Thread: problems with TCP/IP programming

I have a simple server programm with a while loop like this one

Code:

while(recv(sock, &i, sizeof(int), 0) > 0)
{
    cnt++;

    i = i / 10;

    send(sock, &i, sizeof(int), MSG_NOSIGNAL);

    cout << "\nReceived number is: " << i;
    cout << "\nRequest # " << cnt << "\n";
}

The value i is always sent back and correctly received by my client. The client itself only sends an int value once, gets it back, prints it, and then exits. It does this without presenting a single problem.

What happens is: I start the server, and then the client. The first time around the client functions normally, but the server does not increase cnt, nor prints the statements inside the loop to cout. I can't understand because the results I get from the client make it obvious that these lines do execute:

Code:

  i = i / 10;

  send(sock, &i, sizeof(int), MSG_NOSIGNAL);

but the other ones apparently don't, since the second time I execute the client (without having turned off the server), cnt is equal to 1, and not too as it should be. From the second iteration onwards everything works fine, as if the first one had not ocurred at all.

Can anyone help me out? Thanks in advance.

Can I see more of the server code? I'm a little rusty, but you have to make the server listen, and accept the new connection each time the client connects, so maybe something is wrong there?

Code:

#include <iostream>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <unistd.h>

#define PUERTO 25500

bool establecer_socket(int &sock, sockaddr_in &dir);

int mandar(int sock, int *buf, int longitud)
{
  return (send(sock, buf, longitud, MSG_NOSIGNAL));
}

int recibir(int sock, int *buf, int longitud)
{
  return (recv(sock, buf, longitud, 0));
}
	
int main()
{
  using namespace std;
	
  int sock_handle, dimension, nueva_sock;
  sockaddr_in direccion;
  int i, cnt;
  
  cnt = 0;        // contador para el numero de peticiones que resuelva
  
  // preparar a la socket para ser usada antes de bind
  
  if(establecer_socket(sock_handle, direccion))
  {
    cout << "ESCUCHANDO!...\n";
  } 
  else
  {
    cout << "ERROR EN LA CONEXION...";
    exit(1);
  }
 
  listen(sock_handle, 5);
  
  while(true)
  {
    dimension = sizeof(direccion);

    nueva_sock = accept(sock_handle, (sockaddr *) &direccion, (socklen_t *) &dimension);
    
    if(nueva_sock < 0)
    {
      exit(1);
    }

    while(recibir(nueva_sock, &i, sizeof(i)) > 0)
    {
      using namespace std;
	    
      cnt = cnt + 1;
	   
      i = i / 10;

      cout << "\nNumero rebotado: " << i;
      cout << "  # de solicitud: " << cnt;
      
      mandar(nueva_sock, &i, sizeof(i));	
    }
    
    close(nueva_sock);
  } 
  
  return 0;
}

bool establecer_socket(int &sock, sockaddr_in &dir)
{
  bool result = true;
  int opt = 1;        // opciones que se pasan con setsockopt
  
  result = ( (sock = socket(AF_INET, SOCK_STREAM, 0)) >= 0);

  if(result)
  {
    result = ( setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (const char *) &opt, sizeof(opt)) == 0 );
    
    memset(&dir, 0, sizeof(sockaddr_in));
    
    dir.sin_family = AF_INET;
    dir.sin_port = htons(25500);
    dir.sin_addr.s_addr = INADDR_ANY;
  }
    
  if(result)
  {
    result = (bind(sock, (sockaddr *) &dir, sizeof(dir)) == 0); 
  }
 
  return result;
}

Here is the server code. As I had already said, it is fairly short. Let me make the comment once again, that the client itself gets replies from the server every time. Hence I know they are communicating so the problem lies elsewhere. By the way, sorry about the Spanish, I didn't think I'd have to post the code on a forum to seek help so some of the parts are like that, though I thrust it is by no means critical.

You actually posed your question rather well. giving a decent section of code, along with a clear description of the problem. In this case, it is likely that not many are experienced with sockets, and of those that are, even a smaller percentage are experienced with working with sockets under UNIX. Sometimes it will take several hours, or even a day or two, for people with the right expertise to notice a problem like this.

As far as your code goes, it looks nice. Just a few small things to note:
1. #define PEURTO 25500 should be const int PEURTO = 25500. Constants are preferred over DEFINEs in C++ because they offer type safety (plus in more complicated circumstances macros can cause issues).
2. You only need "using namespace std;" once. It's pretty standard to put it after your includes.
3. Instead of using cout.flush(), use "endl" where you put "\n":

Code:

cout << endl << "Numero rebotado: " << i;

It inserts a newline and forces a flush.
4. Use cnt++ instead of cnt = cnt + 1. It is more concise.

Sorry to nitpick. I just thought I'd point out some small things you could change to keep your code cleaner. Besides that, it looks great.

Moved to Networking/Device Communication.