如何使用Java Panama FFI访问MemoryLayout中的C_POINTER

Question

使用JDK16中的FFI预览，我有这样的内存布局：

class FfiTest {
static GroupLayout layout = MemoryLayout.ofStruct(
C_INT.withName("someInt"),
MemoryLayout.ofPaddingBits(32), // So the following pointer is aligned at 64 bits
C_POINTER.withName("somePtr")
);
}

然后，我在本地代码的回调中收到一个指向这样一个结构的指针：

public static void someCallback(MemoryAddress address) {
try (MemorySegment seg = address.asSegmentRestricted(FfiTest.layout.byteSize())) {                
// Works: fetching int from native structure, correct value is returned
VarHandle intHandle = FfiTest.layout.varHandle(int.class, MemoryLayout.PathElement.groupElement("someInt"));
int intResult = (int) vh.get(seg);
// Does not work: get the pointer as a MemoryAddress, fatal JVM crash with Hotspot log
VarHandle badPtrHandle = FfiTest.layout.varHandle(MemoryAddress.class, MemoryLayout.PathElement.groupElement("somePtr"));
// Works: get the pointer as a long, correct value is returned
VarHandle goodPtrHandle = FfiTest.layout.varHandle(long.class, MemoryLayout.PathElement.groupElement("somePtr"));
long longResult = (long) goodPtrHandle.get(seg);
}
}

jdk.internal.foreign.Utils:中JDK代码内部引发异常

public static void checkPrimitiveCarrierCompat(Class<?> carrier, MemoryLayout layout) {
checkLayoutType(layout, ValueLayout.class);
if (!isValidPrimitiveCarrier(carrier))
throw new IllegalArgumentException("Unsupported carrier: " + carrier); // Throws this exception, carrier has the value MemoryAddress
if (Wrapper.forPrimitiveType(carrier).bitWidth() != layout.bitSize())
throw new IllegalArgumentException("Carrier size mismatch: " + carrier + " != " + layout);
}

根据Panama文档，C_POINTER的Java载波应该是MemoryAddress，但这在这里不起作用。

那么，使用long来访问这样的指针是正确的吗？还是别的什么？

Answer 1

Johannes在评论中指出，MemoryHandles::asAddressVarHandle可以用于调整您必须接受并返回MemoryAddress:的long句柄

VarHandle goodPtrHandle = FfiTest.layout.varHandle(long.class, MemoryLayout.PathElement.groupElement("somePtr"));
long longResult = (long) goodPtrHandle.get(seg);
VarHandle addrHandle = MemoryHandles.asAddressVarHandle(goodPtrHandle);
MemoryAddress addrResult = (MemoryAddress) addrHandle.get(seg);