JNA String array issue

[Janko Sokolovic]

I have a fortran subroutine, which I want to wrap via jna.

I have created a dll from fortran file and subroutine's input parameter is string array (i.e. Character*8 arr(3))
So when I try to call it via jna, if Im not doing anything that changes the array's values, it doesn't crash, but when I change anything, i.e.  arr(1) = "new_str" (in fortran) and then try to call it via jna, it throws illegal memory access..

I know that for regular strings i can use byte[]  and it works fine, but JNA doesn't allow byte[][] which would probably handle this issue correctly..

So I've tried with String[] and StringArray, and even Pointer, but whenever I try to change the value of array's elements inside fortran's dll, I get an error with memorry access issue, while passing string as byte[] and changing it, goes well w/o issues..

Here's the example Im working on

FORTRAN:

SUBROUTINE STRARR(arr)

CHARACTER*8 :: arr(3)

arr(1)="newstr11"

arr(2)="newstr22"

arr(3)="newstr33"

END SUBROUTINE

JAVA:

interface F95Test extends com.sun.jna.Library {

        F95Test lib = (F95Test) Native.loadLibrary("strarrtest", F95Test.class); // strarrtest is dll with the fortran subroutine

        void STRARR(StringArray line);

    }

 public static void main(String[] args) throws UnsupportedEncodingException {

        String[] a = {"str_tes1", "str_tes2", "str_tes3"};

       
        StringArray sa =  new StringArray(a);

        F95Test.lib.STRARR(sa); //this is where it crashes with illegal memory access

        for ( String az : a ) System.out.println(az);

    }

Hope it's clear enough, I feel there's a simple solution for this, but I just can't seem to grab it yet..

Cheers

[Timothy Wall]

A writable byte buffer is not the same as a string. You can only use String for `const char*`; if your target writes back to the buffer you need to use `byte[]` or `char[]` instead (depending on your native character type).

After your native call, you can use Native.toString(byte[]) to convert a NUL-terminated byte array back into a String with proper decoding (if any).

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[Timothy Wall]

Oops, sorry I misread that.

Same thing goes for string arrays, though. What you’ve actually got is an array of Pointer, so start with that (Pointer[]) and initialize each element with a com.sun.jna.Memory element of the appropriate size, e.g.

Pointer[] array = new Pointer[] {
new Memory(3), new Memory(3), …, new Memory(3)
}

Pointer.getString(0) on each element will then pull back whatever the native side wrote.

On Jul 30, 2014, at 9:53 AM, Janko Sokolovic <jankoso...@gmail.com> wrote:

[Janko Sokolovic]

But how do I set the values of my strings now and send them as parameters ?
i.e strings are   {"one", "two", "thr"}

now i create Pointer[] array {

    new Memory(3), new Memory(3), new Memory(3)

}

and I tried array[0].setString(0, "one"); // but this gives me outofboundsexception, says that offset is 4 even though it is 0?!

Janko Sokolović

On 31 July 2014 13:49, Timothy Wall <twal...@java.net> wrote:

Oops, sorry I misread that.

Same thing goes for string arrays, though.  What you've actually got is an array of Pointer, so start with that (Pointer[]) and initialize each element with a com.sun.jna.Memory element of the appropriate size, e.g.

Pointer[] array = new Pointer[] {

    new Memory(3), new Memory(3), ..., new Memory(3)

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[Marco Trudel]

Or, using our native interfacing approach [1]:

Container<String8> c = new Container<String8>(3){};

That's all.
Ok, I'll stop doing advertising now :-)

Cheers!
Marco

[1] https://groups.google.com/forum/#!topic/jna-users/tZtKg7e-jTE

On 07/31/2014 01:49 PM, Timothy Wall wrote:
> Oops, sorry I misread that.
>
> Same thing goes for string arrays, though. What you've actually got is an array of Pointer, so start with that (Pointer[]) and initialize each element with a com.sun.jna.Memory element of the appropriate size, e.g.
>
> Pointer[] array = new Pointer[] {

> new Memory(3), new Memory(3), ..., new Memory(3)

[Timothy Wall]

Just allocate one more byte per buffer to account for the NUL terminator in each string. The receiving end won’t likely care that there’s an extra byte available.