[External] : Re: How to return a optional string from native methods?

[Jorn Vernee]

Note that if you just want to dispose of the string after the 
conversion, you can also do:

     private static String readUtf8String(MemorySegment segment) {
         final MemorySegment string = segment.reinterpret(Long.MAX_VALUE);
         String result = string.getUtf8String(0);
         NativeMethods.disposeString(segment);
         return result;
     }

This behaves more or less equivalently to your version. There's no need 
to create a new Arena. MemorySegment::reinterpret doesn't copy the 
underlying data, but MemorySegment::getUtf8String does.

Jorn

On 30/09/2023 05:18, tison wrote:
> Thank you! Finally I work out a string conversion utility:
>
>   private static String readUtf8String(MemorySegment segment) {
>         try (Arena arena = Arena.ofConfined()) {
>             final MemorySegment string = 
> segment.reinterpret(Long.MAX_VALUE, arena, NativeMethods::disposeString);
>             return string.getUtf8String(0);
>         }
>     }
>
> #[no_mangle]
> pub unsafe extern "C" fn dispose_string(s: *mut c_char) {
>     drop(CString::from_raw(s))
> }
>
> Hopefully, this doesn't create too much overhead (I'm not sure the 
> overhead of creating Arena), and no other leaks. It likes 
> JNIEnv#NewString and IIUC JNIEnv#NewString also copy bytes.
>
> > Native functions can just return NULL/0, and the linker will 
> automatically translate that into MemorySegment.NULL.
>
> Good to know!
>
> > So, there is no FFM-specific API for the native code to interact with 
> the Java side
>
> OK. Perhaps I should return an error union in this case and decode & 
> throw exceptions on the Java side.
>
> Best,
> tison.
>
>
> Jorn Vernee <jorn.vernee at oracle.com> 于2023年9月29日周五 23:42写道：
>
>     > 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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