Getting Started with QNX Neutrino - QNX Software Systems

The client’s view
© 2009, QNX Software Systems GmbH & Co. KG.
Client summary
2 Response from the process manager: “Talk to fs-cache first, and then
fs-nfs.”
3 Message to fs-cache: “I’d like to open the file /nfs/home/rk/abc.txt for
write, please.”
4 Response from fs-cache: “Sure, no problem.”
Notice that this time, in step 3, we opened the file for write and not read as we did
previously. It’s not surprising, therefore, that fs-cache allowed the operation this
time (in step 4).
Even more interesting, observe what happens the next time we go to read the file:
1 Message to the process manager: “Who should I talk to about the filename
/nfs/home/rk/abc.txt?”
2 Response from the process manager: “Talk to fs-cache first, and then
fs-nfs.”
3 Message to fs-cache: “I’d like to open the file /nfs/home/rk/abc.txt for
read, please.”
4 Response from fs-cache: “Sure, no problem.”
Sure enough, the caching filesystem handled the request for the read this time (in step
4)!
Now, we’ve left out a few details, but these aren’t important to getting across the basic
ideas. Obviously, the caching filesystem will need some way of sending the data
across the network to the “real” storage medium. It should also have some way of
verifying that no one else modified the file just before it returns the file contents to the
client (so that the client doesn’t get stale data). The caching filesystem could handle
the first read request itself, by loading the data from the network filesystem on the first
read into its cache. And so on.
We’re done with the client side of things. The following are key points to remember:
• The client usually triggers communication with the resource manager via open()
(or fopen()).
• Once the client’s request has “resolved” to a particular resource manager, we never
change resource managers.
• All further messages for the client’s session are based on the file descriptor (or
FILE * stream), (e.g., read(), lseek(), fgets()).
• The session is terminated (or “dissociated”) when the client closes the file
descriptor or stream (or terminates for any reason).
• All client file-descriptor-based function calls are translated into messages.
198 Chapter 5 • Resource Managers April 30, 2009