GMP Wrapper by JNA, but this code is broken

gmp.java

package com.github.tell.gmp;

import java.util.logging.ConsoleHandler;
import java.util.logging.Handler;
import java.util.logging.Level;
import java.util.logging.Logger;

import com.github.tell.gmp.JNAGMP.GMPLib.mpz_struct;
import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.NativeLibrary;
import com.sun.jna.Structure;
import com.sun.jna.ptr.NativeLongByReference;

public class JNAGMP {
    static public final Logger logger = Logger.getLogger("gmp");

    public interface CLibrary extends Library {
        CLibrary INSTANCE = (CLibrary) Native.loadLibrary("c", CLibrary.class);

        void printf(String format, Object... args);
    }

    static private final NativeLibrary libgmp;
    static public final String gmpVersion;

    public interface GMPLib extends Library {
        public class mpz_struct extends Structure {
            public int _mp_alloc;
            public int _mp_size;
            public NativeLongByReference _mp_d;

            protected void initFieldOrder() {
                setFieldOrder(new String[] { "_mp_alloc", "_mp_size", "_mp_d" });
            }

            public mpz_struct() {
                super();
                initFieldOrder();
                allocateMemory();
                ensureAllocated();
                logger.fine(String.format("constructed at: %s", getPointer()));
            }

            public static class ByReference extends mpz_struct implements
                    Structure.ByReference {

            };

            public static class ByValue extends mpz_struct implements
                    Structure.ByValue {

            };
        }

        /*
         * public class mpz_t extends PointerType { public mpz_t() { super();
         * mpz_struct x = new mpz_struct(); setPointer(x.getPointer());
         * logger.fine(String.format("constructed, it has pointer at: %s",
         * getPointer())); }
         * 
         * public mpz_t(Pointer ptr) { super(ptr);
         * setPointer(ptr.getPointer(0));
         * logger.fine(String.format("constructed, it has pointer at: %s",
         * getPointer())); } }
         */

        public void __gmpz_init(mpz_struct x);

        public void __gmpz_clear(mpz_struct x);

        public void __gmpz_set_ui(mpz_struct x, long y);

        public void __gmpz_set_str(mpz_struct x, String numStr, int base);

        public long __gmpz_get_ui(mpz_struct x);

        public String __gmpz_get_str(String repStr, int base, mpz_struct x);
    }

    static public final GMPLib ifgmp;

    static {
        logger.addHandler(new ConsoleHandler());
        logger.setLevel(Level.FINE);
        for (final Handler h : logger.getHandlers()) {
            h.setLevel(Level.FINE);
        }

        GMPLib INSTANCE = (GMPLib) Native.loadLibrary("gmp", GMPLib.class);
        ifgmp = (GMPLib) Native.synchronizedLibrary(INSTANCE);

        libgmp = NativeLibrary.getInstance("gmp");
        gmpVersion = libgmp.getGlobalVariableAddress("__gmp_version")
                .getStringArray(0, 1)[0];
    }

    static public void main(String[] args) {
        CLibrary i = CLibrary.INSTANCE;
        i.printf("Hello World\n");
        i.printf("GMP version is %s\n", gmpVersion);

        mpz_struct x = new mpz_struct();

        final int numOfLoop = 1000;
        for (int j = 0; j < numOfLoop; j++) {
            ifgmp.__gmpz_init(x);
            System.out.printf("init: x = %d%n", ifgmp.__gmpz_get_ui(x));
            System.out.printf("init: x (str) = %s%n",
                    ifgmp.__gmpz_get_str(new String(), 10, x));

            ifgmp.__gmpz_set_ui(x, 10);
            System.out.printf("set 10: x = %d%n", ifgmp.__gmpz_get_ui(x));
            System.out.printf("set 10: x (str) = %s%n",
                    ifgmp.__gmpz_get_str(new String(), 10, x));

            ifgmp.__gmpz_set_str(
                    x,
                    "123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890",
                    10);
            System.out.printf("set str: x (str) = %s%n",
                    ifgmp.__gmpz_get_str(new String(), 10, x));

            ifgmp.__gmpz_clear(x);
            System.out.println("next");
        }
        System.out.println("finished");
    }
}