Linux内核--网络协议栈深入分析(三)--BSD socket和传输层sock

时间:2022-03-03 02:18:15

本文分析基于Linux Kernel 3.2.1

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作者:闫明


Linux内核中协议族有INET协议族,UNIX协议族等,我们还是以INET协议族为例。

下面是内核中的协议族声明:

/* Supported address families. */#define AF_UNSPEC0
#define AF_UNIX1/* Unix domain sockets */
#define AF_LOCAL1/* POSIX name for AF_UNIX*/
#define AF_INET2/* Internet IP Protocol */
#define AF_AX253/* Amateur Radio AX.25 */
#define AF_IPX4/* Novell IPX */
#define AF_APPLETALK5/* AppleTalk DDP */
#define AF_NETROM6/* Amateur Radio NET/ROM */
#define AF_BRIDGE7/* Multiprotocol bridge */
#define AF_ATMPVC8/* ATM PVCs*/
#define AF_X259/* Reserved for X.25 project */
#define AF_INET610/* IP version 6*/
#define AF_ROSE11/* Amateur Radio X.25 PLP*/
#define AF_DECnet12/* Reserved for DECnet project*/
#define AF_NETBEUI13/* Reserved for 802.2LLC project*/
#define AF_SECURITY14/* Security callback pseudo AF */
#define AF_KEY15 /* PF_KEY key management API */
#define AF_NETLINK16
#define AF_ROUTEAF_NETLINK /* Alias to emulate 4.4BSD */
#define AF_PACKET17/* Packet family*/
#define AF_ASH18/* Ash*/
#define AF_ECONET19/* Acorn Econet*/
#define AF_ATMSVC20/* ATM SVCs*/
#define AF_RDS21/* RDS sockets */
#define AF_SNA22/* Linux SNA Project (nutters!) */
#define AF_IRDA23/* IRDA sockets*/
#define AF_PPPOX24/* PPPoX sockets*/
#define AF_WANPIPE25/* Wanpipe API Sockets */
#define AF_LLC26/* Linux LLC*/
#define AF_CAN29/* Controller Area Network */
#define AF_TIPC30/* TIPC sockets*/
#define AF_BLUETOOTH31/* Bluetooth sockets */
#define AF_IUCV32/* IUCV sockets*/
#define AF_RXRPC33/* RxRPC sockets */
#define AF_ISDN34/* mISDN sockets */
#define AF_PHONET35/* Phonet sockets*/
#define AF_IEEE80215436/* IEEE802154 sockets*/
#define AF_CAIF37/* CAIF sockets*/
#define AF_ALG38/* Algorithm sockets*/
#define AF_NFC39/* NFC sockets*/
#define AF_MAX40/* For now.. */

内核中的PF_***和AF_***其实可以混用,它的宏定义如下:

/* Protocol families, same as address families. */#define PF_UNSPECAF_UNSPEC#define PF_UNIXAF_UNIX#define PF_LOCALAF_LOCAL#define PF_INETAF_INET#define PF_AX25AF_AX25#define PF_IPXAF_IPX#define PF_APPLETALKAF_APPLETALK#definePF_NETROMAF_NETROM#define PF_BRIDGEAF_BRIDGE#define PF_ATMPVCAF_ATMPVC#define PF_X25AF_X25#define PF_INET6AF_INET6#define PF_ROSEAF_ROSE#define PF_DECnetAF_DECnet#define PF_NETBEUIAF_NETBEUI#define PF_SECURITYAF_SECURITY#define PF_KEYAF_KEY#define PF_NETLINKAF_NETLINK#define PF_ROUTEAF_ROUTE#define PF_PACKETAF_PACKET#define PF_ASHAF_ASH#define PF_ECONETAF_ECONET#define PF_ATMSVCAF_ATMSVC#define PF_RDSAF_RDS#define PF_SNAAF_SNA#define PF_IRDAAF_IRDA#define PF_PPPOXAF_PPPOX#define PF_WANPIPEAF_WANPIPE#define PF_LLCAF_LLC#define PF_CANAF_CAN#define PF_TIPCAF_TIPC#define PF_BLUETOOTHAF_BLUETOOTH#define PF_IUCVAF_IUCV#define PF_RXRPCAF_RXRPC#define PF_ISDNAF_ISDN#define PF_PHONETAF_PHONET#define PF_IEEE802154AF_IEEE802154#define PF_CAIFAF_CAIF#define PF_ALGAF_ALG#define PF_NFCAF_NFC#define PF_MAXAF_MAX

