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.
Parameter for returning results. Default is transfer full.
The callback is valid only during the call to the method.
Free data after the code is done.
Don't free data after the code is done.