ACE Tutorial 001
A Beginners Guide to Using the ACE Toolkit

From here, we to move on to the main program loop. In a way, we're starting at the final product when we do this, but it is a very simple piece of code and a good place to start.

The main program is really quite simple. The real work is done in the ACE derived classes.

Kirthika Parameswaran offers this abstract of Tutorial 1:

This is a simple logging server example. The Reactor is used to handle more than one client request using a single thread of execution instead of one thread per client. The Reactor reactes to events and demultiplexes the events to the appropriate Event_Handler registered with it, using the "callback" technique. The reactor runs in an infinte event loop handling all the incoming events.

The Logging_Acceptor listens at a SERVER PORT address and passively waits for requests to arrive. The Acceptor is also an Event_Handler and is registered with the Reactor. This way it is simply yet another Event_Handler for the Reactor and hence no special processing is needed for it.

Once a connection request occurs, the Acceptor accepts it and a connection is established. The reactor instance is passed to the handler so that it can register with the Reactor. It does so with an ACE_Event_Handler::ACCEPT_MASK.

The Logging_Client is another Event_Handler which actually handles the client requests in its handle_input() method. It is also registered with the Reactor with the ACE_Event_Handler::READ_MASK.

The Event_Handlers can be unregistered from the Reactor using handle_close() methods or explicitly calling the remove_handler() methods.

This server application builds and executes succesfully waiting for client requests to arrive.

FYI (from Doug):

The ACCEPT_MASK is defined in the ACE_Event_Handler class. It's used to inform the Reactor that you want to register an event handler to "accept" a connection passively. Not surprisingly, the ACE_Acceptor component uses this.

The READ_MASK is also defined in the ACE_Event_Handler class. It's used to inform the Reactor that you want to register an event handler to "read" data from an established connection.

    // page02.html,v 1.14 2000/03/19 20:09:19 jcej Exp

    /* Include the header file where our client acceptor is defined.  */
    #include "ace/Reactor.h"

    /* For simplicity, we create our reactor in the global address space.
      In later tutorials we will do something more clever and
      appropriate. However, the purpose of this tutorial is to introduce a
      connection acceptance and handling, not the full capabilities of a
      reactor.  */
    ACE_Reactor *g_reactor;

    /* Include the header where we define our acceptor object.  An
      acceptor is an abstraction that allows a server to "accept"
      connections from clients.  */
    #include "acceptor.h"

    /* A TCP/IP server can listen to only one port for connection
      requests.  Well-known services can always be found at the same
      address. Lesser-known services are generally told where to listen by
      a configuration file or command-line parameter. For this example,
      we're satisfied with simply hard-coding a random but known value.  */
    static const u_short PORT = ACE_DEFAULT_SERVER_PORT;

    int
    main (int, char *[])
    {
      /* Create a Reactor instance.  Again, a global pointer isn't exactly
        the best way to handle this but for the simple example here, it
        will be OK.  We'll get cute with it later.  Note how we use the
        ACE_NEW_RETURN macro, which returns 1 if operator new fails. */
      ACE_NEW_RETURN (g_reactor,
                      ACE_Reactor,
                      1);

      /* Like the Reactor, I'm skimming over the details of the ADDR
        object. What it provides is an abstraction for addressing services
        in the network. All we need to know at this point is that we are
        creating an address object which specifies the TCP/IP port on
        which the server will listen for new connection requests.  */
      ACE_INET_Addr addr (PORT);

      Logging_Acceptor *peer_acceptor;

      /* We now create an acceptor object.  No connections will yet be
        established because the object isn't "open for business" at this
        time. Which brings us to the next line...  */
      ACE_NEW_RETURN (peer_acceptor,
                      Logging_Acceptor,
                      1);

      /* where the acceptor object is opened.  You'll find that most ACE
        objects have to be open()ed before they're of any use to you.  On
        this open() call, we're telling the acceptor where to listen for
        connections via the 'addr' object.  We're also telling it that we
        want it to be registered with our 'g_reactor' instance.  */
      if (peer_acceptor->open (addr, g_reactor) == -1 )
        ACE_ERROR_RETURN ((LM_ERROR,
                           "Opening Acceptor\n"),
                          -1);

      ACE_DEBUG ((LM_DEBUG,
                  "(%P|%t) starting up server logging daemon\n"));

      /* The reactor's handle_events member function is responsible for
        looking at all registered objects and invoking an appropriate
        member function when anything of interest occurs. When an event is
        processed, the handle_events function returns. In order to get all
        events, we embed this in an infinite loop.

        Since we put ourselves into an infinite loop, you'll need to
        CTRL-C to exit the program.  */

      for (;;)
        g_reactor->handle_events ();

      return 0;
    }

As I said, the main program is really quite simple:

- Create an address for the port we want to listen to
- Create an acceptor which listens on that address
- Register the acceptor with a reactor to respond to the connection requests
- Enter an infinite loop to let the reactor handle the events

On the next page, we will take a look at the acceptor and how it responds to new connection requests.

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