What is the file transfer error in this program while doing a file transfer via Socket in the network?

Question

I have written a C++ demo example. It transfers a file from a server to client.When, I run this program in the local host, it works fine. However, when I run this program over the network the file transfer is incorrect. The image size received is larger than the image size sent. Also, why the same works on the local host? I have tried changing the port number also.

Here is the program -

Server

/** 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;
}

Client socket program -

/** 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

You have to send actual number of bytes read:

count = fread(buffer,1,sizeof(buffer),fp);
cout<<"Read number of bytes ="<<count;
if ( count > 0)
sent = send(sock_active, buffer, count,0);

The same for the client while reading:

while( read_val)
{
  read_val = recv(sock,(char *)msg,sizeof(msg),0);
  if ( read_val > 0) {
      fwrite(msg,1,read_val,fp_w)
  } else if ( red_val == -1) {
      // an error has occured
      break;
  } else {
      // peer has closed the connection
      break;
  {
}

Answer 2

May be you should modify this block of code in your client.

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

recv function will not always receive the number of bytes you want to receive. You have to use the return value of recv to know the number of bytes actually read and use that count to write to file.

I would write this block as,

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.
  }
}

While working with file/socket io APIs, its not good to assume that it read/wrote exact number of bytes you asked to.

Answer 3

You are making the usual mistake of assuming that recv(), fread(), etc. fill the buffer. They aren't required to do that. They return a count of the number of bytes that were actually received. You have to use that count as the length argument when sending,