Getting Started with QNX Neutrino - QNX Software Systems

Porting philosophy
© 2009, QNX Software Systems GmbH & Co. KG.
}
return (sts);
This is an example of using io_msg() (which we’ll define shortly — it’s not a standard
QSS supplied library call!). The io_msg() function does the magic of assembling the
_IO_MSG message. To get around the problems that we discussed about devctl()
having only one “size” parameter, we’ve given io_msg() two size parameters, one for
the input (to the resource manager, sizeof (msg.i)) and one for the output (from
the resource manager, sizeof (msg.o)). Notice how we update the values of *odr
and *ms only if the io_msg() function returns an EOK.
This is a common trick, and is useful in this case because the passed arguments don’t
get modified unless the actual command succeeded. (This prevents the client program
from having to maintain copies of its passed data, just in case the function fails.)
One last thing that I’ve done in the adjust_xyz() function, is that I depend on the
global_fd variable containing the file descriptor of the resource manager. Again, there
are a number of ways that you could handle it:
• Bury the file descriptor within the io_msg() function (this would be useful if you
wanted to avoid having to pass around the file descriptor on each and every call;
useful if you’re ever going to talk to only the one resource manager, and thus most
likely not suitable as a general purpose solution).
Or:
• Pass the file descriptor from the client itself to each function in the API library
(useful if the client’s going to be responsible for talking to the resource manager in
other ways, such as the standard POSIX file descriptor calls like read(), orifthe
client may be talking to multiple resource managers).
Here’s the source for io_msg():
int
io_msg (int fd, int cmd, void *msg, int isize, int osize)
{
io_msg_t io_message;
iov_t rx_iov [2];
iov_t tx_iov [2];
int
sts;
// set up the transmit IOV
SETIOV (tx_iov + 0, &io_msg.o, sizeof (io_msg.o));
SETIOV (tx_iov + 1, msg, osize);
// set up the receive IOV
SETIOV (rx_iov + 0, &io_msg.i, sizeof (io_msg.i));
SETIOV (rx_iov + 1, msg, isize);
// set up the _IO_MSG itself
memset (&io_message, 0, sizeof (io_message));
io_message.type = _IO_MSG;
io_message.mgrid = cmd;
}
return (MsgSendv (fd, tx_iov, 2, rx_iov, 2));
298 Appendix: A • QNX 4 to Neutrino April 30, 2009