套接字程序 - 使用 UDP 的可靠数据传输

Question

我已经用 C 编写了可靠数据传输的代码。在这里，客户端将文件以 1024 字节的块发送到服务器。 服务器将接收到的数据包写入另一个文件。

该程序适用于字符串文件/二进制数据文件。 但是当我尝试复制图像文件时出现了问题。 当我尝试传输 .jpg 文件时，没有编译错误或运行时错误。 但是，当我尝试打开目标文件时，它说“无法打开图像文件”，“解释 jpeg 文件时出错”，“在状态 201 中对 jpeg 库的调用不当”。

任何人都可以查看代码并提出任何解决方案吗？

这是我的客户端代码

#include<stdio.h>
#include<sys/types.h>
#include<netinet/in.h>
#include<netdb.h>
#include<string.h>
#include<stdlib.h>
#include<unistd.h>
#include<errno.h>
#include<arpa/inet.h>

char *itoa(long i,char *s,int dummy_radix)
{
    sprintf(s,"%ld",i);
    return s;
}

int main(int argc, int *argv[])
{
    int sock,length,n;
    struct sockaddr_in server,from;
    struct hostent *hp;
    long int packets =0;
    unsigned char buff[1024] = {0};

    // checking if hostname and the port address is provided //
    if(argc!=3)
    {
        printf("insufficient arguments\n");
        exit(1);
    }

    //create a socket//
    sock = socket(AF_INET,SOCK_DGRAM,0);

    if(sock<0)
    {
        printf("error in opening socket\n");
        return 1;
    }

    //to get  the hostname of the system or the machine//
    hp= gethostbyname(argv[1]);

    if(hp==0)
    {
        printf("Unknown host\n");
        return 1;
    }
    //build the server's IP address //
    bzero((char *)&server,sizeof(server));
    bcopy((char*)hp->h_addr,(char *)&server.sin_addr,hp->h_length);
    server.sin_family = AF_INET;
    server.sin_port =  htons(atoi(argv[2]));
    length = sizeof(server);

    /*open the file that we wish to transfer*/
    FILE *fp = fopen("landscape.jpeg","rb");
    if(fp==NULL)
    {
        printf("file open error");
        return 1;
    }
    fseek(fp,0,SEEK_END); //if exists read the size of the file 
    size_t file_size = ftell(fp);
    fseek(fp,0,SEEK_SET); 

    printf("size of the file is %d\n", file_size);

    /*find the number of packets*/
    packets = (file_size/1024)+1 ;

    /*send the number of packets to the server*/
    itoa(packets,buff,10);
    n = sendto(sock,buff,strlen(buff),0,(struct sockaddr *)&server,sizeof(struct sockaddr));
    if(n<0)
    {
        printf("error in sending message to the serveR");
        return 1;
    }

    /*Read data from file and send it*/
    int packetNum = 0;
    while(1)
    {
        /*First read file in chunks of  1024  bytes */
        int nread = fread(buff,1,1024,fp);
        //printf("Bytes read %d\n",nread);

        /*if read was success ,send data*/
        if(nread>0)
        {
            //printf("data sent now is %s\n",buff);
            n = sendto(sock,buff,strlen(buff),0,(struct sockaddr *)&server,sizeof(struct sockaddr));
            printf("Sending %d, numBytes sent: %d\n", packetNum, n);
            packetNum++;
            if(n<0)
            {
                printf("error in sending message to the server");
                fclose(fp);
                return 1;
            }
        }

        /*There is something tricky going on with the read..
         * Either there was error ,or we reached end of  file.
         */
        if(nread<1024)
        {
            if(feof(fp))
                printf("End of file\n");

            if(ferror(fp))
                printf("Error reading\n");
            break;
        }
    }
    close(sock);
    fclose(fp);

    return 0;
}

这是我的服务器代码

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

int main(int argc, char *argv[])
{
    int sock,length,fromlen,n;
    struct sockaddr_in server;
    struct sockaddr_in from;
    char buf[1024];
    char file_buf[1024];
    int packets = 0;
    int received = 0;
    FILE *newfp;
    newfp = fopen("output.jpeg","wb");
    if(newfp==NULL)
    {
        printf("error opening the file\n");
        return 1;
    }    
    if(argc<2)
    {
        fprintf(stderr, "no port number specified\n");
        exit(0);
    }
    sock = socket(AF_INET,SOCK_DGRAM,0);
    if(sock<0)
    {
        printf("error in opening socket\n");
        return 1;
    }
    length = sizeof(server);
    bzero(&server,length);
    server.sin_family= AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(atoi(argv[1]));
    if(bind(sock,(struct sockaddr*)&server,length)<0)
    {
        printf("cannot bind\n");
        return 1;
    }
    fromlen = sizeof(struct sockaddr_in);
    n = recvfrom(sock,buf,1024,0,(struct sockaddr *)&from,&fromlen);
    if(n<0)
    {
        printf("recvfrom  error\n");
        return 1;
    }

    packets = atoi(buf);
    printf("Num packets expected: %d\n", packets);

    while(received<packets)
    {
        n = recvfrom(sock,buf,sizeof (buf),0,(struct sockaddr *)&from,&fromlen);    

        //printf ("%d\n", n);
        printf("Packet num %d, numBytes received: %d\n", received, n);

        if(n<0)
        {
            printf("recvfrom  error\n");
            return 1;
        }
        //printf("%s",buf);
        if((fwrite(buf,1,n,newfp)) < n)
        {
            printf("error in writing to the file\n");
            return 1;
        }
        received++;
    }
    printf("Finished\n");
    fclose(newfp);
    return 0;
}

Answer 1

1. 您假设fread()空值终止缓冲区。 它没有。
2. 您假设数据不包含其他空值：它可以。
3. 发送文件数据时，您应该使用nread而不是strlen(buffer)作为计数参数。
4. 文件结尾的正确测试不是nread < 1024 ，这可能随时发生：它是nread == 0 。
5. 您不会处理乱序的数据包，或者丢失或重复的数据包。 事实上，这段代码根本不可靠。 您需要数据包中的序列号，在将数据写入文件时将其删除； 你需要一个 ACK​​ 或 NACK 机制来丢失数据包； 您需要一种忽略重复项的机制； ...

你才刚刚开始。