Getting Started with QNX Neutrino - QNX Software Systems

The resource manager library
© 2009, QNX Software Systems GmbH & Co. KG.
dispatch_handler (ctp);
}
}
There you have it! A complete /dev/null resource manager implemented in a few
function calls!
If you were to write this from scratch, and have it support all the functionality that this
one does (e.g., stat() works, chown() and chmod() work, and so on), you’d be looking
at many hundreds if not thousands of lines of C code.
The library really does what we just talked about
By way of introduction to the library, let’s see (briefly) what the calls do in the
/dev/null resource manager.
dispatch_create()
iofunc_attr_init()
iofunc_func_init()
Creates a dispatch structure; this will be used for blocking on
the message reception.
Initializes the attributes structure used by the device. We’ll
discuss attributes structures in more depth later, but for now,
the short story is that there’s one of these per device name, and
they contain information about a particular device.
Initializes the two data structures cfuncs and ifuncs, which
contain pointers to the connect and I/O functions, respectively.
You might argue that this call has the most “magic” in it, as this
is where the actual “worker” routines for handling all the
messages got bound into a data structure. We didn’t actually
see any code to handle the connect message, or the I/O
messages resulting from a client read() or stat() function etc.
That’s because the library is supplying default POSIX versions
of those functions for us, and it’s the iofunc_func_init()
function that binds those same default handler functions into
the two supplied tables.
resmgr_attach()
Creates the channel that the resource manager will use for
receiving messages, and talks to the process manager to tell it
that we’re going to be responsible for “/dev/null.” While
there are a lot of parameters, we’ll see them all in painful detail
later. For now, it’s important to note that this is where the
dispatch handle (dpp), pathname (the string /dev/null), and
the connect (cfuncs) and I/O (ifuncs) message handlers all get
bound together.
dispatch_context_alloc()
Allocates a dispatch internal context block. It contains
information relevant to the message being processed.
dispatch_block()
This is the dispatch layer’s blocking call; it’s where we wait for
a message to arrive from a client.
202 Chapter 5 • Resource Managers April 30, 2009