How to use a struct with a struct in jnr ffi

Question

I have the following c code:

#include <stdio.h>

struct Second {
    int a_number;
};

struct Top {
    struct Second second;
};

void lets_go(struct Top *top) {
    printf("The number is %d\n", top->second.a_number);
}

And I want to do this from Java:

int main(void) {
    struct Top top = {{8}};
    lets_go(&top);
}

I also want to use jnr-ffi, so I looked at the tests and ended up with this:

package structs.playing;

import structs.playing.Program.Test.Top;
import structs.playing.Program.Test.Second;
import jnr.ffi.LibraryLoader;
import jnr.ffi.Runtime;
import jnr.ffi.Struct;

public final class Program {

    public static interface Test {

        void lets_go(Top top);

        public static final class Second extends Struct {               
            public final Signed32 a_number = new Signed32();                
            public Second(final Runtime runtime) {
                super(runtime);
            }
        }

        public static final class Top extends Struct {              
            public Second second;                           
            public Top(final Runtime runtime) {
                super(runtime);
            }
        }
    }

    public static void main(final String[] args) {

        Test test = LibraryLoader.create(Test.class).load("test");
        Runtime runtime = Runtime.getRuntime(test);         
        Top top = new Top(runtime);
        Second second = new Second(runtime);
        top.second = second;
        second.a_number.set(7);         
        test.lets_go(top);
    }    
}

The problem is that the value of a_number is not set at all so I get a junk value in the output, for example:

The number is 46645760

So how do I get the same as in my C code?

Answer 1

I have figured it out (by the way, I am aware that the members should be private and wrapped in properties but I wanted to make the code snippet as small as possible, this is not production quality code)...

If you put a Pointer member variable into the struct you can use it's memory when you construct the sub-ordinate Struct like so...

package structs.playing;

import structs.playing.Program.Test.Top;
import jnr.ffi.LibraryLoader;
import jnr.ffi.Runtime;
import jnr.ffi.Struct;

public final class Program {

    public static interface Test {

        void lets_go(Top top);

        public static final class Second extends Struct {

            public final Signed32 a_number = new Signed32();

            public Second(final Runtime runtime) {
                super(runtime);
            }           
        }

        public static final class Top extends Struct {

            private final Pointer secondPointer = new Pointer();            
            public final Second second;

            public Top(final Runtime runtime) {
                super(runtime);                             
                second = new Second(runtime); 
                second.useMemory(secondPointer.getMemory());
            }           
        }
    }

    public static void main(final String[] args) {

         Test test = LibraryLoader.create(Test.class).load("test");
         Runtime runtime = Runtime.getRuntime(test);         
         Top top = new Top(runtime);
         top.second.a_number.set(8);         
         test.lets_go(top);
    }
}