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.
Free data after the code is done.
Don't free data after the code is done.