在LwIP中，物理网络硬件接口结构保存在一个全局链表中，它们通过结构体中的 next 指针连接。
struct netif {
/// pointer to next in linked list */
struct netif *next;
/// IP address configuration in network byte order */
ip_addr_t ip_addr; //IP地址
ip_addr_t netmask; //子网掩码
ip_addr_t gw; //网关地址
netif_input_fn input; //设备驱动调用该函数传递一个包给TCP/IP协议栈
//IP模块调用该函数传递一个包给网卡，output 函数的第三个參数 ipaddr 是应该接收实际的链路层帧的主机的 IP 地址。
//它不必与 IP 信息包的目的地址同样。特别地，当要发送 IP 信息包到一个并不在本地网络里的主机上时，链路层帧会被发送到网络里的一个路由器上。
//在这样的情况下给output函数的IP地址将是这个路由器的地址。 netif_output_fn output; netif_linkoutput_fn linkoutput; //ARP模块调用该函数传递一个包给网卡 void *state; //由设备驱动设置。指向设备状态信息 #if LWIP_DHCP /// the DHCP client state information for this netif */ struct dhcp *dhcp; #endif // LWIP_DHCP */ #if LWIP_AUTOIP /// the AutoIP client state information for this netif */ struct autoip *autoip; #endif #if LWIP_NETIF_HOSTNAME // the hostname for this netif, NULL is a valid value */ char* hostname; #endif // LWIP_NETIF_HOSTNAME */ u16_t mtu; //一次传送的最大字节数，以太网一般为1500 u8_t hwaddr_len; //物理地址长度，通常是以太网MAC地址长度。6字节 u8_t hwaddr[NETIF_MAX_HWADDR_LEN]; //物理地址，通常是以太网MAC地址 /// flags (see NETIF_FLAG_ above) */ u8_t flags; //使能标志符。比方设置NETIF_FLAG_LINK_UP。接收到数据包才会向上传 /// descriptive abbreviation */ char name[2]; //设备驱动类型 /// number of this interface */ u8_t num; // List of packets to be queued for ourselves. */ struct pbuf *loop_first; //环回。指向发给自己数据包的第一个pbuf struct pbuf *loop_last; //环回，指向发给自己数据包的最后一个pbuf };
/*
* Add a network interface to the list of lwIP netifs.
*
* @param netif a pre-allocated netif structure
* @param ipaddr IP address for the new netif
* @param netmask network mask for the new netif
* @param gw default gateway IP address for the new netif
* @param state opaque data passed to the new netif
* @param init callback function that initializes the interface
* @param input callback function that is called to pass
* ingress packets up in the protocol layer stack.
*
* @return netif, or NULL if failed.
*/
struct netif *
netif_add(struct netif *netif, ip_addr_t *ipaddr, ip_addr_t *netmask,
ip_addr_t *gw, void *state, netif_init_fn init, netif_input_fn input)
{
static u8_t netifnum = 0;
// reset new interface configuration state */
ip_addr_set_zero(&netif->ip_addr);
ip_addr_set_zero(&netif->netmask);
ip_addr_set_zero(&netif->gw);
netif->flags = 0;
// remember netif specific state information data */
netif->state = state;
netif->num = netifnum++;
netif->input = input;
netif_set_addr(netif, ipaddr, netmask, gw);
// call user specified initialization function for netif */
if (init(netif) != ERR_OK) { //用户自己定义初始化函数
return NULL;
}
// add this netif to the list */
netif->next = netif_list;
netif_list = netif;
snmp_inc_iflist();
return netif;
}
//用户自己定义初始化函数
err_t ethernetif_init(struct netif *netif)
{
netif->name[0] = IFNAME0; //比方蓝牙设备（bluetooth）的网络接口能够是bt，随便啦
netif->name[1] = IFNAME1;
netif->output = etharp_output; //IP模块发送数据包函数
netif->linkoutput = low_level_output; // //ARP模块发送数据包函数
low_level_init(netif); //底层硬件初始化函数
return ERR_OK;
}
static void low_level_init(struct netif *netif)
{
netif->hwaddr_len = ETHARP_HWADDR_LEN; //设置MAC地址长度
netif->hwaddr[0] = 'F'; //设置网卡MAC地址
netif->hwaddr[1] = 'O';
netif->hwaddr[2] = 'R';
netif->hwaddr[3] = 'E';
netif->hwaddr[4] = 'S';
netif->hwaddr[5] = 'T';
netif->mtu = 1500; //最大同意传输单元
//同意该网卡的广播和ARP功能。而且该网卡同意有硬件链路连接
netif->flags = NETIF_FLAG_BROADCAST | \
NETIF_FLAG_ETHARP | NETIF_FLAG_LINK_UP;
E1280_init(netif->hwaddr); //详细网卡初始化函数
}
/*
* Send an IP packet to be received on the same netif (loopif-like).
