Generics and defining elements of containers and arrays.
NULL may be passed as the value in, out, in-out; or as a return value.
Parameter for returning results. Default is transfer full.
The callback is valid only during the call to the method.
The caller owns the data container, but not the data inside it.
The caller owns the data, and is responsible for free it.
The data is owned by the callee, which is responsible of freeing it.