通过网卡，让我们能达到通信，对unix是相当的复杂，组要是socket每一层都是用了不同的通信协议，需要好多设置好多的选项。因此在/dev目录下，并没有网卡对应的文件。
socket不仅可以用于各种传输协议的IP的连接。也可以用于内核支持的所有其他地址和协议类型。
套接字是使用socket库函数生成的。然后bind函数等。
socket主要用于协议的选择，通信类型，地址族。
bind函数主要目的是给到戒子分配本地地址。该函数的传递一个sockaddr_type 的参数，定义了本地地址。因为不同地址组的地址类型是不同的。type制定了所需的地址类型。
这个时候，我们来点代码
#ifndef _LINUX_IN_H#define _LINUX_IN_H#include <linux/types.h>#include <linux/socket.h>/* Standard well-defined IP protocols. */enum { IPPROTO_IP = 0,/* Dummy protocol for TCP*/ IPPROTO_ICMP = 1,/* Internet Control Message Protocol*/ IPPROTO_IGMP = 2,/* Internet Group Management Protocol*/ IPPROTO_IPIP = 4,/* IPIP tunnels (older KA9Q tunnels use 94) */ IPPROTO_TCP = 6,/* Transmission Control Protocol*/ IPPROTO_EGP = 8,/* Exterior Gateway Protocol*/ IPPROTO_PUP = 12,/* PUP protocol*/ IPPROTO_UDP = 17,/* User Datagram Protocol*/ IPPROTO_IDP = 22,/* XNS IDP protocol*/ IPPROTO_RSVP = 46,/* RSVP protocol*/ IPPROTO_GRE = 47,/* Cisco GRE tunnels (rfc 1701,1702)*/ IPPROTO_IPV6 = 41,/* IPv6-in-IPv4 tunnelling*/ IPPROTO_ESP = 50, /* Encapsulation Security Payload protocol */ IPPROTO_AH = 51, /* Authentication Header protocol */ IPPROTO_PIM = 103,/* Protocol Independent Multicast*/ IPPROTO_COMP = 108, /* Compression Header protocol */ IPPROTO_SCTP = 132,/* Stream Control Transport Protocol*/ IPPROTO_RAW = 255,/* Raw IP packets*/ IPPROTO_MAX};/* Internet address. */struct in_addr {__u32s_addr;};#define IP_TOS1#define IP_TTL2#define IP_HDRINCL3#define IP_OPTIONS4#define IP_ROUTER_ALERT5#define IP_RECVOPTS6#define IP_RETOPTS7#define IP_PKTINFO8#define IP_PKTOPTIONS9#define IP_MTU_DISCOVER10#define IP_RECVERR11#define IP_RECVTTL12#defineIP_RECVTOS13#define IP_MTU14#define IP_FREEBIND15#define IP_IPSEC_POLICY16#define IP_XFRM_POLICY17/* BSD compatibility */#define IP_RECVRETOPTSIP_RETOPTS/* IP_MTU_DISCOVER values */#define IP_PMTUDISC_DONT0/* Never send DF frames */#define IP_PMTUDISC_WANT1/* Use per route hints*/#define IP_PMTUDISC_DO2/* Always DF*/#define IP_MULTICAST_IF32#define IP_MULTICAST_TTL 33#define IP_MULTICAST_LOOP 34#define IP_ADD_MEMBERSHIP35#define IP_DROP_MEMBERSHIP36#define IP_UNBLOCK_SOURCE37#define IP_BLOCK_SOURCE38#define IP_ADD_SOURCE_MEMBERSHIP39#define IP_DROP_SOURCE_MEMBERSHIP40#define IP_MSFILTER41#define MCAST_JOIN_GROUP42#define MCAST_BLOCK_SOURCE43#define MCAST_UNBLOCK_SOURCE44#define MCAST_LEAVE_GROUP45#define MCAST_JOIN_SOURCE_GROUP46#define MCAST_LEAVE_SOURCE_GROUP47#define MCAST_MSFILTER48#define MCAST_EXCLUDE0#define MCAST_INCLUDE1/* These need to appear somewhere around here */#define IP_DEFAULT_MULTICAST_TTL 1#define IP_DEFAULT_MULTICAST_LOOP 1/* Request struct for multicast socket ops */struct ip_mreq {struct in_addr imr_multiaddr;/* IP multicast address of group */struct in_addr imr_interface;/* local IP address of interface */};struct ip_mreqn{struct in_addrimr_multiaddr;/* IP multicast address of group */struct in_addrimr_address;/* local IP address of interface */intimr_ifindex;/* Interface index */};struct ip_mreq_source {__u32imr_multiaddr;__u32imr_interface;__u32imr_sourceaddr;};struct ip_msfilter {__u32imsf_multiaddr;__u32imsf_interface;__u32imsf_fmode;__u32imsf_numsrc;__u32imsf_slist[1];};#define IP_MSFILTER_SIZE(numsrc) \(sizeof(struct ip_msfilter) - sizeof(__u32) \+ (numsrc) * sizeof(__u32))struct group_req{__u32 gr_interface;/* interface index */struct __kernel_sockaddr_storage gr_group;/* group address */};struct group_source_req{__u32 gsr_interface;/* interface index */struct __kernel_sockaddr_storage gsr_group;/* group address */struct __kernel_sockaddr_storage gsr_source;/* source address */};struct group_filter{__u32 gf_interface;/* interface index */struct __kernel_sockaddr_storage gf_group;/* multicast address */__u32 gf_fmode;/* filter mode */__u32 gf_numsrc;/* number of sources */struct __kernel_sockaddr_storage gf_slist[1];/* interface index */};#define GROUP_FILTER_SIZE(numsrc) \(sizeof(struct group_filter) - sizeof(struct __kernel_sockaddr_storage) \+ (numsrc) * sizeof(struct __kernel_sockaddr_storage))struct in_pktinfo{intipi_ifindex;struct in_addripi_spec_dst;struct in_addripi_addr;};/* Structure describing an Internet (IP) socket address. */#define __SOCK_SIZE__16/* sizeof(struct sockaddr)*/struct sockaddr_in { sa_family_tsin_family;/* Address family*/ unsigned short intsin_port;/* Port number*/ struct in_addrsin_addr;/* Internet address*/ /* Pad to size of `struct sockaddr'. */ unsigned char__pad[__SOCK_SIZE__ - sizeof(short int) -sizeof(unsigned short int) - sizeof(struct in_addr)];};
重点看红色部分， sin_family;地址族 unsigned short int sin_port; 端口号
struct in_addr sin_addr; 网络地址。
在这里考虑到网络字节序还有大端小端的问题。
下面有个简单的例子。
使用了基本的用户函数 socket，bind，listion select 函数
只有他的好身体，没有地方可去，只想到处流浪、人生就像一场旅行，
