lwIp源码解析--udp

最新推荐文章于 2025-08-20 10:58:05 发布

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最新推荐文章于 2025-08-20 10:58:05 发布 · 2.7k 阅读

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#udp #网络协议

本文详细解析lwIp 2.1.2版本的UDP源码，涵盖UDP头部定义、初始化、发送及接收函数。讨论了udp_pcb协议控制块的创建、绑定、连接过程，以及发送函数的调用顺序和接收数据的处理流程。

lwIp源码解析–udp

一、简介
1.1 lwip版本
lwip 2.1.2
1.2 代码路径

lwip-2.1.2\src\core\udp.c

代码行数1.3K，UDP是无连接、无状态的传输层协议，在实现上比TCP要简单很多。

二、源码解析
源码分为四部分，分别是UDP头部定义、UDP初始化函数、UDP发送函数、UDP接收函数。

2.1 udp 头部

struct udp_hdr {
   
   
  PACK_STRUCT_FIELD(u16_t src);
  PACK_STRUCT_FIELD(u16_t dest);  /* src/dest UDP ports */
  PACK_STRUCT_FIELD(u16_t len);
  PACK_STRUCT_FIELD(u16_t chksum);
} PACK_STRUCT_STRUCT;

udp的头部有八个字节，分别是源/目的端口号、UDP头部加上数据的总长度len（此时的总长度和IP头的负载长度的一致）、以及校验和 chksum（此处的校验和要加上12字节的伪头部，包括源/目的IP地址、协议号、UDP长度）

2.2 初始化函数

struct udp_pcb * udp_new        (void);
struct udp_pcb * udp_new_ip_type(u8_t type);
void             udp_remove     (struct udp_pcb *pcb);
err_t            udp_bind       (struct udp_pcb *pcb, const ip_addr_t *ipaddr,
                                 u16_t port);
void             udp_bind_netif (struct udp_pcb *pcb, const struct netif* netif);
err_t            udp_connect    (struct udp_pcb *pcb, const ip_addr_t *ipaddr,
                                 u16_t port);
void             udp_disconnect (struct udp_pcb *pcb);
void             udp_recv       (struct udp_pcb *pcb, udp_recv_fn recv,
                                 void *recv_arg);

2.2.1 创建udp_pcb协议控制块

/**
 * @ingroup udp_raw
 * Creates a new UDP pcb which can be used for UDP communication. The
 * pcb is not active until it has either been bound to a local address
 * or connected to a remote address.
 *
 * @return The UDP PCB which was created. NULL if the PCB data structure
 * could not be allocated.
 *
 * @see udp_remove()
 */
struct udp_pcb *
udp_new(void)
{
   
   
  struct udp_pcb *pcb;

  LWIP_ASSERT_CORE_LOCKED();

  pcb = (struct udp_pcb *)memp_malloc(MEMP_UDP_PCB);
  /* could allocate UDP PCB? */
  if (pcb != NULL) {
   
   
    /* UDP Lite: by initializing to all zeroes, chksum_len is set to 0
     * which means checksum is generated over the whole datagram per default
     * (recommended as default by RFC 3828). */
    /* initialize PCB to all zeroes */
    memset(pcb, 0, sizeof(struct udp_pcb));
    pcb->ttl = UDP_TTL;
#if LWIP_MULTICAST_TX_OPTIONS
    udp_set_multicast_ttl(pcb, UDP_TTL);
#endif /* LWIP_MULTICAST_TX_OPTIONS */
  }
  return pcb;
}

/**
 * @ingroup udp_raw
 * Create a UDP PCB for specific IP type.
 * The pcb is not active until it has either been bound to a local address
 * or connected to a remote address.
 * 
 * @param type IP address type, see @ref lwip_ip_addr_type definitions.
 * If you want to listen to IPv4 and IPv6 (dual-stack) packets,
 * supply @ref IPADDR_TYPE_ANY as argument and bind to @ref IP_ANY_TYPE.
 * @return The UDP PCB which was created. NULL if the PCB data structure
 * could not be allocated.
 *
 * @see udp_remove()
 */
struct udp_pcb *
udp_new_ip_type(u8_t type)
{
   
   
  struct udp_pcb *pcb;

