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<TITLE>ACE Tutorial 008</TITLE>
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<CENTER><B><FONT SIZE=+2>ACE Tutorial 008</FONT></B></CENTER>
<CENTER><B><FONT SIZE=+2>Sending and receiving datagrams</FONT></B></CENTER>
<P>
<HR WIDTH="100%">
<P>In <A HREF="directed_client.cpp">directed_client.cpp</A> we create a
client that knows how to send a datagram to a server on a known host.
This is a good thing if you know where the server lives and want to have
a conversation. The Unix <I>talk</I> utilitiy, for instance,
could be written this way.
<P>
<HR WIDTH="100%">
<PRE>
<font color=red>// $Id$</font>
<font color=blue>#include</font> "<font color=green>ace/SOCK_Dgram.h</font>"
<font color=blue>#include</font> "<font color=green>ace/INET_Addr.h</font>"
<font color=red>/*
Once again, we use the default server port. In a "<font color=green>real</font>" system,
the server's port (or ports) would be published in some way so
that clients would know where to "<font color=green>look</font>". We could even add entries
to the operating system's services file and use a service name
instead of a number. We'll come back to that in some other tutorial
though. For now, let's stay simple.
*/</font>
static const u_short PORT = ACE_DEFAULT_SERVER_PORT;
<font color=red>/*
Our goal here is to develop a client that can send a datagram to
a server running on a known host. We'll use a command-line argument
to specify the hostname instead of hard-coding it.
*/</font>
int main(int argc,char *argv[] )
{
<font color=red>/*
All datagrams must have a point of origin. Since we intend to
transmit instead of receive, we initialize an address with zero
and let the OS choose a port for us. We could have chosen our
own value between 1025 and 65535 as long as it isn't already in
use.
The biggest difference between client and server when datagrams
are used is the fact that servers tend to have a known/fixed
address at which they listen and clients tend to have arbitrary
addresses assigned by the OS.
*/</font>
ACE_INET_Addr local((u_short)0);
<font color=red>/*
And here is our datagram object.
*/</font>
ACE_SOCK_Dgram dgram;
<font color=red>/*
Notice that this looks a lot like the server application. There's
no difference in creating server datagrams an client datagrams.
You can even use a zero-constructed address for your server datagram
as long as you tell the client where you're listening (eg -- by writting
into a file or some such).
*/</font>
if( dgram.open(local) == -1 )
{
ACE_ERROR_RETURN ((LM_ERROR, "<font color=green>%p\n</font>", "<font color=green>datagram open</font>"),-1);
}
<font color=red>/*
Yep. We've seen this before too...
*/</font>
char buf[512];
<font color=red>/*
Ok, now we're doing something different.
*/</font>
sprintf(buf, "<font color=green>Hello World!</font>");
<font color=red>/*
Just like sending a telegram, we have to address our datagram.
Here, we create an address object at the desired port on the
chosen host. To keep us from crashing, we'll provide a default
host name if we aren't given one.
*/</font>
ACE_INET_Addr remote(PORT, argc > 1 ? argv[1] : "<font color=green>localhost</font>" );
ACE_DEBUG ((LM_DEBUG, "<font color=green>(%P|%t) Sending (%s) to the server.\n</font>",buf));
<font color=red>/*
Now we send our buffer of stuff to the remote address. This is
just exactly what the server did after receiving a client message.
Datagrams are rather orthogonal that way: they don't generally make
much of a fuss about being either client or server.
*/</font>
if( dgram.send(buf,strlen(buf)+1,remote) == -1 )
{
ACE_ERROR_RETURN ((LM_ERROR, "<font color=green>%p\n</font>", "<font color=green>send</font>"),-1);
}
<font color=red>/*
Now we've turned around and put ourselves into "<font color=green>server mode</font>" by
invoking the recv() method. We know our server is going to send
us something, so we hang out here and wait for it. Because we
know datagrams are unreliable, there is a chance that the server
will respond but we won't hear. You might consider providing a
timeout on the recv() in that case. If recv() fails due to timeout
it will return -1 and you can then resend your query and attempt
the recv() again.
Like the server application, we have to give the recv() an
uninitialized addr object so that we can find out who is talking
back to us.
*/</font>
if( dgram.recv(buf,sizeof(buf),remote) == -1 )
{
ACE_ERROR_RETURN ((LM_ERROR, "<font color=green>%p\n</font>", "<font color=green>recv</font>"),-1);
}
<font color=red>/*
Find out what the server had to say.
*/</font>
ACE_DEBUG ((LM_DEBUG, "<font color=green>(%P|%t) The server said: %s\n</font>",buf));
<font color=red>/*
Using the "<font color=green>remote</font>" object instance, find out where the server lives.
We could then save this address and use directed datagrams to chat
with the server for a while.
*/</font>
ACE_DEBUG ((LM_DEBUG, "<font color=green>(%P|%t) The server can be found at: (%s:%d)\n</font>",
remote.get_host_name(), PORT ));
return(0);
}
</PRE>
<HR WIDTH="100%">
<P>That's all neat and good but the point of what we're doing here is not
to talk to a server we know about but to discover servers we don't know
about. Now, you could send a directed datagram to every possible
host address on your network but that's not a very nice thing to do.
On the next page, we'll find out the right approach...
<P><HR WIDTH="100%">
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