Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Using message passing
On the server side, (let’s say it’s the filesystem, fs-qnx4), we have a number of 4 KB
cache blocks, and we’d like to efficiently receive the message directly into the cache
blocks. Ideally, we’d like to write some code like this:
// set up the IOV structure to receive into:
SETIOV (iov + 0, &header, sizeof (header.io_write));
SETIOV (iov + 1, &cache_buffer [37], 4096);
SETIOV (iov + 2, &cache_buffer [16], 4096);
SETIOV (iov + 3, &cache_buffer [22], 4096);
rcvid = MsgReceivev (chid, iov, 4, NULL);
This code does pretty much what you’d expect: it sets up a 4-part IOV structure, sets
the first part of the structure to point to the header, and the next three parts to point to
cache blocks 37, 16, and 22. (These numbers represent cache blocks that just
happened to be available at that particular time.) Here’s a graphical representation:
header (12)
header (12)
cache_buffer [37]
data chunk
(12000)
header and
data chunk
cache_buffer [16]
cache_buffer [22]
Client’s address space
Kernel copying data
Driver’s address space
Converting contiguous data to separate buffers.
Then the MsgReceivev() function is called, indicating that we’ll receive a message
from the specified channel (the chid parameter) and that we’re supplying a 4-part IOV
structure. This also shows the IOV structure itself.
(Apart from its IOV functionality, MsgReceivev() operates just like MsgReceive().)
Oops! We made the same mistake as we did before, when we introduced the
MsgReceive() function.
How do we know what kind of message we’re receiving, and how much data is
associated with it, until we actually receive the message?
We can solve this the same way as before:
rcvid = MsgReceive (chid, &header, sizeof (header), NULL);
switch (header.message_type) {
...
case _IO_WRITE:
number_of_bytes = header.io_write.nbytes;
April 30, 2009 Chapter 2 • Message Passing 111