无效的 memory 访问，JNA 从结构内的结构数组读取

Question

I'm tasked with migrating a C# app to a java one. The C# app uses couple DLLs to do its works, comunicating with peripheral devices.

The header for the DLLs looks like this in C#

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
    public struct InnerStructure
    {
        /// COM port used by the device
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = PORT_SIZE)] //7
        public string szPort;            

        /// Specifies whether the device is activated.
        public bool fActivated;         

        /// Name of the device
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAX_NAME_SIZE)]  //248
        public string szName;             //COM
    }

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
    public struct ParamStructure
    {
        /// COM port used by the devices
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 10)]
        public InnerStructure[] USB;
    }

    [DllImport("PCLService.dll", CallingConvention = CallingConvention.Cdecl)]
    public static extern bool startPclService();

    [DllImport("PCLService.dll", CallingConvention = CallingConvention.Cdecl)]
    public static extern bool stopPclService();

    [DllImport("PclUtilities.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Auto)]
    public static extern int getUSBDevices(ref ParamStructure pStructure, ref int pdwSize, ref int pdwEntries);

I tried initially with JNI, but I couldn't manage to load the DLLs (they use .NET, and it was a pain to find the dependencies with DependenciesWalker), so I switched to JNA.

This is my Java code

public class DllDemo {

    @Structure.FieldOrder({ "szPort", "fActivated", "szName" })
    public static class InnerStructure extends Structure {
        public PointerByReference szPort;
        public IntByReference fActivated;
        public PointerByReference szName;

        public InnerStructure() {};
        public InnerStructure(Pointer p) {
            super(p);
            read();
        };
    }

    @Structure.FieldOrder({ "USB" })
    public static class ParamStructure extends Structure implements Structure.ByReference {
        // In the C# code, the array is size 10
        public InnerStructure[] USB = (InnerStructure[])new InnerStructure().toArray(10);

        public ParamStructure() {};
        public ParamStructure(Pointer p) {
            super(p);
            read();
        };
    }

    public interface MyService extends Library {
        MyService INSTANCE = (MyService) Native.load("C:\\IRD\\Documentacion\\VisaNet\\App PCL Demo VisaNet - V2.11\\x32\\PCLService.dll", MyService.class);
        boolean startPclService();
        boolean stopPclService();
    }

    public interface MyUtilities extends Library {
        MyUtilities INSTANCE = (MyUtilities) Native.load("C:\\IRD\\Documentacion\\VisaNet\\App PCL Demo VisaNet - V2.11\\x32\\PclUtilities", MyUtilities.class);
        int getUSBDevices(ParamStructure paramStructure, IntByReference pdwSize, IntByReference pdwEntries);
    }

    public static void main(String args[]) {
        System.out.println("start");

        MyService.INSTANCE.startPclService();
        ParamStructure paramStructure = new ParamStructure();
        paramStructure.write();
        //This value is copied straightforward from the original code as well
        int size = (248 + 8 + 7) * 10;
        IntByReference pdwSize = new IntByReference(size);
        IntByReference pdwEntries = new IntByReference(0);

        int Ret2 = MyUtilities.INSTANCE.getUSBDevices(paramStructure, pdwSize, pdwEntries);
        System.out.println("Ret2 = " + Ret2 + ", pdwEntries = " + pdwEntries.getValue());
        if (pdwEntries.getValue() > 0) {
            for (int i = 0 ; i < pdwEntries.getValue() ; i++) {
                InnerStructure inner = paramStructure.USB[i];
                inner.read();
                System.out.println(i + " => " + inner.toString());
                System.out.println("toString 1 => " + inner.szPort.toString());
                System.out.println("toString 2 => " + inner.szPort.getPointer().toString());
                System.out.println(">" + inner.szPort.getPointer().getString(0, "utf8") + "<");
            }
        }
        paramStructure.clear();
        MyService.INSTANCE.stopPclService();
        System.out.println("stop");
    }
}

And this is the output.

start
Ret2 = 0, pdwEntries = 1
0 => DllDemo$InnerStructure(allocated@0x59b550 (12 bytes) (shared from auto-allocated@0x59b550 (120 bytes))) {
  PointerByReference szPort@0x0=native@0x4f0043 (com.sun.jna.ptr.PointerByReference@4f0043)
  IntByReference fActivated@0x4=native@0x35004d (com.sun.jna.ptr.IntByReference@35004d)
  PointerByReference szName@0x8=null
}
toString 1 => native@0x4f0043 (com.sun.jna.ptr.PointerByReference@4f0043)
toString 2 => native@0x4f0043
Exception in thread "main" java.lang.Error: Invalid memory access
    at com.sun.jna.Native.getStringBytes(Native Method)
    at com.sun.jna.Native.getString(Native.java:2224)
    at com.sun.jna.Pointer.getString(Pointer.java:681)
    at com.ingenico.DllDemo.main(DllDemo.java:65)
Process finished with exit code 1

Line 65 is this

System.out.println(">" + inner.szPort.getPointer().getString(0, "utf8") + "<");

SOMETIMES it doesn't give an error, but the string is empty. I haven't figured out why this happens.

