> value ocaml_getCharWidth(value code)
> {
>   double x, y;
>   Push_roots(r,1);
> 
>   r[0] = alloc_tuple(2);
>   getCharWidth(Int_val(code),&x,&y);
>   Store_double_val(Field(r[0],0),x);
>   Store_double_val(Field(r[0],1),y);
>   Pop_roots();
>   return r[0];
> }

I would write it this way:

value ocaml_getCharWidth(value code)
{
  double x, y;
  value res;
  Push_roots(r,2);

  getCharWidth(Int_val(code),&x,&y);
  r[0] = copy_double(x);
  r[1] = copy_double(y);
  res = alloc_tuple(2);
  Field(res, 0) = r[0];
  Field(res, 1) = r[1];
  Pop_roots();
  return res;
}

Store_double_val is used to store a double in an array of double values 
(which is a special kind of array). In other cases, you have to use 
copy_double to allocate a block containing the double.
So, your code failed because Field(r[0],0) was not a pointer to an array,
but an uninitialized pointer. 
*_roots are used to keep a pointer to both double values. Indeed, these
values can be moved by copy_double or alloc_tuple. The tuple is allocated
last, so that it can be directly filled (without having to use function
modify).

   Jerome