JavaCPP problems compiling the example Callback function

Question

I just wanted to test Callback function example from the webpage.

https://github.com/bytedeco/javacpp#creating-callback-functions

In file foo.cpp

#include <iostream>
#include "jniFoo.h"

int main() {
    JavaCPP_init(0, NULL);
    try {
        foo(6, 7);
    } catch (std::exception &e) {
        std::cout << e.what() << std::endl;
    }
    JavaCPP_uninit();
}

Foo.java where function foo performs

import org.bytedeco.javacpp.*;
import org.bytedeco.javacpp.annotation.*;

@Platform(include="<algorithm>")
@Namespace("std")
public class Foo {
    static { Loader.load(); }

    public static class Callback extends FunctionPointer {
        // Loader.load() and allocate() are required only when explicitly creating an instance
        static { Loader.load(); }
        protected Callback() { allocate(); }
        private native void allocate();

        public @Name("foo") boolean call(int a, int b) throws Exception { 
            throw new Exception("bar " + a * b);
        }
    }

    // We can also pass (or get) a FunctionPointer as argument to (or return value from) other functions
    public static native void stable_sort(IntPointer first, IntPointer last, Callback compare);

    // And to pass (or get) it as a C++ function object, annotate with @ByVal or @ByRef
    public static native void sort(IntPointer first, IntPointer last, @ByVal Callback compare);
}

Building and running this sample code with these commands under Linux x86_64:

javac -cp javacpp.jar Foo.java
java -jar javacpp.jar Foo -header
g++ -I/usr/lib/jvm/java-8-oracle/include/ -I/usr/lib/jvm/java-8-oracle/include/linux/ linux-x86_64/libjniFoo.so foo.cpp -o Foo

In the third command, I got error:

/tmp/ccvrmILI.o: In function `main':
foo.cpp:(.text+0x14): undefined reference to `JavaCPP_init'
foo.cpp:(.text+0x1e): undefined reference to `onAddTwoInteger'
foo.cpp:(.text+0x23): undefined reference to `JavaCPP_uninit'
collect2: error: ld returned 1 exit status  

Why do I get it?

Answer 1

You get the error because the library is placed before the object file at the command line.

The g++ command in the example may work somewhere but it does not work at gcc/linux-linker. The problem is the argument order in the command. If you run

g++ -I/usr/lib/jvm/java-8-oracle/include/ -I/usr/lib/jvm/java-8-oracle/include/linux/ foo.cpp linux-x86_64/libjniFoo.so -o Foo

the it will compile and link successfully.