Two major categories of types exists in json-c, basic and composed.

Basic types

Namely boolean, int32, int64, double and string. All of those types have a new and a get method:

struct json_object * json_object_new_boolean(json_bool b);
json_bool json_object_get_boolean(struct json_object *obj);

struct json_object * json_object_new_int(int32_t i);
int32_t json_object_get_int(struct json_object *obj);

struct json_object * json_object_new_int64(int64_t i);
int64_t json_object_get_int64(struct json_object *obj);

struct json_object * json_object_new_double(double d);
struct json_object * json_object_new_double_s(double d, const char *ds);
double json_object_get_double(struct json_object *obj);

struct json_object * json_object_new_string(const char *s);
struct json_object * json_object_new_string_len(const char *s, int len);
const char * json_object_get_string(struct json_object *obj);
int json_object_get_string_len(struct json_object *obj);

Note: don’t mix-up json_object_get_string() and json_object_to_json_string(), json_object_get_string() should be used only to retrieve a string and the second only to print a whole json_object.

json_object_new_double_s() allows a much more accurate representation of the double.

json_object_new_string_len() create a new json_object from the supplied string, the buffer size for the string is of size len (with str[len+1] = '\0').

Composed types

Composed types are arrays and objects in json, and can contain every combination of types you can think of.

struct json_object * json_object_new_array(void);
struct array_list * json_object_get_array(struct json_object *obj);
int json_object_array_length(struct json_object *obj);
void json_object_array_sort(struct json_object *jso, int(*sort_fn)(const void *, const void *));
int json_object_array_add(struct json_object *obj, struct json_object *val);
int json_object_array_put_idx(struct json_object *obj, int idx, struct json_object *val);
struct json_object * json_object_array_get_idx(struct json_object *obj, int idx);

struct json_object * json_object_new_object(void);
struct lh_table * json_object_get_object(struct json_object *obj);
int json_object_object_length(struct json_object *obj);
void json_object_object_add(struct json_object *obj, const char *key, struct json_object *val);
json_bool json_object_object_get_ex(struct json_object *obj, const char *key, struct json_object **value);
void json_object_object_del(struct json_object *obj, const char *key);
#define json_object_object_foreach(obj, key, val)

I never actually used json_object_get_array() and json_object_get_object(), it’s unlikely you will ever have to… json_object_array_sort() don’t have much of documentation but it’s in fact a direct call to the qsort() system fonction (man 3 qsort). json_object_object_foreach() is a handy foreach loop, key and val will be declared by the macro, you can name those variable as you please.

Warning: in version 0.10 of json-c json_object_object_add() doesn’t work properly if you try to overwrite the value of an existing key! I strongly recommend you to use a newer version than 0.10. If you can’t, apply the patch from this commit

Identify the type of a json object

As you might have noticed, every type is a json_object! To differentiate types two functions are available:

enum json_type {
    json_type_null, json_type_boolean, json_type_double, json_type_int,
    json_type_object, json_type_array, json_type_string
} json_type;

int json_object_is_type(struct json_object *obj, enum json_type type);
enum json_type json_object_get_type(struct json_object *obj);

json_type_null correspond to a NULL json_object pointer.


After all those mind mind-numbing prototypes it’s example time. I didn’t wish to demonstrate how to use every single function, after all it’s almost identical from a boolean to any other basic type…

And the output:

key: "message", type of val: val is a string	->	"We have been made!"
key: "security-code", type of val: val is an array

Details of the security code:
security-code[0] = 12345
security-code[1] = true

Json in plain text:
{ "message": "We have been made!", "security-code": [ 12345, true ] }

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