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 {
__u32 s_addr;
};
#define IP_TOS 1
#define IP_TTL 2
#define IP_HDRINCL 3
#define IP_OPTIONS 4
#define IP_ROUTER_ALERT 5
#define IP_RECVOPTS 6
#define IP_RETOPTS 7
#define IP_PKTINFO 8
#define IP_PKTOPTIONS 9
#define IP_MTU_DISCOVER 10
#define IP_RECVERR 11
#define IP_RECVTTL 12
#define IP_RECVTOS 13
#define IP_MTU 14
#define IP_FREEBIND 15
#define IP_IPSEC_POLICY 16
#define IP_XFRM_POLICY 17
/* BSD compatibility */
#define IP_RECVRETOPTS IP_RETOPTS
/* IP_MTU_DISCOVER values */
#define IP_PMTUDISC_DONT 0 /* Never send DF frames */
#define IP_PMTUDISC_WANT 1 /* Use per route hints */
#define IP_PMTUDISC_DO 2 /* Always DF */
#define IP_MULTICAST_IF 32
#define IP_MULTICAST_TTL 33
#define IP_MULTICAST_LOOP 34
#define IP_ADD_MEMBERSHIP 35
#define IP_DROP_MEMBERSHIP 36
#define IP_UNBLOCK_SOURCE 37
#define IP_BLOCK_SOURCE 38
#define IP_ADD_SOURCE_MEMBERSHIP 39
#define IP_DROP_SOURCE_MEMBERSHIP 40
#define IP_MSFILTER 41
#define MCAST_JOIN_GROUP 42
#define MCAST_BLOCK_SOURCE 43
#define MCAST_UNBLOCK_SOURCE 44
#define MCAST_LEAVE_GROUP 45
#define MCAST_JOIN_SOURCE_GROUP 46
#define MCAST_LEAVE_SOURCE_GROUP 47
#define MCAST_MSFILTER 48
#define MCAST_EXCLUDE 0
#define MCAST_INCLUDE 1
/* 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_addr imr_multiaddr; /* IP multicast address of group */
struct in_addr imr_address; /* local IP address of interface */
int imr_ifindex; /* Interface index */
};
struct ip_mreq_source {
__u32 imr_multiaddr;
__u32 imr_interface;
__u32 imr_sourceaddr;
};
struct ip_msfilter {
__u32 imsf_multiaddr;
__u32 imsf_interface;
__u32 imsf_fmode;
__u32 imsf_numsrc;
__u32 imsf_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
{
int ipi_ifindex;
struct in_addr ipi_spec_dst;
struct in_addr ipi_addr;
};
/* Structure describing an Internet (IP) socket address. */
#define __SOCK_SIZE__ 16 /* sizeof(struct sockaddr) */
struct sockaddr_in {
sa_family_t sin_family; /* Address family */
unsigned short int sin_port; /* Port number */
struct in_addr sin_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
<strong><span style="font-size: 18px;">首先 运行在linux 运行gcc -g -o select select.c</span></strong><pre class="cpp" name="code">/ *******select.c*********/
/ *******Using select() for I/O multiplexing */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
/* port we're listening on */
#define PORT 2020
int main(int argc, char *argv[])
{
/*定义描述符集合 */
fd_set master;
/* 定义select函数可读的描述符*/
fd_set read_fds;
/* 服务器地址 */
struct sockaddr_in serveraddr;
/* 客户地址 */
struct sockaddr_in clientaddr;
/* 定义最大描述符数 */
int fdmax;
/* 监听描述符 */
int listener;
/* accept描述符 */
int newfd;
/* 缓冲 */
char buf[1024];
int nbytes;
/* 设置 setsockopt() SO_REUSEADDR */
int yes = 1;
int addrlen;
int i, j;
/* 清除 */
FD_ZERO(&master);
FD_ZERO(&read_fds);
/* 连接 */
if((listener = socket(AF_INET, SOCK_STREAM, 0)) == -1)
{
perror("Server-socket() error lol!");
exit(1);
}
printf("Server-socket() is OK...\n");
/*判断地址以用 */
if(setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1)
{
perror("Server-setsockopt() error lol!");
exit(1);
}
printf("Server-setsockopt() is OK...\n");
/* bind */
serveraddr.sin_family = AF_INET;
serveraddr.sin_addr.s_addr = INADDR_ANY;
serveraddr.sin_port = htons(PORT);
memset(&(serveraddr.sin_zero), '\0', 8);
if(bind(listener, (struct sockaddr *)&serveraddr, sizeof(serveraddr)) == -1)
{
perror("Server-bind() error lol!");
exit(1);
}
printf("Server-bind() is OK...\n");
/* listen */
if(listen(listener, 10) == -1)
{
perror("Server-listen() error lol!");
exit(1);
}
printf("Server-listen() is OK...\n");
/* 把 listener 加到 master set */
FD_SET(listener, &master);
/* keep track of the biggest file descriptor */
fdmax = listener; /* so far, it's this one*/
/* 循环了 */
for(;;)
{
/* copy it */
read_fds = master;
if(select(fdmax+1, &read_fds, NULL, NULL, NULL) == -1)
{
perror("Server-select() error lol!");
exit(1);
}
printf("Server-select() is OK...\n");
/*从存在的描述符中查找要读的数据*/
for(i = 0; i <= fdmax; i++)
{
if(FD_ISSET(i, &read_fds))
{ /* 得到一个... */
if(i == listener)
{
/* h处理这个的连接 */
addrlen = sizeof(clientaddr);
if((newfd = accept(listener, (struct sockaddr *)&clientaddr, &addrlen)) == -1)
{
perror("Server-accept() error lol!");
}
else
{
printf("Server-accept() is OK...\n");
FD_SET(newfd, &master); /* add to master set */
if(newfd > fdmax)
{ /* keep track of the maximum */
fdmax = newfd;
}
printf("%s: New connection from %s on socket %d\n", argv[0], inet_ntoa(clientaddr.sin_addr), newfd);
}
}
else
{
/* 处理从客户端来的数据*/
if((nbytes = recv(i, buf, sizeof(buf), 0)) <= 0)
{
/* 没有数据 */
if(nbytes == 0)
/* 断开连接。 */
printf("%s: socket %d hung up\n", argv[0], i);
else
perror("recv() error lol!");
/* 关闭... */
close(i);
/* 删除这个连接符 */
FD_CLR(i, &master);
}
else
{
/* we got some data from a client*/
for(j = 0; j <= fdmax; j++)
{
/* 发给每个人! */
if(FD_ISSET(j, &master))
{
/* 除了监听的和自己的。 */
if(j != listener && j != i)
{
if(send(j, buf, nbytes, 0) == -1)
perror("send() error lol!");
}
}
}
}
}
}
}
}
return 0;
}</pre><br>
<pre></pre>
<pre class="cpp" name="code"><strong><span style="font-size: 18px;">然后在运行 ./select & 或者ctrl+z 让他后台运行</span></strong></pre><pre class="cpp" name="code"><span style="font-size: 18px;"><strong>开一个窗口 运行telnet localhostip 2020</strong> </span></pre><pre class="cpp" name="code"><span style="font-size: 18px;">程序如下</span></pre><pre class="cpp" name="code"></pre>
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