以后的分析就是以INET协议族为例来分析的。

下面的结构体就是在系统初始化时用来管理协议族初始化的结构体:

struct net_proto_family {intfamily;int(*create)(struct net *net, struct socket *sock,  int protocol, int kern);struct module*owner;};
第一个属性就是协议族的宏定义,如PF_INET;

第二个属性就是协议族对应的初始化函数指针;

INET协议族对应该结构的定义如下:

static const struct net_proto_family inet_family_ops = {.family = PF_INET,.create = inet_create,.owner= THIS_MODULE,};

下面结构体是协议族操作集结构体定义:

struct proto_ops {intfamily;struct module*owner;int(*release)   (struct socket *sock);int(*bind)     (struct socket *sock,      struct sockaddr *myaddr,      int sockaddr_len);int(*connect)   (struct socket *sock,      struct sockaddr *vaddr,      int sockaddr_len, int flags);int(*socketpair)(struct socket *sock1,      struct socket *sock2);int(*accept)    (struct socket *sock,      struct socket *newsock, int flags);int(*getname)   (struct socket *sock,      struct sockaddr *addr,      int *sockaddr_len, int peer);unsigned int(*poll)     (struct file *file, struct socket *sock,      struct poll_table_struct *wait);int(*ioctl)     (struct socket *sock, unsigned int cmd,      unsigned long arg);#ifdef CONFIG_COMPATint (*compat_ioctl) (struct socket *sock, unsigned int cmd,      unsigned long arg);#endifint(*listen)    (struct socket *sock, int len);int(*shutdown)  (struct socket *sock, int flags);int(*setsockopt)(struct socket *sock, int level,      int optname, char __user *optval, unsigned int optlen);int(*getsockopt)(struct socket *sock, int level,      int optname, char __user *optval, int __user *optlen);#ifdef CONFIG_COMPATint(*compat_setsockopt)(struct socket *sock, int level,      int optname, char __user *optval, unsigned int optlen);int(*compat_getsockopt)(struct socket *sock, int level,      int optname, char __user *optval, int __user *optlen);#endifint(*sendmsg)   (struct kiocb *iocb, struct socket *sock,      struct msghdr *m, size_t total_len);int(*recvmsg)   (struct kiocb *iocb, struct socket *sock,      struct msghdr *m, size_t total_len,      int flags);int(*mmap)     (struct file *file, struct socket *sock,      struct vm_area_struct * vma);ssize_t(*sendpage)  (struct socket *sock, struct page *page,      int offset, size_t size, int flags);ssize_t (*splice_read)(struct socket *sock,  loff_t *ppos,       struct pipe_inode_info *pipe, size_t len, unsigned int flags);};

INET协议族中TCP和UDP协议对应的上述操作集的定义不同:

TCP协议z在INET层操作集inet_stream_ops

const struct proto_ops inet_stream_ops = {.family   = PF_INET,.owner   = THIS_MODULE,.release   = inet_release,.bind   = inet_bind,.connect   = inet_stream_connect,.socketpair   = sock_no_socketpair,.accept   = inet_accept,.getname   = inet_getname,.poll   = tcp_poll,.ioctl   = inet_ioctl,.listen   = inet_listen,.shutdown   = inet_shutdown,.setsockopt   = sock_common_setsockopt,.getsockopt   = sock_common_getsockopt,.sendmsg   = inet_sendmsg,.recvmsg   = inet_recvmsg,.mmap   = sock_no_mmap,.sendpage   = inet_sendpage,.splice_read   = tcp_splice_read,#ifdef CONFIG_COMPAT.compat_setsockopt = compat_sock_common_setsockopt,.compat_getsockopt = compat_sock_common_getsockopt,.compat_ioctl   = inet_compat_ioctl,#endif};
UDP协议在INET层操作集inet_dgram_ops
const struct proto_ops inet_dgram_ops = {.family   = PF_INET,.owner   = THIS_MODULE,.release   = inet_release,.bind   = inet_bind,.connect   = inet_dgram_connect,.socketpair   = sock_no_socketpair,.accept   = sock_no_accept,.getname   = inet_getname,.poll   = udp_poll,.ioctl   = inet_ioctl,.listen   = sock_no_listen,.shutdown   = inet_shutdown,.setsockopt   = sock_common_setsockopt,.getsockopt   = sock_common_getsockopt,.sendmsg   = inet_sendmsg,.recvmsg   = inet_recvmsg,.mmap   = sock_no_mmap,.sendpage   = inet_sendpage,#ifdef CONFIG_COMPAT.compat_setsockopt = compat_sock_common_setsockopt,.compat_getsockopt = compat_sock_common_getsockopt,.compat_ioctl   = inet_compat_ioctl,#endif};

