本文原创为freas_1990，转载请标明出处：http://blog.csdn.net/freas_1990/article/details/11264237

大家都知道TCP是面向stream，而UDP是面向datagram的。

那，到底什么是stream呢？

我们来看一下以下代码

	/* Now we need to check if we have a half built packet. */
	if ((skb = tcp_dequeue_partial(sk)) != NULL) {
	        int hdrlen;

	         /* IP header + TCP header */
		hdrlen = ((unsigned long)skb->h.th - (unsigned long)skb->data)
		         + sizeof(struct tcphdr);

		/* Add more stuff to the end of skb->len */
		if (!(flags & MSG_OOB)) {
			copy = min(sk->mss - (skb->len - hdrlen), len);
			/* FIXME: this is really a bug. */
			if (copy <= 0) {
			  printk("TCP: **bug**: \"copy\" <= 0!!\n");
			  copy = 0;
			}
	  
			memcpy_fromfs(skb->data + skb->len, from, copy);
			skb->len += copy;
			from += copy;
			copied += copy;
			len -= copy;
			sk->write_seq += copy;
		      }
		if ((skb->len - hdrlen) >= sk->mss ||
		    (flags & MSG_OOB) ||
		    !sk->packets_out)
			tcp_send_skb(sk, skb);
		else
			tcp_enqueue_partial(skb, sk);
		continue;
	}

明白了吧？