NULL is OK, both for passing and for returning.
Generics and defining elements of containers and arrays.
NULL must not be passed as the value in, out, in-out; or as a return value.
NULL may be passed as the value in, out, in-out; or as a return value.
NULL may be passed instead of a pointer to a location.
Parameter for returning results. Default is transfer full.
Out parameter, where caller must allocate storage.
The callback is valid only during the call to the method.
Exposed in C code, not necessarily available in other languages.
The intention of a Stable interface is to enable arbitrary third parties to develop applications to these interfaces, release them, and have confidence that they will run on all minor releases of the product (after the one in which the interface was introduced, and within the same major release). Even at a major release, incompatible changes are expected to be rare, and to have strong justifications.
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.