The presence or absence of the constructors in both classes and the lines paramStructure.write() and inner.read() make no difference.

For whatever is worth, this is how it looks in the debugger from IntelliJ

debugger

I have tried changing the inner structure like this

public static class InnerStructure extends Structure implements Structure.ByReference {
    public PointerByReference szPort;
    public IntByReference fActivated;
    public PointerByReference szName;

    public InnerStructure() {};
    public InnerStructure(Pointer p) { super(p); };
}

Or even like this.

public static class InnerStructure extends Structure implements Structure.ByReference {
    public String szPort;
    public int fActivated;
    public String szName;

    public InnerStructure() {};
    public InnerStructure(Pointer p) { super(p); };
}

In both cases, I get

Exception in thread "main" java.lang.Error: Invalid memory access
    at com.sun.jna.Native._getPointer(Native Method)
    at com.sun.jna.Native.getPointer(Native.java:2211)
    at com.sun.jna.Pointer.getPointer(Pointer.java:642)
    at com.sun.jna.Pointer.getValue(Pointer.java:390)
    at com.sun.jna.Structure.readField(Structure.java:732)
    at com.sun.jna.Structure.read(Structure.java:591)
    at com.sun.jna.Structure.autoRead(Structure.java:2141)
    at com.sun.jna.Structure.conditionalAutoRead(Structure.java:561)
    at com.sun.jna.Structure.updateStructureByReference(Structure.java:690)
    at com.sun.jna.Pointer.readArray(Pointer.java:492)
    at com.sun.jna.Pointer.getValue(Pointer.java:450)
    at com.sun.jna.Structure.readField(Structure.java:732)
    at com.sun.jna.Structure.read(Structure.java:591)
    at com.sun.jna.Structure.autoRead(Structure.java:2141)
    at com.sun.jna.Function.invoke(Function.java:381)
    at com.sun.jna.Library$Handler.invoke(Library.java:265)
    at com.sun.proxy.$Proxy3.getUSBDevices(Unknown Source)
    at com.ingenico.DllDemo.main(DllDemo.java:49) <- getUSBDevices

Answer 1

You're not correctly mapping the strings.

You've defined szPort as a PointerByReference , which is a pointer to a memory location holding a pointer. Then you are trying to call it with inner.szPort.getPointer() . That's still the same "pointer to the pointer". A good rule of thumb with JNA is that if you are using a ByReference class and never accessing the getValue() method on it, you're probably doing someting wrong. You probably meant inner.szPort.getValue().getString(...) . But that would have failed, because you don't actually have a real pointer. The first 4 bytes that fill the szPort element are actually unicode characters.

You've defined the structure with 3 4-byte elements (3 x 4-byte pointers) totaling 12 bytes (per your debug image). You actually need a 7-character fixed width string, 4-byte boolean, and 248-character fixed width string. either 259 bytes if your encoding is ASCII, or in your case, 514 bytes for Unicode strings.

Looking at your debug image, we can see what's happening. The first 4 bytes contain 0x0043004f which you're treating like a pointer (address 0x4f0043) and trying to read data from. But it's really the unicode characters 0x0043 ("C") and 0x004f ("O"). When you try to read from that address, you don't own the the memory. If you're lucky the memory is zero and you never read anything, and your code returns an zero-length null terminated string. But if that memory is anything other than null, you get the error.

The next 4 bytes are 0x004d0035 which you're referring to as an IntByReference "pointer" (0x35004d) but it's really the characters "M" and "5". Since the szName field has a null pointer, it appears the next 4 bytes are 0x00000000 and you've hit the null terminator. So it looks like your szPort String is "COM5", hiding right there in plain sight!

So now we know why your code is breaking. How do you fix it?

Your C# mapping defines the type of szPort is UnmanagedType.ByValTStr which, according to MS Docs , is:

A fixed-length array of characters; the array's type is determined by the character set of the containing structure.

You probably want a byte array in that structure (that you can toss into a String constructor with the appropriate encoding), with a length of PORT_SIZE x character width (1 if your encoding is ASCII, 2 for Unicode). You should map szName similarly with charwidth x MAX_NAME_SIZE .

Finally, IntByReference is also a pointer (4-bytes on your system) but the original variable in C# is a bool . Without marshaling it, it maps to the Windows BOOL which is a plain 4-byte C int . Java's boolean is also 4 bytes so you can just use that here. So you probably want something like:

int CHAR_WIDTH = W32APITypeMapper.DEFAULT == W32APITypeMapper.UNICODE ? 2 : 1;
int PORT_SIZE = 7;
int MAX_NAME_SIZE = 248;

class InnerStructure ... {
    public byte[] szPort = new byte[CHAR_WIDTH * PORT_SIZE];
    public boolean fActivated;
    public byte[] szName = new byte[CHAR_WIDTH * MAX_NAME_SIZE];
...
}