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.
The callback is valid until first called.
The callback is valid only during the call to the method.
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.