SPIN Operating System Documentation

The SocketRW interface

Motivation

The socketRW extension gives you an easy way to use sockets within the SPIN kernel. It makes a TCP socket connection behave like a reader stream paired with a writer stream.

Overview

We ported this code from the tcp directory of the SRC Modula-3 distribution. All that it really does is provide the veneer of a reader and a writer over the underlying socket. It must be linked with the SPIN networking extensions which actually implement the sockets.

Using socketRW

The basic idea is that you create a TCP.T object (think of it as a socket) and then use it to produce one reader and one writer which communicate over the socket. The socket can be either a server or a client side socket. On the server side use:

  VAR
    listener: TCP.Connector;
    socket: TCP.T;
    rd: Rd.T;
    wr: Wr.T;
  BEGIN
    (* TCP.NewConnector calls the socket listen procedure. *)
    listener := TCP.NewConnector(IP.Endpoint{IP.NullAddress, port});
    (* TCP.Accept is just like the Unix accept(). *)
    socket := TCP.Accept(listener);

    rd := ConnRW.NewRd(socket);
    wr := ConnRW.NewWr(socket);

    IO.Put("blah", wr);
  END;

To create a client side socket you call TCP.Connect like this:

  VAR
    address: IP.Address;
    socket: TCP.T;
    rd: Rd.T;
    wr: Wr.T;
  BEGIN
    IP.GetHostByName("spinoff", address);
    (* TCP.Connect works like the Unix accept(). *)
    socket := TCP.Connect(IP.Endpoint{address, 80});

    rd := ConnRW.NewRd(socket);
    wr := ConnRW.NewWr(socket);

    IO.Put("You talkin' to me?", wr);
  END;

Socket readers and writers can be used just like any other readers and writers. For example, you can redirect a thread's input and output to them. That is how the telnet_shell command works.

Using telnet_shell:

telnet_shell [port#]  -  Listen on specified port. When a connection 
			 comes in a thread is forked which executes 
	 		 the SpinShell main loop and has its standard 
			 reader and writer connected to the socket.

From a unix box, run telnet to connect to the machine which is running telnet_shell. You will be prompted for a password which is the same as the rconsole password. After entering the correct password you should see the prompt "spin>" and you should be able to run all of the normal spin shell commands. Note that echoing is done by your telnet program on the unix machine, not by the spin program.

Remaining Issues

We were able to port the code with minimal changes, but there is some future work connected with the socketRW extension. First, when the "select" procedure is stable, we will use that to implements timeouts. Second, the Network Objects library from SRC will be ported to SPIN as an extension which uses the TCP services of socketRW.

garrett@cs.washington.edu