c++ - 在网络中通过 Socket 进行文件传输时，此程序中的文件传输错误是什么？

Question

我已经写了一个 C++ 演示示例。它将文件从服务器传输到客户端。当我在本地主机上运行这个程序时，它工作正常。但是，当我通过网络运行该程序时，文件传输不正确。接收到的图像尺寸大于发送的图像尺寸。另外，为什么同样适用于本地主机？我也尝试过更改端口号。

这是程序-

服务器

/** man 2 socket **/
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>

#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <iostream>

using namespace std;

int main(int argc, char *argv[])
{

 char msg[] = "Shreyas..first socket prog";
 int sock , sock_active;
 struct sockaddr_in server, client;
 int sent,ret;
 unsigned int len;
 char buffer[1024];


   FILE *fp;

   if ( (sock = socket(AF_INET,SOCK_STREAM,0))  == -1 )
   //if ( (sock = socket(AF_INET,SOCK_DGRAM ,0))  == -1 )
   {
    perror("Sock:");

   }

  server.sin_family = AF_INET;
  server.sin_port = htons(15000);
  //server.sin_addr.s_addr = INADDR_ANY;
  inet_aton("136.170.195.17",  &(server.sin_addr));

  bzero(&server.sin_zero, 8);

  len = sizeof(struct sockaddr_in);

  if( ret = (bind( sock, (struct sockaddr *)&server, len)) == -1 )
  {
     perror("bind :");
  }

  ret = listen(sock, 0);


 while(1)
  {

 if( (sock_active = accept(sock, (struct sockaddr *)&client, &len)) == -1 )
    {
      perror("Problem in active socket:");

    }


  fp = fopen("./Tiger.JPG","rb");

  if( fp == NULL )
  {
     cout<<"Error open file";
     return -1;

  }



  memset(buffer, 1024,0);
  int packets = 0;
  int count;

 //while ( fgets(buffer,1024,fp ) != NULL )

  while( ! feof(fp) )
    {
       packets++;
      cout<<"Client IP address is "<<inet_ntoa(client.sin_addr)<<endl;
      cout<<"Client Port address is "<<ntohs(client.sin_port)<<endl;
      /** Fread is reliable when using to find out the EOF , in feof(fp **/
       count = fread(buffer,1,sizeof(buffer),fp);
      /** fgets doesn't move the FP correctly */
      //fgets(buffer,sizeof(buffer),fp);
      cout<<"Read number of bytes ="<<count;
      sent = send(sock_active, buffer, sizeof(buffer),0);
      cout<<"The number of bytes sent ="<<sent<<"packet number = "<<packets<<endl;
      memset(buffer, 1024,0);

     }
    cout<<"CLose current socket"<<endl;
    close(sock_active);
    fclose(fp);
}
  cout<<"CLosing socket now" <<endl;

  close(sock);

  return 0;
}

客户端套接字程序-

/** man 2 socket **/
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>

#include <netinet/in.h>
#include <arpa/inet.h>

#include <iostream>
#include <unistd.h>


using namespace std;

int main(int argc, char *argv[])
{
   struct sockaddr_in server;
   int ret; 
   int sock;
   int read_val = 1;
   unsigned int len;

   if ( (sock = socket(AF_INET,SOCK_STREAM,0))  == -1 )
   {
    perror("Sock:");

   }

   if( argc != 2 )

     {

       cout<<"Pass the Server IP "<<endl;
       exit(EXIT_FAILURE);

     }

  server.sin_family = AF_INET;
  server.sin_port = htons(15000);
  inet_aton(argv[1],  &(server.sin_addr));
  bzero(&server.sin_zero, 8);
  len = sizeof(server);


  if ( (ret = connect(sock, (struct sockaddr *)&server,sizeof(server))) == -1)
  {
     perror("Connect failed:"); 
     exit(-1); 
  }

  char msg[1024];
  memset(msg,0,1024);
  FILE *fp_w;

  fp_w = fopen("./try.JPG","wb");

  while( read_val)
   {
     read_val = recv(sock,(char *)msg,sizeof(msg),0);
     fwrite(msg,1,sizeof(msg),fp_w);
   }
     cout<<"Read is complete"<<endl;
     fclose(fp_w);
     close(sock);

     return 0;

}

Answer 1

也许您应该在您的客户端中修改这段代码。

while( read_val)
{
  read_val = recv(sock,(char *)msg,sizeof(msg),0);
  fwrite(msg,1,sizeof(msg),fp_w);
}

recv 函数不会总是收到您想要接收的字节数。您必须使用 recv 的返回值来了解实际读取的字节数，并使用该计数写入文件。

我会把这个 block 写成，

while( read_val)
{
  read_val = recv(sock,(char *)msg,sizeof(msg),0);
  if ( read_val > 0)
  {
     fwrite(msg,1,read_val,fp_w); // I am using `read_val` while writing.
  }
}

在使用文件/套接字 io API 时，假设它读/写了您要求的确切字节数是不好的。