C# out IntPtr equivalent in Java

Question

I have a methods in C# which I call from .dll

[DllImport("somedll.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int find([MarshalAs(UnmanagedType.AnsiBStr, SizeConst = 64)] string atr, out IntPtr int);

[DllImport("somedll.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int getData(IntPtr int, int dataId, byte[] dataBuffer, ref int dataBufferSize);

In C# call of this methods looks like this

static IntPtr number = IntPtr.Zero;
static int res = 0;
try{
    number = IntPtr.Zero;
    res = find(null, out number);   
    if (number == IntPtr.Zero)
                throw new ApplicationException("Something is wrong");
    uint dataBufferSize = 1024;
    res = getData(number, 1, null, ref dataBufferSize);
}

I didn't find what would be equivalent in Java.

If I do it like this:

public int find(String atr, Pointer int);

It says

java.lang.Error: Invalid memory access
at com.sun.jna.Native.invokeInt(Native Method)
at com.sun.jna.Function.invoke(Function.java:419)
at com.sun.jna.Function.invoke(Function.java:354)
at com.sun.jna.Library$Handler.invoke(Library.java:244)

If I do it like this:

 public int find(String atr, IntByReference int);

Nothing happens.

Java code

IntByReference iref = new IntByReference();                     
res = find(null, iref);                                             
Pointer pointer = iref.getPointer();                            
int dataBufferSize = 1024;
byte[] dataBuffer = new byte[dataBufferSize];
res = getData(Pointer.nativeValue(pointer), 1, dataBuffer, dataBufferSize);

find returns 0, which means OK, but getData returns 6 which means that memory address is not good. By nothing happens I mean any other res than 0.

So, maybe you should drop the C# tag and add the JNI, or JNA tag? — Mike Nakis, Apr 11 '17 at 07:19
(I mean, you don't have to add any tags at all, and you may even use completely misleading tags if you so please, but then the people who might be able to provide an answer might not see your question.) — Mike Nakis, Apr 11 '17 at 07:21
Possible duplicate of [How to do the equivalent of pass by reference for primitives in Java](http://stackoverflow.com/questions/5614562/how-to-do-the-equivalent-of-pass-by-reference-for-primitives-in-java) — kennytm, Apr 11 '17 at 13:02
An `IntPtr` is 32 bit on 32 bit and 64 bit on 64 bit systems, you might be looking for `NativeLongByReference` instead (although I'm not sure it behaves like that on Windows). — Mark Rotteveel, Apr 12 '17 at 19:15

Daniel Widdis · Accepted Answer · 2017-04-14T17:48:02.113

IntByReference is a possible correct mapping (depending on what the pointer is pointing to); however, the value you're looking for is the native "peer" value inside the corresponding Pointer. You may want to use PointerType instead. Either way, the Invalid Memory Access is related to trying to access Native-side memory that you have not allocated. A plain Pointer doesn't allocate any memory Native-side: normally you would use a new Memory() to initialize a Pointer and allocate the memory it's pointing to. (If you create one of the ByReference types, it allocates the appropriate size there, which is why that error goes away.)

An IntPtr is an integer value representing the actual memory address, which you have to extract from the IntByReference or PointerType by using getPointer() to get the Pointer object, and then passing that to Pointer.nativeValue() to return the (long) memory address corresponding to your IntPtr.

Updated in response to your edited code:

A few wrong mappings that may be causing problems:

The signature for getData() requires dataBufferSize to be a reference to an int, so you should use IntByReference for that variable.

You cannot pass a plain array from Java to the native side. You need to allocate memory for it. For example:

Pointer dataBuffer = new Memory(dataBufferSize);
// call getData();
byte[] foo = dataBuffer.getByteArray(0, dataBufferSize);

Additional update:

It appears that the find() method call returns a pointer to the memory address you want, so you take the resulting IntByReference from find() and use getValue() to pass that integer to getData().

You may need to further process the resulting byte[] array. You haven't given the documentation for the API but if it returns a different value for dataBufferSize you may need to truncate your byte array to match. Since you named the IntByReference you passed to getData() as iref, this would be:

byte[] dataByteArray = new byte[iref.getValue()];
System.arraycopy( dataBuffer, 0, dataByteArray, 0, iref.getValue());
String data = new String(dataByteArray, "UTF-8");

If the C string is encoded differently than ASCII you may need to use a different conversion.

Now I have another problem, how can I get string from dataBuffer? `String str=new String(dataBuffer,"UTF8");` produces strange characters — vuis, Apr 13 '17 at 12:40
I don't know how your data is encoded. Is it ASCII? UTF-8 has a hyphen. — Daniel Widdis, Apr 13 '17 at 15:21
Values in C# byte array and Java are not the same. In C# I get 4 bytes, but in Java I get much more. — vuis, Apr 14 '17 at 06:55
`Pointer memory = new Memory(dataBufferSize.getValue()); byte[] buffer = memory.getByteArray(0, dataBufferSize.getValue()); String bufferContent = new String(buffer,"UTF-8"); ` bufferContent is full of some characters before calling getData(). `res = getData(number.getValue(),1, array,iref); byte[] dataBufferAfter=array.getByteArray(0,dataBufferSize);String bufferContentAfter= new String(dataBufferAfter,"UTF-8");` bufferContent and bufferContentAfter are the same — vuis, Apr 14 '17 at 10:34
It's really hard to read code in a comment. Since this outside of the scope of the original question you might consider a new question. General debugging steps are to output the byte array values as integers and see if you can figure out what string is encoded. C uses null terminated strings while Java ignores the zeroes in the `new String()` code you use. You can `trim()` if it's all trailing zeroes but the best thing is to find the string length (I assume `iref.getValue()` tells you this) and truncate the array and create the string with that. — Daniel Widdis, Apr 14 '17 at 16:16

vuis · Answer 2 · 2017-04-14T07:43:27.607

It works if I pass number.getValue() to getData(). Now res is 0.

Complete code:

IntByReference number=new IntByReference(0);                        
res = find(null, number);                                   
int dataBufferSize = 1024;
IntByReference iref=new IntByReference(dataBufferSize);
Pointer array = new Memory(dataBufferSize);                         
res = getData(number.getValue(),1, array,iref);                 
byte[] dataBuffer=array.getByteArray(0,dataBufferSize);

C# out IntPtr equivalent in Java

2 Answers2