recv选项MSG_PEEK

如果在recv的时候，flag字段设置了MSG_PEEK，则读取数据包的时候，不会把该数据包从缓存队列

中删除；下次读取时还是这个数据包。

tcp_recvmsg源代码

 

 

if (!(flags & MSG_PEEK)) {
   sk_eat_skb(sk, skb, copied_early);
   copied_early = 0;
 }

 

/**
 * sk_eat_skb - Release a skb if it is no longer needed
 * @sk: socket to eat this skb from
 * @skb: socket buffer to eat
 * @copied_early: flag indicating whether DMA operations copied this data early
 *
 * This routine must be called with interrupts disabled or with the socket
 * locked so that the sk_buff queue operation is ok.
*/

 

static inline void sk_eat_skb(struct sock *sk, struct sk_buff *skb, int copied_early)
{
 __skb_unlink(skb, &sk->sk_receive_queue);
 __kfree_skb(skb);
}