Linux协议栈之BSD和INET socket层（一）
Linux的BSD和INET socket层分别对应于ISO的表示层和会话层，其中有两个比较重要的数据结构：
struct socket
{
socket_state state;
unsigned long flags;
struct proto_ops *ops;
struct inode *inode;
struct fasync_struct *fasync_list; /* Asynchronous wake up list */
struct file *file; /* File back pointer for gc */
struct sock *sk;
wait_queue_head_t wait;
short type;
unsigned char passcred;
};
其中state表示socket当前的状态，file和inode指向文件系统的文件描述符和节点信息。每个文件描述符都要对应一个inode节点，其中file->f_inode指向inode节点。
struct proto_ops *ops是一个L6层的处理函数，pops数组将在sock_register中初始化，对应于不同的作用域将被初始化为不同的值。fasync_list用于异步唤醒，对应的函数是sock_wake_async()。
而struct sock *sk 就是我们要说的另外一个非常重要的数据结构，相对于socket只应用于L6层，sock贯穿于L2～L5层并且做过各层之间贯穿的一个纽带。
struct sock {
/* Socket demultiplex comparisons on incoming packets. */
__u32 daddr; /* Foreign IPv4 addr */
__u32 rcv_saddr; /* Bound local IPv4 addr */
__u16 dport; /* Destination port */
unsigned short num; /* Local port */
int bound_dev_if; /* Bound device index if != 0 */
/* Main hash linkage for various protocol lookup tables. */
struct sock *next;
struct sock **pprev;
struct sock *bind_next;
struct sock **bind_pprev;
volatile unsigned char state, /* Connection state */
zapped; /* In ax25 & ipx means not linked */
__u16 sport; /* Source port */
unsigned short family; /* Address family */
unsigned char reuse; /* SO_REUSEADDR setting */
unsigned char shutdown;
atomic_t refcnt; /* Reference count */
socket_lock_t lock; /* Synchronizer... */
int rcvbuf; /* Size of receive buffer in bytes */
wait_queue_head_t *sleep; /* Sock wait queue */
struct dst_entry *dst_cache; /* Destination cache */
rwlock_t dst_lock;
atomic_t rmem_alloc; /* Receive queue bytes committed */
struct sk_buff_head receive_queue; /* Incoming packets */
atomic_t wmem_alloc; /* Transmit queue bytes committed */
struct sk_buff_head write_queue; /* Packet sending queue */
atomic_t omem_alloc; /* "o" is "option" or "other" */
int wmem_queued; /* Persistent queue size */
int forward_alloc; /* Space allocated forward. */
__u32 saddr; /* Sending source */
unsigned int allocation; /* Allocation mode */
int sndbuf; /* Size of send buffer in bytes */
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,
broadcast,
bsdism;
unsigned char debug;
unsigned char rcvtstamp;
unsigned char use_write_queue;
unsigned char userlocks;
/* Hole of 3 bytes. Try to pack. */
int route_caps;
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;
/* Error queue, rarely used. */
struct sk_buff_head error_queue;
struct proto *prot;
#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;
unsigned char localroute; /* Route locally only */
unsigned char protocol;
struct ucred peercred;
int rcvlowat;
long rcvtimeo;
long sndtimeo;
#ifdef CONFIG_FILTER
/* Socket Filtering Instructions */
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
struct netlink_opt *af_netlink;
#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
#if defined(CONFIG_WAN_ROUTER) || defined(CONFIG_WAN_ROUTER_MODULE)
struct wanpipe_opt *af_wanpipe;
#endif
} protinfo;
/* This part is used for the timeout functions. */
struct timer_list timer; /* This is the sock cleanup timer. */
struct timeval stamp;
/* Identd and reporting IO signals */
struct socket *socket;
/* RPC layer private data */
void *user_data;
/* Callbacks */
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);
};
write_queue表示待发送到队列，send_head和send_tail表示队列数据包已经发送出去尚未收到应答；receive_queue为读队列其不同于back_log在于后者缓冲从网络层传上来的数据包然后移至receive_queue队列，如果此时此时缓冲区太小则直接丢弃从back_log队列上摘下的数据包。