compose the parsing and validation API:
Validate and unpack the JSON value root according to the format string fmt. Returns 0 on success and -1 on failure.
Validate and unpack the JSON value root according to the format string fmt. If an error occurs and error is not NULL, write error information to error. flags can be used to control the behaviour of the unpacker, see below for the flags. Returns 0 on success and -1 on failure.
The following unpacking flags are available:
Examples:
/* root is the JSON integer 42 */
int myint;
json_unpack(root, "i", &myint);
assert(myint == 42);
/* root is the JSON object {"foo": "bar", "quux": true} */
const char *str;
int boolean;
json_unpack(root, "{s:s, s:b}", "foo", &str, "quux", &boolean);
assert(strcmp(str, "bar") == 0 && boolean == 1);
/* root is the JSON array [[1, 2], {"baz": null} */
json_error_t error;
json_unpack_ex(root, &error, JSON_VALIDATE_ONLY, "[[i,i], {s:n}]", "baz");
/* returns 0 for validation success, nothing is extracted */
/* root is the JSON array [1, 2, 3, 4, 5] */
int myint1, myint2;
json_unpack(root, "[ii!]", &myint1, &myint2);
/* returns -1 for failed validation */
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.
By default, Jansson uses malloc() and free() for memory allocation. These functions can be overridden if custom behavior is needed.
A typedef for a function pointer with malloc()‘s signature:
typedef void *(*json_malloc_t)(size_t);
A typedef for a function pointer with free()‘s signature:
typedef void (*json_free_t)(void *);
Use malloc_fn instead of malloc() and free_fn instead of free(). This function has to be called before any other Jansson’s API functions to ensure that all memory operations use the same functions.
Examples:
Use the Boehm’s conservative garbage collector for memory operations:
json_set_alloc_funcs(GC_malloc, GC_free);
Allow storing sensitive data (e.g. passwords or encryption keys) in JSON structures by zeroing all memory when freed:
static void *secure_malloc(size_t size)
{
/* Store the memory area size in the beginning of the block */
void *ptr = malloc(size + 8);
*((size_t *)ptr) = size;
return ptr + 8;
}
static void secure_free(void *ptr)
{
size_t size;
ptr -= 8;
size = *((size_t *)ptr);
guaranteed_memset(ptr, 0, size);
free(ptr);
}
int main()
{
json_set_alloc_funcs(secure_malloc, secure_free);
/* ... */
}
For more information about the issues of storing sensitive data in memory, see http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/protect-secrets.html. The page also examplains the guaranteed_memset() function used in the example and gives a sample implementation for it.