add_buf
#define next_buf(bufnr) add_buf(bufnr, 1)
q->u.in.batch_start = add_buf(q->u.in.batch_start, overlap);
set_buf_state(q, add_buf(start, count - 1), SLSB_P_INPUT_ACK);
q->first_to_check = add_buf(start, count);
q->first_to_check = add_buf(start, count);