上面两个操作集是属于INET协议族层次,可以由协议族层套接字socket来管理,下面是协议族层析的套接字结构体(BSD Socket)定义:

/** *  struct socket - general BSD socket *  @state: socket state (%SS_CONNECTED, etc) *  @type: socket type (%SOCK_STREAM, etc) *  @flags: socket flags (%SOCK_ASYNC_NOSPACE, etc) *  @ops: protocol specific socket operations *  @file: File back pointer for gc *  @sk: internal networking protocol agnostic socket representation *  @wq: wait queue for several uses */struct socket {socket_statestate;kmemcheck_bitfield_begin(type);shorttype;kmemcheck_bitfield_end(type);unsigned longflags;struct socket_wq __rcu*wq;struct file*file;struct sock*sk;const struct proto_ops*ops;};

最后一个属性就指向了上面所述的操作集。若使用TCP协议,ops就是inet_stream_ops,若是UDP协议,ops就是inet_dgram_ops。

short type属性的取值可以是如下值:

enum sock_type {SOCK_DGRAM= 1,SOCK_STREAM= 2,SOCK_RAW= 3,SOCK_RDM= 4,SOCK_SEQPACKET= 5,SOCK_DCCP= 6,SOCK_PACKET= 10,};

传输层的协议操作集结构体定义:

struct proto {void(*close)(struct sock *sk, long timeout);int(*connect)(struct sock *sk,        struct sockaddr *uaddr, int addr_len);int(*disconnect)(struct sock *sk, int flags);struct sock *(*accept) (struct sock *sk, int flags, int *err);int(*ioctl)(struct sock *sk, int cmd, unsigned long arg);int(*init)(struct sock *sk);void(*destroy)(struct sock *sk);void(*shutdown)(struct sock *sk, int how);int(*setsockopt)(struct sock *sk, int level, int optname, char __user *optval,unsigned int optlen);int(*getsockopt)(struct sock *sk, int level, int optname, char __user *optval, int __user *option);   #ifdef CONFIG_COMPATint(*compat_setsockopt)(struct sock *sk,int level,int optname, char __user *optval,unsigned int optlen);int(*compat_getsockopt)(struct sock *sk,int level,int optname, char __user *optval,int __user *option);int(*compat_ioctl)(struct sock *sk,unsigned int cmd, unsigned long arg);#endifint(*sendmsg)(struct kiocb *iocb, struct sock *sk,   struct msghdr *msg, size_t len);int(*recvmsg)(struct kiocb *iocb, struct sock *sk,   struct msghdr *msg,size_t len, int noblock, int flags, int *addr_len);int(*sendpage)(struct sock *sk, struct page *page,int offset, size_t size, int flags);int(*bind)(struct sock *sk, struct sockaddr *uaddr, int addr_len);int(*backlog_rcv) (struct sock *sk, struct sk_buff *skb);/* Keeping track of sk's, looking them up, and port selection methods. */void(*hash)(struct sock *sk);void(*unhash)(struct sock *sk);void(*rehash)(struct sock *sk);int(*get_port)(struct sock *sk, unsigned short snum);void(*clear_sk)(struct sock *sk, int size);/* Keeping track of sockets in use */#ifdef CONFIG_PROC_FSunsigned intinuse_idx;#endif/* Memory pressure */void(*enter_memory_pressure)(struct sock *sk);atomic_long_t*memory_allocated;/* Current allocated memory. */struct percpu_counter*sockets_allocated;/* Current number of sockets. *//* * Pressure flag: try to collapse. * Technical note: it is used by multiple contexts non atomically. * All the __sk_mem_schedule() is of this nature: accounting * is strict, actions are advisory and have some latency. */int*memory_pressure;long*sysctl_mem;int*sysctl_wmem;int*sysctl_rmem;intmax_header;boolno_autobind;struct kmem_cache*slab;unsigned intobj_size;intslab_flags;struct percpu_counter*orphan_count;struct request_sock_ops*rsk_prot;struct timewait_sock_ops *twsk_prot;union {struct inet_hashinfo*hashinfo;struct udp_table*udp_table;struct raw_hashinfo*raw_hash;} h;struct module*owner;charname[32];struct list_headnode;#ifdef SOCK_REFCNT_DEBUGatomic_tsocks;#endif};
该结构体和proto_ops的区别是:该结构体和具体的传输层协议相关,其中的函数指针指向对应的协议的相应的操作函数。


