The first thing we want to look at is server.cpp. This is a pretty simple application that listens for datagrams at a known port and sends back a response. In order to implement a true "discovery" mechanism, the server will have to be a little bit more picky about who it responds to. We'll tackle that issue in the next tutorial though...
/*
Our datagram server will, of course, need to create
a datagram.
We'll also need an address object so that we know
where to listen.
*/
#include "ace/SOCK_Dgram.h"
#include "ace/INET_Addr.h"
/*
Use the typical TCP/IP port address for receiving
datagrams.
*/
static const u_short PORT = ACE_DEFAULT_SERVER_PORT;
int main(int,char**)
{
/*
This is where we'll listen
for datagrams coming from the
clients. We'll give
this address to the open() method
below to enable the listener.
*/
ACE_INET_Addr local(PORT);
/*
A simply constructed datagram
that we'll listen with.
*/
ACE_SOCK_Dgram dgram;
/*
Like most ACE objects, the
datagram has to be opened before
it can be uses. Of course,
-1 on failure.
A datagram will fail to open
if there is already a datagram
listening at the port we've
chosen. It *is* OK to open
a datagram at a port where
there is an ACE_SOCK_Stream
though. This is because
datagrams are UDP and SOCK_Stream
is TCP and the two don't cross
paths.
*/
if( dgram.open(local) == -1 )
{
ACE_ERROR_RETURN ((LM_ERROR,
"%p\n", "open"),-1);
}
/*
Create a simple buffer to
receive the data. You generally need
to provide a buffer big enough
for the largest datagram you
expect to receive. Some
platforms will let you read a little
and then some more later but
other platforms will throw out
whatever part of the datagram
you don't get with the first
read. (This is on a
per-datagram basis BTW.) The theoretical
limit on a datagram is about
64k. The realistic limit (because
of routers & such) is
much smaller. Choose your buffer size
based on your application's
needs.
*/
char buf[512];
/*
Unlike ACE_SOCK_Stream, datagrams
are unconnected. That is,
there is no "virtual circuit"
between server and client.
Because of this, the server
has to provide a placeholder
for the OS to fill in the
source (client) address information
on the recv. You can
initialize this INET_Addr to anything,
it will be overwritten when
the data arrives.
*/
ACE_INET_Addr remote;
ACE_DEBUG ((LM_DEBUG, "(%P|%t) starting up server daemon\n"));
/*
Receive datagrams as long
as we're able.
*/
while( dgram.recv(buf,sizeof(buf),remote) !=
-1 )
{
/*
Display
a brief message about our progress. Notice how we
use
the 'remote' object to display the address of the client.
With
an ACE_SOCK_Stream we used get_remote_addr() to get the
address
the socket is connected to. Because datagrams are
unconnected,
we use the addr object provided to recv().
*/
ACE_DEBUG ((LM_DEBUG,
"(%P|%t) Data (%s) from client (%s)\n", buf, remote.get_host_name()));
/*
To
respond to the client's query, we have to become a client
ourselves.
To do so, we need an anonymous local address from
which
we'll send the response and a datagram in which to send
it.
(An anonymous address is simply one where we let the OS
choose
a port for us. We really don't care what it is.O
*/
ACE_INET_Addr
local((u_short)0);
ACE_SOCK_Dgram client;
/*
Open
up our response datagram as always.
*/
if( client.open(local)
== -1 )
{
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "client open"),-1);
return(0);
}
/*
Build
a witty response...
*/
sprintf(buf,"I am here");
/*
and
send it to the client. Notice the symetry with the recv()
method.
Again, the unconnected nature of datagrams forces
us
to specify an address object with each read/write operation.
In
the case of read (recv()) that's where the OS stuffs the
address
of the datagram sender. In the case of write (send())
that
we're doing here, the address is where we want the network
to
deliver the data.
Of
course, we're assuming that the client will be listening
for
our reply...
*/
if( client.send(buf,strlen(buf)+1,remote)
== -1 )
{
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "send"),-1);
return(0);
}
}
return(0);
}
And that's really all there is to it. Obviously there is some room for improvement. The most blatant is the somewhat small buffer size for receiving the datagram. I've never been able to get a solid answer on datagram sizes. The theoretical limit is just under 64k but you have to deal with fragmentation. Some readings indicate that 8k is a reasonable size, others go much smaller. My general rule of thumb is to keep datagrams relatively small (eg -- under 8k or so) and test a lot. If you find that your routers are fragmenting your larger datagrams, back off to something smaller. Of course, if you must send 100k and can only do so 1k at a time, you'll have to worry about retransmissions & reordering. At that point, you might consider going to TCP. Remember: datagrams are unreliable! Don't try to make 'em do something they werent' designed for!