<!-- page04.html,v 1.13 2000/11/27 17:56:43 othman Exp --> <HTML> <HEAD> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> <META NAME="GENERATOR" CONTENT="Mozilla/4.04 [en] (X11; I; Linux 2.0.32 i486) [Netscape]"> <META NAME="Author" CONTENT="James CE Johnson"> <TITLE>ACE Tutorial 008</TITLE> </HEAD> <BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#000FFF" VLINK="#FF0F0F"> <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>// page04.html,v 1.13 2000/11/27 17:56:43 othman Exp</font> <font color=blue>#include</font> "<A HREF="../../../ace/Log_Msg.h">ace/Log_Msg.h</A>" <font color=blue>#include</font> "<A HREF="../../../ace/SOCK_Dgram_Bcast.h">ace/SOCK_Dgram_Bcast.h</A>" <font color=blue>#include</font> "<A HREF="../../../ace/INET_Addr.h">ace/INET_Addr.h</A>" static const u_short PORT = ACE_DEFAULT_SERVER_PORT; int main (int argc,char *argv[]) { ACE_UNUSED_ARG(argc); ACE_UNUSED_ARG(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[BUFSIZ]; 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, <font color=#008888>ACE_OS::strlen</font> (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 response could come from any one of them. </blockquote> <P><HR WIDTH="100%"> <CENTER>[<A HREF="../online-tutorials.html">Tutorial Index</A>] [<A HREF="page05.html">Continue This Tutorial</A>]</CENTER>