TCP协议的操作集定义如下:

struct proto tcp_prot = {.name= "TCP",.owner= THIS_MODULE,.close= tcp_close,.connect= tcp_v4_connect,.disconnect= tcp_disconnect,.accept= inet_csk_accept,.ioctl= tcp_ioctl,.init= tcp_v4_init_sock,.destroy= tcp_v4_destroy_sock,.shutdown= tcp_shutdown,.setsockopt= tcp_setsockopt,.getsockopt= tcp_getsockopt,.recvmsg= tcp_recvmsg,.sendmsg= tcp_sendmsg,.sendpage= tcp_sendpage,.backlog_rcv= tcp_v4_do_rcv,.hash= inet_hash,.unhash= inet_unhash,.get_port= inet_csk_get_port,.enter_memory_pressure= tcp_enter_memory_pressure,.sockets_allocated= &tcp_sockets_allocated,.orphan_count= &tcp_orphan_count,.memory_allocated= &tcp_memory_allocated,.memory_pressure= &tcp_memory_pressure,.sysctl_mem= sysctl_tcp_mem,.sysctl_wmem= sysctl_tcp_wmem,.sysctl_rmem= sysctl_tcp_rmem,.max_header= MAX_TCP_HEADER,.obj_size= sizeof(struct tcp_sock),.slab_flags= SLAB_DESTROY_BY_RCU,.twsk_prot= &tcp_timewait_sock_ops,.rsk_prot= &tcp_request_sock_ops,.h.hashinfo= &tcp_hashinfo,.no_autobind= true,#ifdef CONFIG_COMPAT.compat_setsockopt= compat_tcp_setsockopt,.compat_getsockopt= compat_tcp_getsockopt,#endif};


UDP协议的操作集则为:

struct proto udp_prot = {.name   = "UDP",.owner   = THIS_MODULE,.close   = udp_lib_close,.connect   = ip4_datagram_connect,.disconnect   = udp_disconnect,.ioctl   = udp_ioctl,.destroy   = udp_destroy_sock,.setsockopt   = udp_setsockopt,.getsockopt   = udp_getsockopt,.sendmsg   = udp_sendmsg,.recvmsg   = udp_recvmsg,.sendpage   = udp_sendpage,.backlog_rcv   = __udp_queue_rcv_skb,.hash   = udp_lib_hash,.unhash   = udp_lib_unhash,.rehash   = udp_v4_rehash,.get_port   = udp_v4_get_port,.memory_allocated  = &udp_memory_allocated,.sysctl_mem   = sysctl_udp_mem,.sysctl_wmem   = &sysctl_udp_wmem_min,.sysctl_rmem   = &sysctl_udp_rmem_min,.obj_size   = sizeof(struct udp_sock),.slab_flags   = SLAB_DESTROY_BY_RCU,.h.udp_table   = &udp_table,#ifdef CONFIG_COMPAT.compat_setsockopt = compat_udp_setsockopt,.compat_getsockopt = compat_udp_getsockopt,#endif.clear_sk   = sk_prot_clear_portaddr_nulls,};


