请问unsigned int ip_hl:4;这里的:4意思？

interestbird

在网络编程中/netinet/ip.h中的一个数据结构里：
struct ip
  {
#if __BYTE_ORDER == __LITTLE_ENDIAN
    unsigned int ip_hl:4;               /* header length */
    unsigned int ip_v:4;                /* version */
#endif
#if __BYTE_ORDER == __BIG_ENDIAN
    unsigned int ip_v:4;                /* version */
    unsigned int ip_hl:4;               /* header length */
#endif
    u_int8_t ip_tos;                    /* type of service */
    u_short ip_len;                     /* total length */
    u_short ip_id;                      /* identification */
    u_short ip_off;                     /* fragment offset field */
#define IP_RF 0x8000                    /* reserved fragment flag */
#define IP_DF 0x4000                    /* dont fragment flag */
#define IP_MF 0x2000                    /* more fragments flag */
#define IP_OFFMASK 0x1fff               /* mask for fragmenting bits */
    u_int8_t ip_ttl;                    /* time to live */
    u_int8_t ip_p;                      /* protocol */
    u_short ip_sum;                     /* checksum */
    struct in_addr ip_src, ip_dst;      /* source and dest address */
  };
我用一个指针p指向这个结构，但是一旦调用p->ip_hl，编译时就会出现 error: dereferencing pointer to incomplete type 这个错误。请高人指教，应该怎么样正确使用这个数据。谢谢！

弥敦路九号

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C offers the capability of defining and accessing fields within a word directly rather than by bitwise logical operators. A bit-field, or field for short, is a set of adjacent bits within a single implementation-defined storage unit that we will call a ``word.'' For example:
   struct {
       unsigned int is_keyword : 1;
       unsigned int is_extern  : 1;
       unsigned int is_static  : 1;
   } flags;
This defines a variable table called flags that contains three 1-bit fields. The number following the colon represents the field width in bits. The fields are declared unsigned int to ensure that they are unsigned quantities.
Individual fields are referenced in the same way as other structure members: flags.is_keyword, flags.is_extern, etc. Fields behave like small integers, and may participate in arithmetic expressions just like other integers. Thus the previous examples may be written more naturally as
   flags.is_extern = flags.is_static = 1;
to turn the bits on;
   flags.is_extern = flags.is_static = 0;
to turn them off; and
   if (flags.is_extern == 0 && flags.is_static == 0)
      ...
to test them.
Almost everything about fields is implementation-dependent. Whether a field may overlap a word boundary is implementation-defined. Fields need not be names; unnamed fields (a colon and width only) are used for padding. The special width 0 may be used to force alignment at the next word boundary.
Fields are assigned left to right on some machines and right to left on others. This means that although fields are useful for maintaining internally-defined data structures, the question of which end comes first has to be carefully considered when picking apart externally-defined data; programs that depend on such things are not portable. Fields may be declared only as ints; for portability, specify signed or unsigned explicitly. They are not arrays and they do not have addresses, so the & operator cannot be applied on them.

interestbird

恩。懂了，难怪，这样要自己再另外定义一个数据结构了。谢谢！