How to return a optional string from native methods?

[Jorn Vernee]

 > 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).

Native functions can just return NULL/0, and the linker will 
automatically translate that into MemorySegment.NULL.

As for throwing a Java exception from native code, similar to what is 
possible with JNI Throw(New) [1], this is not supported. The Linker 
operates on the general assumption that the native code it is 
interacting with has no knowledge about Java at all. So, there is no 
FFM-specific API for the native code to interact with the Java side.

Jorn

[1]: 
https://docs.oracle.com/en/java/javase/21/docs/specs/jni/functions.html#throw

On 29/09/2023 16:08, tison wrote:
> 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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