Question

The objective of my program is to use select to manage multiple sockets. However, I thought of trying it with one socket first. Now, the problem that I am facing is that initially client sends data to server, and server receives it and displays it, but then when client again sends some data, the server code remains still at select command.

here are some snippets that will give you an idea of how I am initializing the socket.

if((master_socket = socket(PF_INET, SOCK_STREAM, 0)) < 0)
    {
      exit(1);
    }

    if((bind(master_socket, (struct sockaddr *)&req, sizeof(req))) < 0)
    {
      exit(1);
    }

    listen(master_socket, 5);
    FD_SET(master_socket,&listening);

    /* wait for connection, then receive and print text */
    len = sizeof(struct sockaddr);
    while(1)
     {
            FD_ZERO(&listening);  //Flush out everything in socket
        FD_SET(master_socket,&listening); // Add master
        if(f_client>0)           // Add client if any
        {
            FD_SET(f_client,&listening);
        }
        printf("Checking for new connection \n");
           //Timeout is null, so waiting indefinitely
        rc = select(FD_SETSIZE, &listening, NULL, NULL, NULL);

     if (FD_ISSET(master_socket, &listening))
        {
          printf("Master side invoked\n");
          if((f_client = accept(master_socket, (struct sockaddr *)&req, &len)) < 0)
          {
             exit(1);
              }
            }

    else if (FD_ISSET(f_client,&listening))
    {
         if ((valread = read( f_client , buf, 1024)) == 0)
         {
            close(f_client);
            f_client=0;
         }
         else
         {
            fputs(buf, stdout);  
         }

    }

}

Basically in above program, it connects to the server, maintains a file descriptor for client f_client and add it. And in every round, it clears the socket, add master socket, and client socket if any, and then checks. Problem here is, first time it works, but second time when client sends some data. it gets hang to rc = select(FD_SETSIZE, &listening, NULL, NULL, NULL);

I am not to understand what is wrong here. Can anyone help?

Answer 1

     if ((valread = read( f_client , buf, 1024)) == 0)
     {
        close(f_client);
        f_client=0;
     }
     else
     {
        fputs(buf, stdout);  
     }

This code is broken. The fputs function can only be used with a C-style string. You just have arbitrary data with no particular structure. Since you ignore valread, you also have no idea how many bytes you read. (Think about it, how could fputs possibly know how many bytes to output? That information is only in valread, and you don’t pass it that information.)

You’ve already received the data, this broken code just threw it away. If you log valread, you’ll see that you actually already read it in your last call to read before the call to select that hung.

instead of fputs, you could use something like this:

for (int i = 0; i < valread; ++i)
   putchar(buf[i]);