Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Writing a resource manager
The third parameter is different, however. It’s an ocb, which stands for “Open Context
Block.” It holds the context that was bound by the connect message handler (e.g., as a
result of the client’s open() call), and is available to the I/O functions.
As discussed above, when it comes time to fill in the two tables, we recommend that
you use the iofunc_func_init() function to load up the tables with the POSIX-layer
default handler routines. Then, if you need to override some of the functionality of
particular message handlers, you’d simply assign your own handler function instead of
the POSIX default routine. We’ll see this in the section “Putting in your own
functions.”
The resmgr_context_t internal context block
Finally, one data structure is used by the lowest layer of the library to keep track of
information that it needs to know about. You should view the contents of this data
structure as “read-only,” (except for the iov member).
Here’s the data structure (from ):
typedef struct _resmgr_context {
int
rcvid;
struct _msg_info info;
resmgr_iomsgs_t *msg;
dispatch_t
*dpp;
int
id;
unsigned
msg_max_size;
int
status;
int
offset;
int
size;
iov_t iov [1];
} resmgr_context_t;
As with the other data structure examples, I’ve taken the liberty of deleting reserved
fields.
Let’s look at the contents:
rcvid
info
msg
dpp
The receive ID from the resource manager library’s
MsgReceivev() function call. Indicates who you should reply to (if
you’re going to do the reply yourself).
Contains the information structure returned by MsgReceivev() in
the resource manager library’s receive loop. Useful for getting
information about the client, including things like the node
descriptor, process ID, thread ID, and so on. See the
documentation for MsgReceivev() for more details.
A pointer to a union of all possible message types. This isn’t very
useful to you, because each of your handler functions get passed
the appropriate union member as their second parameter.
A pointer to the dispatch structure that you passed in to begin
with. Again, not very useful to you, but obviously useful to the
resource manager library.
April 30, 2009 Chapter 5 • Resource Managers 209