Homework 8.10 TFTP protocol download function

TFTP协议 下载功能

实现代码

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <fcntl.h>

//Printing error new macro function
#define ERR_MSG(msg) do{\ fprintf(stderr, " __%d__ ", __LINE__);\ perror(msg);\ }while(0)

int main(int argc, const char *argv[])
{
    
	if(argc < 4)
	{
    
		fprintf(stderr, "请输入IP port\n");
		return -1;
	}
	//The port number string that will be obtained,转换成整形
	int port = atoi(argv[2]);

	//Create a newspaper socket
	int sfd = socket(AF_INET, SOCK_DGRAM, 0);
	if(sfd < 0)
	{
    
		ERR_MSG("socket");
		return -1;
	}	

	//Bind the client's own address information structure ---> not required binding 
	
	//填充服务器的IP地址以及端口号 -->Because the client wants to actively send packets to the server
	struct sockaddr_in sin;
	sin.sin_family 		= AF_INET;
	sin.sin_port 		= htons(port);
	sin.sin_addr.s_addr = inet_addr(argv[1]);

	struct sockaddr_in rcv_addrmsg; 	//Stores where received packets came from
	socklen_t addrlen = sizeof(rcv_addrmsg);

	char buf[128] = "";         //读写请求
	char buf2[1024]= "";        //数据包
	char buf3[16]="";           //ACK
	char *ptr=buf;
	short int* pa =(short int*)ptr;
	*pa=htons(1);               //Opcode for read and write requests
	char *pb=ptr+2;             //The filename for read and write requests
	strcpy(pb,"5.png");
	char *pc =pb+strlen(pb);
	char *pd=pc+1;
	strcpy(pd,"octet");         //The mode of read and write requests
	size_t size=2+strlen(pb)+1+strlen("octet")+1;   //Read and write request length

	//将数据包发送给服务器,Therefore, the address information structure needs to be filled with server information
	if(sendto(sfd, buf,size,0,(struct sockaddr*)&sin, sizeof(sin)) < 0)
	{
    
		ERR_MSG("sendto");
		return -1;
	}
	//printf("sendto success\n");
	
	short int num=1;            //块编号
	ssize_t res;                //The number of bytes successfully read by the packet
	int fd=open(argv[3],O_WRONLY|O_TRUNC|O_CREAT,0777);     //打开指定文件
	if(fd<0)
	{
    
		ERR_MSG("open");
		return -1;
	}

	while(1)
	{
    
		bzero(buf2, sizeof(buf2));     // Packet empty

		//Receive data packets sent by the server
		if((res=recvfrom(sfd, buf2, sizeof(buf2), 0, (struct sockaddr*)&rcv_addrmsg,&addrlen))< 0)
		{
    
			ERR_MSG("recvfrom");
			return -1;
		}
		char *ptr2=buf2;
		short int *err;
		if(ntohs(*(err=(short int*)ptr2))==5)  //The opcode is equal to5,ERROR
		{
    
			printf("error:%s\n",(char*)(err+2));       //Print error message
			return -1;
		}

		//Opcodes are not5,即为3,是数据包,ptr2Points to the first address of the data packet
		write(fd,ptr2+4,res-4);             //The data of the packet is written to the file
		short int *pa2=(short int*)ptr2;    //Get the block number sent by the server
		if(ntohs(*(pa2+1))!=num)            //Compare if the block numbers are the same,i.e. the order is correct
		{
    
			continue;
		}
		num++;
		
		bzero(buf3, sizeof(buf3));       //清空ACK
		char *ptr3=buf3;                 //ACK首地址
		short int* pa3=(short int*)ptr3;
		*pa3=htons(4);                   //ACK操作码
		*(pa3+1)=*(pa2+1);               //ACK块编号

		//将ACK发送给服务器
		if(sendto(sfd, buf3,4,0,(struct sockaddr*)&rcv_addrmsg, addrlen) < 0)
		{
    
			ERR_MSG("sendto");
			return -1;
		}
		
		if(res<516)      //The data in the packet is less than512字节
		{
    
			break;                //退出循环
		}
	}
	printf("下载完成\n");
	//关闭套接字
	close(sfd);
	return 0;
}

运行结果

服务器
IP地址：192.168.31.22
端口：69
从服务器下载文件5.png