Socket programming: Basics

• Creating a socket (at both client and server)

    sockfd = socket(AF_INET, SOCK_STREAM, 0);
  

• Bind, listen at server

  
    struct sockaddr_in server_addr;
  
    //fill in server address
  
   ret = bind(sockfd, (struct sockaddr *) &server_addr, sizeof(server_addr));
  
   listen(sockfd, 10);
  
  

• Connect at client

  ret = connect(sockfd, (struct sockaddr *) &server_addr, sizeof(server_addr) );
  

• Accept at server

    struct sockaddr_in server_addr, client_addr;
    client_addr_len = sizeof(client_addr);
  
    newsockfd = accept(sockfd, (struct sockaddr *) & client_addr, &client_addr_len);
  
    //client address is filled, newsockfd is new socket for this client
  
  
  

• Read and write at both client (from sockfd) and server (from newsockfd)

  n = read(sockfd, inputbuf , 255);
  
  n = write(sockfd,inputbuf,strlen(inputbuf));
  

Socket programming: Select system call

• At server, bind and listen as usual.
• Create a list of sockets that must be monitored. Initially, this list includes only the main server listening socket.

  
    fd_set master_fds;
  
    FD_SET(sockfd, &master_fds);
    fdmax = sockfd;
  

• Keep running the select procedure, and see if any sockets are set. At the start of every round, make a copy of the master list of sockets and pass in the system call. The returned list will tell you which sockets have events pending.

   
   fd_set read_fds;
    
   for( ; ; )
      {
  
        //give list of sockets to check
        read_fds = master_fds;
        
         ret = select(fdmax+1, &read_fds, NULL, NULL, NULL);
  
        //read_fds now has those sockets that have events
         ...........
       }
  
  

• Iterate over all sockets in the list (from 0 to fdmax). Check each socket in the returned list to see if it is "set", meaning it has an event pending. If the main server socket has an event pending, it means a new client connection has arrived. In that case, we accept the client connection, create a new socket, and add it to the master list for the next round.

  if(FD_ISSET(i, &read_fds) && (i == sockfd)) //new connection
  { 
  
  newsockfd = accept(sockfd, (struct sockaddr *) & client_addr, &client_addr_len);
  
  FD_SET(newsockfd, &master_fds);
  
  if(newsockfd > fdmax)
       fdmax = newsockfd;
  }
  
  

• Otherwise, if it is a normal client socket that is set, read from it and do the usual processing.
• Once all sockets in the list have been checked for events, repeat and run select again with the updated master list.