24 Networking

In this chapter, you will learn how to send data over the network using the ROOT socket classes.

24.1 Setting-up a Connection

On the serverside, we create a TServerSocket to wait for a connection request over the network. If the request is accepted, it returns a full-duplex socket. Once the connection is accepted, we can communicate to the client that we are ready to go by sending the string “go”, and we can close the server socket.

{ // server
   TServerSocket *ss = new TServerSocket(9090,kTRUE);
   TSocket *socket = ss->Accept();
   socket->Send("go");
   ss->Close();
}

On the clientside, we create a socket and ask the socket to receive input.

{ // client
   TSocket *socket = new TSocket("localhost",9090);
   Char str[32];
   socket->Recv(str,32);
}

24.2 Sending Objects over the Network

We have just established a connection and you just saw how to send and receive a string with the example “go”. Now let’s send a histogram.

To send an object (in our case on the client side) it has to derive from TObject class because it uses the Streamers to fill a buffer that is then sent over the connection. On the receiving side, the Streamers are used to read the object from the message sent via the socket. For network communication, we have a specialized TBuffer, a descendant of TBuffer called TMessage. In the following example, we create a TMessage with the intention to store an object, hence the constant kMESS_OBJECT in the constructor. We create and fill the histogram and write it into the message. Then we call TSocket::Send to send the message with the histogram.

...
// create an object to be sent
TH1F *hpx = new TH1F("hpx","px distribution",100,-4,4);
hpx->FillRandom("gaus",1000);

// create a TMessage to send the object
TMessage message(kMESS_OBJECT);

// write the histogram into the message buffer
message.WriteObject(hpx);

// send the message
socket->Send(message);
...

On the receiving end (in our case the server side), we write a while loop to wait and receive a message with a histogram. Once we have a message, we call TMessage::ReadObject, which returns a pointer to TObject. We have to cast it to a TH1 pointer, and now we have a histogram. At the end of the loop, the message is deleted, and another one is created at the beginning.

while (1) {
   TMessage *message;
   socket->Recv(message);
   TH1 *h = (TH1*)message->ReadObject(message->GetClass());
   delete message;
}

24.3 Closing the Connection

Once we are done sending objects, we close the connection by closing the sockets at both ends.

Socket->Close();

This diagram summarizes the steps we just covered:

Server - Client setting-up and closing the connection

Server - Client setting-up and closing the connection

24.4 A Server with Multiple Sockets

Chances are that your server has to be able to receive data from multiple clients. The class we need for this is TMonitor. It lets you add sockets and the TMonitor::Select method returns the socket with data waiting. Sockets can be added, removed, or enabled and disabled. Here is an example of a server that has a TMonitor to manage multiple sockets:

{
   TServerSocket *ss = new TServerSocket (9090, kTRUE);

   // Accept a connection and return a full-duplex communication socket.
   TSocket *s0 = ss->Accept();
   TSocket *s1 = ss->Accept();

   // tell the clients to start
   s0->Send("go 0");
   s1->Send("go 1");

   // Close the server socket (unless we will use it
   // later to wait for another connection).
   ss->Close();

   TMonitor *mon = new TMonitor;
   mon->Add(s0);
   mon->Add(s1);

   while (1) {
      TMessage *mess;
      TSocket  *s;
      s = mon->Select();
      s->Recv(mess);
      ...
   }
}

The full code for the example above is in $ROOTSYS/tutorials/net/hserv.C and

$ROOTSYS/tutorials/net/hclient.C.