Is cast (NonJavaObject*) to (long) legal for cni callback purposes?

Fri Jun 23 15:47:00 GMT 2006

Hi, I'm currently using a procedure described below to do c2j2c 
callbacks with cni.
It seems to work for now, but I'm not quite sure it's supposed to ALWAYS 
work.
I was wondering if maybe somebody more knowledgable could tell me if 
it's ok.
============================
I want to be able to
1. Create an instance of NonJavaClass in cpp code.
2. Have cpp call java using cni.
3. Have java do a callback to the original NonJavaClass instance using cni.
============================
The way I do it is as follows:
1. I cast the pointer to the NonJavaClass instance to a long and pass 
that as a jlong to java.
2. When java wants to do the NonJavaClass callback,
 it provides the long, which in cpp then gets cast back to 
(NonJavaClass*)
More explicitly I have a file CppImplementation.java which
implements the public method doIt() using native cpp:
------------------------------------------------------
public class CppImplementation
{
 public CppImplementation(long aLong) {theLong = aLong;}
 public final void doIt() {this.doItNative(theLong);}
 private final native void doItNative(long anAddress);
 private long theLong;
}
-----------------------------------------------------------
And the private native method is implemented in cpp as follows:
---------------------------------------------------------
#include "CppImplementation.h" // generated by gcjh
#include <stdio.h>
#include "NonJavaObject.cc"
void CppImplementation::doItNative (jlong anAddress )
{
 long myLong = (long) anAddress;
 NonJavaClass* myObject = (NonJavaClass*) myLong;
 printf("c2j2c callback: %d, %f\n", myLong, myObject->theBla);
}
----------------------------------------------------------
The whole thing gets called as follows:
----------------------------------------------------------
void myMain()
{
 NonJavaClass myNonJavaObject;
 myNonJavaObject.theBla = 5.0;
 long myLong = (long) (&myNonJavaObject);
 CppImplementation* myObject = new CppImplementation((jlong) myLong);
 myObject->doIt();
}
=====================================
My question is: Can I do that? Or can it happen that the operating system
every now and then relocates memory and updates a known set of pointers
in a smart way and that the 'long' variable wouldn't get updated 
accordingly?
Actually the question has nothing to do with java or cni, I might as well
have asked the question in a pure cpp context, namely whether
it's safe in the first place to cast from pointer to long and back a 
litter later.
I would guess that casts are just compile-time concepts, of which nothing is
visible in the object code, but I was wondering if maybe
the casting leaves some sneaky trace in the symbol tables of .o files or
something?
Thanks, Olivier