Thread: socket programming cannot connect

Hi everyone, I am writing client C socket program in Linux and server C# socket
program in window. I am also running these 2 programs in different PCs. When I
run client program it shows that "Cannot connect".
my client C program in
Linux is

Code:

#include <sys/socket.h>  //for socket(), connect(), sendto() and recvform()
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>  // for printf() and fprintf()
#include <string.h>  //for memset()
#include <stdlib.h>   //for atoi() and exit()
#include <unistd.h>  //for close()
#include <errno.h>
#include <arpa/inet.h>  //for sockaddr_in and inet_addr()
 

#define SERVERPORT 1234	

int main ( )                      
{
	int sockfd, n;
	struct sockaddr_in serv_addr;
	struct hostent *serverINFO;
	
	char buffer[256];
	char hostname[1024];
	//struct hostent* h;
	gethostname(hostname, sizeof(hostname));
	serverINFO = gethostbyname(hostname);
	
	if(serverINFO == NULL)
	{
		printf("Problem interpreting host\n");
		return 1;  
	}
	sockfd = socket(AF_INET, SOCK_STREAM, 0);
	
	if (sockfd < 0)
		{
			printf("Cannot create socket\n");
			return 1;
		}
	inet_pton(AF_INET, "192.168.0.1",&(serv_addr.sin_addr));
	//serv_addr.sin_family = serverINFO ->h_addrtype;
	//memcpy((char *) &serv_addr.sin_addr, serverINFO->h_addr_list[0], serverINFO->h_length);
	//serv_addr.sinport = htons(SERVERPORT);
	//bzero((char *) &serv_addr, sizeof(serv_addr));
	//serv_addr.sin_family = AF_INET;
	//memcpy(&serv_addr.sin_addr, server->h_addr, server->h_length);
	//bcopy((char *)server->h_addr,(char *)&serv_addr.sin_addr.s_addr,server->h_length);
	serv_addr.sin_port = htons (SERVERPORT);
	
	if(connect(sockfd, (struct sockaddr *)&serv_addr, sizeof (serv_addr)) < 0) 
		{
			printf("Cannot connect\n");
			return 1;
		}
	else
		printf("Connected\n");
	/*printf("Please enter the message: ");
	bzero(buffer, 256);
	fgets(buffer,255,stdin);
	n = write(sockfd, buffer, strlen(buffer));
	
	if (n < 0)
		printf ("ERROR writing to socket");
	bzero(buffer, 256);
	n = read (sockfd, buffer, 255);
	
	if (n < 0)   
		printf ("ERROR reading from socket");
	printf("here we are %s\n", buffer);
	close(sockfd);*/
}

my server C# program in window is

Code:

namespace server_window
{
    public partial class Form1 : Form
    {
          static TcpListener tcpListener = new TcpListener(IPAddress.Parse("192.168.0.2"), 1234);
        TcpClient socketForClient;
       EndPoint ep;
       public Form1()
        {
            InitializeComponent();
            BW_Connection.RunWorkerAsync();
        }
      
        private void BW_Connection_DoWork(object sender, DoWorkEventArgs e)
        {
            tcpListener.Start();
            socketForClient = tcpListener.AcceptTcpClient();
            ep = socketForClient.Client.RemoteEndPoint;
              this.Invoke(new MethodInvoker(delegate { TB_text.Text = "Client connected..."; }));
        }
           private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            socketForClient.Close();
            
        }

    }
}

If my program is wrong, pls kindly let me know. Coz I am a bit new to socket
programming and Linux. Thanks a lot..

Originally Posted by c99tutorial

Are you sure the server is actually reachable on this port from your client? Usually you can use telnet to verify this, e.g.:

telnet 192.168.0.2 1234

1. Also notice your server is listening on port 1234 but nowhere in your client do you say which port you want
2. Why do you comment out lots of lines in your code? If its not related to your question theres no reason to post it here

I had used ping to each IP address. So there is connection between them. And I used

Code:

serv_addr.sin_port = htons(SERVERPORT);

which is port no 1234.

so sorry to delete unused code.

Originally Posted by vart

I do not understand - on server you are usign TcpListener and on client you create socket with type 0 which is IP and not TCP

Have you tried to put into the 3rd parameter of the socket() call IPPROTO_TCP?

my understanding is that SOCK_STREAM is for TCP. 0 is just default right?

Originally Posted by c99tutorial

ping does not verify that a particular port is open. The question is whether you can in fact make a connection using port 1234 from your client to your server

I cannot make connection. I run my server first and later client. At that time client program prompt that cannot connect. so that's y

I used teraterm to check whether the port is ok or not. But it showed connection refused...

Originally Posted by c99tutorial

ping does not verify that a particular port is open. The question is whether you can in fact make a connection using port 1234 from your client to your server

I used telent now. I type telnet myclientip 1234 . it showed that connecting to myclientip.. could not open to the host, on port 1234: Connect failed.. means I cannot use that port no.. ???

Originally Posted by vart

Is server running on windows? check windows firewall settings

Hi thanks for your answer. I had checked my firewall settings. It was off.

Originally Posted by Barney McGrew

Firstly, gethostbyname is considered obsolete by POSIX, so you might use getaddrinfo instead. But neither are really needed by your program so you can get rid of gethostbyname, along with the call to gethostname.

Your problem is with the call to connect -- some of the fields used by serv_addr are uninitialised in your call and sin_family should be assigned AF_INET. Call memset(&serv_addr, 0, sizeof serv_addr) before assigning to any of the fields.

Also, for the call to socket, though not the call to connect, it may be necessary to check that the return value is equal to -1, rather than less than 0. I'm not sure, but I think you can have file descriptors with negative values.

Hi thanks for your answer. Now I changed my program a little bit. I just wanna test whether it can connect or not.
my program is

Code:

#include
<sys/socket.h>//for socket(), connect(), sendto() and recvform()

#include
<sys/types.h>

#include
<netinet/in.h>

#include
<netdb.h>

#include
<stdio.h>// for printf() and fprintf()

#include
<string.h>//for memset()

#include
<stdlib.h>//for atoi() and exit()

#include
<unistd.h>//for close()

#include
<errno.h>

#include
<arpa/inet.h>//for sockaddr_in and inet_addr()

 


#define
 SERVERPORT 1234	


int
 main(void)

{

	
int sockfd =0;

	
struct sockaddr_in serv_addr;

	
if((sockfd=socket(AF_INET, SOCK_STREAM, 0)) < 0)

	{

		printf(
"Cannot Connect\n");

		
return 1;

	
else

		printf(
"Connected\n");

	}

}

but still cannot connect

Originally Posted by Salem

> printf("Cannot Connect\n");
Perhaps replacing printf() with perror() will shed enlightenment on to the REASON for the failure, as opposed to the ultimate shoulder shrug "it didn't work" you have at the moment.
perror

Hi I am using perror as you said. I started to run my server program from window and run client program in Linux. At that time, the terminal showed Cannot connect : connection refused. Why is it so?