Getting Started with QNX Neutrino - QNX Software Systems

Message passing over a network
© 2009, QNX Software Systems GmbH & Co. KG.
srcmsglen
(Qnet likes to transfer 8 KB).
indicates how much data the client wanted to transfer (determined by
the client).
Some notes on NDs
So, if the client wanted to transfer 1 megabyte of data over the network, the server’s
MsgReceive() would unblock and msglen would be set to 8192 (indicating that 8192
bytes were available in the buffer), while srcmsglen would be set to 1048576
(indicating that the client tried to send 1 megabyte).
The server then uses MsgRead() to get the rest of the data from the client’s address
space.
The other “funny” thing that we haven’t yet talked about when it comes to message
passing is this whole business of a “node descriptor” or just “ND” for short.
Recall that we used symbolic node names, like /net/wintermute in our examples.
Under QNX 4 (the previous version of the OS before Neutrino), native networking
was based on the concept of a node ID, a small integer that was unique on the network.
Thus, we’d talk about “node 61,” or “node 1,” and this was reflected in the function
calls.
Under Neutrino, all nodes are internally referred to by a 32-bit quantity, but it’s not
network unique! What I mean by that is that wintermute might think of spud as
node descriptor number “7,” while spud might think of magenta as node descriptor
number “7” as well. Let me expand that to give you a better picture. This table shows
some sample node descriptors that might be used by three nodes, wintermute, spud,
and foobar:
Node wintermute spud foobar
wintermute 0 7 4
spud 4 0 6
foobar 5 7 0
Notice how each node’s node descriptor for itself is zero. Also notice how
wintermute’s node descriptor for spud is “7,” as is foobar’s node descriptor for
spud. But wintermute’s node descriptor for foobar is “4” while spud’s node
descriptor for foobar is “6.” As I said, they’re not unique across the network,
although they are unique on each node. You can effectively think of them as file
descriptors — two processes might have the same file descriptor if they access the
same file, but they might not; it just depends on who opened which file when.
Fortunately, you don’t have to worry about node descriptors, for a number of reasons:
1 Most of the off-node message passing you’ll typically be doing will be through
higher-level function calls (such as open(), as shown in the example above).
128 Chapter 2 • Message Passing April 30, 2009