Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Examples
Step 3
A simple io_read() example
Data area size considerations
io_func.io_read = my_io_read;
This will override the POSIX-layer default function that got put into the table by
iofunc_func_init() with a pointer to your function, my_io_read().
You probably don’t want your resource manager called /dev/whatever, so you
should select an appropriate name. Note that the resmgr_attach() function is where
you bind the attributes structure (the attr parameter) to the name — if you wish to
have multiple devices handled by your resource manager, you’d call resmgr_attach()
multiple times, with different attributes structures (so that you could tell the different
registered names apart at runtime).
To illustrate how your resource manager might return data to a client, consider a
simple resource manager that always returns the constant string "Hello,
world!\n". There are a number of issues involved, even in this very simple case:
• matching of client’s data area size to data being returned
• handling of EOF case
• maintenance of context information (the lseek() index)
• updating of POSIX stat() information
In our case, the resource manager is returning a fixed string of 14 bytes — there is
exactly that much data available. This is identical to a read-only file on a disk that
contains the string in question; the only real difference is that this “file” is maintained
in our C program via the statement:
char
*data_string = "Hello, world!\n";
Handling of EOF case
The client, on the other hand, can issue a read() request of any size — the client could
ask for one byte, 14 bytes, or more. The impact of this on the io_read() functionality
you’re going to provide is that you must be able to match the client’s requested data
size with what’s available.
A natural fallout of the way you handle the client’s data area size considerations is the
corner case of dealing with the End-Of-File (EOF) on the fixed string. Once the client
has read the final “\n” character, further attempts by the client to read more data
should return EOF.
April 30, 2009 Chapter 5 • Resource Managers 255