While we currently have good support for passing Java methods wrapped as function pointers to native code so they can be called there, we are lacking a bit on the opposite front where function pointers (in the form of MemoryAddresses) are passed from native code to Java, and the user would like to invoke this function pointer.

While it is technically possible to do this today by manually invoking CLinker.downcallHandle with the function pointer and an appropriate MethodType and FunctionDescriptor, the user has to manually figure out the needed type information, as it is currently not exposed in the generated jextract bindings.

Even if the type information were to be exposed, it would have to be supplied manually. In this case it would be more useful if jextract generated helper methods in the functional interface types it generates, that can convert a MemoryAddress into a MethodHandle, or an instance of the functional interface, such as:

    public interface myHeader$foo$f {
        ...
        public static myHeader$foo$f ofAddress(MemoryAddress);
    }

That way, when the user receives a function pointer from native code and would like to invoke it, they can easily do so using this helper method.