How to return a optional string from native methods?

[tison]

I found reinterpret that can help in the first step where length=0 for the
returned memory. So we may return a leading long with how many bytes is
followed?

final MemorySegment version = (MemorySegment)
datafusionVersionMethodHandle.invokeExact();
return version.reinterpret(10, arena).getUtf8String(0);

Best,
tison.


tison <wander4096 at gmail.com> 于2023年9月29日周五 22:08写道：

> Here is my native method written in Rust:
>
> #[no_mangle]
> pub unsafe extern "C" fn datafusion_version() -> *const c_char {
>     CString::new(datafusion::DATAFUSION_VERSION).unwrap().into_raw()
> }
>
> And below is my attempt to bind it with FFM APIs:
>
>     static MethodHandle createMethodHandle(String name, FunctionDescriptor
> descriptor) {
>         final MemorySegment fp = LOADER.lookup.find(name).orElseThrow();
>         return LOADER.linker.downcallHandle(fp, descriptor);
>     }
>
>     private static final MethodHandle datafusionVersionMethodHandle =
> NativeLoader.createMethodHandle(datafusionVersionMethodName,
> datafusionVersionMethodDesc);
>
>     @SneakyThrows
>     public static String datafusionVersion() {
>         final MemorySegment version = (MemorySegment)
> datafusionVersionMethodHandle.invokeExact();
>         return version.getUtf8String(0);
>     }
>
> I noticed that the returned MemorySegment is always with length 0 and thus
> any access with return OutOfBoundException.
>
> In the demos from JEP, all memory segments are allocated from the Java
> side and the native code only move or modify those allocated memory
> segments instead of allocate/shrink memory.
>
> I wonder what is the formal method to allocate a string from native
> methods and pass back to the Java world.
>
> And, if we think of error handling, how do we properly return a NULL from
> native methods, and how do we throw exceptions from native methods?
>
> MemorySegment.NULL seems an Object without overriding equals, and JNIEnv's
> throw counterpart is missing or I don't find it (perhaps upcall helps but
> it's still a bit away from real-world usage).
>
> Best,
> tison.
>
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