Getting Started with QNX Neutrino - QNX Software Systems

The client’s view
© 2009, QNX Software Systems GmbH & Co. KG.
Obviously, as hinted at in my choice of parameters for the open() call above, it may be
a clever idea to allow some “traditional” drivers to be opened with additional
parameters past the “usual” name. However, the rule of thumb here is, “If you can get
away with it in a design review meeting, knock yourself out.” Some of my students,
upon hearing me say that, pipe up with “But I am the design review committee!” To
which I usually reply, “You are given a gun. Shoot yourself in the foot. :-)”
Union’d filesystems
Take a closer look at the diagram we’ve been using:
Resource managers
Pathname space
(stored by the Process Manager)
Resource managers
(0,1,1,1) procnto / fsys-qnx4 (0,4965,1,1)
(0,1,1,1) procnto
proc dev
procnto (0,1,1,1)
(0,1,1,1)
procnto
boot ser1 devc-ser8250 (0,44,1,1)
(0,1,1,1)
procnto
1 ser2 devc-ser8250 (0,44,1,2)
(0,1,1,1)
procnto
4965 con devc-con (0,725,1,1)
...
...
Neutrino’s namespace.
Notice how both fs-qnx4 and the process manager have registered themselves as
being responsible for “/”? This is fine, and nothing to worry about. In fact, there are
times when it’s a very good idea. Let’s consider one such case.
Suppose you have a very slow network connection and you’ve mounted a networked
filesystem over it. You notice that you often use certain files and wish that they were
somehow magically “cached” on your system, but alas, the designers of the network
filesystem didn’t provide a way for you to do that. So, you write yourself a caching
filesystem (called fs-cache) that sits on top of the network filesystem. Here’s how it
looks from the client’s point of view:
Resource managers
Pathname space
(stored by the Process Manager)
/
Resource managers
(0,326,1,1) fs-cache
nfs
fs-nfs (0,77625,1,1)
Overlaid filesystems.
196 Chapter 5 • Resource Managers April 30, 2009