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_statestate;unsigned longflags;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_twait;shorttype;unsigned charpasscred;};?
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. */__u32daddr;/* Foreign IPv4 addr*/__u32rcv_saddr;/* Bound local IPv4 addr*/__u16dport;/* Destination port*/unsigned shortnum;/* Local port*/intbound_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 charstate,/* Connection state*/zapped;/* In ax25 & ipx means not linked*/__u16sport;/* Source port*/unsigned shortfamily;/* Address family*/unsigned charreuse;/* SO_REUSEADDR setting*/unsigned charshutdown;atomic_trefcnt;/* Reference count*/socket_lock_tlock;/* Synchronizer...*/intrcvbuf;/* Size of receive buffer in bytes*/wait_queue_head_t*sleep;/* Sock wait queue*/struct dst_entry*dst_cache;/* Destination cache*/rwlock_tdst_lock;atomic_trmem_alloc;/* Receive queue bytes committed*/struct sk_buff_headreceive_queue;/* Incoming packets*/atomic_twmem_alloc;/* Transmit queue bytes committed*/struct sk_buff_headwrite_queue;/* Packet sending queue*/atomic_tomem_alloc;/* "o" is "option" or "other" */intwmem_queued;/* Persistent queue size */intforward_alloc;/* Space allocated forward. */__u32saddr;/* Sending source*/unsigned intallocation;/* Allocation mode*/intsndbuf;/* 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 chardead,done,urginline,keepopen,linger,destroy,no_check,broadcast,bsdism;unsigned chardebug;unsigned charrcvtstamp;unsigned charuse_write_queue;unsigned charuserlocks;/* Hole of 3 bytes. Try to pack. */introute_caps;intproc;unsigned long lingertime;inthashent;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_tcallback_lock;/* Error queue, rarely used. */struct sk_buff_headerror_queue;struct proto*prot;#if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)union {struct ipv6_pinfoaf_inet6;} net_pinfo;#endifunion {struct tcp_optaf_tcp;#if defined(CONFIG_INET) || defined (CONFIG_INET_MODULE)struct raw_opttp_raw4;#endif#if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)struct raw6_opttp_raw;#endif /* CONFIG_IPV6 */#if defined(CONFIG_SPX) || defined (CONFIG_SPX_MODULE)struct spx_optaf_spx;#endif /* CONFIG_SPX */} tp_pinfo;interr, err_soft;/* Soft holds errors that don't cause failure but are the cause of a persistent failure not just 'timed out' */unsigned shortack_backlog;unsigned shortmax_ack_backlog;__u32priority;unsigned shorttype;unsigned charlocalroute;/* Route locally only */unsigned charprotocol;struct ucredpeercred;intrcvlowat;longrcvtimeo;longsndtimeo;#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_optaf_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_sockaf_at;#endif#if defined(CONFIG_IPX) || defined(CONFIG_IPX_MODULE)struct ipx_optaf_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;#endifstruct 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_listtimer;/* This is the sock cleanup timer. */struct timevalstamp;/* 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。
?