Question 414902 in Ubuntu Question posted in
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Ubuntu – Access a C++ struct from Java using JNA

I’m new to JNA and trying to put together a very minimal proof-of-concept for accessing structs from a C++ shared library. Unfortunately, it doesn’t run and I cannot figure it out.

Here’s my code:

Main.java:

import com.sun.jna.Structure;
import com.sun.jna.Native;
import com.sun.jna.Library;
import com.sun.jna.Structure.FieldOrder;

public class Main {
    public interface MyCppLibrary extends Library {
        MyCppLibrary INSTANCE = (MyCppLibrary) Native.load("mycpplib", MyCppLibrary.class);

        @FieldOrder({"x", "y"})
        class MyStruct extends Structure {
            public int x;
            public int y;
        }

        MyStruct getStruct();
    }

    public static void main(String[] args) {
        MyCppLibrary lib = MyCppLibrary.INSTANCE;
        MyCppLibrary.MyStruct myStruct = lib.getStruct();

        System.out.println("x: " + myStruct.x);
        System.out.println("y: " + myStruct.y);
    }
}

MyStruct.cpp:

#include <iostream>

struct MyStruct {
    int x;
    double y;
};

extern "C" {
    MyStruct* getStruct() {
        return new MyStruct{10, 20.5};
    }
}

I compile the C++ code into a shared library (using WSL2 Ubuntu 20.04):

g++ -v -shared -o libmycpplib.so my_struct.cpp

But when I run the Java class (of course including a recent JNA jar, jna-5.15.0.jar, to my CLASSPATH) I get the following error:

Exception in thread "main" java.lang.UnsatisfiedLinkError: Error looking up function 'getStruct': /home/h3007/.vscode-server/data/User/workspaceStorage/af05c638282256cbf9a3f960fefbba02/redhat.java/jdt_ws/hassan-jna-struct_c38993c4/bin/libmycpplib.so: undefined symbol: getStruct
        at com.sun.jna.Function.<init>(Function.java:255)
        at com.sun.jna.NativeLibrary.getFunction(NativeLibrary.java:618)
        at com.sun.jna.NativeLibrary.getFunction(NativeLibrary.java:594)
        at com.sun.jna.NativeLibrary.getFunction(NativeLibrary.java:580)
        at com.sun.jna.Library$Handler.invoke(Library.java:248)
        at jdk.proxy1/jdk.proxy1.$Proxy0.getStruct(Unknown Source)
        at Main.main(Main.java:21)

I’m assuming I’ve done something silly. Apologies, but pointers to my error would be appreciated.

Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    Apologies, problem was as simple as a missing EOF character to the my_struct.cpp file ! At least the above should work for others interesting in trying out JNA!


  2. 0

    I managed to make your proposal working with command line with two modifications.

    First, I had to compile MyStruct.cpp with the -fPIC option:

    g++  -shared -o libmycpplib.so MyStruct.cpp -fPIC

    Second, you have to be careful in the Main.java file since the type of y should be coherent with the type double in the cpp file:

    import com.sun.jna.Structure;
import com.sun.jna.Native;
import com.sun.jna.Library;
import com.sun.jna.Structure.FieldOrder;

public class Main 
{
    public interface MyCppLibrary extends Library 
    {
        MyCppLibrary INSTANCE = (MyCppLibrary) Native.load("mycpplib", MyCppLibrary.class);

        @FieldOrder({"x", "y"})
        class MyStruct extends Structure 
        {
            public int x;
            public double y;
        }

        MyStruct getStruct();
    }

    public static void main(String[] args) 
    {
        MyCppLibrary lib = MyCppLibrary.INSTANCE;
        MyCppLibrary.MyStruct myStruct = lib.getStruct();

        System.out.println("x: " + myStruct.x);
        System.out.println("y: " + myStruct.y);
    }
}

    Then compile and launch with:

    javac -classpath /usr/share/java/jna.jar  Main.java
java  -classpath /usr/lib/java/jna.jar:.  Main

    Possible output:

    x: 10
y: 20.5
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