  LWIP_ASSERT_CORE_LOCKED();

  pcb = udp_new();
#if LWIP_IPV4 && LWIP_IPV6
  if (pcb != NULL) {
   
   
    IP_SET_TYPE_VAL(pcb->local_ip,  type);
    IP_SET_TYPE_VAL(pcb->remote_ip, type);
  }
#else
  LWIP_UNUSED_ARG(type);
#endif /* LWIP_IPV4 && LWIP_IPV6 */
  return pcb;
}

以上两个函数都是创建udp_pcb协议控制块，差别在于udp_new_ip_type() 可以传入type指定pcb对应的IP类型为IPv6、IPv4或IPv6/v4。而 udp_new() 默认创建的pcb对应IP类型为IPv4,因为IPv4的IP TYPE值是0，如下所示:

enum lwip_ip_addr_type {
   
   
  /** IPv4 */
  IPADDR_TYPE_V4 =   0U,
  /** IPv6 */
  IPADDR_TYPE_V6 =   6U,
  /** IPv4+IPv6 ("dual-stack") */
  IPADDR_TYPE_ANY = 46U
};

2.2.2 绑定和连接
创建了udp_pcb之后，必须通过bind()或者connect()操作，将pcb加入到pcb链表struct udp_pcb *udp_pcbs中，才能由内核管理接收和发送数据。其中插入链表采用的是头插法。

/**
 * @ingroup udp_raw
 * Bind an UDP PCB.
 * 
 * @param pcb UDP PCB to be bound with a local address ipaddr and port.
 * @param ipaddr local IP address to bind with. Use IP_ANY_TYPE to
 * bind to all local interfaces.
 * @param port local UDP port to bind with. Use 0 to automatically bind
 * to a random port between UDP_LOCAL_PORT_RANGE_START and
 * UDP_LOCAL_PORT_RANGE_END.
 *
 * ipaddr & port are expected to be in the same byte order as in the pcb.
 *
 * @return lwIP error code.
 * - ERR_OK. Successful. No error occurred.
 * - ERR_USE. The specified ipaddr and port are already bound to by
 * another UDP PCB.
 *
 * @see udp_disconnect()
 */
err_t
udp_bind(struct udp_pcb *pcb, const ip_addr_t *ipaddr, u16_t port)
{
   
   
  struct udp_pcb *ipcb;
  u8_t rebind;
#if LWIP_IPV6 && LWIP_IPV6_SCOPES
  ip_addr_t zoned_ipaddr;
#endif /* LWIP_IPV6 && LWIP_IPV6_SCOPES */

  LWIP_ASSERT_CORE_LOCKED();

#if LWIP_IPV4
  /* Don't propagate NULL pointer (IPv4 ANY) to subsequent functions */
  if (ipaddr == NULL) {
   
   
    ipaddr = IP4_ADDR_ANY;
  }
#else /* LWIP_IPV4 */
  LWIP_ERROR("udp_bind: invalid ipaddr", ipaddr != NULL, return ERR_ARG);
#endif /* LWIP_IPV4 */

  LWIP_ERROR("udp_bind: invalid pcb", pcb != NULL, return ERR_ARG);

  LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE, ("udp_bind(ipaddr = "));
  ip_addr_debug_print(UDP_DEBUG | LWIP_DBG_TRACE, ipaddr);
  LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE, (", port = %"U16_F")\n", port));

  rebind = 0;
  /* Check for double bind and rebind of the same pcb */
  for (ipcb = udp_pcbs; ipcb != NULL; ipcb = ipcb->next) {
   
   
    /* is this UDP PCB already on active list? */
    if (pcb == ipcb) {
   
   
      rebind = 1;
      break;
    }
  }

#if LWIP_IPV6 && LWIP_IPV6_SCOPES
  /* If the given IP address should have a zone but doesn't, assign one now.
   * This is legacy support: scope-aware callers should always provide properly
   * zoned source addresses. Do the zone selection before the address-in-use
   * check below; as such we have to make a temporary copy of the address. */
  if (IP_IS_V6(ipaddr) && ip6_addr_lacks_zone(ip_2_ip6(ipaddr)<