Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Summary
Summary
Writing a resource manager is by far the most complicated task that we’ve discussed
in this book.
A resource manager is a server that receives certain, well-defined messages. These
messages fall into two broad categories:
Connect messages
Related to pathname-based operations, these may establish a
context for further work.
I/O messages
Always arrive after a connect message and indicate the actual work
that the client wishes to have done (e.g., stat()).
The operations of the resource manager are controlled by the thread pool functions
(discussed in the Processes and Threads chapter) and the dispatch interface functions.
QSS provides a set of POSIX helper functions in the resource manager library that
perform much of the work of dealing with the client’s Connect and I/O messages that
arrive.
There are a number of data structures relating to the clients and devices manifested by
the resource manager to keep in mind:
OCB
Attributes structure
Mount structure
Allocated on a per-open basis, this contains the context for the
client (e.g., current lseek() position)
Allocated on a per-device basis, this contains information about
the device (e.g., size of the device, permissions, etc.)
Allocated on a per-resource-manager basis, and contains
information about the characteristics of the entire resource
manager.
The clients communicate with the resource manager via message passing by resolving
the pathname (via the open() and other calls) into a node descriptor, process ID,
channel ID, and handle.
Finally you supply the functionality you wish to actually do in your resource manager
by overriding some of the callouts in the Connect and I/O functions table.
April 30, 2009 Chapter 5 • Resource Managers 283