etc.c

#ifdef HAVE_GETGRENT
static VALUE
setup_group(grp)
    struct group *grp;
{
    VALUE mem;
    char **tbl;

    mem = rb_ary_new();
    tbl = grp->gr_mem;
    while (*tbl) {
	rb_ary_push(mem, safe_setup_str(*tbl));
	tbl++;
    }

etc.rb.ffi

  def mem
    ptr = self[:gr_mem].read_pointer

    ary = []
    
    2.times do |x|
      unless ptr.null?
        s = ptr.read_string
        ary << s
      end
      ptr.address = ptr.address + s.length + 1
    end
    ary
  end