Thread: Server-client file transfer using UDP in C

I have to make a server-client file transfer using UDP . I have created a basic server which receives message sent by client . That's all. Now comes the major part :- 1. The message sent by client is the name of file
.2. Now the server checks whether there exists this file or not .
3. If there exists it sends the file to the client and it also keeps the count of the number of requests of file made by the client . Here is the Server Code

Code:

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


int main()
{


  char buff[2000];
  char file_buffer[2000];
  int sd,connfd,len;


  struct sockaddr_in servaddr,cliaddr;


  sd = socket(AF_INET, SOCK_DGRAM, 0);


  if(sd==-1)
    {
      printf(" socket not created in server\n");
      exit(0);
    }
  else
    {
      printf("socket created in  server\n");
    }


  bzero(&servaddr, sizeof(servaddr));


  servaddr.sin_family = AF_INET;
  servaddr.sin_addr.s_addr = INADDR_ANY;
  servaddr.sin_port = htons(7802);


  if ( bind(sd, (struct sockaddr *)&servaddr, sizeof(servaddr)) != 0 )
    printf("Not binded\n");
  else
    printf("Binded\n");






  len=sizeof(cliaddr);


  recvfrom(sd,buff,1024,0,
	   (struct sockaddr *)&cliaddr, &len);


  printf("%s\n",buff);
  /* */
  FILE *fp;
  fp=fopen(buff,"r");
  if(fp==NULL)
    {
      printf("file does not exist\n");
    }


  fseek(fp,0,SEEK_END);
  size_t file_size=ftell(fp);
  fseek(fp,0,SEEK_SET);
  if(fread(file_buffer,file_size,1,fp)<=0)
    {
      printf("unable to copy file into buffer\n");
      exit(1);
    }
  if(sendto(sd,file_buffer,strlen(file_buffer),0,  (struct sockaddr *)&cliaddr, &len)<0){
    printf("error in sending the file\n");
    exit(1);
  }
  bzero(file_buffer,sizeof(file_buffer));




  /* */
  close(sd);
   
  return(0);
}

Client code:-

Code:

#include <stdio.h>
#include <errno.h>
#include <sys/socket.h>
#include <resolv.h>
#include<netinet/in.h>
#include<sys/types.h>




int main()
{
  char buff[2000];
  int sockfd,connfd,len;


  struct sockaddr_in servaddr,cliaddr;


  // create socket in client side
  sockfd = socket(AF_INET, SOCK_DGRAM, 0);


  if(sockfd==-1)
    {
      printf(" socket not created in client\n");
      exit(0);
    }
  else
    {
      printf("socket created in  client\n");
    }




  bzero(&servaddr, sizeof(servaddr));


  servaddr.sin_family = AF_INET;
  servaddr.sin_addr.s_addr = INADDR_ANY; // ANY address or use specific address
  servaddr.sin_port = htons(7802);  // Port address




  printf("Type ur  UDP client message\n");
  scanf("%s",buff);


  // send  msg to server


  sendto(sockfd, buff, strlen(buff)+1, 0,
	 (struct sockaddr *)&servaddr, sizeof(struct sockaddr));
  /* */
  char file_buffer[2000];
  if (recvfrom(sockfd,file_buffer,2000,0,  (struct sockaddr *)&servaddr, sizeof(struct sockaddr))<0)
    {
      printf("error in recieving the file\n");
      exit(1);
    }


  char new_file[]="copied";
  strcat(new_file,buff);
  FILE *fp;
  fp=fopen(new_file,"w+");
  if(fwrite(file_buffer,1,sizeof(file_buffer),fp)<0)
    {
      printf("error writting file\n");
      exit(1);
    }


  /**/








  //close client side connection
  close(sockfd);


  return(0);
}

The server reads the data from file and writes into it and send to the client , On the other end client receive the data and make duplicate of this file and write into it. But on compiling it gives error in reading the file . Please do heelp me

> But on compiling
Compiling is when you do
gcc -o server server.c

Running (or executing) the program is when you do this.
./server


> it gives error in reading the file
None of your posted code contains the error message "error in reading the file".

Copy/paste your console logs, don't guess and paraphrase.

> char new_file[]="copied";
> strcat(new_file,buff);
How many characters can you append to new_file?
If you think the answer is anything other than zero, then you need to go back to some C basics.

Originally Posted by keypad

Sorry about that . Yes as you are saying it gives the error in sending the file.
Yes this is an error but i am new to C so please help me out?

I don't interpret what Salem said as an error in sending the file. I assume you are talking about your invalid use of string literals that he pointed out. Basically, what you did is store a constant string in memory, then you tried to write to read-only data. To fix this, do one of two things, either dynamically allocate memory for more characters to a character pointer that won't be constant, or specify how many characters you want the new_file array to be able to hold.

Why is it too hard to simply copy/paste whatever is printed in your console when it crashes?

> if(sendto(sd,file_buffer,strlen(file_buffer),0, (struct sockaddr *)&cliaddr, &len)<0)
Here are some things wrong with this.
1. fread() does NOT append a \0 to the buffer, which is a problem if you come along and try and do strlen() on it. You'll get a length all right (or a crash), but it'll be pure dumb luck if it's the answer you want. You've already measured the length, so use it.
2. Rather than just saying "it's an error", try printing the actual error.
Eg.

Code:

  if(sendto(sd,file_buffer,strlen(file_buffer),0,  (struct sockaddr *)&cliaddr, &len)<0){
    perror("error in sending the file\n");
    exit(1);
  }

perror() will tell you WHY it failed - at least enough to go and look up the reason in the manual page for sendto()

> if(fread(file_buffer,file_size,1,fp)<=0)
How are you making sure the file length is less than the buffer length?

> bzero(file_buffer,sizeof(file_buffer));
It's too late for that now, you've already finished with the buffer.


> Basically, what you did is store a constant string in memory, then you tried to write to read-only data.
This is way off base as well.
It's not a question of the string being read-only, it's a question of it having NO space to be able to append any data to it.

This is bad
char new_file[]="copied";
strcat(new_file,buff);

This is better
char new_file[100]="copied";
strcat(new_file,buff);

Better still would be range-checking the length of the data to be copied.