I'm having trouble passing a struct to a library function.

The C declaration for S is: struct CS { unsigned char *barray; long *iarray; }

I need (at least for now) to be able to write Java that looks like the following
C code, with f() being in a .dll library:

struct CS cs; cs.barray = new unsigned char[len]; cs.iarray = 0; int rv = f(cs);

I have code that looks like the following, and it is failing. During invocation
the field s.memory is still null, causing an exception. What am I doing wrong?
(I hope that during simplification of my code to get the snippet below, I didn't
discard the problem...) I think the problem is not barray, but rather iarray.
How do I set it null?

public class S extends Structure { public byte[] barray; public int[] iarray; public S(byte []obs) { barray = obs; iarray = null; } }

byte[] obs = new[len]; S s = new S(obs); // s.size == CALCULATE_SIZE on exit from ctor. int rv = Lib.INSTANCE.f(s); // null pointer exception here, because s.memory ==
null, because s.size == CALCULATE_SIZE still, because for iarray both
value==null and type.isArray() are true in s.calculateSize().