Funneling Objects through void*

[Maurizio Cimadamore]

On 15/06/2023 17:18, Johannes Kuhn wrote:
> Yes, I have read the thread. That StackOverflow question (and the 
> thread) was just an other trigger to discuss a IMHO a more fundamental 
> primitive: Passing an java object through a void* pointer.


If you can create a JNI reference from an object and get a segment for 
it, you can also pass it to a foreign function, no?

It seems to me that you want to be able to pass the object _directly_ to 
the function, which, to me, seems like a lot of accidental complexity 
for the linker machinery, with not a lot of upside (if you have some 
_other_ way to go Object -> segment).

>
>
> I just want a pair of methods that turn an object into a pointer and 
> back - preferably with a deterministic "deallocation".
> How that is implemented is a different story.

In the thread I quoted, there's this:

> native long makeGlobalRef(Object o) // call NewGlobalRef, cast result to
> jlong and return
> native void destroyGlobalRef(long ref) // cast to jobject, then call
> DeleteGlobalRef
Wouldn't that pair of functions allow you to do what you want? It is 
fairly simple to provide FFM bindings for these. For instance, few weeks 
ago I created this class (note, JNIUtils define the native methods to 
bind to the functionality described above):

```java
import java.lang.foreign.Arena;
import java.lang.foreign.MemorySegment;

public class JNIUtilsWrapper {
     public static MemorySegment newGlobalRef(Object o, Arena arena) {
         return MemorySegment.ofAddress(JNIUtils.makeGlobalRef(o))
                 .reinterpret(arena, (ref) -> 
JNIUtils.destroyGlobalRef(ref.address()));
     }

     public static Object getRef(MemorySegment ref) {
         return JNIUtils.readGlobalRef(ref.address());
     }
}
```

Which I then used like so:

```java
try (Arena arena = Arena.ofConfined()) {
       MemorySegment str = JNIUtilsWrapper.newGlobalRef("hello", arena);
       System.out.println(JNIUtilsWrapper.getRef(str));
}
```

Maurizio
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