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.
Typed errors. Similar to throws in Java.
Parameter for input. Default is transfer none.
Parameter for returning results. Default is transfer full.
The callback is valid until first called.
The caller owns the data, and is responsible for free it.
The data is owned by the callee, which is responsible of freeing it.
