BridJ callback from Native to Java: Is it possible that values in native are overriden?

Question

I want to write a Java wrapper to a native library (with BridJ). The method I want to wrap takes a struct of operations (callbacks):

typedef struct _operations {
   int (*op1) (int, ...);
   int (*op2) (float, ...);
} operations;

int method(*operations ops);

The operations should obviously be written in Java, so method calls back to Java space (using BridJ's Callback mechanism).

However, after executing the Java callbacks and returning to method, local variables in the C code have changed:

int method(*operations ops) {
  void* uselesspointer;
  DbgPrint("useless pointer before callback is %d\n", uselesspointer); 
  // prints x
  // execute callbacks in Java here
  DbgPrint("useless pointer after callback is %d\n", uselesspointer); 
  // prints something different from x
}

How can I prevent this from happening?

// see https://github.com/Maxhy/dokany/issues/7 for details

// edit thats interesting: the offset of useless pointer seems to be constant, in my application the second debug message is always "192" instead of "0"