现在介绍struct socket结构体中一个属性struct sock类型的结构体指针,这个结构体就是传输层的套接字,所有套接字通过该结构来使用网络协议的所有服务。定义如下:

struct sock {/* * Now struct inet_timewait_sock also uses sock_common, so please just * don't add nothing before this first member (__sk_common) --acme */struct sock_common__sk_common;#define sk_node__sk_common.skc_node#define sk_nulls_node__sk_common.skc_nulls_node#define sk_refcnt__sk_common.skc_refcnt#define sk_tx_queue_mapping__sk_common.skc_tx_queue_mapping#define sk_dontcopy_begin__sk_common.skc_dontcopy_begin#define sk_dontcopy_end__sk_common.skc_dontcopy_end#define sk_hash__sk_common.skc_hash#define sk_family__sk_common.skc_family#define sk_state__sk_common.skc_state#define sk_reuse__sk_common.skc_reuse#define sk_bound_dev_if__sk_common.skc_bound_dev_if#define sk_bind_node__sk_common.skc_bind_node#define sk_prot__sk_common.skc_prot#define sk_net__sk_common.skc_netsocket_lock_tsk_lock;struct sk_buff_headsk_receive_queue;/* * 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. * Note : rmem_alloc is in this structure to fill a hole * on 64bit arches, not because its logically part of * backlog. */struct {atomic_trmem_alloc;intlen;struct sk_buff*head;struct sk_buff*tail;} sk_backlog;#define sk_rmem_alloc sk_backlog.rmem_allocintsk_forward_alloc;#ifdef CONFIG_RPS__u32sk_rxhash;#endifatomic_tsk_drops;intsk_rcvbuf;struct sk_filter __rcu*sk_filter;struct socket_wq __rcu*sk_wq;#ifdef CONFIG_NET_DMAstruct sk_buff_headsk_async_wait_queue;#endif#ifdef CONFIG_XFRMstruct xfrm_policy*sk_policy[2];#endifunsigned long sk_flags;struct dst_entry*sk_dst_cache;spinlock_tsk_dst_lock;atomic_tsk_wmem_alloc;atomic_tsk_omem_alloc;intsk_sndbuf;struct sk_buff_headsk_write_queue;kmemcheck_bitfield_begin(flags);unsigned intsk_shutdown  : 2,sk_no_check  : 2,sk_userlocks : 4,sk_protocol  : 8,sk_type      : 16;kmemcheck_bitfield_end(flags);intsk_wmem_queued;gfp_tsk_allocation;intsk_route_caps;intsk_route_nocaps;intsk_gso_type;unsigned intsk_gso_max_size;intsk_rcvlowat;unsigned long        sk_lingertime;struct sk_buff_headsk_error_queue;struct proto*sk_prot_creator;rwlock_tsk_callback_lock;intsk_err,sk_err_soft;unsigned shortsk_ack_backlog;unsigned shortsk_max_ack_backlog;__u32sk_priority;struct pid*sk_peer_pid;const struct cred*sk_peer_cred;longsk_rcvtimeo;longsk_sndtimeo;void*sk_protinfo;struct timer_listsk_timer;ktime_tsk_stamp;struct socket*sk_socket;void*sk_user_data;struct page*sk_sndmsg_page;struct sk_buff*sk_send_head;__u32sk_sndmsg_off;intsk_write_pending;#ifdef CONFIG_SECURITYvoid*sk_security;#endif__u32sk_mark;u32sk_classid;void(*sk_state_change)(struct sock *sk);void(*sk_data_ready)(struct sock *sk, int bytes);void(*sk_write_space)(struct sock *sk);void(*sk_error_report)(struct sock *sk);  int(*sk_backlog_rcv)(struct sock *sk,  struct sk_buff *skb);  void                    (*sk_destruct)(struct sock *sk);};
若sk_family是PF_INET,则sk_type可以取值:SOCK_STREAM,SOCK_DGRAM,SOCK_RAW。其中sk_prot就是指向具体协议的操作集,如TCP协议就为tcp_prot。

若要将协议族操作集和具体协议操作集整合起来为IP协议提供接口,就需要下面的结构体定义:

struct inet_protosw {struct list_head list;        /* These two fields form the lookup key.  */unsigned short type;   /* This is the 2nd argument to socket(2). */unsigned short protocol; /* This is the L4 protocol number.  */struct proto *prot;const struct proto_ops *ops;  char             no_check;   /* checksum on rcv/xmit/none? */unsigned char flags;      /* See INET_PROTOSW_* below.  */};

