NULL is OK, both for passing and for returning.
Parameter points to an array of items.
Parameter for input. Default is transfer none.
NULL may be passed as the value in, out, in-out; or as a return value.
Parameter for returning results. Default is transfer full.
Exposed in C code, not necessarily available in other languages.
The caller owns the data, and is responsible for free it.
The data is owned by the callee, which is responsible of freeing it.