Memory Returned by json_object_to_json_string not freed

[vaasu1234]

const char * json_str = json_object_to_json_string(jsonobj);
json_object_put(jsonobj);
printf("%s\n", json_str);
json_object_object_add(jsonobj, "key", json_object_new_int64(value)); //

I can still print the string even after freeing the object (line 3)?

To check if obj is really freed, I tried adding an object and i'm hitting seg fault which proved object is freed. So, why json_str is still valid after object freed?

[vaasu1234]

Looks like a side-effect of free. free(ptr) doesn't necessarily erase memory. So the contents of the memory might still be valid until the block is reassigned.