Help with C struct by value on OSX 64bits

[Johan Hernandez]

Hi. I'm having trouble calling a C function passing a structure 
by value, printing a member and getting it back in D but I must 
be doing something wrong.

I have a C file called native.c that defines a function called 
'filter' taking a buf_t structure, I compile it using GCC to 
native.o. From D, I use extern(C) to access the 'filter' function 
defined in native.o.

##Makefile

app: native.o
        dmd -ofbin/app app.d native.o
        chmod +x bin/app
        bin/./app

native.o: native.c
        gcc -c native.c -o native.o

##app.d

import std.stdio;

extern(C) struct buf_t {
    void* base;
    size_t len;
  }

extern(C) buf_t filter(const buf_t value);

void main() {
  buf_t buf;
  buf.len = 10;
  auto r = filter(buf);
  writefln("D: len %d", r.len);
}

***native.c***

#include <sys/types.h>
#include <stdio.h>

typedef struct {
  void* base;
  size_t len;
} buf_t;

buf_t filter(buf_t value) {
  printf("C: len: %zu\n", value.len);
  return value;
}


***The output***

C: len: 140734905718328
D: len 0


"len" should be 10 in both cases. What am I doing wrong?

I'm using:
OS: Mac OSX Lion, 64bits.
D compiler: DMD64 D Compiler v2.058
C compiler: gcc version 4.2.1 (Based on Apple Inc. build 5658) 
(LLVM build 2336.1.00)

Thanks,
Johan