Null Callbacks

[Jorn Vernee]

Hi Mark,

An API for getting a null callback is currently in the process of being 
added [1].

In the meantime, there is a way to work around this, but it's tricky 
since there doesn't seem to be a public API for parsing a Function 
descriptor, or a public API to easily get a layout for a NativeCallback 
class (will make a note of that as well). AFAICT it currently requires 
manually constructing the Function descriptor for the callback. But, you 
should be able to use something like this:

```
     @NativeCallback("()v")
     public interface FI1 {
         void run();
     }

     public static <T> T pointerLiteral(LayoutType<T> type, long value) {
         try(Scope scope = Scope.newNativeScope()) {
             Pointer<Long> ptr = scope.allocate(NativeTypes.UINT64);
             ptr.set(value);
             return ptr.cast(NativeTypes.VOID).cast(type).get();
         }
     }

     public static void main(String[] args) {
         Function f = Function.ofVoid(false, NativeTypes.VOID.layout());
         Address a = Address.ofFunction(64, f);
         Callback<FI1> nullCB = pointerLiteral(LayoutType.ofFunction(a, 
FI1.class), 0);
         System.out.println(nullCB.entryPoint().isNull()); // prints 
'true'
     }
```

Hope that helps,
Jorn

[1] : 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-February/004373.html

Mark Hammons schreef op 2019-02-16 15:53:
> Hi,
> 
> I'm back with a new wayland problem. More specifically, my problem is
> with a helper library called wlroots. One of its functions,
> wlr_backend_autocreate takes a function pointer as an argument. The
> big problem here is that jextract represents this as a Callback<Fi1>,
> and part of the valid input for this parameter is the null pointer. So
> the question is, how do I create a null callback? Can it even be done
> with foreign as it's currently made?
> 
> Thanks,
> 
> Mark Hammons