circ_count_to_end
n = min_t(size_t, len, circ_count_to_end(crc));
n = min_t(int, sz, circ_count_to_end(&rd->fifo));
len = circ_count_to_end(crc);
len = circ_count_to_end(crc);
if (circ_count_to_end(crc) == 0)