Issue with gethostbyname() calls.

[mcmillhj]

Hi everyone,

I am writing a C program that uses sockets(UDP) to both send and receive in the same program.
I call gethostbyname() on my client program to get the hostname and dynamic port, then i call gethostbyname() again on the server program's data. But for some reason my initial gethostbyname() call which was stored is being overwritten. I was wondering if anyone could tell me why.

struct hostent *hostptr;
struct hostent *destptr;

hostptr = gethostbyname( hostname );
  if ( hostptr == NULL ) 
    {
      perror( "recvUDP:gethostbyname()" );
      return -1;
    }

  /*
   * Bind a name to the socket
   */
  val = bind( socketFD, (struct sockaddr *)&srcAddr, (socklen_t)srclen);
  if ( val < 0 )
    {
      perror( "recvUDP:bind()" );
      return -1;
    }

  /* update the structure with the dynamic port number */
  int retval; /* return value for getsockname */  
  retval = getsockname(socketFD, (struct sockaddr *)&srcAddr, (socklen_t *)&srclen );
  if( retval < 0 )
    {
      perror( "recvUDP:getsockname()" );
      return -1;  
 }

  /*
   * Announce this programs IP and PORT
   */
  printf("Host name: %s\n", hostptr->h_name);                  !!!!!!!! This one prints the 
  printf("Host port: %d\n\n\n", ntohs(srcAddr.sin_port));    !!!!!!!! correct hostname


  //server info
  destptr = gethostbyname(argv[1]);
   if ( destptr == NULL )
  {
    perror("sendUDP:gethostbyname()");
    return -1;
  }
  


  /*
   * Announce this programs IP and PORT
   */
  printf("Host name: %s\n", hostptr->h_name);    !!! This one is incorrect
  printf("Host port: %d\n", ntohs(srcAddr.sin_port));

  //Annouce server's IP and PORT
  printf("Server name: %s\n", destptr->h_name);
  printf("Server port: %d\n", ntohs(destAddr.sin_port));

I'm pretty sure my second call to gethostbyname() is overwriting the first, i just dont know why. Any help would be great. This is for an assignment so please no solutions, just hints.

Thanks.
Hunter

[anduril462]

Check the RETURN VALUE and NOTES section of the man page for gethostbyname.

[mcmillhj]

Thanks for your prompt reply, the man pages on our server doesnt have the notes section for some reason.

So to how would I perform a deep copy, would I have to copy each member of the struct into a corresponding struct?

i.e.

struct hostent *hostptr;
struct hostent *temp;
temp = gethostbyname();

for all member of temp:
      hostptr.member_name = temp.member_name;
end for

?

Thanks.
Hunter

[anduril462]

That's the right idea, though memcpy would be easier. The only problem I forsee is if the pointer members of a hostent also refer to static variables, in which case simple address assignment wouldn't work.

[mcmillhj]

hmm well I'm running into trouble with memcpy, this is what I have tried:

  memcpy(&(destptr->h_name), (int *)tempptr->h_name, sizeof(char *));
  memcpy(&(destptr->h_length), (int *)tempptr->h_length, sizeof(int));
  memcpy(&(destptr->h_addr), tempptr->h_addr, sizeof(char *));

 and

  memcpy(destptr->h_name, tempptr->h_name, sizeof(char *));
  memcpy(destptr->h_length, tempptr->h_length, sizeof(int));
  memcpy(destptr->h_addr, tempptr->h_addr, sizeof(char *));

neither of which seem to be working.
any ideas?

Thanks,
Hunter

[anduril462]

I didn't mean memcpy each element. Copy the whole struct, so the src address for memcpy is the address returned by gethostbyname. The dest is whatever you declare to hold the data, and the size is sizeof(hostent).

[mcmillhj]

hmm copying the whole structure like you suggested is causing a segmentation fault.

/*
   * Get DNS information about this hosts's name
   */
  memcpy(hostptr, gethostbyname(hostname), sizeof(struct hostent));
  if (hostptr == NULL) 
    {
      perror( "recvUDP:gethostbyname()" );
      return -1;
    }

 
  //server info
  /*
   * Get DNS information for the destination hosts name
   */

  memcpy(destptr, gethostbyname(argv[1]), sizeof(struct hostent));
  if ( destptr == NULL )
    {
      perror( "recvUDP:gethostbyname()" );
      return -1;
    }

I have intently looked at the file, and can't find anyplace where I am accessing memory out of turn.

Hunter

[anduril462]

You need actual space reserved for the memcpy. hostptr is just a pointer (4 bytes on a 32-bit system), and probably an uninitialized one at that, meaning you're copying too much data into god knows where. You need a hostent object, and memcpy to the address, like so:

hostent host;
memcpy(&host, ...

Note that gethostbyname could return NULL, so you should check for that before memcpy'ing.