內核網(wǎng)絡四層結構：BSD socket層、INET socket層、IP層、數(shù)據(jù)鏈路層

sock結構工作在INET socket層，所有BSD socket層的操作都通過struct sock
及其域字段prot指針轉化為prot鎖指向的函數(shù)處理，所以sock結構是維系
BSD socket層和INET socket層的紐帶，其重要性是不言而喻的。

下面是對sock結構的簡單注釋，有些沒有的待我弄懂后再來修改，^_^

struct sock {
 /* Socket demultiplex comparisons on incoming packets. */
 __u32   daddr;  /* 外部IP地址   */
 __u32   rcv_saddr; /* 綁定的本地IP地址  */
 __u16   dport;  /* 目標端口   */
 unsigned short  num;  /* 本地端口    */
 int   bound_dev_if; /* 綁定網(wǎng)卡接口的索引 不為0  */

 /* 用于各種協(xié)議查找表的哈希鏈表 */
 struct sock  *next;
 struct sock  **pprev;
 struct sock  *bind_next;
 struct sock  **bind_pprev;

 volatile unsigned char state,  /* 連接狀態(tài)   */
    zapped;  /* In ax25 & ipx means not linked */
 __u16   sport;  /* 源端口    */

 unsigned short  family;  /* 地址簇   */
 unsigned char  reuse;  /* SO_REUSEADDR設置，表示地址重用 */
 unsigned char  shutdown; /* 半關閉標志 SEND_SHUTDOWN和RCV_SHUTDOWN掩碼 */
 atomic_t  refcnt;  /* 引用計數(shù)   */

 socket_lock_t  lock;  /* 同步鎖.   */
 int   rcvbuf;  /* 接受緩沖區(qū)長度(單位：字節(jié)) */

 wait_queue_head_t *sleep;  /* 等待隊列，通常指向socket的wait域 */
 struct dst_entry *dst_cache; /* 目的地緩存，用于路由查找結果  */
 rwlock_t  dst_lock; /* 目的地緩存鎖*/
 atomic_t  rmem_alloc; /* 已接收隊列字節(jié)數(shù) */
 struct sk_buff_head receive_queue; /* 接收包隊列   */
 atomic_t  wmem_alloc; /* 已發(fā)送字節(jié)數(shù) */
 struct sk_buff_head write_queue; /* 包發(fā)送隊列   */
 atomic_t  omem_alloc; /* "o"是"option"(選項)或"other"(其它)的意思 */
 int   wmem_queued; /* 穩(wěn)定的隊列長度 */
 int   forward_alloc; /* 提前分配的空間 */
 __u32   saddr;  /* 發(fā)送源地址   */
 unsigned int  allocation; /* 分配模式   */
 int   sndbuf;  /* 發(fā)送緩沖區(qū)長度(單位：字節(jié))  */
 struct sock  *prev;

 /* Not all are volatile, but some are, so we might as well say they all are.
  * XXX Make this a flag word -DaveM
  */
 volatile char  dead,
    done,
    urginline,
    keepopen,
    linger,
    destroy,
    no_check, /*SO_NO_CHECK標志，表示是否檢查包*/
    broadcast,
    bsdism;
 unsigned char  debug;
 unsigned char  rcvtstamp;
 unsigned char  userlocks;/*SO_SNDBUF和SO_RCVBUF標志 */
 int   proc;
 unsigned long         lingertime;

 int   hashent;
 struct sock  *pair;

 /* The backlog queue is special, it is always used with
  * the per-socket spinlock held and requires low latency
  * access.  Therefore we special case it's implementation.
  */
 struct {
  struct sk_buff *head;
  struct sk_buff *tail;
 } backlog;

 rwlock_t  callback_lock;/*用于結構末尾的回調函數(shù)互斥*/

 /* 錯誤隊列，很少使用 */
 struct sk_buff_head error_queue;

 struct proto  *prot;/*網(wǎng)絡地址簇內部協(xié)議處理*/

#if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)
 union {
  struct ipv6_pinfo af_inet6;
 } net_pinfo;
#endif

 union {
  struct tcp_opt  af_tcp;
#if defined(CONFIG_INET) || defined (CONFIG_INET_MODULE)
  struct raw_opt  tp_raw4;
#endif
#if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)
  struct raw6_opt  tp_raw;
#endif /* CONFIG_IPV6 */
#if defined(CONFIG_SPX) || defined (CONFIG_SPX_MODULE)
  struct spx_opt  af_spx;
#endif /* CONFIG_SPX */

