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<META NAME="Author" CONTENT="James CE Johnson">
<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%">
<P><TT>#include "ace/SOCK_Dgram.h"</TT>
<BR><TT>#include "ace/INET_Addr.h"</TT>
<P><TT>/*</TT>
<BR><TT> Once again, we use the default server port.
In a "real" system,</TT>
<BR><TT> the server's port (or ports) would be published in
some way so</TT>
<BR><TT> that clients would know where to "look". We
could even add entries</TT>
<BR><TT> to the operating system's services file and use a
service name</TT>
<BR><TT> instead of a number. We'll come back to that
in some other tutorial</TT>
<BR><TT> though. For now, let's stay simple.</TT>
<BR><TT> */</TT>
<BR><TT>static const u_short PORT = ACE_DEFAULT_SERVER_PORT;</TT>
<P><TT>/*</TT>
<BR><TT> Our goal here is to develop a client that can send
a datagram to</TT>
<BR><TT> a server running on a known host. We'll use
a command-line argument</TT>
<BR><TT> to specify the hostname instead of hard-coding it.</TT>
<BR><TT> */</TT>
<BR><TT>int main(int argc,char *argv[] )</TT>
<BR><TT>{</TT>
<BR><TT> /*</TT>
<BR><TT> All
datagrams have to have a point of origin. Since we intend to</TT>
<BR><TT> transmit
instead of receive, we initialize an address with zero</TT>
<BR><TT> and
let the OS choose a port for us. We could have chosen our</TT>
<BR><TT> own
value between 1025 and 65535 as long as it isn't already in use.</TT>
<BR><TT> */</TT>
<BR><TT> ACE_INET_Addr
local((u_short)0);</TT>
<P><TT> /*</TT>
<BR><TT> And
here is our datagram object.</TT>
<BR><TT> */</TT>
<BR><TT> ACE_SOCK_Dgram dgram;</TT>
<BR><TT> </TT>
<BR><TT> /*</TT>
<BR><TT> Notice
that this looks a lot like the server application. There's</TT>
<BR><TT> no
difference in creating server datagrams an client datagrams.</TT>
<BR><TT> You
can even use a zero-constructed address for your server datagram</TT>
<BR><TT> as
long as you tell the client where you're listening (eg -- by writting</TT>
<BR><TT> into
a file or some such).</TT>
<BR><TT> */</TT>
<BR><TT> if( dgram.open(local)
== -1 )</TT>
<BR><TT> {</TT>
<BR><TT>
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "datagram open"),-1);</TT>
<BR><TT> }</TT>
<P><TT> /*</TT>
<BR><TT> Yep.
We've seen this before too...</TT>
<BR><TT> */</TT>
<BR><TT> char buf[512];</TT>
<P><TT> /*</TT>
<BR><TT> Ok,
now we're doing something different.</TT>
<BR><TT> */</TT>
<BR><TT> sprintf(buf, "Hello
World!");</TT>
<P><TT> /*</TT>
<BR><TT> Just
like sending a telegram, we have to address our datagram.</TT>
<BR><TT> Here,
we create an address object at the desired port on the</TT>
<BR><TT> chosen
host. To keep us from crashing, we'll provide a default</TT>
<BR><TT> host
name if we aren't given one.</TT>
<BR><TT> */</TT>
<BR><TT> ACE_INET_Addr
remote(PORT, argc > 1 ? argv[1] : "localhost" );</TT>
<P><TT> ACE_DEBUG ((LM_DEBUG,
"(%P|%t) Sending (%s) to the server.\n",buf));</TT>
<BR><TT> /*</TT>
<BR><TT>
Now we send our buffer of stuff to the remote address. This is</TT>
<BR><TT>
just exactly what the server did after receiving a client message.</TT>
<BR><TT>
Datagrams are rather orthogonal that way: they don't generally make</TT>
<BR><TT>
much of a fuss about being either client or server.</TT>
<BR><TT> */</TT>
<BR><TT> if( dgram.send(buf,strlen(buf)+1,remote)
== -1 )</TT>
<BR><TT> {</TT>
<BR><TT>
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "send"),-1);</TT>
<BR><TT> }</TT>
<P><TT> /*</TT>
<BR><TT> Now
we've turned around and put ourselves into "server mode" by</TT>
<BR><TT> invoking
the recv() method. We now our server is going to send</TT>
<BR><TT> us
something, so we hang out here and wait for it. Because we</TT>
<BR><TT> know
datagrams are unreliable, there is a chance that the server</TT>
<BR><TT> will
respond but we won't hear. You might consider providing a</TT>
<BR><TT> timeout
on the recv() in that case. If recv() fails due to timeout</TT>
<BR><TT> it
will return -1 and you can then resend your query and attempt</TT>
<BR><TT> the
recv() again.</TT>
<BR><TT> */</TT>
<BR><TT> if( dgram.recv(buf,sizeof(buf),remote)
== -1 )</TT>
<BR><TT> {</TT>
<BR><TT>
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "recv"),-1);</TT>
<BR><TT> }</TT>
<P><TT> /*</TT>
<BR><TT> Find
out what the server had to say.</TT>
<BR><TT> */</TT>
<BR><TT> ACE_DEBUG ((LM_DEBUG,
"(%P|%t) The server said: %s\n",buf));</TT>
<P><TT> return(0);</TT>
<BR><TT>}</TT>
<P>
<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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