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")
    );
}

然后我在本机代码的回调中收到指向此类结构的指针：

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 中的 JDK 代码内部抛出异常：

    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);
    }

根据巴拿马文档，C_POINTER 的 Java 承运人应该是 MemoryAddress，但这在这里不起作用。

那么使用 longs 来访问这样的指针是否正确？或者别的什么？

Answer 1

正如 Johannes 在评论中指出的那样，MemoryHandles::asAddressVarHandle 可用于调整您必须接受的 long 句柄和 return 一个 MemoryAddress:

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);

如何使用 Java Panama FFI 访问 MemoryLayout 中的 C_POINTER

How to access a C_POINTER in a MemoryLayout using Java Panama FFI

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ffi

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