Getting Started with QNX Neutrino - QNX Software Systems

Examples
© 2009, QNX Software Systems GmbH & Co. KG.
“What if I wanted to set the sample rate to some arbitrary number n, how would I do
that?” That will be answered in the next io_devctl() example below.
Step 3
This step is simply good defensive programming. We return an error code of ENOSYS
to tell the client that we didn’t understand their request.
Step 4
Finally, we clear out the return structure and set up a one-part IOV to point to it. Then
we return a value to the resource manager library encoded by the macro
_RESMGR_NPARTS() telling it that we’re returning a one part IOV. This is then
returned to the client. We could alternatively have used the _RESMGR_PTR() macro:
// instead of this
// 4) tell the client it worked
memset (&msg -> o, 0, sizeof (msg -> o));
SETIOV (ctp -> iov, &msg -> o, sizeof (msg -> o));
return (_RESMGR_NPARTS (1));
// we could have done this
// 4) tell the client it worked
memset (&msg -> o, 0, sizeof (msg -> o));
return (_RESMGR_PTR (ctp, &msg -> o, sizeof (msg -> o)));
The reason we cleared out the return structure here (and not in the io_read() or
io_write() examples) is because in this case, the return structure has actual contents!
(In the io_read() case, the only data returned was the data itself and the number of
bytes read — there was no “return data structure,” and in the io_write() case, the only
data returned was the number of bytes written.)
An io_devctl() example that deals with data
In the previous io_devctl() example, above, we raised the question of how to set
arbitrary sampling rates. Obviously, it’s not a good solution to create a large number
of DCMD_AUDIO_SET_SAMPLE_RATE_* constants — we’d rapidly use up the
available bits in the dcmd member.
From the client side, we’ll use the dev_data_ptr pointer to point to the sample rate,
which we’ll simply pass as an integer. Therefore, the nbytes member will simply be
the number of bytes in an integer (4 on a 32-bit machine). We’ll assume that the
constant DCMD_AUDIO_SET_SAMPLE_RATE is defined for this purpose.
Also, we’d like to be able to read the current sampling rate. We’ll also use the
dev_data_ptr and nbytes as described above, but in the reverse direction — the
resource manager will return data into the memory location pointed to by
dev_data_ptr (for nbytes) instead of getting data from that memory location. Let’s
assume that the constant DCMD_AUDIO_GET_SAMPLE_RATE is defined for this
purpose.
Let’s see what happens in the resource manager’s io_devctl(), as shown here (we
won’t discuss things that have already been discussed in the previous example):
266 Chapter 5 • Resource Managers April 30, 2009