NULL is OK, both for passing and for returning.
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.
Parameter for returning results. Default is transfer full.
Rename the original symbol's name to SYMBOL.
The callback is valid only during the call to the method.
The callback is valid until the GDestroyNotify argument is called.
Exposed in C code, not necessarily available in other languages.
Free data after the code is done.