This parameter is a 'user_data', for callbacks; many bindings can pass NULL here.
This parameter is a 'destroy_data', for callbacks.
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.
The callback is valid only during the call to the method.
The caller owns the data, and is responsible for free it.
The data is owned by the callee, which is responsible of freeing it.