 } tp_pinfo;

 int   err, err_soft; /* Soft holds errors that don't
         cause failure but are the cause
         of a persistent failure not just
         'timed out' */
 unsigned short  ack_backlog;
 unsigned short  max_ack_backlog;
 __u32   priority;
 unsigned short  type;  /*socket類型，如：SOCK_STREAM*/
 unsigned char  localroute; /* 僅用于本地路由 */
 unsigned char  protocol; /*socket 所屬網(wǎng)絡地址簇的協(xié)議*/
 struct ucred  peercred; /*對端sock信息 SO_PEERCRED標志設置*/
 int   rcvlowat; /*SO_RCVLOWAT標志位*/
 long   rcvtimeo; /*SO_RCVTIMEO標志位*/
 long   sndtimeo; /*SO_SNDTIMEO標志位*/

#ifdef CONFIG_FILTER
 /* Socket過濾指令 */
 struct sk_filter       *filter;
#endif /* CONFIG_FILTER */

 /* This is where all the private (optional) areas that don't
  * overlap will eventually live.
  */
 union {
  void *destruct_hook;
    struct unix_opt af_unix;
#if defined(CONFIG_INET) || defined (CONFIG_INET_MODULE)
  struct inet_opt af_inet;
#endif
#if defined(CONFIG_ATALK) || defined(CONFIG_ATALK_MODULE)
  struct atalk_sock af_at;
#endif
#if defined(CONFIG_IPX) || defined(CONFIG_IPX_MODULE)
  struct ipx_opt  af_ipx;
#endif
#if defined (CONFIG_DECNET) || defined(CONFIG_DECNET_MODULE)
  struct dn_scp           dn;
#endif
#if defined (CONFIG_PACKET) || defined(CONFIG_PACKET_MODULE)
  struct packet_opt *af_packet;
#endif
#if defined(CONFIG_X25) || defined(CONFIG_X25_MODULE)
  x25_cb   *x25;
#endif
#if defined(CONFIG_AX25) || defined(CONFIG_AX25_MODULE)
  ax25_cb   *ax25;
#endif
#if defined(CONFIG_NETROM) || defined(CONFIG_NETROM_MODULE)
  nr_cb   *nr;
#endif
#if defined(CONFIG_ROSE) || defined(CONFIG_ROSE_MODULE)
  rose_cb   *rose;
#endif
#if defined(CONFIG_PPPOE) || defined(CONFIG_PPPOE_MODULE)
  struct pppox_opt *pppox;
#endif
#ifdef CONFIG_NETLINK
  struct netlink_opt *af_netlink;
#endif
#if defined(CONFIG_ECONET) || defined(CONFIG_ECONET_MODULE)
  struct econet_opt *af_econet;
#endif
#if defined(CONFIG_ATM) || defined(CONFIG_ATM_MODULE)
  struct atm_vcc  *af_atm;
#endif
#if defined(CONFIG_IRDA) || defined(CONFIG_IRDA_MODULE)
  struct irda_sock        *irda;
#endif
 } protinfo;   /*私有數(shù)據(jù)區(qū)，網(wǎng)絡地址簇規(guī)范*/ 

 /* This part is used for the timeout functions. */
 struct timer_list timer;  /* sock清除定時器. */
 struct timeval  stamp;  /*最后到達的包的時間戳*/

 /* Identd and reporting IO signals */
 struct socket  *socket; /*BSD socket層套接字*/

 /* RPC層私有數(shù)據(jù) */
 void   *user_data;
 
 /* 回調函數(shù) */
 void   (*state_change)(struct sock *sk);
 void   (*data_ready)(struct sock *sk,int bytes);
 void   (*write_space)(struct sock *sk);
 void   (*error_report)(struct sock *sk);

   int   (*backlog_rcv) (struct sock *sk,
      struct sk_buff *skb); 
 void                    (*destruct)(struct sock *sk);
};