JNA program function lookup failure

Question

I'm newbee with JNA programing, the task I want to accomplish is:

1. C++ library exposes functionality to "put" a buffer in to a file and "lookup" a buffer. I compiled a shared object (.so) for this library with the header file providing the function definitions under "extern "C" " to make it C compiler friendly.

2. Test java program to access the buffers.

The code looks like this:

C/C++ code:

extern "C"
{
int get(int length, char *buffer);
}

#include <iostream>
#include <string.h>

int get(int length, char *buffer)
{
    char *newBuff = new char[length];
    for (int i = 0; i < length; ++i)
    {
        newBuff[i] = 'a';
    }

    memcpy(newBuff, buffer, length);
    delete newBuffer;
    return length;
}

java code:

import com.sun.jna.Library;
import com.sun.jna.Memory;
import com.sun.jna.Native;

public class TestJna
{
    public static interface TestNative extends Library
    {
        int get(int length, Memory buffer);
    }
    private static final TestNative lib_ = (TestNative)Native.loadLibrary("libsample.so", TestNative.class);
    public static void main(String[] args)
    {
        int length = 1024;
        Memory buffer = new Memory(length);
        int ret = lib_.get(length, buffer);
        System.out.println("ret:" + ret + ":buffer:" + buffer.toString());
    }
}

On running the program I get below error message on invocation of "lib.get()" method:

Exception in thread "main" java.lang.UnsatisfiedLinkError: Error looking up function 'get': dlsym(0x7f8d08d1e7d0, get): symbol not found

Answer 1

I was able to make it work by modifying code as follows:

public static interface TestNative extends Library
    {
        int get(int length, Pointer buffer);
    }

Pointer was obtained by:

Pointer bfPtr = Native.getDirectBufferPointer(buffer); // buffer points to ByteBuffer allocated as direct NIO buffer.

Answer 2

Your exported symbol (according to nm) is mangled. You need to add extern "C" prior to your function definition in addition to its declaration, i.e.

extern "C" get(int length, char* buffer) {
    ...
}

The first extern "C" syntax you use is typically used for groups of declarations in a header file. You also must explicitly unmangle the definition.