位域与iphdr定义

这是linux对IP头的定义 /usr/include/linux/ip.h 或 linux/include/linux/ip.h)
struct iphdr {
#if __BYTE_ORDER == __LITTLE_ENDIAN
uint8_t ihl:4,
version:4;
#elif __BYTE_ORDER == __BIG_ENDIAN
uint8_t version:4,
ihl:4;
#endif
uint8_t tos;
uint16_t tot_len;
uint16_t id;
uint16_t frag_off;
uint8_t ttl;
uint8_t protocol;
uint16_t check;
uint32_t saddr;
uint32_t daddr;
/*The options start here. */
};

VxWorks  的target\h\netinet\Ip.h

/*
 * Structure of an internet header, naked of options.
 *
 * We declare ip_len and ip_off to be short, rather than u_short
 * pragmatically since otherwise unsigned comparisons can result
 * against negative integers quite easily, and fail in subtle ways.
 */
struct ip {
#if _BYTE_ORDER == _LITTLE_ENDIAN
 u_int ip_hl:4,  /* header length */
  ip_v:4,   /* version */
#endif
#if _BYTE_ORDER == _BIG_ENDIAN
 u_int ip_v:4,   /* version */
  ip_hl:4,  /* header length */
#endif
  ip_tos:8,  /* type of service */
  ip_len:16;  /* total length */
 u_short ip_id;   /* identification */
 short ip_off;   /* fragment offset field */
#define IP_DF 0x4000   /* dont fragment flag */
#define IP_MF 0x2000   /* more fragments flag */
#define IP_OFFMASK 0x1fff  /* mask for fragmenting bits */
 u_char ip_ttl;   /* time to live */
 u_char ip_p;   /* protocol */
 u_short ip_sum;   /* checksum */
 struct in_addr ip_src,ip_dst; /* source and dest address */
};

VC下的例子：

 struct BitField
 {
  int first:3;
  int second:2;
  int third:3;
 }field;

 field.first = 4;
 field.second = 2;
 field.third = 2;

 char *pChar = (char *)&field;

 printf("bit domain test:%d\n", *pChar);

 

field：100 10 010

打印出 010 10 100

 

总结：在小端环境下，先分配 least significant 3 bits，再分配middle 2 bits，最后分配most significant 3 bits。

 

推测：iphdr 中version和ihl区分大端，小端 方便对他们的处理

     现在有了字节序和位域序的概念，听说还有bit序待进一步了解