网络.c

本程序通过两个文件中传递IP报文来模拟网络中IP数据报的传送

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<malloc.h>

#define max_ip_data   2048
#define max_varible_head  40

#define   varible_head  ip_data.Varible_head
#define   data          ip_data.Data

struct ip_head
{
	char ver_head_length;
	char ser_type;
	int total_len;
	int iden;
	int flag_offset;
	char survival_time;
	char protocol;
	int head_check;
	long source_addr;
	long dest_addr;
	union 
	{
		char Varible_head[max_varible_head];    //optional portion,in this program i set it null
		char Data[max_ip_data]; 
	}ip_data; 
	char *data_start;   
};
struct ip_head head;

FILE *fp,*fp1;

int send_data(struct ip_head *ip,long s,long d,int var)
{
	int count,i;
	if(var)
	{
		for(i=0;i<var;i++)
			ip->varible_head[i]=0;
		ip->ver_head_length=(char)(0x40+20+(char)var);
		ip->data_start=ip->data+var;
	}
	else
	{
		ip->ver_head_length=(char)(0x40+20);
		ip->data_start=ip->data;
	}
	ip->ser_type=(char)0x51;
	ip->iden=0;              //NULL
	ip->survival_time=30;    //30 seconds,no use in the program
	ip->protocol=0;          //NULL
	ip->head_check=0;        //not realizing
	ip->source_addr=s;
	ip->dest_addr=d;
	count=strlen(fgets(ip->data_start,max_ip_data-var,fp));
	ip->flag_offset=count%8+(count==(count/8)*8 ? 0:1);    
	if(count==max_ip_data-var-1)
		ip->flag_offset=ip->flag_offset|0x2000;
	ip->total_len=count;   //the total_len is this ip datagram length,not all,for it is some difficult to realize
	return count;
}

int recevie_data(struct ip_head *ip,long d)
{
	if(ip->dest_addr!=d) return 0;
	return 1;
}

int send_pro(long s,long d)
{
	int t;
	t=send_data(&head,s,d,0);
	return t;
}

int rece_pro(struct ip_head *ip,long d,char *ver_head,int *total,long *s)
{
	int t;
	t=recevie_data(&head,d);
	if(!t)return -1;          //error
	*ver_head=ip->ver_head_length;
	*total+=ip->total_len;
	*s=ip->source_addr;
	fputs(ip->data_start,fp1);
	if(ip->flag_offset&0x2000&&!(ip->flag_offset&0x4000)) return 1;   //have more data
	else return 0;    //finish transfering data
} 

void main(void)
{
	char *p2="192.168.1.6",*p3="192.168.2.10";
	char ver_head;
	long sour=192<<12+168<<8+1<<4+6;
	long dest=192<<12+168<<8+2<<4+10;
	long s;
	int tt,count,total=0;
	tt=0;
	if((fp=fopen("send.txt","r"))==NULL)
	{
		printf("can't read\n");
		exit(1);
	}
	if((fp1=fopen("receive_data.txt","w"))==NULL)
	{
		printf("can't write");
		exit(1);
	}
	while(1)
	{
		count=send_pro(sour,dest);
		tt+=count;
		if((rece_pro(&head,dest,&ver_head,&total,&s))!=1)
			break;
	}
	fclose(fp);
	fclose(fp1);
	printf("\nthe source address:");
	printf("%s",p2);
	printf("\nthe destination address:");
	printf("%s",p3);
	printf("\nversion:");
	printf("%d",(int)((ver_head&0xf0)>>4));
	printf("\nthe ip head length:");
	printf("%d",(int)(ver_head&0x0f));
	printf("\nthe total data length:\n");
	printf("%d",tt);
	printf("\nsuccess!!:\n");
}