WRAP-JSON facility

The facility wrap-json is based on the pack/unpack API on the library jansson. The two chapters below are copied from the documentation of jansson library copyrighted by Petri Lehtinen (see at end).

Building Values

This section 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 strings and a boolean as its items:

/* Create the JSON integer 42 */
wrap_json_pack(&result, "i", 42);

/* Create the JSON array ["foo", "bar", true] */
wrap_json_pack(&result, "[ssb]", "foo", "bar", 1);

Here's the full list of format specifiers. 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 or arguments.

s (string) [const char *]

Convert a null terminated UTF-8 string to a JSON string.
s? (string) [const char *]

Like s, but if the argument is NULL, output a JSON null value.
s* (string) [const char *]

Like s, but if the argument is NULL, do not output any value. This format can only be used inside an object or an array. If used inside an object, the corresponding key is additionally suppressed when the value is omitted. See below for an example.
s# (string) [const char *, int]

Convert a UTF-8 buffer of a given length to a JSON string.
s% (string) [const char *, size_t]

Like s# but the length argument is of type size_t.
+ [const char *]

Like s, but concatenate to the previous string. Only valid after s, s#, + or +#.
+# [const char *, int]

Like s#, but concatenate to the previous string. Only valid after s, s#, + or +#.
+% (string) [const char *, size_t]

Like +# but the length argument is of type size_t.
y (byte array) [const uint8_t *, size_t]

Convert the byte array whose length is given to its base64url string representation.
Y (byte array) [const uint8_t *, size_t]

Like 'y' but output is base64.
y?, Y? (byte array or null) [const uint8_t *, size_t]

Like 'y' or 'Y' but allows to output a JSON null value either when the buffer is NULL or when the size is 0.
y*, y* (optional byte array) [const uint8_t *, size_t]

Like 'y' or 'Y' but do not put JSON value either when the buffer is NULL or when the size is 0. This format can only be used inside an object or an array. If used inside an object, the corresponding key is additionally suppressed when the value is omitted. See below for an example.
n (null)

Output a JSON null value. No argument is consumed.
b (boolean) [int]

Convert a C int to JSON boolean value. Zero is converted to false and non-zero to true.
i (integer) [int]

Convert a C int to JSON integer.
I (integer) [json_int_t]

Convert a C json_int_t to JSON integer.
f (real) [double]

Convert a C double to JSON real.
o (any value) [json_t *]

Output any given JSON value as-is. If the value is added to an array or object, the reference to the value passed to o is stolen by the container.
O (any value) [json_t *]

Like o, but the argument's reference count is incremented. This is useful if you pack into an array or object and want to keep the reference for the JSON value consumed by O to yourself.
o?, O? (any value) [json_t *]

Like o and O, respectively, but if the argument is NULL, output a JSON null value.
o*, O* (any value) [json_t *]

Like o and O, respectively, but if the argument is NULL, do not output any value. This format can only be used inside an object or an array. If used inside an object, the corresponding key is additionally suppressed. See below for an example.
[fmt] (array)

Build an array with contents from the inner format string. fmt may contain objects and arrays, i.e. recursive value building is supported.
{fmt} (object)

Build an object with contents from the inner format string fmt. The first, third, etc. format specifier represent a key, and must be a string (see s, s#, + and +# above), as object keys are always strings. The second, fourth, etc. format specifier represent a value. Any value may be an object or array, i.e. recursive value building is supported.

Whitespace, : and , are ignored.

More examples:

/* Build an empty JSON object */
wrap_json_pack(&result, "{}");

/* Build the JSON object {"foo": 42, "bar": 7} */
wrap_json_pack(&result, "{sisi}", "foo", 42, "bar", 7);

/* Like above, ':', ',' and whitespace are ignored */
wrap_json_pack(&result, "{s:i, s:i}", "foo", 42, "bar", 7);

/* Build the JSON array [[1, 2], {"cool": true}] */
wrap_json_pack(&result, "[[i,i],{s:b}]", 1, 2, "cool", 1);

/* Build a string from a non-null terminated buffer */
char buffer[4] = {'t', 'e', 's', 't'};
wrap_json_pack(&result, "s#", buffer, 4);

/* Concatenate strings together to build the JSON string "foobarbaz" */
wrap_json_pack(&result, "s++", "foo", "bar", "baz");

/* Create an empty object or array when optional members are missing */
wrap_json_pack(&result, "{s:s*,s:o*,s:O*}", "foo", NULL, "bar", NULL, "baz", NULL);
wrap_json_pack(&result, "[s*,o*,O*]", NULL, NULL, NULL);

Parsing and Validating Values

This section describes functions that help to validate complex values and extract, or unpack, data from them. Like building values <apiref-pack>, 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 specifier ! or by using the flag JSON_STRICT. See below for details.

Here's the full list of format specifiers. The type in parentheses denotes the JSON type, and the type in brackets (if any) denotes the C type whose address should be passed.

s (string) [const char *]

Convert a JSON string to a pointer to a null terminated UTF-8 string. The resulting string is extracted by using json_string_value() internally, so it exists as long as there are still references to the corresponding JSON string.
s% (string) [const char *, size_t *]

Convert a JSON string to a pointer to a null terminated UTF-8 string and its length.
y (byte array) [uint8_t **, size_t *]

Convert an input string base64url encoded to its byte array representation. The result and its length are stored. The returned buffer must be freed by the caller.
Y (byte array) [uint8_t **, size_t *]

Like 'y' but input is base64.
n (null)

Expect a JSON null value. Nothing is extracted.
b (boolean) [int]

Convert a JSON boolean value to a C int, so that true is converted to 1 and false to 0.
i (integer) [int]

Convert a JSON integer to C int.
I (integer) [json_int_t]

Convert a JSON integer to C json_int_t.
f (real) [double]

Convert a JSON real to C double.
F (integer or real) [double]

Convert a JSON number (integer or real) to C double.
o (any value) [json_t *]

Store a JSON value with no conversion to a json_t pointer.
O (any value) [json_t *]

Like O, but the JSON value's reference count is incremented.
[fmt] (array)

Convert each item in the JSON array according to the inner format string. fmt may contain objects and arrays, i.e. recursive value extraction is supported.
{fmt} (object)

Convert each item in the JSON object according to the inner format string fmt. The first, third, etc. format specifier represent a key, and must be s. The corresponding argument to unpack functions is read as the object key. The second fourth, etc. format specifier 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 supported.
!

This special format specifier is used to enable the check that all object and array items are accessed, on a per-value basis. It must appear inside an array or object as the last format specifier before the closing bracket or brace.
*

This special format specifier is the opposite of !. This is the default. It must appear inside an array or object as the last format specifier before the closing bracket or brace.

Whitespace, : and , are ignored.

Examples:

/* root is the JSON integer 42 */
int myint;
wrap_json_unpack(root, "i", &myint);
assert(myint == 42);

/* root is the JSON object {"foo": "bar", "quux": true} */
const char *str;
int boolean;
wrap_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} */
wrap_json_check(root, "[[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;
wrap_json_unpack(root, "[ii!]", &myint1, &myint2);
/* returns -1 for failed validation */

/* root is an empty JSON object */
int myint = 0, myint2 = 0, myint3 = 0;
wrap_json_unpack(root, "{s?i, s?[ii]}",
            "foo", &myint1,
            "bar", &myint2, &myint3);
/* myint1, myint2 or myint3 is no touched as "foo" and "bar" don't exist */

Copyright

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