NULL is OK, both for passing and for returning.
Parameter points to an array of items.
This parameter is a 'user_data', for callbacks; many bindings can pass NULL here.
Generics and defining elements of containers and arrays.
Parameter for returning results. Default is transfer full.
The callback is valid until first 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.