Getting Started with QNX Neutrino - QNX Software Systems

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© 2009, QNX Software Systems GmbH & Co. KG.
}
atoz_attrs [i].inode = i + 1;
atoz_attrs [i].nbytes = 1;
// add our functions; we’re interested only in
// io_open and io_read
connect_func.open = my_open;
io_func.read = my_read;
// establish a name in the pathname space
if (resmgr_attach (dpp, &resmgr_attr, "/dev/atoz",
_FTYPE_ANY, _RESMGR_FLAG_DIR,
&connect_func, &io_func,
&attr) == -1) {
perror ("Unable to resmgr_attach\n");
exit (EXIT_FAILURE);
}
// allocate a context
ctp = dispatch_context_alloc (dpp);
// wait here forever, handling messages
while (1) {
if ((ctp = dispatch_block (ctp)) == NULL) {
perror ("Unable to dispatch_block\n");
exit (EXIT_FAILURE);
}
dispatch_handler (ctp);
}
}
// you’ll never get here
return (EXIT_SUCCESS);
my_open()
While my_open() is very short, it has a number of crucial points. Notice how we
decide if the resource being opened is a “file” or a “directory” based only on the
pathname length. We can do this “trick” because we know that there are no other
directories in this resource manager apart from the main one. If you want to have
multiple directories below the mountpoint, you have to do more complicated analysis
of the path member of the msg structure. For our simple example, if there’s nothing in
the pathname, we know it’s the directory. Also, notice the extremely simplified
pathname validation checking: we simply compare to make sure that there’s only one
character passed to us, and that the character lies within the range “a” through “z”
inclusive. Again, for more complex resource managers, you’d be responsible for
parsing the name past the registered mountpoint.
Now, the most important feature! Notice how we used the POSIX-layer default
functions to do all the work for us! The iofunc_open_default() function is usually
installed in the connect functions table at the same spot that our new my_open()
function is now occupying. This means that it takes the identical set of arguments! All
we have to do is decide which attributes structure we want to have bound with the
OCB that the default function is going to create: either the directory one (in which
case we pass attr), or one of the 26 different ones for the 26 different files (in which
278 Chapter 5 • Resource Managers April 30, 2009