lwip通过结构体解析数据，通讯原始数据与结构体

涉及数据格式

下面一段代码在ip4.h

PACK_STRUCT_BEGIN
/* The IPv4 header */
struct ip_hdr {/* version / header length */PACK_STRUCT_FLD_8(u8_t _v_hl);/* type of service */PACK_STRUCT_FLD_8(u8_t _tos);/* total length */PACK_STRUCT_FIELD(u16_t _len);/* identification */PACK_STRUCT_FIELD(u16_t _id);/* fragment offset field */PACK_STRUCT_FIELD(u16_t _offset);
#define IP_RF 0x8000U        /* reserved fragment flag */
#define IP_DF 0x4000U        /* don't fragment flag */
#define IP_MF 0x2000U        /* more fragments flag */
#define IP_OFFMASK 0x1fffU   /* mask for fragmenting bits *//* time to live */PACK_STRUCT_FLD_8(u8_t _ttl);/* protocol*/PACK_STRUCT_FLD_8(u8_t _proto);/* checksum */PACK_STRUCT_FIELD(u16_t _chksum);/* source and destination IP addresses */PACK_STRUCT_FLD_S(ip4_addr_p_t src);PACK_STRUCT_FLD_S(ip4_addr_p_t dest);
} PACK_STRUCT_STRUCT;
PACK_STRUCT_ENDstruct 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;//用到的定义
#define PACK_STRUCT_FLD_8(x) PACK_STRUCT_FIELD(x)
#define PACK_STRUCT_FIELD(x) x
//与udp数据对接
udphdr = (struct udp_hdr *)p->payload;

err_t
ip4_input(struct pbuf *p, struct netif *inp)
{struct ip_hdr *iphdr;/* identify the IP header */iphdr = (struct ip_hdr *)p->payload;...

PACK_STRUCT_STRUCT attribute ((packed))

串口数据传输数据时，共用体和结构体如何转换
实际使用时，会出现问题，可以参考LWIP修改

struct data_m
{char add;short code;char end;
};int main()
{struct data_m *st;st = data;printf("add:%x,code:%x,end:%x",st->add,st->code,st->end);return 0;
}