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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%">
In <A HREF="broadcast_client.cpp">broadcast_client.cpp</A> we
find out how to send a single datagram to every host on our (sub)network.
I have to say <I>(sub)network</I> because broadcast datagrams typically
are not passed through routers. So, if your network admin has divided
up your network into subnets, your broadcasts will likey stay on the
subnet you're a part of.
<P>I've only commented the parts that are different from the directed_client.
<HR WIDTH="100%">
<PRE>
<font color=red>// $Id$</font>
<font color=blue>#include</font> "<font color=green>ace/SOCK_Dgram_Bcast.h</font>"
<font color=blue>#include</font> "<font color=green>ace/INET_Addr.h</font>"
static const u_short PORT = ACE_DEFAULT_SERVER_PORT;
int main(int argc,char *argv[] )
{
ACE_INET_Addr local((u_short)0);
<font color=red>/*
Instead of creating the ACE_SOCK_Dgram we created last time,
we'll create an ACE_SOCK_Dgram_Bcast. "<font color=green>Bcast</font>" means, of course,
"<font color=green>Broadcast</font>". This ACE object is clever enough to go out to the
OS and find all of the network interfaces. When you send()
on a Dgram_Bcast, it will send the datagram out on all of those
interfaces. This is quiet handy if you do it on a multi-homed
host that plays router...
*/</font>
ACE_SOCK_Dgram_Bcast dgram;
if( dgram.open(local) == -1 )
{
ACE_ERROR_RETURN ((LM_ERROR, "<font color=green>%p\n</font>", "<font color=green>datagram open</font>"),-1);
}
char buf[512];
sprintf(buf, "<font color=green>Hello World!</font>");
<font color=red>/*
The only other difference between us and the directed client
is that we don't specify a host to receive the datagram.
Instead, we use the magic value "<font color=green>INADDR_BROADCAST</font>". All hosts
are obliged to respond to datagrams directed to this address
the same as they would to datagrams sent to their hostname.
Remember, the Dgram_Bcast will send a datagram to all interfaces
on the host. That's true even if the address is for a specific
host (and the host address makes sense for the interface).
The real power is in using an INADDR_BROADCAST addressed datagram
against all interfaces.
*/</font>
ACE_INET_Addr remote(PORT,INADDR_BROADCAST);
ACE_DEBUG ((LM_DEBUG, "<font color=green>(%P|%t) Sending (%s) to the server.\n</font>",buf));
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);
}
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);
}
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> About that subnet thing:
<BLOCKQUOTE>If you run this client on a host that has multiple network
interfaces, the broadcast will go to all of those (sub)networks.
What do you do, though, if you need to get past a router? My advice
is to write a server that will run on hosts on both sides of your router.
When a server on one side of the router receives a broadcast, it would
send a directed datagram to it's counterpart on the other side of the router.
The counterpart would then re-broadcast the original datagram on that sub-net.
Cheap, simple and effective.</BLOCKQUOTE>
One final word of warning:
<BLOCKQUOTE>When creating your broadcast datagrams you may see something
like this: <I>ACE_SOCK_Dgram_Bcast::mk_broadcast: Broadcast is not
enable for this interface.: Unknown error</I>. There are some interfaces
(ppp, slip) that don't support broadcast datagrams. That's what you're
seeing here.</BLOCKQUOTE>
Ok, one more warning:
<blockquote>If you happen to have multiple servers running on your
network when you invoke this client, the resopnse could come from any
one of them.
</blockquote>
<P><HR WIDTH="100%">
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