Getting Started with QNX Neutrino - QNX Software Systems

The client’s view
© 2009, QNX Software Systems GmbH & Co. KG.
The process manager receives the request and looks through its tree structure to see if
there’s a match (let’s assume for now that we need an exact match). Sure enough, the
pathname “/dev/ser1” matches the request, and the process manager is able to reply
to the client: “I found /dev/ser1. It’s being handled by node descriptor 0, process
ID 44, channel ID 1, handle 1. Send them your request!”
Remember, we’re still in the client’s open() code!
So, the open() function creates another message, and a connection to the specified
node descriptor (0, meaning our node), process ID (44), channel ID (1), stuffing the
handle into the message itself. This message is really the “connect” message — it’s
the message that the client’s open() library uses to establish a connection to a resource
manager (step 3 in the picture below). When the resource manager gets the connect
message, it looks at it and performs validation. For example, you may have tried to
open-for-write a resource manager that implements a read-only filesystem, in which
case you’d get back an error (in this case, EROFS). In our example, however, the serial
port resource manager looks at the request (we specified O_WRONLY; perfectly legal
for a serial port) and replies back with an EOK (step 4 in the picture below).
(3) Connect
Client
(4) Reply
Process
Manager
The _IO_CONNECT message.
Finding the process manager
Finally, the client’s open() returns to the client with a valid file descriptor.
Really, this file descriptor is the connection ID we just used to send a connect message
to the resource manager! Had the resource manager not givenusanEOK, we would
have passed this error back to the client (via errno and a -1 return from open()).
(It’s worthwhile to note that the process manager can return the node ID, process ID
and channel ID of more than one resource manager in response to a name resolution
request. In that case, the client will try each of them in turn until one succeeds, returns
an error that’s not ENOSYS, ENOENT, orEROFS, or the client exhausts the list, in
which case the open() fails. We’ll discuss this further when we look at the “before”
and “after” flags, later on.)
Now that we understand the basic steps used to find a particular resource manager, we
need to solve the mystery of, “How did we find the process manager to begin with?”
Actually, this one’s easy. By definition, the process manager has a node descriptor of 0
(meaning this node), a process ID of 1, and a channel ID of 1. So, the ND/PID/CHID
triplet 0/1/1 always identifies the process manager.
194 Chapter 5 • Resource Managers April 30, 2009