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.

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.

An interface that can be used within the GNOME stack itself, but that is not documented for end-users. Such functions should only be used in specified and documented ways.

The callback is valid only during the call to the method.

Exposed in C code, not necessarily available in other languages.

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.