Null Callbacks

[Mark Hammons]

I'll keep that in mind Jorn, thanks. I found an alternative workaround 
(I found the default function that was being used in place of the 
callback if the callback was null, and just turned that into my 
callback), but now it seems I'm truly blocked by the issue I posted 
about yesterday: Exception in thread "main" 
java.lang.UnsupportedOperationException: bitsSize on Unresolved.

Using jextract on wlroots introduced a lot of new headers, and a lot of 
cross-header dependencies. There's a struct defined as:

```

@NativeStruct("[${wl_signal}(destroy)${wl_signal}(new_input)${wl_signal}(new_output)](anon$backend_h$444)")
     public interface anon$backend_h$444 extends 
Struct<backend.anon$backend_h$444> {
         @NativeLocation(
             file = "/usr/include/wlr/backend.h",
             line = 23,
             column = 20
         )
         @NativeGetter("destroy")
         wl_signal destroy$get();

         @NativeSetter("destroy")
         void destroy$set(wl_signal var1);

         @NativeAddressof("destroy")
         Pointer<wl_signal> destroy$ptr();

         @NativeLocation(
             file = "/usr/include/wlr/backend.h",
             line = 25,
             column = 20
         )
         @NativeGetter("new_input")
         wl_signal new_input$get();

         @NativeSetter("new_input")
         void new_input$set(wl_signal var1);

         @NativeAddressof("new_input")
         Pointer<wl_signal> new_input$ptr();

         @NativeLocation(
             file = "/usr/include/wlr/backend.h",
             line = 27,
             column = 20
         )
         @NativeGetter("new_output")
         wl_signal new_output$get();

         @NativeSetter("new_output")
         void new_output$set(wl_signal var1);

         @NativeAddressof("new_output")
         Pointer<wl_signal> new_output$ptr();
     }

```

and it cannot be instantiated. The ```val l2 = 
Libraries.bind(MethodHandles.lookup(), classOf[wlroots.backend])``` 
trick that worked yesterday doesn't work for this either so I'm kinda 
roadblocked.

Mark

On 2/16/19 4:31 PM, Jorn Vernee wrote:
> 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