NULL is ok, both for passing and for returning.
This parameter is a 'user_data', for callbacks; many bindings can pass NULL here.
This parameter is a 'destroy_data', for callbacks.
Generics and defining elements of containers and arrays.
Parameter for returning results. Default is transfer full.
The callback is valid until first called.
The callback is valid until the GDestroyNotify argument is called.
Free data container after the code is done.
Free data after the code is done.
Don't free data after the code is done.