& | 与&& || 的区别

& 和 | 总是要计算两个操作数，将操作符两边的操作数按位与/或运算。
&& 和 || 先检查第一个操作数的值，再根据该值进行操作，可能根本就不处理第二个操作数，进行的是布尔运算。
在其他语言中也被叫做短路布尔表达式（Short-circult boolean expressions)。
x = (y != 0) && ( z / y > 5 );
如果y为0则会导致“除0错误”，所以可以先判断y的值，如果y为0则不会进行除法运算。

例：

/*
     * The condition for this PT_WAIT_UNTIL is a little tricky: the
     * protothread will wait here until all data has been acknowledged
     * (data_acked() returns true) and until all data has been sent
     * (send_data() returns true). The two functions data_acked() and
     * send_data() must be called in succession to ensure that all
     * data is sent. Therefore the & operator is used instead of the
     * && operator, which would cause only the data_acked() function
     * to be called when it returns false.
     */
    PT_WAIT_UNTIL(&s->psockpt, data_acked(s) & send_data(s));

static char data_acked(register struct psock *s)
{
  if(s->state == STATE_DATA_SENT && uip_acked()) {
    if(s->sendlen > uip_mss()) {
      s->sendlen -= uip_mss();
      s->sendptr += uip_mss();
    } else {
      s->sendptr += s->sendlen;
      s->sendlen = 0;
    }
    s->state = STATE_ACKED;
    return 1;
  }
  return 0;
}

static char send_data(register struct psock *s)
{
  if(s->state != STATE_DATA_SENT || uip_rexmit()) {
    if(s->sendlen > uip_mss()) {
      uip_send(s->sendptr, uip_mss());
    } else {
      uip_send(s->sendptr, s->sendlen);
    }
    s->state = STATE_DATA_SENT;
    return 1;
  }
  return 0;
}

uip中 psock_send() 函数为了实现连续发送，需要每次检测上一次发送的是否收到了收到确认，并会发送数据（如果data_acked() 返回为false则为重发，否则发送下一段数据）。