* The pbuf is simply copied and handed back to netif->input.
* In multithreaded mode, this is done directly since netif->input must put
* the packet on a queue.
* In callback mode, the packet is put on an internal queue and is fed to
* netif->input by netif_poll().
*
* @param netif the lwip network interface structure
* @param p the (IP) packet to 'send'
* @param ipaddr the ip address to send the packet to (not used)
* @return ERR_OK if the packet has been sent
* ERR_MEM if the pbuf used to copy the packet couldn't be allocated
*/
err_t
netif_loop_output(struct netif *netif, struct pbuf *p,
ip_addr_t *ipaddr)
{
struct pbuf *r;
err_t err;
struct pbuf *last;
SYS_ARCH_DECL_PROTECT(lev);
LWIP_UNUSED_ARG(ipaddr);
// Allocate a new pbuf */
r = pbuf_alloc(PBUF_LINK, p->tot_len, PBUF_RAM);
if (r == NULL) {
return ERR_MEM;
}
// Copy the whole pbuf queue p into the single pbuf r */
if ((err = pbuf_copy(r, p)) != ERR_OK) { //拷贝要发送的数据到r中
pbuf_free(r);
return err;
}
// Put the packet on a linked list which gets emptied through calling
netif_poll(). */
//能够调用netif_poll函数将当前loop链表上全部数据提交给IP层
// let last point to the last pbuf in chain r */
for (last = r; last->next != NULL; last = last->next);
SYS_ARCH_PROTECT(lev);
if(netif->loop_first != NULL) { //原来的loop中还有数据
netif->loop_last->next = r; //接到原来数据的后面
netif->loop_last = last;
} else {
netif->loop_first = r;
netif->loop_last = last;
}
SYS_ARCH_UNPROTECT(lev);
return ERR_OK;
}
/*
* This function should be called when a packet is ready to be read
* from the interface. It uses the function low_level_input() that
* should handle the actual reception of bytes from the network
* interface. Then the type of the received packet is determined and
* the appropriate input function is called.
*
* @param netif the lwip network interface structure for this ethernetif
*/
static void
ethernetif_input(struct netif *netif)
{
struct ethernetif *ethernetif;
struct eth_hdr *ethhdr;
struct pbuf *p;
ethernetif = netif->state;
// move received packet into a new pbuf */
p = low_level_input(netif);
// no packet could be read, silently ignore this */
if (p == NULL) return;
// points to packet payload, which starts with an Ethernet header */
ethhdr = p->payload;
switch (htons(ethhdr->type)) {
// IP or ARP packet?
*/ case ETHTYPE_IP: case ETHTYPE_ARP: #if PPPOE_SUPPORT // PPPoE packet? */ case ETHTYPE_PPPOEDISC: case ETHTYPE_PPPOE: #endif // PPPOE_SUPPORT */ // full packet send to tcpip_thread to process */ if (netif->input(p, netif)!=ERR_OK) { LWIP_DEBUGF(NETIF_DEBUG, ("ethernetif_input: IP input error\n")); pbuf_free(p); p = NULL; } break; default: pbuf_free(p); p = NULL; break; } } /* * This function should be called when a packet is ready to be read * from the interface. It uses the function low_level_input() that * should handle the actual reception of bytes from the network * interface. Then the type of the received packet is determined and * the appropriate input function is called. * * @param netif the lwip network interface structure for this ethernetif */ static void ethernetif_input(struct netif *netif) { struct ethernetif *ethernetif; struct eth_hdr *ethhdr; struct pbuf *p; ethernetif = netif->state; // move received packet into a new pbuf */ p = low_level_input(netif); //拷贝接收到的数据到一个新的pbuf // no packet could be read, silently ignore this */ if (p == NULL) return; // points to packet payload, which starts with an Ethernet header */ ethhdr = p->payload; switch (htons(ethhdr->type)) { // IP or ARP packet?
*/ case ETHTYPE_IP: case ETHTYPE_ARP: // full packet send to tcpip_thread to process */ if (netif->input(p, netif)!=ERR_OK) //调用上层（IP/ARP）函数进行处理 { pbuf_free(p); p = NULL; } break; default: pbuf_free(p); p = NULL; break; } }