Linux Kernel网络部分代码剖析(2)

union { //指向传输层
  struct tcphdr  *th;
  struct udphdr  *uh;
  struct icmphdr *icmph;
  struct igmphdr *igmph;
  struct iphdr   *ipiph;
  struct ipv6hdr *ipv6h;
  unsigned char  *raw;
 } h;

union { //指向网络层
  struct iphdr   *iph;
  struct ipv6hdr *ipv6h;
  struct arphdr  *arph;
  unsigned char  *raw;
 } nh;

union { //指向链路层
    unsigned char  *raw;
 } mac;

struct  dst_entry *dst; //目的表项(指向一个路由cache，在后面文章详解)
 struct sec_path   *sp;  //只被xfrm使用(这个讨论基本上没用到)

/*
  * This is the control buffer. It is free to use for every
  * layer. Please put your private variables there. If you
  * want to keep them across layers you have to do a skb_clone()
  * first. This is owned by whoever has the skb queued ATM.
  */
 char   cb[40];

unsigned int  len,      //真实数据区的长度
               data_len, //数据区的长度
               mac_len,  //mac长度
               csum;     //checknum
 __u32   priority;

//以下都是作为flag用,只用了一个byte的5个bit
 __u8    local_df:1, //允许local分片标志
         cloned:1,   //允许clone操作标志
         ip_summed:2,
         nohdr:1;
         /* 3 bits spare */

 __u8     pkt_type; //packet的类型，在if_packet.h中定义取值
 __be16   protocol;

void   (*destructor)(struct sk_buff *skb);
............

............
 /* These elements must be at the end, see alloc_skb() for details.  */
 unsigned int  truesize; //buffer的大小(后面详解)
 atomic_t      users;    //user计数,atomic_t��明该变量只能"原子"操作

//在后面详细介绍
 unsigned char  *head,
                *data,
                *tail,
                *end;
};