ffi s390x issues, callback with non standard integral type

Makefile

all:
	gcc -c ret16bit.c
	gcc -shared ret16bit.o -o ret16bit.so
	python test.py

ret16bit.so

#include <stdio.h>
#include <inttypes.h>
#include <string.h>

int test(int (*cb)(int,int), int x, int y) {
    int i = 0;
    // let's pollute the stack!!!
    uint64_t * sp = (uint64_t*)__builtin_frame_address(0);
    while (i < 3000) {
        *(sp-(i+160/8)) = 0x0102030440302010;
        i++;
    }
    return cb(x,y);
}

test.py

import ctypes
import os

me = os.path.abspath(os.path.dirname(__file__))
d = ctypes.cdll.LoadLibrary(os.path.join(me, "ret16bit.so"))

def callback(a, b):
    return a+b

func = d.test
i = 1213
j = 1213
v = func(ctypes.CFUNCTYPE(*[ctypes.c_int16]*3)(callback), i, j)
assert v == (i + j), "failed! %d != %d + %d" % (v, i, j)