add.c

#include <json-c/json.h>
#include <stdio.h>

int main() {
    // Write
    struct json_object *root = json_object_new_object();
    json_object_object_add(root, "name", json_object_new_string("Bob"));
    json_object_object_add(root, "age", json_object_new_int(25));
    FILE *fp = fopen("data.json", "w");
    fputs(json_object_to_json_string_ext(root, JSON_C_TO_STRING_PRETTY), fp);
    fclose(fp);
    json_object_put(root);

    // Read
    fp = fopen("data.json", "r");
    fseek(fp, 0, SEEK_END);
    long len = ftell(fp);
    rewind(fp);
    char *data = malloc(len + 1);
    fread(data, 1, len, fp);
    data[len] = '\0';
    fclose(fp);

    struct json_object *parsed = json_tokener_parse(data);
    free(data);
    struct json_object *name, *age;
    json_object_object_get_ex(parsed, "name", &name);
    json_object_object_get_ex(parsed, "age", &age);

    printf("Name: %s\n", json_object_get_string(name));
    printf("Age: %d\n", json_object_get_int(age));

    json_object_put(parsed);
    return 0;
}