LwIP 接收函数udp_input精简版

void
udp_input(struct pbuf *p, struct netif *inp)
{
  struct udp_hdr *udphdr;
  struct udp_pcb *pcb, *prev;
  struct udp_pcb *uncon_pcb;
  u16_t src, dest;
  u8_t broadcast;
  u8_t for_us = 0;


  /* Check minimum length (UDP header) 检查最小长度，不能小于UDP首部*/
  if (p->len < UDP_HLEN) {
    /* drop short packets */
    pbuf_free(p);
    goto end;
  }
  //指向UDP报文首部，并且强制转换成udp_hdr类型，方便操作
  udphdr = (struct udp_hdr *)p->payload;

  /* is broadcast packet ? 判断是不是广播包*/
  broadcast = ip_addr_isbroadcast(ip_current_dest_addr(), ip_current_netif());


  /* convert src and dest ports to host byte order 得到UDP首部中的源地址和目标主机端口号*/
  src = lwip_ntohs(udphdr->src);
  dest = lwip_ntohs(udphdr->dest);



  pcb = NULL;
  prev = NULL;
  uncon_pcb = NULL;
  /* Iterate through the UDP pcb list for a matching pcb.遍历UDP链表，找到对应的端口号，如果找不到，
   * 'Perfect match' pcbs (connected to the remote port & ip address) are就用链表的第一个未使用的UDP控制块
   * preferred. If no perfect match is found, the first unconnected pcb that
   * matches the local port and ip address gets the datagram. */
  for (pcb = udp_pcbs; pcb != NULL; pcb = pcb->next) {

    /* compare PCB local addr+port to UDP destination addr+port */
    if ((pcb->local_port == dest) &&   //将UDP控制块本地地址+端口与UDP首部目标地址+端口进行比较
        (udp_input_local_match(pcb, inp, broadcast) != 0)) {
      if (((pcb->flags & UDP_FLAGS_CONNECTED) == 0) &&
          ((uncon_pcb == NULL))) {
        /* the first unconnected matching PCB 如果未找到则使用第一个UDP控制块*/
        uncon_pcb = pcb;
      }

      /* compare PCB remote addr+port to UDP source addr+port将UDP控制块的目标地址+端口号与UDP首部控制块源地址+端口进行比较*/
      if ((pcb->remote_port == src) &&
          (ip_addr_isany_val(pcb->remote_ip) ||
          ip_addr_cmp(&pcb->remote_ip, ip_current_src_addr()))) {
        /* the first fully matching PCB */
        if (prev != NULL) {
          /* move the pcb to the front of udp_pcbs so that is
             found faster next time */
          prev->next = pcb->next;
          pcb->next = udp_pcbs;
          udp_pcbs = pcb;
        } else {
          UDP_STATS_INC(udp.cachehit);
        }
        break;
      }
    }

    prev = pcb;
  }
  /* no fully matching pcb found? then look for an unconnected pcb */
  if (pcb == NULL) {
    pcb = uncon_pcb;
  }

  /* Check checksum if this is a match or if it was directed at us. */
  if (pcb != NULL) {
    for_us = 1;
  } else {
    if (!ip_current_is_v6()) {
      for_us = ip4_addr_cmp(netif_ip4_addr(inp), ip4_current_dest_addr());
    }
  }

  if (for_us) {
    IF__NETIF_CHECKSUM_ENABLED(inp, CHECKSUM_CHECK_UDP) {
      if (ip_current_header_proto() == IP_PROTO_UDPLITE) {
        /* Do the UDP Lite checksum */
        u16_t chklen = lwip_ntohs(udphdr->len);
        if (chklen < sizeof(struct udp_hdr)) {
          if (chklen == 0) {
            /* For UDP-Lite, checksum length of 0 means checksum
               over the complete packet (See RFC 3828 chap. 3.1) */
            chklen = p->tot_len;
          } else {
            /* At least the UDP-Lite header must be covered by the
               checksum! (Again, see RFC 3828 chap. 3.1) */
            goto chkerr;
          }
        }
        if (ip_chksum_pseudo_partial(p, IP_PROTO_UDPLITE,
                     p->tot_len, chklen,
                     ip_current_src_addr(), ip_current_dest_addr()) != 0) {
          goto chkerr;
        }
      } else
	//校验和通过，则将报文中的数据递交给用户程序处理，先调整 payload 指针
    if (pbuf_header(p, -UDP_HLEN)) {   //指向报文中的数据区
      /* Can we cope with this failing? Just assert for now */
      pbuf_free(p);   //调整失败，则删除整个数据报
      goto end;
    }

    if (pcb != NULL) {
      MIB2_STATS_INC(mib2.udpindatagrams);
      /* callback */
      if (pcb->recv != NULL) {
        /* now the recv function is responsible for freeing p */
        pcb->recv(pcb->recv_arg, pcb, p, ip_current_src_addr(), src);
      } else {
        /* no recv function registered? then we have to free the pbuf! */
        pbuf_free(p);
        goto end;
      }
    } else {

#if LWIP_ICMP || LWIP_ICMP6
      /* No match was found, send ICMP destination port unreachable unless
         destination address was broadcast/multicast. */
      if (!broadcast && !ip_addr_ismulticast(ip_current_dest_addr())) {
        /* move payload pointer back to ip header */
        pbuf_header_force(p, (s16_t)(ip_current_header_tot_len() + UDP_HLEN));
        icmp_port_unreach(ip_current_is_v6(), p);
      }
#endif /* LWIP_ICMP || LWIP_ICMP6 */
      pbuf_free(p);
    }
  } else {
    pbuf_free(p);
  }
end:
  PERF_STOP("udp_input");
  return;
  
#if CHECKSUM_CHECK_UDP
chkerr:
  LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
              ("udp_input: UDP (or UDP Lite) datagram discarded due to failing checksum\n"));
  UDP_STATS_INC(udp.chkerr);
  UDP_STATS_INC(udp.drop);
  MIB2_STATS_INC(mib2.udpinerrors);
  pbuf_free(p);
  PERF_STOP("udp_input");
#endif /* CHECKSUM_CHECK_UDP */
}