Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Message passing over a network
2 Node descriptors are not to be cached — if you get one, you’re supposed to use
it immediately and then forget about it.
3 There are library calls to convert a pathname (like /net/magenta) to a node
descriptor.
To work with node descriptors, you’ll want to include the file
because it includes a bunch of netmgr_*() functions.
You’d use the function netmgr_strtond() to convert a string into a node descriptor.
Once you have this node descriptor, you’d use it immediately in the ConnectAttach()
function call. Specifically, you shouldn’t ever cache it in a data structure! The reason
is that the native networking manager may decide to reuse it once all connections to
that particular node are disconnected. So, if you got a node descriptor of “7” for
/net/magenta, and you connected to it, sent a message, and then disconnected,
there’s a possibility that the native networking manager will return a node descriptor
of “7” again for a different node.
Since node descriptors aren’t unique per network, the question that arises is, “How do
you pass these things around the network?” Obviously, magenta’s view of what node
descriptor “7” is will be radically different from wintermute’s. There are two
solutions here:
• Don’t pass around node descriptors; use the symbolic names (e.g.,
/net/wintermute) instead.
• Use the netmgr_remote_nd() function.
The first is a good general-purpose solution. The second solution is reasonably simple
to use:
int
netmgr_remote_nd (int remote_nd, int local_nd);
This function takes two parameters: the remote_nd is the node descriptor of the target
machine, and local_nd is the node descriptor (from the local machine’s point of view)
to be translated to the remote machine’s point of view. The result is the node
descriptor that is valid from the remote machine’s point of view.
For example, let’s say wintermute is our local machine. We have a node descriptor
of “7” that is valid on our local machine and points to magenta. What we’d like to
find out is what node descriptor magenta uses to talk to us:
int
int
remote_nd;
magenta_nd;
magenta_nd = netmgr_strtond ("/net/magenta", NULL);
printf ("Magenta’s ND is %d\n", magenta_nd);
remote_nd = netmgr_remote_nd (magenta_nd, ND_LOCAL_NODE);
printf ("From magenta’s point of view, we’re ND %d\n",
remote_nd);
This might print something similar to:
April 30, 2009 Chapter 2 • Message Passing 129