mfd_assert_size
mfd_assert_size(fd, mfd_def_size / 2);
mfd_assert_size(fd, 0);
mfd_assert_size(fd, mfd_def_size * 2);
mfd_assert_size(fd, mfd_def_size * 4);
mfd_assert_size(fd, mfd_def_size * 8);