INET三种套接字定义的inetsw_array数组如下:

static struct inet_protosw inetsw_array[] ={{.type =       SOCK_STREAM,.protocol =   IPPROTO_TCP,.prot =       &tcp_prot,.ops =        &inet_stream_ops,.no_check =   0,.flags =      INET_PROTOSW_PERMANENT |      INET_PROTOSW_ICSK,},{.type =       SOCK_DGRAM,.protocol =   IPPROTO_UDP,.prot =       &udp_prot,.ops =        &inet_dgram_ops,.no_check =   UDP_CSUM_DEFAULT,.flags =      INET_PROTOSW_PERMANENT,       },       {.type =       SOCK_DGRAM,.protocol =   IPPROTO_ICMP,.prot =       &ping_prot,.ops =        &inet_dgram_ops,.no_check =   UDP_CSUM_DEFAULT,.flags =      INET_PROTOSW_REUSE,       },       {       .type =       SOCK_RAW,       .protocol =   IPPROTO_IP,/* wild card */       .prot =       &raw_prot,       .ops =        &inet_sockraw_ops,       .no_check =   UDP_CSUM_DEFAULT,       .flags =      INET_PROTOSW_REUSE,       }};
不过,在初始化的时候我们会将上面数组中的的元素按套接字类型插入inetsw链表数组中。其定义如下:

static struct list_head inetsw[SOCK_MAX];


那内核中套接字struct socket、struct sock、struct inet_sock、struct tcp_sock、struct raw_sock、struct udp_sock、struct inet_connection_sock、struct inet_timewait_sock和struct tcp_timewait_sock的关系是怎样的呢?

*struct socket这个是BSD层的socket,应用程序会用过系统调用首先创建该类型套接字,它和具体协议无关。

*struct inet_sock是INET协议族使用的socket结构,可以看成位于INET层,是struct sock的一个扩展。它的第一个属性就是struct sock结构。

*struct sock是与具体传输层协议相关的套接字,所有内核的操作都基于这个套接字。

*struct tcp_sock是TCP协议的套接字表示,它是对struct inet_connection_sock的扩展,其第一个属性就是struct inet_connection_sock inet_conn。

*struct raw_sock是原始类型的套接字表示,ICMP协议就使用这种套接字,其是对struct sock的扩展。

*struct udp_sock是UDP协议套接字表示,其是对struct inet_sock套接字的扩展。

*struct inet_connetction_sock是所有面向连接协议的套接字,是对struct inet_sock套接字扩展。

后面两个是用于控制超时的套接字。

就拿struct inet_sock和struct sock为例来说明,为什么内核中可以直接将sock结构体首地址强制转换成inet_sock的首地址?并且inet_sock的大小要大于sock,直接进行如下强制转换

inet = inet_sk(sk);

static inline struct inet_sock *inet_sk(const struct sock *sk){return (struct inet_sock *)sk;}

不会发生内存非法访问吗?!那就是在分配的时候并不只是分配的struct sock结构体大小的存储空间!

可以细看sock结构体分配的代码:

struct sock *sk_alloc(struct net *net, int family, gfp_t priority,      struct proto *prot){struct sock *sk;sk = sk_prot_alloc(prot, priority | __GFP_ZERO, family);if (sk) {sk->sk_family = family;sk->sk_prot = sk->sk_prot_creator = prot;sock_lock_init(sk);sock_net_set(sk, get_net(net));atomic_set(&sk->sk_wmem_alloc, 1);sock_update_classid(sk);}return sk;}
紧接着调用sk_prot_alloc函数分配:

static struct sock *sk_prot_alloc(struct proto *prot, gfp_t priority,int family){struct sock *sk;struct kmem_cache *slab;slab = prot->slab;if (slab != NULL) {sk = kmem_cache_alloc(slab, priority & ~__GFP_ZERO);..............................} elsesk = kmalloc(prot->obj_size, priority);.....................return sk;......................}
上面的代码中首先判断高速缓存中是否可用,如果不可用,直接在内存分配空间,不过大小都是prot->obj_size。

如果是TCP协议中的tcp_prot中指明该属性的大小为.obj_size= sizeof(struct tcp_sock)。

所以,程序中给struct sock指针分配的不是该结构体的实际大小,而是大于其实际大小,以便其扩展套接字的属性占用。
以图例说明tcp_sock是如何从sock强制转换来的:

Linux内核--网络协议栈深入分析(三)--BSD socket和传输层sock

下篇将分析套接字的绑定、连接等一系列操作的实现。


下篇将分析套接字的操作函数。