NULL is ok, both for passing and for returning.

This parameter is a 'user_data', for callbacks; many bindings can pass NULL here.

Don't free data after the code is done.

Generics and defining elements of containers and arrays.

Parameter for input. Default is transfer none.

Free data container after the code is done.

Free data after the code is done.

The callback is valid until the GDestroyNotify argument is called.