Test.java

import java.lang.foreign.*;
import java.lang.invoke.*;
import java.util.Arrays;

import static java.lang.foreign.ValueLayout.*;

public class Test {
    @FunctionalInterface
    public interface Callback {
        int invoke(MemorySegment a, MemorySegment b);
    }

    private static final MethodHandle callbackMethodHandle;
    private static final MethodHandle qsortHandle;

    static {
        try {
            callbackMethodHandle = MethodHandles.publicLookup()
                    .findVirtual(Callback.class, "invoke", MethodType.methodType(int.class, MemorySegment.class, MemorySegment.class));

            qsortHandle = Linker.nativeLinker().downcallHandle(
                    Linker.nativeLinker().defaultLookup().find("qsort").get(),
                    FunctionDescriptor.ofVoid(ADDRESS, JAVA_LONG, JAVA_LONG, ADDRESS)
            );
        } catch (NoSuchMethodException | IllegalAccessException e) {
            throw new InternalError(e);
        }
    }

    public static void qsort(MemorySegment array, Callback callback) {
        try (var arena = Arena.ofConfined()) {
            MemorySegment comparator = Linker.nativeLinker().upcallStub(
                    callbackMethodHandle.bindTo(callback),
                    FunctionDescriptor.of(JAVA_INT, ADDRESS.withTargetLayout(JAVA_INT), ADDRESS.withTargetLayout(JAVA_INT)),
                    arena);

            qsortHandle.invokeExact(
                    array,
                    (long) (array.byteSize() / Integer.BYTES),
                    (long) Integer.BYTES,
                    comparator);
        } catch (Throwable e) {
            throw new InternalError(e);
        }
    }

    public static void main(String[] args) throws Exception {
        try (var arena = Arena.ofConfined()) {
            var array = arena.allocateArray(JAVA_INT, 0, 9, 3, 4, 6, 5, 1, 8, 2, 7);
            qsort(array, (a, b) -> Integer.compare(a.get(JAVA_INT, 0), b.get(JAVA_INT, 0)));
            System.out.println(Arrays.toString(array.toArray(JAVA_INT)));
        }
    }
}