This is a variant of g_object_get_qdata() which returns a 'duplicate' of the value. dup_func defines the meaning of 'duplicate' in this context, it could e.g. take a reference on a ref-counted object.
If the quark is not set on the object then dup_func will be called with a NULL argument.
Note that dup_func is called while user data of object is locked.
This function can be useful to avoid races when multiple threads are using object data on the same key on the same object.
a GQuark, naming the user data pointer
function to dup the value
passed as user_data to dup_func
the result of calling dup_func on the value associated with quark on object, or NULL if not set. If dup_func is NULL, the value is returned unmodified.