Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. The resource manager’s view
The resource manager’s view
Registering a pathname
Let’s look at things from the resource manager’s perspective. Basically, the resource
manager needs to tell the process manager that it’ll be responsible for a certain part of
the pathname space (it needs to register itself). Then, the resource manager needs to
receive messages from clients and handle them. Obviously, things aren’t quite that
simple.
Let’s take a quick overview look at the functions that the resource manager provides,
and then we’ll look at the details.
The resource manager needs to tell the process manager that one or more pathnames
are now under its domain of authority — effectively, that this particular resource
manager is prepared to handle client requests for those pathnames.
The serial port resource manager might handle (let’s say) four serial ports. In this case,
it would register four different pathnames with the process manager: /dev/ser1,
/dev/ser2, /dev/ser3, and /dev/ser4. The impact of this is that there are now
four distinct entries in the process manager’s pathname tree, one for each of the serial
ports. Four entries isn’t too bad. But what if the serial port resource manager handled
one of those fancy multiport cards, with 256 ports on it? Registering 256 individual
pathnames (i.e., /dev/ser1 through /dev/ser256) would result in 256 different
entries in the process manager’s pathname tree! The process manager isn’t optimized
for searching this tree; it assumes that there will be a few entries in the tree, not
hundreds.
As a rule, you shouldn’t discretely register more than a few dozen pathnames at each
level — this is because a linear search is performed. The 256 port registration is
certainly beyond that. In that case, what the multiport serial resource manager should
do is register a directory-style pathname, for example /dev/multiport. This
occupies only one entry in the process manager’s pathname tree. When a client opens
a serial port, let’s say port 57:
fp = fopen ("/dev/multiport/57", "w");
The process manager resolves this to the ND/PID/CHID/handle for the multiport serial
resource manager; it’s up to that resource manager to decide if the rest of the pathname
(in our case, the “57”) is valid. In this example, assuming that the variable path
contains the rest of the pathname past the mountpoint, this means that the resource
manager could do checking in a very simple manner:
devnum = atoi (path);
if ((devnum = 256)) {
// bad device number specified
} else {
// good device number specified
}
This search would certainly be faster than anything the process manager could do,
because the process manager must, by design, be much more general-purpose than our
resource manager.
April 30, 2009 Chapter 5 • Resource Managers 199